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PUBLICLY AVAILABLE

SPECIFICATION

IEC PAS 61975

Pre-Standard First edition2004-08

System tests for high-voltagedirect current (HVDC) installations

Reference number

IEC/PAS 61975:2004(E)

C o p y r i gh t e d m a t er i al l i c en s e d t oE l e c t r i c i t y of V i e t n am b y T h om s on S c i en t i f i c ,I n c

. ( www. t e c h s t r e e t . c om ) .T h i s c o p y d ownl o

a d e d on2 0 1 4 - 0 1 - 0 6 0 8 : 2 8 : 2 8 - 0 6 0 0 b y a u t h or i z e d u s er P h am V an c h i .

N of ur t h er r e pr o d u c t i on or d i s t r i b u t i oni s p er mi t t e d .

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Publication numbering

As from 1 Janua ry 1997 al l IEC publ icati ons are issued wi th a des ignat ion in the60000 series. For example, IEC 34-1 i s now referred to as IEC 60034-1.

Consolidated editions

The IEC is now publishing consolidated versions of its publications. For example,edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, thebase publication incorporating amendment 1 and the base publication incorporatingamendments 1 and 2.

Further information on IEC publications

The technical content of IEC publications is kept under constant review by the IEC,thus ensuring that the content reflects current technology. Information relating tothis publication, including its validity, is available in the IEC Catalogue ofpublications (see below) in addition to new editions, amendments and corrigenda.Information on the subjects under consideration and work in progress undertakenby the technical committee which has prepared this publication, as well as the list

of publications issued, is also available from the following:

• IEC Web Site (www.iec.ch)

• Catalogue of IEC publications

The on-line catalogue on the IEC web site (www.iec.ch/searchpub) enables you tosearch by a variety of criteria including text searches, technical committeesand date of publication. On-line information is also available on recently issuedpublications, withdrawn and replaced publications, as well as corrigenda.

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• Customer Service Centre

If you have any questions regarding this publication or need further assistance,please contact the Customer Service Centre:

Email: [email protected] Tel: +41 22 919 02 11Fax: +41 22 919 03 00





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7/25/2019 iecpas61975{ed1.0}en.pdf


PUBLICLY AVAILABLESPECIFICATION

IEC PAS 61975

Pre-Standard First edition2004-08

System tests for high-voltagedirect current (HVDC) installations

PRICE CODE

IEC 2004 Copyright - all rights reserved

No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the publisher.

International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland

Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: [email protected] Web: www.iec.ch

XGFor price, see current catalogue

Commission E lectrotechnique InternationaleInternational Electrotechnical Com mission





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– 2 – PAS 61975 © IEC:2004 (E)

CONTENTS

Part 0: Executive Summary......................................................................... 4

Part 1: General........................................................................................... 11

1.1 Statement of Purpose ...................................................................... 131.2 Structure of the HVDC System......................................................... 151.3 Structure of the Control and Protection System................................ 161.4 Logical Steps of Commissioning ...................................................... 171.5 Structure of System Testing............................................................. 19

Part 2: Off-Site Tests ................................................................................. 20

2.1 Steady State Performance of the Controls ....................................... 232.1.1 Measurements................................................................................. 242.1.2 Control and Protective Sequences................................................... 242.1.3 Steady Stale Performance Tests...................................................... 252.2 Dynamic Performance Tests............................................................ 272.2.1 Controls-Step Responses ................................................................ 292.2.2 Control Mode Transfer..................................................................... 312.2.3 AC System Interaction/Control......................................................... 332.2.4 Commutation Failures and Valve Misfires ........................................ 34

2.2.5 AC Filter, Transformer and Reactive Element Switching.................. 362.2.6 AC and DC System Faults ............................................................... 372.2.7 Islanding ......................................................................................... 412.3 Functional Performance Tests ......................................................... 432.4 Type Tests on the Control and Protection Equipments .................... 46

Part 3: Converter Tests ............................................................................. 50

3.1 Converter Unit Tests ....................................................................... 523.2 Converter Station Tests................................................................... 54

3.2.1 HV Energization or AC Filters, Capacitor Banks and Shunt Reactors .. 543.2.2 Open Line Test of the DC Switchyard.............................................. 553.2.3 Load Tests..................................................................................... 573.3 Open Line Tests on the DC Transmission Circuit...... ......... ......... ..... 61

Part 4: End-to-End-Tests........................................................................... 63

4.1 Changing the DC System Configuration, Off Voltage ....................... 674.2 Start and Stop Sequences and Steady State Operation at Minimum Power...................................................................................................... 69

4.3 Protective Blocking and Tripping Sequences ................................... 734.4 Power and Current Ramping ........................................................... 764.5 Reduced Voltage Operation............................................................. 78





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PAS 61975 © IEC:2004 (E) – 3 –

Part 5: Steady-State Performance and Interference Tests....................... 81

5.1 Harmonic Performance and Filter Components Rating ..................... 835.2 Audible Noise.................................................................................. 865.3 Overload/Temperature Rise ............................................................ 88

5.4 Interference .................................................................................... 905.5 Earth Electrode................................................................................ 92

Part 6: Operation and Integration Tests .................................................... 9

6.1 Changes of DC Configuration.......................................................... 986.1.1 Tests from Monopolar Metallic Return Operation............................ 1006.1.2 Tests from Monopolar Earth Return Mode...................................... 1016.1.3 Tests from Bipolar Operation ......................................................... 1016.2 Control Performance...................................................................... 103

6.2.1 Step Response .............................................................................. 1056.2.2 Control Mode Transfer................................................................... 1106.2.3 AC System Interaction / Control..................................................... 1136.2.4 Commutation Failure ..................................................................... 1166.3 Switching AC Side Fillers and Transformers................................... 1196.4 Loading Tests................................................................................ 1216.5 AC and DC System Staged Fault Tests.......................................... 1246.6 Loss of Telecommunications, Auxiliaries, or Redundant Equipment 1296.6.1 Loss of Telecommunications between Terminals ........................... 1296.6.2 Loss of Auxiliary Power Supplies ................................................... 1326.6.3 Loss of Redundant Equipment....................................................... 134

Part 7: Trial Operation............................................................................. 135

Part 8: System Test Plan and Documentation....................................... 138

8.1 Plant Documentation and Operating Manuals ................................ 1398.2 System Study Reports and Technical Specifications ...................... 1398 3 Inspection and Test Plan ............................................................... 1398.4 System Test Program.................................................................... 1418.5 Test Procedures for each Test....................................................... 1428.6 Documentation of System Test Results.......................................... 1428.7 Deviation Reports .......................................................................... 143





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INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________

SYSTEM TESTS FOR HIGH-VOLTAGE DIRECT CURRENT (HVDC)INSTALLATIONS

FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprisingall national electrotechnical committees (IEC National Committees). The object of IEC is to promoteinternational co-operation on all questions concerning standardization in the electrical and electronic fields. Tothis end and in addition to other activities, IEC publishes International Standards, Technical Specifications,Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IECPublication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interestedin the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closelywith the International Organization for Standardization (ISO) in accordance with conditions determined by

agreement between the two organizations.2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international

consensus of opinion on the relevant subjects since each technical committee has representation from allinterested IEC National Committees.

3) IEC Publications have the form of recommendations for international use and are accepted by IEC NationalCommittees in that sense. While all reasonable efforts are made to ensure that the technical content of IECPublications is accurate, IEC cannot be held responsible for the way in which they are used or for anymisinterpretation by any end user.

4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publicationstransparently to the maximum extent possible in their national and regional publications. Any divergencebetween any IEC Publication and the corresponding national or regional publication shall be clearly indicated inthe latter.

5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for anyequipment declared to be in conformity with an IE C Publication.

6) All users should ensure that they have the latest edition of this publication.

7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts andmembers of its technical committees and IEC National Committees for any personal injury, property damage orother damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) andexpenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IECPublications.

8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications isindispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject ofpatent rights. IEC shall not be held responsible for i dentifying any or all such patent rights.

A PAS is a technical specification not fulfi ll ing the requirements for a standard but madeavailable to the public.

IEC-PAS 61975 was submitted by the CIGRÉ (International Council on Large ElectricSystems) and has been processed by subcommittee 22F: Power electronics for electricaltransmission and distribution systems, of IEC technical committee 22: Power electronic systemsand equipment.

The text of this PAS i s based on thefollowing document:

This PAS was approved forpublication by the P-members of thecommittee concerned as indicated in

the following document

Draft PAS Report on voting

22F/96/NP 22F/101/RVN

Following publication of this PAS, which is a pre-standard publication, the technical committeeor subcommittee concerned will transform it into an International Standard.





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PAS 61975 © IEC:2004 (E) – 5 –

An IEC-PAS licence of copyright and assignment of copyright has been signed by the IEC andCIGRÉ and is recorded at the Central Office.

This PAS shall remain valid for an initial maximum period of three years starting from 2004-08. The validity may be extended for a single three-year period, following which it shall be

revised to become another type of normative document or shall be withdrawn.





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PART 0: EXECUTIVE SUMMARY

Abstract

This document which gives guidance on all aspects of system tests for HVDC

installations (excluding multiterminal HVDC systems), has been prepared by

CIGRE WG 14.12. It is structured in eight parts.

The guide should give potential users guidance, regarding which course of

action should be taken in planning commissioning activities.

Structure of the tests and a brief statement of the purpose of the individual group

of tests is presented.

Introduction

Commissioning an HVDC system is a very complex task which may affect more

than the actual contract parties. The complexity and the diversified areas of

concern during system testing require thorough planning and scheduling,

cooperation of all parties involved, and complete and structured documentation.

System testing completes commissioning of an HVDC system.

It allows the supplier to verify the suitability of the station equipment installed

and the functional completeness of the system; adjustments and optimization

can be made.

The user is shown that the requirements and stipulations in the contract are met

and that there is correlation with studies and previous off-site testing.





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PAS 61975 © IEC:2004 (E) – 7 –

In adapting the HVDC system to the "real world" (the connected AC systems)

various constraints may exist, which require coordination within the economic

schedules of the AC system operators.

System testing proves to the public that tolerable values of phenomena

concerning public interest are not exceeded.

Five (5) major aspects are subject to system testing:

- HVDC station equipment and DC line/cable/bus incl. earth electrode, if any

- HVDC controls and protection

- Environmental considerations

- AC/DC system interaction

- System performance

The following diagram shows the interrelation between these aspects:





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Acceptance tests shall be defined between supplier and user in advance and

may be performed at an appropriate time during the test schedule.

The testing sequence is best scheduled starting at local level with simple tests

before Involving additional locations and the transmission system and more

complex tests,

A system test plan has proven itself as a good means for planning and

scheduling.

Complete and organized documentation of the system tests is to the benefit ofboth the supplier and the user, it shall form part of the project documentation

and contain al! necessary oscillograms, logs, etc, and if necessary a

commentary and references.

Structure of System Testing

System testing should follow the structure of the HVDC system, starting from the

smallest, least complex operational unit and shall end with the total system in

operation.

The first step, to ensure proper function, is to debug and to test the control

system during off-site tests. Because of the complex nature of the HVDC

system, this requires a simulator. Where applicable it is recommended to run

commissioning tests and acceptance tests during the off-site tests in a similar

way to those performed later at site. In such a way off-site tests can serve as

reference for the site tests.





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PAS 61975 © IEC:2004 (E) – 9 –

System Tests for HVDC Installations

Before system commissioning can begin at site, preconditions concerning

subsystem tests, operator training and safety Instructions, system test plan and

test procedures, and all necessary test equipment must be fulfilled.

After all preconditions are fulfil led, each converter unit is commissioned

separately during the converter unit test. Open-circuit tests and/or short-circuit

tests are possible for this purpose. Converter station tests also include

energization of the AC filter, DC yard energization and back-to-back tests.

Back-to-back tests allow full active power with the nominal DC voltage, firing

angles, harmonics, etc. whilst still disconnected from the second AC system.Certain control, relaying and instrumentation changes as welt as temporary DC

switchyard changes may be required for back-to-back tests.

Before end-:o-end tests are performed, it is advisable to perform an open line

test and shorted line tests with the DC transmission line. This test can be

repeated from both ends to verify the integrity of the DC line.





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End-to-end tests involve both stations and the transmission Tine. With this

operation, power is transmitted for the first time. This test usually start on a

monopolar basis, with full bipolar operation being the final step.

Having the complete system running properly, steady state verification tests can

be performed. With normal operating ramp settings and automatic switching

sequences in place the effect of a number of disturbances on the DC side of the

system as well as in the AC systems may be checked.

Operation and Integration tests verify the transient and fault recovery behaviour

of the HVDC system-Correct operation of the HVDC system over an extendedperiod of time is checked during the trial operation.

The HVDC system tests are now completed, all functions have been verified and

the HVDC system is ready to be handed over to the owners. The acceptance

tests necessary to verify whether acceptance criteria have been met may have

been performed all or in part during the commissioning period.





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PAS 61975 © IEC:2004 (E) – 11 –

PART 1: GENERAL

Introduction

This document deals with all aspects of system tests of HVDC systems. System

tests start when all relevant subsystems have been precommissioned and are

ready for operation. They end with full acceptance of the system for operation in

the power systems.

This document provides background information for IEC to produce standards

for system testing. It is structured in eight parts:

1. General

2. Off-site Tests

3. Converter Tests

Commissioning of converter units, verification of steady state performance

of units, switching tests

- converter unit tests

- converter station tests.

4. End-to-End Tests

Commissioning of the transmission system, verification of station

coordination.

5. Steady-State Performance and Interference Tests

Verification of steady-state performance and interference caused by the

HVDC-system.





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6. Operation and Integration Tests

Operational and fault tests, verification of dynamic performance and

interaction between the DC and AC systems.

7. Trial operation

8. System test plan and documentation

The guide also covers interrelation with off-site system tests. Preconditions of

system tests wit] be established.

Part 1 General will address the purpose of this document, the HVDC systemstructure, the control and protection structure, the logical steps of commissioning

and the structure of system testing of HVDC system. Parts 2 to 7 comprise

individual paragraphs on general test objectives, information on test procedures,

as well as detailed descriptions of the individual tests, including as appropriate

the following;

- Specific objectives per test

- Test procedures

- Test acceptance criteria

- Preconditions for the test

- References to system studies/specifications

- References to off-site tests

- Special conditions

Part 8 describes the documentation normally required to adequately perform the

system tests. This primarily consists of the following:

- Plant documentation

- Inspection and test plan (ITP)

- System study reports/technical specifications

- System test program

- Test procedures for each test

- Documentation of system test results





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PAS 61975 © IEC:2004 (E) – 13 –

- Deviation report

The guide should give potential users guidance, regarding which course of

action should be taken in planning commissioning activities. The tests described

in the guide may not be applicable to all projects, but represent a range of

possible tests which should be considered.

1.1 Statement of Purpose

System testing completes the commissioning of an HVDC system.

It allows the supplier to verify the suitability of the station equipment installed

and the functional completeness of the system; adjustments and optimizationcan be made.

The user is shown that the requirements and stipulations in the contract are met

and that there is correlation with studies and previous off-site testing.

For the user, the completion of system testing marks the beginning of

commercial operation of the HVDC system.

In adapting the HVDC system to the "real world" (the connected AC systems)

various constraints may exist, which require coordination within the economic

schedules of the AC system operators.

System testing proves to the public that tolerable values of phenomena

concerning public interest are not exceeded.

Five (5) major aspects are subject to system testing:

- HVDC station equipment and DC line/cable/bus incl. earth electrode, if any

- HVDC controls and protection

- Environmental considerations

- AC/DC system interaction –






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The following diagram shows the interrelation between these aspects:

Thorough and complete system testing of the above components can be

achieved with the tests described in the eight parts of the guide.

Acceptance tests shall be defined between supplier and user in advance and

may be performed at an appropriate time during the test schedule.

System testing may affect more than the actual contract parties. Those parties

shall be informed in time.

The complexity and the diversified areas of concern during system testing

require thorough planning and scheduling, cooperation of all parties involved, as

well as complete and organized documentation.

The testing sequence is best scheduled starting at local level with simple tests

before involving additional locations and the transmission system and more

complex tests. A system test plan (probably as part of a site test plan) has

proven itself as a good means for planning and scheduling.





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PAS 61975 © IEC:2004 (E) – 15 –

Complete and organized documentation of the system tests is to the benefit of

both the supplier and the user, it shall form part of the project documentation

and contain all necessary oscillograms, logs, etc., and if necessary a

commentary and references.

1.2 Structure of the HVDC System

From a functional point of view an HVDC system consists of a sending terminal

and a receiving terminal, each connected to an AC-system. The two terminals

have one or several converters connected in series on the DC side and in

parallel on the AC side. The terminals are connected by a transmission line or

cable or a short piece of busbar (Back-to-Back station). Multiterminal systems

are not addressed in this document





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1.3 Structure of the Control and Protection System

Each of the converter units can be controlled individually. To make the system

function as a transmission system the converter units should be controlled in acoordinated way by a second level of the control system. Coordinated controls

and protection are essential for the proper function of HVDC systems.





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PAS 61975 © IEC:2004 (E) – 17 –

1.4 Logical Steps of Commissioning

System commissioning should follow the structure of the HVDC system, starting

from the smallest, least complex operational unit, usually a 12-pulse converter,and shall end with the total system in operation.

The first step, to ensure proper function, is to debug and test the control system

during factory system tests. Because of the complex nature of the HVDC

system, this requires a simulator. Where applicable, it is recommended to run

commissioning tests and acceptance tests in addition to ail limiting design cases

on the simulator in a similar way to those done later at site. In such a way

simulator tests can serve as reference for the site tests.

Before system commissioning can begin at site the following preconditions

should be fulfilled.

- All subsystems tested and commissioned including AC filters and the

converter transformers with special attention to possible transformer/AC filter

resonance during energizing

- Sufficient training of operating personnel- Operating instructions for the station available to the operators

- Personnel, plant safety and security instructions made

- System test plan and documentation (part 8) ready and agreed upon

- AC/DC power profiles agreed for each test

- Any AC/DC system operating restrictions identified

- Operator voice communications available

- All necessary test equipment calibrated and operational

- Procedures for the preparation and evaluation of test results agreed upon

After all preconditions are fulfil led, each converter unit is commissioned

separately. Open-circuit tests and/or short-circuit tests are possible for this

purpose. Steady-state performance and interference tests may start at this

instant; however, they can be performed one by one at any convenient other

place dying the system test program, in order to minimize duplication of tests.





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If possible, as a next step the converters in each station should be connected

back-to-back. This allows full active power with the nominal DC voltage, firing

angles, harmonics, etc. whilst still disconnected from the second AC system.

Certain control, relaying and instrumentation changes as well as temporary DC

switchyard changes may be required for back-to-back tests.

Before full end-to-end tests are performed, it is advisable to perform an open line

test and shorted line tests with the DC transmission line. This test can be

repeated from both ends to verify the integrity of the DC line.

End-to-end tests will then be conducted. These tests involve both stations and

the transmission line. With this operation power is transmitted for the first time.

This test is usually done on a monopolar basis first, with full bipolar operating

being the final step.

Having the complete system running properly in steady state operation, with

normal operating ramp settings and automatic switching sequences in place the

effect of a number of disturbances on the DC side of the system as well as in the

AC systems may be checked. Operation and integration tests verify the transient

and fault recovery behaviour of the HVDC system.

The HVDC system tests are now completed, all functions have been verified and

the HVDC system is ready to be handed over to the owners. The acceptance

tests necessary to verify whether acceptance criteria have been met may have

been performed all or In part during the commissioning period- To avoid

unnecessary duplication of such tests, careful consideration should be given in

advance as to when acceptance tests are carried out.

If acceptance tests are still outstanding or acceptance tests have to be repeated

due to modifications they should be performed at this time, or following trial

operation, as appropriate.

Finally it has to be ensured that the system also operates correctly over an

extended period of time. This is checked during the trial operation.





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PAS 61975 © IEC:2004 (E) – 19 –

1.5 Structure of System Testing





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PART 2: OFF-SITE TESTS

General

Introduction

This part describes the testing of the control equipment prior to it being shipped

to site. The following tests are outlined:

2.1 Steady state performance of the controls

2.2 Dynamic performance tests

2.3 Functional performance tests

2.4 Type tests on the control and protection equipments

Subsequent to the Routine Testing of the HVDC-System Control and Protection

equipment it is normal practice to check the steady state, the functional and the

dynamic performance of this equipment prior to it being shipped to site. Thesetests provide the opportunity to set up the parameters of the control circuits

(though these set tings may have to be fine tuned later at site) and to obtain a

preliminary check on the performance of the equipment relative to the specified

requirements.

Performance of the protective functions of the converter, during various

simulated faults, can also be checked. This enables the equipment to be partly

commissioned off-site. It also provides the opportunity to detect and correct

hardware and software errors or deficiencies in the control and protection

systems.





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PAS 61975 © IEC:2004 (E) – 21 –

To carry out the dynamic performance tests it is necessary to have a real time

HVDC simulator which includes representation of the AC systems, AC filters,

converter equipment, DC smoothing reactors, DC lines and DC filters. The

extent of the system representations should be sufficient to replicate resonances

as determined from previous studies. In general, a comprehensive simulation will

enable thorough fault tracing to be done and allow effective commissioning at

site.

As defined in Part 1 of this guide the control equipment is arranged in a

hierarchical structure and only the representative sections which will affect the

dynamic performance would be used for the dynamic tests. These controls would

be the closed loop control sections of the HVDC System Controls (Master

Control), Station Control, Pole Controls and Converter Control. The

telecommunication system and the valve base electronics would be appropriate.

The Valve Base Electronics would be appropriately simulated. Conventional

protection equipment would be omitted i.e. that for AC filters, DC filters and

converter transformers but the protection for the DC system would be included.

The control equipment as defined above needs not necessarily be that supplied

for the contract.

The real time simulator may also be used for the functional performance tests

but other forms of simulation e.g. by software models, are possible. For the

functional performance tests the complete control system shall be tested. Fault

recorders and Sequence of Event Recorders which are "stand alone

equipments" may not be included for the Functional Performance Tests. If these

recorders are not used the validity of output signals to these equipments wouldbe checked during the tests.

Finding and correcting hardware and software errors in the control system is an

important function of off-site testing. Such faults are easier to find and correct

off-site rather than during commissioning. Correcting such faults reduces the

probability of disturbing the Customers' power system during site commissioning.





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The principles for off-site testing of the control system are as follows:

a) Controls, as defined above, should be present and connected in an identical

manner to the final site configuration. Possible exceptions are simple interfaceequipment, Fault Recorders and Sequence of Events Recorders.

b) It is desirable that the test team acts independently from the equipment

design team to verify correct operation of the control system. The test team

shall include representatives from the commissioning group, the design

group and the test group. It is recommended that the customer takes a

significant role in this testing team in order to provide valuable experience

in training customers staff.

c) During the test period the equipment being tested shall be under the control

of the test team leader and no changes should be made without his

approval. Changes to the equipment should be recorded in conformity with

the defined Quality Assurance (QA) procedures, thus ensuring that the

tests are carried out on a known state of hardware and software.

d) An off-site test plan including AC system representations, DC configurations

and tests to be performed shall be mutually agreed between the manufacturer

and the customer in advance of the commencement of the tests.

Type testing, if required, can be carried out to demonstrate performance over the

specified environment, with variations of power supply voltage and simulation of

faults on auxiliary systems, to demonstrate the stability of the control and the

accuracy of protection settings. In addition it may be possible to demonstrate

interference immunity of the control and protection equipment, together with the

communication interface.

General Test Objectives

1. To check the steady state, the functional and the dynamic performance

of the control equipment and, if required, to carry out some of the type

tests for the control and protection equipment,

2. To make preliminary settings of the control parameters.





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PAS 61975 © IEC:2004 (E) – 23 –

3. To provide confirmation of the design specifications and study results for

the control and protection equipment and also to provide test data for

comparison with that obtained during the operation and integration tests

defined in PART 6 of the guide.

4. To find and correct errors and deficiencies in the control and protection

hardware and software.

Preconditions for the Tests

1. For the functional performance tests the control and protection

equipment shall have passed all its routine tests.

2. For the functional performance tests the control and protection cubicles

shall be interconnected In the "as site" configuration.

3. An off-site test plan including AC system representations, DC

configurations and tests to be performed shall be mutually agreed

between the manufacturer and the customer in advance of the

commencement of the tests.

4. The studies defining the control strategies shall have been completed.

2.1 Steady State Performance of the Controls

Introduction

Before the functional and the dynamic performance test can be performed, the

steady state performance of the control equipment shall have been

demonstrated.





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2.1.1 Measurements

General

The following measurements should only be regarded as typical since they will

vary with different control system designs.

Test Objectives

To confirm that the correct measurements are transmitted to the appropriate

points within the control equipment.

Test Procedure

All measurements shall be checked for appropriate level, polarity, phasing and

sequence at both source and destination.

- Line side voltage/test supply for valve firing;

- Valve winding currents;

- DC current and voltage;

- di/dt if applicable;

- Alpha and Gamma responses;

- Active and reactive power;

- Frequency.

2.1.2 Control and Protective Sequences

General

The sequences described in the Test Procedure can be checked with the

converter system off-load.





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Test Objectives

To ensure that the operational sequences of the valve control and protection are

correct.

Test Procedure

Check that the correct sequence of events, with appropriate timings take place

during deblocking/blocking of the converter system.

Check that the valve firing pulse sequences sent to the firing controls are correct

Check that forced retard, valve refire and blocking signals are generated in the

correct locations and are transmitted to the correct locations.

Formation of bypass pairs should be checked if applicable.

2.1.3 Steady State Performance Tests Test Objectives

To ensure that the basic control functions meet the designed performance. The

following tests should be regarded as typical since they may vary with different

designs of control equipment.

Test Procedure

Verify the DC voltage and current static characteristics.





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With the converter system deblocked check:

- Alpha and gamma order calibration;

- With the voltage and frequency of the AC systems varied over their normal

ranges of operation confirm that the current order can be varied over the full

range, DC voltage and current can be confirmed at different levels of currents

order, and that any prescribed limits are maintained;

- that the power order can be varied over the full range, the derived current

order may be confirmed at different levels of power order;

- block the rectifier and confirm the current error calibration and the correct re-

sponse of the gamma control loop.

Reduced DC Voltage Operation

Manual or automatic reduction of DC voltage may be required to reduce stresses

on DC cables when the power transfer level is being reduced, or to reduce the

possibility of flashover of overhead DC lines during extreme weather conditions

or conditions of excessive contamination.

Manual reduction will simply be cone by a selector switch operating the tap

changer control. With automatic operation check that the reduction occurs under

the designed conditions.

Protective shut-down

With minimum current order setting and the converter system deblocked apply a

protective blocking signal at the rectifier. Repeat with maximum continuous

current order setting. Confirm that correct blocking sequence occurs followed bycircuit breaker tripping if required. Repeat for protective blocking at the inverter

and with reversed power flow if appropriate.

Test Acceptance Criteria

For all the above tests the performance should conform to the system studies

and the design parameters.





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2.2 Dynamic Performance Tests

Introduction

For the dynamic performance tests only the closed loop controls of the HVDC

System Control (Master Control), Station Control, Pole Controls and Converter

Controls would be used. The telecommunication system shall be adequately

simulated. Protection for the DC system would also be included. Tests would be

performed using a real time HVDC Simulator. The control equipment hardware

used for the representation need not necessarily be that supplied for the

Contractor, but should be functionally identical. If the idea is to check software

and hardware problems, they should be supplied by the Contractor's equipment.

Test Objectives

1. To check that measurements for the controls are of the correct

magnitude and phasing.

2. To check that the sequences for deblocking/blocking, the firing sequencefor the valves, and the signals for forced retard, refire bypassing and

blocking are correct.

3. To check the stability and response of the controls during transient

disturbances.

4. To make preliminary settings of the control parameters.

5. To check the correct operation of the protective functions for various

types of faults in the DC system and the associated AC systems.

6. To find and correct any hardware deficiencies and software errors.

7. To check the interaction between the AC and DC systems under all

relevant operating conditions.

8. Crosscheck against digital studies for consistency.





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Preconditions for the Tests

In addition to the general preconditions the following must be fulfilled.

1. The preliminary control parameters as defined by the controls design

study have been installed.

2. The set points, thresholds and time delays of the protective relays as

defined by the protection co-ordination study have been checked by

injection tests.

3. The steady state performance defined in section 2.1 shall have been

demonstrated.

Test Procedures

The procedure for each of the following tests will be described separately in

each section.

2.2.1 Control - Step Response

2.2.2 Control Mode Transfer

2.2.3 AC System Interaction/Control

2.2.4 Commutation Failures and Valve Misfires

2.2.5 AC Filter, Transformer and Reactive Element Switching

2.2.6 AC and DC System Faults

2.2.7 Islanding

All tests for optimization and verification of the controls and protection dynamic

performance require a similar recording and monitoring set-ups as listed below,

in cases where additional or different set-ups are required these are listed in the

individual sections.

The results of these tests will be used as references for the end-to-end tests and

the operation and integration tests at site and it is desirable that similar

recording equipment and test report formats are used for both test sequences.





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Where practical the following should be monitored:

- Current or power order

- DC current

- DC voltage

- Alpha order

- Alpha response

- Gamma response

- Control mode identification

- AC busbar voltages and frequency

- Valve winding AC currents- DC power

- Reactive power

- Forced retard, blocking and initiation commands

- Valve firing sequence

- Tap position indications (if available)

2.2.1 Controls-Step Responses General

The corresponding site tests are described in 6.2.1 of the operation and

integration tests.

Test Objectives

To confirm that the control equipment operates in a stable manner during

changes of current order, power order and converter control angle.

Test Procedure

The AC systems should be set up with the strongest rectifier system together

with the weakest inverter system applicable, since these typically represent the

most onerous operating conditions. However, other cases should be considered.





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With current order setting of 0,1 pu, deblock and block the converter system.

Confirm that the speed of response to achieve the ordered value conforms to the

specification and that no significant overshoot occurs. The procedure should be

repeated for both power flow directions, if appropriate.

If a prescribed ramping rate for current order is required apply a step change of

0,5 pu or 1,0 pu current order to confirm that the correct rates are achieved.

Current order step response

With the converter system in DC current control and current order settings of

0,15 pu, 0,5 pu or 1,0 pu apply a 0,05 pu step reduction in current order followedby a 0,05 pu step up. Sufficient time should be allowed between changes to

allow stable operation to be achieved.

Inverter extinction angle step response

With the rectifier in its normal mode of control and the inverter in minimum

constant extinction angle control, apply a step change in gamma to increase the

reference. An equal step reduction in gamma to the original value should beapplied.

Inverter current control step responses

With the rectifier in constant firing angle control (alpha minimum) and the

inverter in constant current control mode apply a 0,05 pu step increase in current

order, followed by a corresponding reduction after a suitable time period.

Power order step responses

With the converter system in power control and the order set at 0,15 pu, 0,5 pu

or 1,0 pu apply a step down in order of 0,05 pu followed by a corresponding

increase after a suitable period.

NOTE Other control loops, for example, constant DC voltage, may be tested similarly to

the above, the general principle being that step changes shall be small enough that

converter firing angles do not reach limits.





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All controller settings should be adjusted such that the response and recovery

times as specified or as defined by system studies are achieved.

No instability should be apparent in the step response tests and the responses

shall be well damped without significant overshoot.


General

The most common transfers between control modes are:

- at the rectifier, from constant power control mode to constant current order

mode and back;

- at the inverter, from constant extinction angle control mode to constant

current control mode and back;

- at the rectifier, from normal alpha control mode to minimum alpha mode.

Additional control modes may be used:

- at the inverter, constant DC voltage control may be used for weak AC

system applications;

- for both rectifier and inverter, control of AC system voltage may be used

during temporary load rejection conditions.


integration tests.





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Test Objectives

Control mode transfer tests are performed to verify that the change from one

control mode to another can be achieved without adverse interaction.

Test Procedure

The transfer from constant power control to constant current control and back

can be done manually. The change from inverter constant extinction angle

control to constant current control may be activated automatically by reducing

the rectifier AC system voltage thus forcing the rectifier to minimum alphaoperation. To demonstrate return to the inverter constant extinction angle control

mode the rectifier AC system voltage should be increased. Similarly a change

from inverter constant DC voltage control to constant current control can be

activated automatically.

To activate the AC system voltage control either the "remote" converter should

be temporarily blocked, or a three phase solid short-circuit should be applied to

its AC busbars.


Control mode transfers should occur without inducing adverse interaction.

Step changes in power shall not occur during transfer from constant power

control to constant current control.

Transfer from constant extinction angle control or constant DC voltage control to

constant current control at the inverter must be stable.

Activation of the AC system voltage control should occur during temporary DC

system shutdown.





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2.2.3 AC System Interaction/Control

General

The flexibility of the controls of HVDC systems enables them to be used to

enhance the performance of the associated AC systems. Examples of such

features are:

- control of AC system frequency;

- modulation of transmitted power to assist in fault recovery;

- limitation of overvoltage during load rejection on the DC system;

- limitation of reactive power demand from the AC systems during load changes;

- AC system voltage control.


integration tests.

Test Objectives

To check the performance of specific control functions which may berequirements of the Contract Specification.

Test Procedure

Power/Current Control Modulation

For AC system damping acquire an appropriate level of signal and frequency

and inject this into the modulation control loop and confirm that the response

varies in compliance with system studies. Inject an appropriate level of signalinto the frequency control loop and confirm that the response varies in

compliance with the specified requirements. Alternatively a frequency change

may be induced by tripping a generator.

Limitations of AC System Overvoltage

With the maximum rating of reactive power elements connected, and at the

maximum power transfer level block the inverter for a defined duration and check

that the dynamic voltage is limited to the design level. Repeat by blocking the

rectifier under the same operating conditions.





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Control of Reactive Power Exchange with the AC Systems

Confirm that the reactive power elements are switched in or out, at the

prescribed power transmission levels, as the power order is increased to its

maximum value and then reduced to its minimum value. Check that the control

angles at the rectifier and inverter remain within the defined bands during the

power changes.


For all the above tests the performance should conform with the system studies

and the design parameters.

No adverse interactions should occur

2.2.4 Commutation Failures and Valve Misfires

General

Because of the interaction of the DC system with both of the AC systems towhich it is connected, various forms of valve faults should be applied to

demonstrate either the adequacy of the recovery performance of the DC system

after fault clearance, or that the correct protective actions leading to shutdown of

part or all of the DC system are correctly performed.

The corresponding site tests are described in 6.2.4 of the operations and inte-

gration tests.

Test Objectives

To simulate commutation failure of a converter valve, the recovery procedure

and performance.

To simulate misfiring of a converter valve and the operation of any specialized

protection if this is applied.





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Test Procedure

Some extra monitoring may be applied:

- timing signals allowing synchronization between recordings;- one valve voltage on each converter group;

- crucial protection signals.

Commutation Failure

Valve commutation failure (which is most likely to occur at an inverter) can be

simulated by blocking the start pulse at one valve. The duration of blocking the

start pulse should be such that a single commutation failure occurs, then for a

duration sufficient to activate valve overload protection (i.e. voltage dependent

current limit), and finally for a duration sufficient to activate the persistent

commutation failure protection.

Valve Misfire

Valve misfire at a rectifier can be simulated by blocking the start pulse to one

valve. The duration should be sufficient to ensure that specialized protection,

e.g. asymmetry protection or excessive harmonic protection, will operate.


The acceptance criteria are:

- for valve commutation failures the corrective action by the control or

protective systems should be initiated and be successful over the entire

range of power transfer. Operation of the line fault detection should not

occur.

- for valve misfire the correct protective shutdown should occur.





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2.2.5 AC Filter, Transformer and Reactive Element Switching

General

The corresponding site tests are described in 6.3 of the operation and integration

tests.

Test Objectives

To confirm that switching of such elements will not have any adverse effect on

the operation of the DC system, that AC system voltage disturbances are within

the prescribed limits and that if resonances occur they are adequately damped.

Proper recovery from commutation failure, if any, should be verified.

Test Procedure

The short circuit ratios of the AC systems should be at the specified minimum

values or for resonant investigations at an appropriate value.

Additional recordings may be made of transformer primary current, and AC filter,

shunt capacitor and shunt reactor currents during the appropriate parts of thetest sequence.

If the system is bipolar then the transformer switching tests can be carried out in

monopolar configuration using one of the transformers from the second pole. For

a monopolar scheme a representation of a relevant system transformer of

suitable rating should be used.

!f the scheme has two or more converter groups per pole then switching of onetransformer with the remaining groups in service is possible provided the

scheme filters are designed for this mode of operation.

Switching on and off each type of filter should be done in turn with the converter

equipment energized.

If capacitor banks or shunt reactors form part of the overall scheme then

switching of these elements singly or in appropriate combinations should bedone with the converters energized.





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The switching tests should be repeated for both rectifier and inverter terminals

and with reversed power flow, if appropriate.


In all switching tests the HVDC system should continue in stable operation. The

AC system voltage disturbances should be within the specified limits.

During a switching operation commutation failure may occur but should be

limited to only one event

2.2.6 AC and DC System Faults

General

Because of the interaction of the DC system with both of the AC systems to

which it is connected, various forms of AC and DC faults should be applied to

demonstrate either the adequacy of the recovery performance of the DC system

after fault clearance, or that the correct protective actions leading to shutdown of

part of all of the DC system are correctly performed.

Local AC system faults, which give 100 % voltage reduction in one phase, or all

3 phases, and remote AC system faults which give, say, 30 % voltage reduction

in one phase, or all 3 phases, should be applied, each being applied for the

specified fault duration.

Due to current inrush conditions during recovery period the voltages may be

severely distorted, but this should not affect the correct performance of the

control and protection systems.

During some AC system faults commutation failure of the converter system is

inevitable, but this condition must be of limited duration and must not adversely

affect the recovery performance of the complete system.





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For DC line faults the normal clearance procedure is to suppress the DC voltage

by means of converter control action, with a preset time to allow deionization at

the fault and then reapply the DC voltage.

If the fault is not cleared, a preset number of such sequences can be repeated

including (if applicable) a restart at reduced voltage before permanent shutdown

is applied. For DC cable faults it is assumed that such faults are permanent and

shutdown of the affected cable is immediate.

The corresponding site tests are described in 6-5 of the operation and integra-

tion tests.

Test Objectives

To apply single-phase and three-phase faults to either AC system, in locations

close to or distant from the converter terminals, in order to demonstrate the

performance of the converter system during the faults and the recovery

performance subsequent to the faults.

To apply DC line (or cable) faults at different locations to demonstrate theprotective actions taken by the converter controls are correct.

To apply simulated faults at different points within the DC terminals and

demonstrate correct protective action and shutdown.

To verify that the correct protection co-ordination was achieved.

To check that the pole loss compensation system (if applicable) operates

correctly.

Test Procedure

Some extra monitoring may be applied:

- timing signals allowing synchronization between recordings;

- one valve voltage on each converter group;

- crucial protection signals.





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AC System Faults

The sequence of faults should be as follows.

- 1-phase busbar fault at the rectifier, repeated at 0,1 pu and 1,0 pu load;

- remote 1-phase fault at the rectifier, repeated at 0,1 pu and 1,0 pu load;

- local 3-phase fault at the rectifier, repeated at 0,1 pu and 1,0 pu load;

- distant 3-phase fault at the rectifier, repeated at 0,1 pu and 1,0 pu load;

- repeat of the above 1- and 3-phase faults applied at the inverter AC system;

- repeat of the above 1- and 3-phase faults at specified locations within either

of the AC systems if there are locations of special interest;

- repeat all of the above tests if the system has reversed power flow;

- repeat some of the tests with a different fault duration and with the specified

power overload.

DC System Faults

The sequence of faults should be as follows.

- temporary fault application to the DC line at a point adjacent to the rectifier

terminal, at the mid-point of the line and at a point adjacent to the inverter

terminal. For cable connections permanent faults should be applied at these

three locations;

- apply permanent faults to demonstrate bushing flashovers of say DC reactor,

HVDC wall bushing or transformer valve winding bushing of the star and

delta windings;

- apply a short circuit between phases on the valve side of the converter trans-

former,

- verify that back-up protection operates correctly if the primary protection isdisabled.





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AC System Faults


- the DC system shall remain stable during and after clearing of the fault;

- the time for DC power recovery shall within the specified limits;

- no spurious operation of protection relays should occur;

- transient AC and DC voltages should be contained within the specified

levels;

- permanent faults should result in correct shutdown of the remote station

without the use of telecommunication signals;

- inverter AC system faults should not cause inadvertent operation of the DC

line protection.

DC System Faults


- For DC line faults the correct fault clearance sequence should occur and

operation of protection on the converter side of the DC line reactors or the

line protection in the unfaulted pole should not occur.

- For DC cable faults safe protective shutdown should occur.

- For bushing flashovers within the converter terminals the correct protective

shutdown should occur at both stations with and without telecommunication.

- For transformer valve winding faults the correct protective shutdown should

occur.

- The pole loss compensation scheme operates correctly.





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2.2.7 Islanding

General

The term Islanding can cover several operating conditions associated with

HVDC systems. Two examples of islanding are:

- isolation of the inverter terminal of the HVDC system due to switching out of

a single AC transmission line which connects the HVDC system to the main

AC system;

- loss of the main AC system leaving one or more rotating machines

connected to either terminal of the HVDC system.

In the first example power transmission would continue in the HVDC until the

problem was identified and corrected and would result in severe overvoltage on

the AC filters, other components used for reactive compensation, the converter

transformers and valves at the inverter terminal, if no protective action was

taken.

In the second example, loss of the main system would result in the remaining

machines being unsynchronized. Depending on whether the main system had

been supplying power to or absorbing power from the HVDC system the

remaining machines could quickly reduce or increase speed unless the HVDC

system had suitable controls to prevent such changes in frequency.

Test Objectives

To demonstrate the effective action of the protection or control action of the

HVDC system when some form of Islanding occurs.





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Test Procedure

In the first example given above, the HVDC system should be run at minimum

power transfer with the AC system at minimum short circuit capacity and the ACcircuit breaker at the remote end of the single AC transmission line, which is

connected to the inverter terminal, should be opened.

The action of the HVDC system will depend on whether or not auto-reclose of

the AC circuit breaker is applicable. If auto-reclose of the breaker is applicable,

the protection of the HVDC system should detect that the breaker has opened

and initiate action in the HVDC system controls to avoid any excessive

overvoltage. The HVDC system should then be held in readiness for power

restoration following reclosure of the breaker, if auto-reclose is not applicable

then the HVDC system protection should initiate action in the HVDC system

controls to avoid excessive overvoltage and then safely shut down the HVDC

system. The test should be repeated at other levels of power transfer.

In the second example the HVDC system should be run at a power level

consistent with the rating of the connected AC machines, at the appropriate

converter terminal, plus low power being supplied by the connected main AC

system. The breaker connecting the main AC system should be opened. The

HVDC system controls should operate to regulate the frequency of the machines

to the prescribed value. The test should be repeated with higher levels of power

being supplied by the main AC system and then with power being absorbed by

the main AC system.


Depending upon the type of islanding applicable to the HVDC system, protection

and/or control action shall be taken by the HVDC system to ensure, if it is

appropriate to the application, that overcurrent or overvoltage damage, or

significant frequency deviation of the connected AC machines do not occur.





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2.3 Functional Performance Test Introduction

As discussed in the general introduction for the functional performance test the

complete control system including telecommunication interface will be testedagainst some form of simulation of the HVDC system including the DC

switchgear and the AC switchgear. The appropriate telecommunication time

delays shall be simulated.

General

The control system hardware and software shall be the actual and completedeliverables.

The simulation can be by a real time simulator, software modules or a

combination of both.

Test Objectives

1. To check that all individual control cubicles function properly.

2. To check that all control cubicles interact properly.

3. To check that all interfaces between the control cubicles and all other

equipment are correct.

4. To check that all transfers to redundant controls are smooth and do not

affect the operation of the on-line equipment.

5. To verify that power supplies with redundant elements do not affect

controls or protection operation in case one element is shut down.

6. To verify that failed elements can be removed and replaced without

affecting the operation of the on-line equipment.





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7. Where redundancy is applied it shall be verified that:

- single contingency failures do not cause a shutdown;

- failure of one element of a redundant system initiates changeover to

the standby element;

- failure of all redundant elements should result in safe shutdown.

Preconditions

1, The simulator must represent the defined HVDC system, the DCswitchgear and the AC switchgear and be operational.

2. The control equipment to be tested must have passed the factory routine

test and must be available for operation at the test site.

3. The test arrangement shall be defined and agreed upon between the

user and the manufacturer.

4, A test plan shall be mutually agreed upon between the user and the

manufacturer before commencement of tests.

5. Metering and recording equipment shall be available and connected to

the test.

6, The deliverables for the control system including cubicle interconnections

shall be installed and connected to the simulator.

7. The steady state performance defined in 2.1 shall have been demonstrated.

Test Procedure

Each cubicle and the operators interface shall first be commissioned separately

and then be connected with each other.





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The function listed below should be tested for normal operating conditions. It will

also be necessary to test some of these functions in contingency operation

conditions. Normal operation applies to undisturbed operation of the HVDC

System. Contingency operation exists if a single failure occurs fn the HVDC-

System but the System can continue operation within specified limits or a

disturbance occurs in either AC-System which requires a specified action in the

HVDC System.

In order to create contingency operating conditions malfunctions shall be

deliberately simulated at the simulator. Typical functions are:

- change of HVDC system configuration;- change status of operation;

- change energy transfer level and energy direction;

- change preselections;

- steady state converter control;

- switching of reactive power compensation elements;

- loss of redundancy and power supplies for the Controls and Protection;

- apply various invalid inputs to the control system and check that invalid

signals are alarmed to the operator;

- runback functions;

- power modulation;

- loss of end-to-end telecommunication;

- change of control location.


- all functions of the control system must work properly. Initiated signals must

follow the designated signal path, generate the appropriate command and

create the checkback signal to indicate the execution of the correct action on

the operators interface.





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- loss of a redundant element for control or protection should not affect the DC

system operation.

- loss of a redundant element of the power supplies should not affect the DC

system operation;- for loss of both elements of a power supply system with redundancy the DC

system should shut down safely;

- normal transitions between redundant elements for control or protection

should not disturb the AC systems. Non-allowable combinations should

either be impossible or result in safe shutdown of the DC system;

- failure of a single input measurement, where measurements are redundant,

should not result in disturbance to the AC or DC systems;

- failure of an unduplicated measurement, or both duplicated measurements,

should result in safe shutdown of the DC system.

2.4 Type Tests on the Control and Protection Equipments

Introduction

Some of the type tests, if specifically required for the contract, may be carried

out on the control and protection equipment, together with communication

interfaces. These tests would demonstrate the equipment performance over the

specified environmental conditions together with specified variation of the

auxiliary supply voltage. In addition the immunity of each equipment to

electromagnetic and electrostatic disturbances may be demonstrated.

Since these tests are carried out with the control and protection equipment in an

operational state it may be convenient to carry out these type tests white the

equipment is connected to the HVDC simulator, though alternative methods

would be equally acceptable.





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Test Objective

To demonstrate that the control performance is not affected by operation at

various environmental conditions and within the prescribed operating range ofthe power supply.

To demonstrate that the protection settings and timings are not affected by

operation at temperatures and within the prescribed operating range of the

power supply.

To demonstrate that the specified levels of electromagnetic and electrostatic

disturbances do not cause misoperation of control and protection equipments-

To demonstrate that hand held communication equipment does not cause

misoperation of control and protection equipment.

Test Procedure

The following procedure covers type testing with respect to Control Cubicles:

- temperature;

- power supply variations;

- electromagnetic and electrostatic disturbance;

- hand held communication equipment.

Enclose the cubicle to be tested within a thermally insulated housing with a heat

source.

Operate the HVDC system at 1,0 pu load.

With the cubicle temperature initially at room ambient:

- Check that the DC voltage and current measurements are stable during all

control modes;

- Check the accuracy of all the control loops by comparing their respective

settings with their responses;

- Check the transient response by applying step changes in current order.- Vary the voltage (and frequency if appropriate) of the power supply to the





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control equipment, to the specified maximum and minimum levels and at

each voltage level recheck the steady state and transient performance and

compare results;

- Raise the temperature within the housing to the highest specified ambienttemperature, preferably for several hours;

- Recheck the steady state accuracy and transient performance and compare

results;

- Vary the voltage (and frequency if appropriate) of the power supply to the

control equipment to the specified maximum and minimum levels and at each

voltage level recheck the steady state accuracy and transient performance

and compare results;- Reduce the temperature within the housing to nominal ambient;

- Recheck the steady state and transient performance and compare results.

With the DC system operating at 1,0 pu load, subject the control equipment to

the appropriate electromagnetic and electrostatic interference, and demonstrate

that there is no disturbance to the operation of the DC system.

Protection Cubicles

Enclose the protection cubicle to be tested within a thermally insulated housing

with a heat source.

With the cubicle temperature at room ambient:

- Check in turn each protection operation for setting and timing;


protection cubicle to the specified maximum and minimum levels and at eachvoltage level recheck in turn each protection operation for setting and

timing;.

- Raise the temperature within the housing to the highest specified ambient

temperature, preferably for several hours.

- Recheck in turn each protection operation for setting and timing.


protection cubicle to the specified maximum and minimum levels and at each

voltage level recheck in turn each protection operation for setting and timing;





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- Compare the results with the previous study;

- Lower the temperature to normal ambient and recheck in turn each

protection operation for setting and timing.

With the DC system operating at 1,0 pu load, subject the protection equipment to

the appropriate electromagnetic and electrostatic interference and demonstrate

that misoperation of the protection does not occur.


The steady state accuracy and transient performance results for all the tests

shall be within the design tolerances.

The setting and timing results for each protection shall be within the design

tolerances.





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PART 3: CONVERTER TESTS

General

Introduction

This part describes the testing of each converter station as a unit and the

verification of the HVDC transmission line prior to transmitting power. This group

of tests precedes the end-to-end testing. The following tests are outlined:

3.1 Converter unit tests

• HV energization of converter transformer

3.2 Converter station tests

• HV energization of AC-filters and shunt banks

• Open line test (HV energization) of the DC switchyard

• Load tests: Back-to-Back testing

Short circuit testing (optional)

3.3 Open line test of the DC transmission circuit





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During the test program, conformance with environmental specifications should

be included where applicable. Preliminary observations of audible noise, radio

and PLC interference levels may be done. This is described in Part 5 of the

guide. Temperature rise of major equipment can be monitored. However, the

actual measurement of the above mentioned quantities are conducted during

end-to-end operation.

Test of Back-to-Back stations is covered in part 4 of this guide whereas back-to-

back testing referred to in section 3.2 is accomplished by a temporary

connection between the converters of a bipolar long distance HVDC station.

Statement of Purpose

The purpose of the Converter Tests is to verify the correct operation of an

individual converter station and the proper insulation of all main circuit

equipment before starting the end-to-end testing.


The converter tests may be divided into high voltage energization and load tests.The test objective of the HV energization is to verify that proper voltage

insulation is achieved in the AC and DC main circuit equipment.

Load tests (back-to-back or short circuit tests) may be conducted to get a

provisional verification of the control system, the valve cooling capability and the

main circuit with respect to temperature rise, audible noise and radio

interference. Final verification will be made during end-to-end testing.

General Preconditions

Before beginning the Converter tests, the following equipment must be verified

off voltage and be available:





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- AC switchgear;

- AC filters, capacitor banks and shunt reactors;

- DC filters and switchgear;

- Converter transformers;

- Thyristor valves and cooling system;

- Station auxiliary service;

- Fire protection system;

- AC and DC protection systems;

- Control system;

- DC line or cable (for open line test);- Sequency of event ;

- Recorder and Alarm System;

- Transient Fault Recorders.

In order to verify the converter firing control, the valve base electronics and the

valve main circuit connections, prior to HV energization, it is recommended to

conduct a low voltage energization test The test may be performed by applying

0.5 - 10 kV to the primary side or to the valve side of the converter transformers

with all but a few thyristors short-circuited in each valve position. An appropriate

resistor may serve as a load on the DC-side.

Prior to the converter tests detailed procedures and plans should be prepared.

As the tests may involve some disturbance or increased risk to the connected

AC systems, the operators of the systems should be consulted.

3.1 Converter Unit Tests Introduction

This test is the HV energization of the converter transformer with blocked valves

and is performed on each individual converter unit in a HVDC station.

Test Objectives

The test objective is to verify that insulation voltage withstand is achieved and to

check that the electrical phasing is correct.

Preconditions





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Before conducting the HV energization the following preconditions shall be

fulfilled.

- All controls and protections associated with the high voltage equipment shall

be verified and in service. A trip test shall be made shortly before highvoltage energization.

- Monitoring instrumentation shall be connected and ready.

- All clamp joints shall have been tightened and the insulators wiped clean.

- The HVDC transmission line disconnect switch shall be opened and locked.

- All safety procedures shall have been carried out.

- A final visual inspection of the high voltage equipment shall be performed

and the arrester counter numbers shall be recorded.

- The low voltage side of the valves shall be grounded.

Test Procedures

During the test, AC voltage, steady-state and inrush-currents may be recorded.

Inspect the equipment during the test for abnormal sounds and corona

discharge. The following test sequence is recommended:

- Energize the converter transformer with the valves blocked and check the

electrical phasing through the control system.

- The converter transformer tap changers should initially be at highest position

(lowest valve side voltage) and then stepped to rated voltage.

- Keep the transformer energized for a minimum of 24 hours.


No abnormal sounds or corona discharge shall occur in the energized

equipment. No protections shall operate improperly.





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3.2 Converter Station Tests Introduction

This group of tests includes HV energization of AC fillers, capacitor banks and

shunt reactors, open line test of the DC switchyard and load tests of theconverter station and back-to-back or short circuit test. If there are multiple

groups in each pole, they should be initially energized individually and then

together.

3.2.1 HV Energization of AC filters, Capacitor Banks and Shunt Reactors

Introduction

This section deals with the first energization of AC filters, capacitor banks andshunt reactors individually for the first time. Combined switching of these

elements is dealt with in 6.3.

Test Objectives

The test objective of the HV energization is to verify that proper voltage

insulation is achieved and that the AC filters, capacitor banks and shunt reactors

are balanced between the three phases and each phase for itself. Confirmationof the onload currents and voltages of the protections may be conducted at this

time.

Preconditions

Before conducting the HV energization the following preconditions shall be

fulfilled.

- All controls and protections (main and backup) associated with the highvoltage equipment shall be verified and in service. A trip test shall be made

shortly before high voltage energization.

- AC filter tuning shall be completed.

- AC filters and shunt capacitors shall have been balanced.


- All safety procedures shall have been carried out





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- A final visual inspection of the high voltage equipment shall be performed,

and the arrester counter numbers shall be recorded.

Test Procedure

During the test, AC voltage, steady-state and inrush-currents may be recorded.

Inspect the equipment during the test for abnormal sounds, corona discharge or

partial discharge arcing. The following test sequence is recommended.

- Energize the AC filters, capacitor banks and shunt reactors one by one.

- Keep each AC filter, capacitor bank and shunt reactor energized for a

minimum of 2 hours.


No abnormal sounds or corona discharge arcing shall occur in the energized

equipment. No protections shall operate improperly. The AC filters and capacitor

banks shall be balanced within design tolerances.

3.2.2 Open Line Test of the DC Switchyard Test Objectives

The test objective of the open line test of the DC switchyard is to verify that

proper insulation voltage withstand is achieved. This open line test will also

show that the converter firing control and the valve base electronics function

properly.

Preconditions

Before conducting the open line test the following preconditions must be fulfilled:

- The converter transformers shall have been energized,

- All controls and protections associated with the converter transformers and

the DC switchyard including the DC voltage dividers shall be verified and in

service. A trip test shall be made shortly before high voltage energization.

Preferably a low voltage energization shall have been performed.





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- Monitoring instrumentation shall be connected and ready.


- The HVDC transmission fine disconnect switch shall be opened and locked.


- A final visual inspection of the DC switchyard shall be performed and the

arrester counter numbers shall be recorded.

- DC filter tuning shall be completed.

Test Procedures

During the test, AC and DC voltage and current shall be recorded. Inspect the

equipment during the test for abnormal sounds, corona discharge or partial

discharge arcing. The following test sequence is recommended:

- Connect the neutral side of the converter to ground or the electrode (if

available).

- Energize the converter transformer at the lowest valve side voltage and step

the tap changers to rated voltage,

- Deblock the converter in the open fine test status of operation and ramp the

DC voltage slowly to rated value.

- Keep the DC equipment energized for a minimum of 2 h.

- After the successful completion of the test, decrease the DC voltage to zero

and block the converter.

- Repeat the open line test of the DC switchyard with the appropriate DC filters

connected one by one and then together.

- If there are multiple valve groups in each pole they should be initially

energized individually and then together.


No abnormal sounds or corona discharge shall occur in the energized

equipment. No protections shall operate improperly.

Confirm that the designed DC voltage can be achieved.





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3.2.3 Load Tests

3.2.3. 1 Back-to-Back Test

Introduction

For bipolar HVDC systems back-to-back testing may be considered since this

provides the least disturbance to the connected AC system. Back-to-back testing

is accomplished by operating a bipolar terminal with one converter as rectifier

and the other as inverter. The connection between the converters may be within

the station or may include part or all of the DC transmission circuit. In the latter

case it is recommended to conduct an open tine test of the transmission circuit,as described in 3.3, prior to the test. Back-to-back testing gives an effective

general check of converter operation up to the specified maximum load and is

preferably conducted at all converter terminate.

It shall be noted that the telecommunication system is not required for these

tests. Adjustments to the control system may be necessary.

Test Objectives

The test objective of the back-to-back test is to get a provisional verification of

the control system, the valve cooling capability and the main circuit with respect

to temperature rise.

The back-to-back tests do not reduce the end-to-end test requirements. The

tests should permit troubleshooting at the individual terminals to provide for a

more efficient end-to-end test procedure. Current control, power control andreactive power control checkout may be performed with each converter pole

operating both as rectifier and inverter.

Preconditions

Before beginning the tests, the following preconditions must be fulfilled:

- The energization tests of the AC filters, capacitor banks and shunt reactors

shall have been successfully completed.





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- The converter transformers, the valves (and the DC switchyard filters if

available) and switchgear shall have been energized.

- A temporary connection between the converter or a part of the DC

transmission circuit is required.- The DC-reactor shall be included in series with the converter units.

- Some control parameters may have to be temporarily adjusted for

back-to-back testing if appropriate.


Consideration must be given to a possible trip of the converters during testing. If

a trip occurs, a protection scheme shall ensure that the station AC bus is

adequately protected by limiting the level and duration of the voltage rise to

acceptable values. This may be achieved by putting a limit on maximum

allowable direct current. For AC systems with low short circuit ratio (refer to

CIGRE Planning Guide for weak AC-systems, Publication Number ..,) it is

recommended to install a tripping arrangement (if not already included) that

removes all filters should a converter block.

Test Procedure

The two converters must be temporarily interconnected. The interconnection

must be adequate for maximum test current. The reactive power should vary

with direct current similarity to expected values during end-to-end operation. The

harmonic AC filters and shunt banks shall be utilized to reduce maximum

variation of reactive power exchange with the AC network. Tests and

measurements should be made to verify:

- Deblocking and blocking of converters.

- Ramping up to maximum test current including check of current measuring

circuits.

- Proper functioning of current control, power control, automatic tap changer

control and reactive power control.

- The measured quantities for the AC-filters protection and for the transformer

protection. Make adjustments if necessary.

- Acceptable equipment temperatures and absence of hot spots.





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- Steady state and transient properties of the cooling system of the valves are

correct.

- Switching sequences in the auxiliary power system are correct and these

should be done initially at low DC-current levels.- Correct operation of redundancy arrangements, for instance by simulation of

faults in units or functions, where redundancy is built into the equipment

During back-to-back operation, even though the rectifier angle is kept close to

nominal, the angle may vary with different levels of direct currents and thus vary

the phase angle between the harmonics from the rectifier and the inverter. At

some point of operation some harmonics may be in phase opposition to each

other, which will result in the filter current being very small. This should be kept

in mind during the observation of loading of the AC filters.


Demonstration of proper converter operation.

Equipment temperatures shall be within specified limits. No hot spots may exist.

Initial assessment of the cooling capability of the thyristor valves, transformers,

reactors, building and valve hall shall demonstrate no abnormalities.

No protection shall operate improperly.

3.2.3.2 Short Circuit Test Introduction

The short circuit test may be conducted on a monopolar DC system or in caseswhere schedules or constraints do not allow both converters of a bipolar station

to be used for back-to-back testing.

Test Objective

To provide minimal verification of the control system before end-to-end testing.





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Preconditions

The preconditions for the short circuit test are the same as for the back-to-back

test, described in sections 3.2.3.1. The DC smoothing reactor must be includedin series with the converter unit when creating the short circuit.

The DC circuit must be operated short circuited with alpha at approximately 90

degree (Ud = 0) during this test. The large delay angle results in much higher

than normal losses in the valve circuits. As a precaution it is recommended to

limit the duration of the test and to have adequate cool down time between tests.

Control and protection systems that would have operated as a result of theconditions during the test must be disabled in a safe manner.

Test Procedure

The test should be made with the converter transformer tap changer in highest

position (lowest valve side voltage). Tests may be made to verify:

- Deblocking and blocking;- Current control stability;

- Response to a step change.


Demonstration of response and stability of the current control system.





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3.3 Open Line Test of the DC Transmission Circuit Introduction

Open line test of the DC transmission circuit can be performed from either end of

the HVDC system but one at a time, it is normally done from the converter thatbecomes first available but preferably from the terminal with the highest

operating voltage. The open line test is an excellent test to demonstrate the

condition of the HVDC line and cables. Both short-circuit and open line

conditions can be detected over the entire length of the circuit.

The open line test is mainly conducted to verify the insulation of the DC

transmission circuit prior to transmitting power. The test is not only important for

the initial start up of the System but may also be used for test purposes following

a permanent fault on the system before returning to commercial operation.

Test Objectives

The test objective is to verify that the HVDC transmission circuit is not opened or

grounded anywhere over its entire length. The open line test should also verify

that proper voltage insulation of the DC transmission circuit is achieved.

Preconditions

Before conducting the open line lest the following preconditions shall be fulfilled.

- The DC switchyard shall have been successfully energized.

- All control and protection systems shall have been verified and be

operational.

- The DC transmission circuit shall be available.

- Check that the other terminal is isolated.

- Voice communication system shall be in service.


- The neutral side of the converter shall be connected to the earth electrode

and the high voltage side to the line or cable.





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Test Procedures

During the test AC and DC voltage and current shall be recorded. Inspect the

switchyard equipment during the test for abnormal sounds or corona discharge.The following test sequence is recommended.

- Apply a ground to the remote end of the DC line or cable,

- Energize the converter transformer and step the tap changers to rated

voltage.

- Deblock the converter in open line test status of operation and try to ramp

the DC voltage. If this is possible, the DC transmission circuit is opened

somewhere.

- Block the converter, deenergize the converter transformer and remove the

grounding at the remote end.

- Energize the converter transformer and step the tap changer to rated

voltage.

- Deblock the converter in open line test status of operation and slowly ramp

the DC voltage up to the desired level. Make sure that the DC voltage level

does not exceed the voltage insulation level at the other end.

- After the successful completion of the test, decrease the DC voltage to zero

and block the converter.

- Repeat the test for each line or cable and each polarity, if applicable.

- Keep the DC transmission circuit energized for a minimum of 2 h.


No protections shall operate improperly.

The DC voltage shall build up when the fine ground connection at the remote

end is removed.





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PART 4: END-TO-END-TESTS

General

Introduction

The end-to-end tests constitute a basic set of verifications at low power that are

to be conducted when the converter terminals are connected for the first time by

the DC transmission circuit

Since power will be transmitted, tests must be arranged to limit the potentialimpact to the HVDC system equipment and to the interconnected AC systems.

It will be necessary to perform end-to-end tests in all applicable HVDC system

configurations. The general test procedure is shown in the flow chart of

Figure 4.1.

It is necessary that some verifications are done off voltage or "dry" prior to

testing at any power level In particular the transfer between different DCsystem configurations and the verification of protective tripping sequences

should be done "dry", before tests at minimum power transfer level can start.

The HVDC system may be operated in the following different configurations:

- Monopolar Earth Return;

- Monopolar Metallic Return;

- Bipolar Operation;- Integration of Parallel or Series Converters.





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The following tests are outlined:

4.1 Changing the DC system configurations, off voltage.

4.2 Start and stop sequences and Steady State Operation at minimum power.4.3 Protective Blocking and Tripping Sequences.

4.4 Power and current ramping.

4.5 Reduced Voltage Operation.

If no back-to-back test has been performed, verification of the performance of

the valve cooling system must be done as power is increased.

The operating conditions of the HVDC system can be:

- Normal operation. This includes synchronous and islanded operation if

applicable.

- Contingency operation, which exists if a single failure occurs but the HVDC

system can continue operation within specified limits. Loss of telecom

represents a typical example.

- Emergency operation as per project specific definitions. This does not

normally need to be tested in the actual system during commissioning.

However it is advisable to test during off-site testing.

Operator control actions can be initiated from various locations. Such typical

locations are:

- Local control, which applies when the control functions can be initiated from

the local control room at either converter terminal.

- Remote control, which applies when the control functions can be initiated

from a control centre remote from either converter terminal.

The operator can accomplish a change of operational status by using one of two

different control methods:

- Automatic Control

- Manual Control





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In automatic control the operator initiates a change of operational status, a

change of configuration or a change of power (current) reference selection and

the respective controls execute the correct order. The successful completion is

verified by monitoring.

In Manual, the control of the converter terminal is performed in discrete steps

from one operational status to another. Compared to the automatic control

mode, the operator is offered a choice of additional intermediate steps and

intervention in the control sequences. Completion of any control sequence will

be verified by monitoring.

The control level denotes the hierarchical level of HVDC system control. A

distinction is made between:

- System control level (Joint control available).

- Station or Bipole control level (Joint or Separate control Available).

- Pole or Converter control level (Joint or Separate control available).

At the System control level the complete HVDC system can be controlled jointly

from one of the remote or local operator control locations, hencetelecommunication is required at this level.

At the Station (or Bipole) control levels each HVDC station/Bipole is controlled

separately when telecommunication is not available. The stations can be jointly

controlled from one location when telecommunication is available.

At the Pole (or Converter) control level each pole/converter is controlled

separately when telecommunication is not available. The poles of the twostations can be jointly controlled from one location when telecommunication is

available.





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The following activities must have been completed before the end-to-end tests

can proceed: Off-site Tests.

Converter Tests for both stations.

Back-to-back Test or Short Circuit Tests when possible,

Open Line Test of the DC transmission circuit.

Electrodes and electrode lines checked and cleared.

Telecommunication system and telephone system operational.

The overall test procedures, safety rules, dispatch coordination and test

responsibilities have been established.

Fire protection and detection systems have been fully checked and in service.

All Control, Protection, Metering, Sequence of Events and Fault Recording

systems must be in service.


End-to-end tests are intended to verify the proper co-ordination and interstation

interlocking of the basic HVDC control and protection functions at low power

levels. The tests shall be performed with various AC and DC system

configurations and for applicable contingencies. Changing the DC system

configuration (as described in part 6) or the status of operation for the first time

must be tested without high voltage (Dry Run Test or Off Voltage Test),

The tests should be organized in such a way that all the control modes, control

levels and control locations will be tested without unnecessary duplication of

tests.





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4.1 Changing the DC System Configuration, OFF-Voltage

Introduction

Changing the DC system configuration can be relatively simple in a point to point

system which has a single converter per pole. The switching operations become

more complicated if the station poles have parallel or series converters. This test

is intended to demonstrate that equipment, breakers, disconnects or grounding

switches are operated in the correct sequence and, are properly interlocked.

Changing the DC system configuration on load is described in part 6.

Transfers between different DC system configurations can be initiatedautomatically and jointly or manually from local control or from a remote dispatch

centre. In manual or separate control, the operators need to coordinate these

actions via voice communication.

Test Objectives

The purpose of the tests is to verify that the DC system configuration can be

safely changed as specified prior to transmitting power for the first time.

Preconditions

Before conducting the test the following preconditions must be fulfilled:

- Off-site tests verifying DC system configuration change shall have beencompleted.

- All necessary AC and DC switching equipment shall be operational.

- Operator instructions and test procedures shall be available.





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Test Procedures

The transfer between specified DC System configurations shall be

demonstrated. The most logical approach is to perform the changes ofconfiguration from the initial conditions defined below:

- Monopolar earth return.

- Monopolar metallic return.

- Bipolar operation.

- Integration of parallel or series converters.

The tests shall first be performed with telecommunication out of service, andthen with telecommunication in service:

- Establish voice communication with the operator of the other terminal.

- Switch off the telecommunication.

- Reset all alarms.

- Select the initial DC system configuration in accordance with the operating

instructions. The procedure will have to be done in steps involving operators

at both terminals.

- Verify the appropriate switching action on the operator control interface and

sequence of events recorder. Simulate failures in the Switching Sequence.

- Repeat the test for all applicable DC system configurations.

- Restore telecommunication.

- Repeat the tests, now in automatic and joint control mode.


- Switching sequences shall correctly transfer between all applicable DC

system configurations, with and without telecommunication.

- All Switching sequences shall be safely and correctly completed.

- The interlocking of disconnectors, switches and breakers of the AC and DC

yards shall be in accordance with the technical specifications.

- It should be possible to initiate a change of DC system configuration from

local and, if available, from remote control locations.

- An incomplete sequence shall be terminated in a safe condition.





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4.2 Start and Stop Sequences and Steady State Operation at Minimum

Power

Introduction

Great care must be exercised when going through the start and stop sequences

for the first time. The thyristor valves will be deblocked at the rectifier and

inverter end and current will be established in the DC transmission circuit.

The sequences for starting and stopping the converters are not identical for all

HVDC systems and can vary depending upon the design philosophies of the

supplier and the system requirements of the customer. The general sequence of

bringing a converter from STOPPED status to DEBLOCKED is defined below.

- The STOPPED status is generally characterized by the following conditions:

Converter control and protection may be energized or de-energized;

Valve cooling system switched off;

Transformer and smoothing reactor cooling systems switched off;

Tap changer control inhibited;

Converter transformer circuit breaker open;

DC filter disconnector can be open;

AC filter circuit breaker open;

Firing pulses to the converter blocked.

- The STAND-BY (or DE-ENERGIZED) status is generally characterized by

the following conditions: Converter Control and protection must be energized;

Valve cooling system switched on;

Transformer and smoothing reactor cooling systems switched on;

Power or current control setpoint selection available;

Tap changer operational and in correct position;

AC filter disconnector closed, circuit breaker open;





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DC filter disconnector closed;

Converter Transformer disconnector closed, circuit breaker open.

- The BLOCKED (or READY FOR OPERATION) status is generally

characterized by the following conditions:

Converter transformer circuit breaker closed;

Tap changer control activated;

AC filter circuit breaker open or closed ;

Thyristor precheck completed.

- The DEBLOCKED status is generally characterized by the following

conditions:

AC filter circuit breakers closed;

Valve Firing pulses deblocked;

Rectifier and/or inverter in operation.

Test Objectives

The general purpose of this test is to deblock both converters of the HVDC

transmission system and transmit minimum power for the first time, in both

directions if applicable. Other purposes are to:

Verify for each pole, that control actions associated with each status change,

start and stop sequences are executed in the right order.

Verify, that minimum power can be established and stopped smoothly and

reliably in all applicable HVDC system configurations.

Verify correct system measurements during steady state operation at

minimum power.





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Preconditions

Before conducting the tests some preconditions must be fulfilled:

The DC system configuration has been established by means of a

successful off-voltage test.

Off-site tests for start and stop sequences have been successfully completed.

Converter tests for both stations have been successfully completed.

Appropriate AC and DC filters are available.

The valve cooling equipment has been verified and is operational.

No alarm are present.

Thyristor monitoring shows thyristor redundancy is not exceeded.

Protective blocking and tripping sequences should have been successfully

tested off voltage. All protective lockouts shall have been reset.

Operator's instructions and lest procedures shall be available.

The connected AC systems shall be capable of delivering or accepting the

transmitted power without affecting their stable operation.

Test Procedure

The start and stop test must be preceded by establishment of one of the AC and

DC system configurations off voltage. The test must then be repeated for all

applicable system configurations and in both synchronous and islanded operation.

- When the HVDC system configuration is established, the operational status

can be tested beginning with "stopped" or "stand-by". This change of status

is done off voltage. By changing the operational status for both ends to

"blocked", voltage will be applied to the converters.

- Establish voice communication between the operators at each end and put

telecommunication out of service.

- Establish operation at minimum power by first deblocking the inverter and

then the rectifier.

- Remain at minimum power for a short period. Then block the rectifier and the

inverter.- Restore telecommunication.





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- Establish operation at minimum power in automatic and joint mode. Remain

at minimum power for at least 1 h and verify measurements of DC currents,

voltage etc. It is advisable to perform switching of redundant controls and

other elements at minimum power level in order to verify that there is no

impact on the HVDC transmission or the AC network.

- Special attention shall be given throughout the test to verify that no hot spots

develop. The equipment inside the valve hall, the converter transformers,

smoothing reactors and the wall bushings are among the critical items to

check. Also verify that the valve cooling system maintains the valve

temperature within specified limits.

- If applicable, transmission of power in the reverse direction shall be tested.


- Starting and stopping at minimum power must be safely and reliably

accomplished in each applicable HVDC system configuration.

- All control actions shall take place in correct order and timing. Interstation

interlocking shall function properly.

- No malfunction may occur.

- Equipment rating shall not be exceeded.

- No AC and DC system disturbance may occur.

- The cooling system shall keep the valve temperature within specified limits.

- Switching between redundant elements shall have no impact on the HVDC

transmission.





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4.3 Protective Blocking and Tripping Sequences

Introduction

HVDC systems can be exposed to various types of faults of permanent or

temporary nature.

Depending on the type of fault, the protective blocking and tripping sequence

may result in some combination of the following actions:

- Instantaneous advancing of the inverter firing angle (commutation failures);

- Transfer to redundant control (if applicable);

- Retarding the firing angle of the rectifier;

- Blocking of the converter valves blocking (with or without selection of by-

pass pairs);

- AC breaker tripping (converter transformers and possibly AC filters);

- Pole isolation by opening DC switches;

- Blocking of remote station.

Protective control sequences may also be initiated as a consequence of load

rejection.

Test Objectives

Verify that proper blocking or protective tripping sequences take place

selectively when clearing a fault or equipment malfunction.

Verify that DC protective actions are properly co-ordinated with the AC

breaker

operation and other protections,

Verify that control actions reestablish system transmission as specified.





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Preconditions

The co-ordination between the protective systems and the control systems

shall have been demonstrated during off site tests.

The AC and DC protection systems have been exercised during subsystem

and converter station tests.

All redundant control and protection systems are in service.

All monitoring and alarm systems are in service.

Test personnel are well informed on each protection and on the protective

actions to expect.

Telecommunication system is in service.

The connected AC systems shall be capable of delivering or accepting the

transmitted power without affecting their stable operation.

The AC and DC equipment are energized.

Test Procedure

In order to verify the proper function of a protective blocking or tripping sequence

the faults will have to be simulated.

It is advisable that testing the protective sequences begins with tests off voltage.

After successful completion of the off voltage tests, trips shall be simulated at

minimum power in all applicable system configurations and with the

telecommunication system in and out of operation.

The following signals shall be recorded:

- AC voltages and currents on each phase;

- DC voltages on both poles;

- AC currents in the Valve winding on each phase;

- DC currents on each pole and on each DC neutral connection;

- AC currents in filter banks;

- Main sequencing signals;





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- Protection signals;

- Firing and extinction angles order and measurements;

- Current order.

During each simulated fault, test personnel must check that the correct circuit

breaker trips and that the corresponding alarm and sequence of event recorder

signals are initiated.

Operators must confirm that both terminals are stopped safely.


For each simulated fault, the corresponding protective blocking and tripping

sequence shall operate in accordance with the technical specifications and

system studies.

The consequential outages of a fault must be limited to the smallest possible

zone and properly isolated from the rest of the HVDC system.

Correct alarms are announced, sequence of event recorder and transient faultrecorder should show that the correct sequence has taken place.

Safe isolation of a pole and/or converter is accomplished with and without

telecommunication.

The impact to the HVDC system and the connected AC systems must be within

specified Performance Criteria.





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4.4 Power and Current Ramping

Introduction

This test is intended to demonstrate that the HVDC system power can be

smoothly ramped up from minimum power to approximately 0,3 pu. and down, in

both power and current control. The test is normally conducted with

telecommunication in service. However, if applicable, ramping shall also be

demonstrated with telecommunication out of service.

Test Objectives

The general purpose of this test is to verily that smooth ramping at different rates

can be executed for each pole and, if applicable, for the bipole. It is also to verify

that:

- Transfer between operation with and without telecommunication is smooth.

- Transfer between power and current control is smooth.

Preconditions

To perform the ramping test, the following preconditions should apply:

- The HVDC system has been energized and operated at minimum rated

current;

- The protective blocking and tripping sequences have been verified;

- The connected AC systems shall be capable of delivering or accepting thetransmitted power without affecting their stable operation.

Test Procedure

The ramping test shall be performed in monopolar metallic and ground return

configuration for each pole and then, if applicable, in bipolar operation.

If possible, ramping shall first be tested without telecommunication in service.





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Having established voice communication operators deblock the converters

individually and allow the HVDC system current to stabilize at its minimum rated

value.

- Operate at this current level for a short time. The current can then be ramped

to the next preselected level. The system is again allowed to settle. This

procedure is repeated until the current has reached 0,3 pu.

- Special attention shall be given throughout the test to all current paths to

verify that no hot spots develop. The equipment inside the valve hall, the

converter transformers, smoothing reactors and the wall bushings are among

the critical items to check. Also verity that the valve cooling system maintains

the valve temperature within specified limits.

- Ramp down to minimum current and restore telecommunication.

- Repeat the above test in automatic control mode and joint control.

- Transfer to power control and repeat the test.


The following test acceptance criteria shall be fulfilled.

- Smooth ramping of the HVDC system current and power shall be possible for

all applicable DC system configurations.

- AC and DC currents and voltages shall at all times be stable and remain

within specified limits. Verify the control system keeps the firing angle within

specified limits and that transformer tap changer control operates correctly.

- No hot spots should occur in the AC and DC yard equipment or in the valvehall.

- The valve cooling system maintains the valve temperature within specified

limits. Smooth transfer between current and power control or vice versa.

- If applicable, it should also be verified that the control system schedules the

necessary reactive power elements in the appropriate order to keep the AC

voltage and reactive power interchange within the specified limits.





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4.5 Reduced Voltage Operation

Introduction

Manual or Automatic reduction of the DC voltage may be required to reduce

stresses on cables when the power transfer level is reduced, or to minimize the

possibility of flashover of overhead DC lines during adverse weather conditions

or conditions of excessive contamination.

Test Objectives

The purpose of the test is to verify that transfer to and from reduced voltage

operation, and that steady state operation at reduced voltage can take place in a

stable manner.

Preconditions

Before conducting reduced voltage tests the following preconditions must be

fulfilled.

- Off-site tests with reduced voltage operation successfully completed.

- The DC transmission has been operated at nominal voltage.

- The power and current ramping tests have been completed.

- The telecommunication should be in service.





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Test Procedure

Reduced voltage operation shall be tested in all AC and DC system

configurations.- Start the DC transmission to minimum current and nominal voltage. Transfer

to reduced voltage operation and maintain steady state condition for one

hour. Transfer back to nominal voltage operation. Both manual and

automatic initiated transfer to reduced voltage operation (e.g. after DC line

faults) should be tested.

- Repeat the test at a higher current level.

- Repeat the test in power control.

- Correct functioning of the control system and adequate performance of the

valve cooling system should be observed for all current levels.


- Transfer to and from reduced voltage operation takes place smoothly and

does not disturb the connected AC network.- Steady state operation with reduced voltage is stable.





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PART 5: STEADY-STATE PERFORMANCE AND INTERFERENCE TESTS

General

Introduction

In this part HVDC system steady-state performance and interference tests are

described- These are system tests which seek to verify steady-state

performance parameters of the HVDC system and verify that electrical and

audible noise interference caused by the HVDC system are within specified

limits. It is important to coordinate in advance with all parties that may be

affected by operation of the HVDC system. For these steady-state and

interference tests, such parties may include telephone utilities, railroad

companies, pipeline companies, etc.. Steady-state performance and interference

tests are normally performed prior to the operation and integration tests.

Previous commissioning tests, including converter tests and end-to-end tests,

have verified that the DC equipment being commissioned functions correctly.

The following tests are outlined:

5.1 Harmonic performance and filter components rating

5.2 Audible noise

5.3 Overload/temperature rise

5.4 Interference

5.5 Earth electrode





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The following activities should have been completed before the Steady-State

Performance and Interference Tests can proceed:

• Commissioning tests of ail converter units.

• Commissioning tests of transmission system as required for stable operation.

• Commissioning tests of reactive power control system.


The purpose of the Steady-State Performance and Interference Tests is to verify

stable operation of the HVDC system within the limits given by the specifications

and verification of environmental considerations.


Steady-State Performance and Interference Tests are intended to verify that the

AC and DC harmonic levels, audible noise/corona and interference levels

conform with the specified levels. In addition, the tests verify the

overload/temperature rise and earth electrode performance conform with the

design targets.





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5.1 Harmonic Performance and Filter Components Rating

Introduction

The power conversion process in HVDC systems results in the generation of harmon-

ic currents and voltages which can affect the interconnected AC systems, DC

network, and third party electrical systems. Factors considered in the design and

harmonic studies of HVDC systems to minimize the level of harmonics propagated

from the HVDC system into the AC and DC network include the following:

AC system network impedance.

Harmonic frequencies of concern (characteristic and uncharacteristic).

Harmonic magnification (resonance) on the AC network.

Valve firing angle.

Converter transformer reactance and firing angle unbalance.

Unbalanced impedances of AC system and harmonic filters.

Transformer saturation effects and stray capacitance,

AC system distortion and interference limits.

Sensitivity of adjacent open-wire systems.

HVDC control system instabilities and interactions with other active devices.

Pre-existing harmonics on the AC network.

HVDC configuration and ground resistivity.

Ambient temperature range.

Phase angle relationship between AC systems.

AC and DC filter design.

AC harmonic filters connected to the AC bus reduce the harmonic voltages

appearing on the interconnected AC network and the harmonic currents injected

into the AC network to specified levels.

For HVDC systems with overhead transmission lines, smoothing reactors

together with DC filtering circuits are typically installed in the DC circuit to

reduce the harmonic currents to levels which will prevent interference with third

party telephone or electrical systems.





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The loading of the AC and DC harmonic filters must be checked and the filter

components ratings verified.

Test Objectives

The objective of the harmonic tests is to confirm that the harmonic voltages and

currents produced by the HVDC system are reduced by the AC and DC filters to

conform to specified limits and verify the harmonics do not cause unacceptable

interference with third party telephone or electrical systems. Additionally, the

tests demonstrate that harmonic performance is acceptable when operated in

various configurations and under required contingency modes.

The loading of the filter components shall be verified to be within the individual

component ratings.

Preconditions

Before conducting the tests the following preconditions must be fulfilled:

AC network conditions within specified limits.

Study results available which estimate the levels and spectrum of harmonics

from the HVDC system.

AC and DC filter design study results available.

Inductive coordination study results available.

Measurements available which establish pre-existing background levels and

spectrum of harmonic levels.

Precommissioning test data available for the AC and DC filter parameters.

Test instrumentation and data acquisition system available.

Specialized harmonic measuring equipment available.

Test Procedures

The harmonic measurements should be performed in those HVDC system

configurations and modes of operation which are specified. A test plan which

identifies these configurations and modes of operation along with the physical

locations for the harmonic measurements should be established, considering the





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following HVDC system operating conditions:

Stand-by operation.

Operation at minimum and rated power and at each power level which

initiates switching of a filter or reactive power component, and overloadHVDC power transfer levels.

Various HVDC and AC system configurations.

Any special condition generating maximum harmonics.

Reduced HVDC voltage operation.

Normal operation with specified filter configurations.

Operation at larger-than-normal firing angles. Operation with filter banks or reactive power banks unavailable.

Steady-state range of AC power frequency and voltage.

Extremes of ambient (as far as possible).

Automatic filter tuning.

DC harmonics testing should consider the following additional items:

Equipment safety from induced voltage.

Data transmission and railway signalling circuit effects.

Voice communication circuit effects.

Excitation of resonance conditions between the HVDC line and electrode

line.

Converter transformer DC currents in the neutral.

Harmonic impact on minimum current operation.

Bipolar and monopolar operation (with and without ground or metallic return).

During the harmonic measurement tests, the environmental conditions need to

be noted and recorded due to the sensitivity of the harmonic filters to ambient

temperature conditions or weather.





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The results of the performance measurements should verify that the AC and DC

harmonic voltages and currents are within the limits required and interference tothird party electrical systems is within acceptable limits. Additionally, the tests

should confirm that operating restrictions of the HVDC system and AC and DC

filter configurations are within specified limits. The AC and DC filter components

must not be overloaded.

5.2 Audible Noise

Introduction

HVDC systems emit noise in the audible-frequency spectrum. The source of the

audible noise (AN) in HVDC systems is from terminal equipment and HVDC

transmission lines and can be categorized as follows.

• Component generated audible noise:

Related to converter transformers, DC smoothing reactors, shunt reactors, AC and DC harmonic filter reactors and capacitors, thyristor valves, cooling

system, and auxiliary equipment. AN will vary with loading conditions and

firing angle changes.

• Conductor generated audible noise:

Related to corona phenomena on HVDC transmission lines, substation

buswork, and outdoor equipment. This source of AN is associated with

ionization phenomena near conductive surfaces when the electric fieldstrength is high enough to cause a breakdown of the surrounding air. As

such, corona generated AN can vary with environmental and ambient

conditions due to conductive surface irregularities and contamination. AN

from DC generated corona is usually higher under dry ambient conditions.

• Impact generated audible noise:

Related to operation of equipment such as power circuit breakers,

disconnect switches, etc.





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AN limits within the converter station facility, including building interiors, and

along the perimeter of the HVDC system are specified to ensure applicable

regulations and codes of practice are met. In some cases, special noise

abatement measures may be required to reduce the levels and spectrum of AN

from the HVDC system.

Test Objectives

The objectives of the AN tests are to measure and verify that the AN caused by

the HVDC system is within specified limits.

Preconditions

Before conducting AN tests the following preconditions must be fulfilled:

Defined locations for pre- and post-construction measurements.

Measurements available which establish preconstruction background levels

and spectrum of AN levels.

Study results available which estimate the levels and spectrum of AN fromthe HVDC system,

Factory AN test results available of applicable equipment


Test Procedures

AN tests should be performed at predetermined locations in those HVDC system

configurations and modes of operation specified. A test plan which identifies

these configurations and physical locations for the AN measurements should be

established, considering the following HVDC system operating conditions:

Stand-by operation.

Minimum, intermediate, rated, and applicable overload HVDC power transfer

levels.

Any special condition generating maximum AN. Long term measurement considerations (when required).





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The presence of corona can create AN. Corona can be observed by visual

inspection with the use of binoculars or located with the use of ultra sonic corona

detection devices.

During the AN tests, the environmental conditions need to be noted and

recorded due to the sensitivity of AN measurements to ambient conditions, such

as air temperature, barometric pressure, relative humidity, wind speed and

direction, and background acoustic noise.

Instrumentation and AN measurement procedures should follow the specified

requirements or applicable standards.


Test criteria for AN levels resulting from HVDC systems are project specific and

depend to some degree on surrounding environments. The AN levels measured

with the HVDC system in operation must be within the design limits specified.

5.3 Overload/Temperature Rise

Introduction

HVDC systems may be designed and allowed to operate in an overload

condition where the HVDC power transfer level is greater than the rated value.

Overload operation may result in reduced performance of the HVDC system for

the following reasons:

Equipment life expectancy reduced due to increased thermal stresses.

Reduced reliability due to use of redundant equipment during overload

operation.

Limited ambient temperature operating range.

Restricted performance range for such critical operating parameters as

reactive power compensation and harmonic filter performance.

Control and instrumentation limits.





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Operating conditions and design requirements of the AC and HVDC system

should be able to support the overload operation without component damage.

Major equipment which can be directly affected by overload operation include

the thyristor valves, valve cooling system, converter transformers, harmonic

filters, smoothing reactor, current and voltage transformers, bushings, bus work,

and transmission systems.

Test Objectives

The objective of the overload/temperature rise tests are to verify the HVDC

systems overload performance capability and confirm the temperature rise of the

individual equipment is within acceptance limits.

Preconditions

Before conducting the tests the following preconditions shall be fulfilled.

Factory heat-run test results of major equipment available.

AC and DC transmission systems prepared for overload tests.

Temperature monitoring equipment inplace.

Ambient conditions are compatible with requirements.

Test Procedures

The HVDC system should be operated at rated load prior to commencement ofthe overload tests for a period of time necessary to achieve thermal equilibrium

of major electrical components such as converter transformers, DC smoothing

reactors, thyristor valves, AC and DC harmonic filters, and valve cooling system.

After the temperature of the major equipment items has thermally stabilized at

full load (typically 12-18 hours), the HVDC system can be operated under the

overload condition which is compatible with the existing ambient conditions and

the temperature rise of major electrical components recorded. In addition, the

valve cooling system performance should be monitored and temperature





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monitoring devices should be employed to check the buswork, connection

points, terminations, neutral connections, and switch contacts.


The maximum operating temperatures and temperature rise of the equipment

shall be within specified limits. The maximum operating temperature of

equipment being tested may have to be corrected to maximum ambient

temperature to compensate for lower ambient temperatures during test.

5.4 Interference

Introduction

HVDC systems produce voltages and currents in conductors which may cause

interference from both the conducted energy and radiated energy. Conductor

corona pulses and partial discharges on insulators are also potential sources of

electrical interference from HVDC systems.

Filtering devices, shielding, and noise suppression techniques are implemented

to minimize interference. Since electrical interference has the potential of

affecting third party electrical systems, the commissioning test program should

include coordinated tests with operators of interference-sensitive equipment-

Design criteria for interference levels resulting from HVDC systems are project

specific and depend on the surrounding environments and regulations.

Interference limits imposed typically consider the following:

Radio interference (Rl).

Television interference (TVI).

Telephone carrier interference (TCI).

Microwave communication system interference (MCSI),

Railroad signal interference (RSI),

Power line carrier interference (PLCI).





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Test Objectives

The objectives of the interference tests are to measure and verify that the inter-

ference levels caused by the HVDC system are within required limits andestablish that there is no degradation of low-level electric circuits (e.g. telephone

networks, computers, radio and television systems, railroad signal equipment,

and other electronic apparatus).

Preconditions

Before conducting interference tests the following preconditions shall be fulfilled.

Measurements available which establish pre-existing background

interference levels.

Study results available which estimate the levels of interference expected as

a result of HVDC system operations.

Coordination established with operators of interference-sensitive equipment.

HVDC system operating with all interference mitigating equipment in service.

HVDC station controls and protection verified to be immune from

interference.


Test Procedures

Interference tests for the HVDC system can be broadly classified into the

following separate groups:

HVDC converter station tests with measurements taken within and adjacent

to the fenced boundary of the facilities.

Transmission line tests with measurements taken along a perimeter of the

AC and DC transmission lines servicing the HVDC system.





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The interference tests should be performed at predetermined locations and in

those HVDC system configurations and modes of operation specified. A test plan

which identifies these configurations and levels along with the physical locations

for the interference measurements should be established, considering the

following HVDC system operating conditions:

Stand-by operation,

Minimum, intermediate, rated and applicable overload HVDC power transfer

levels.

Any special condition generating maximum interference,

Number of points and number of measurements.

Test plan which identifies the bandwidths and measuring techniques,

correction factors, accuracy, antenna types, standards (where applicable),

frequency scans and spectrums, operating configurations and modes of the

HVDC system, ambient weather conditions, voltages, conductor

configuration, structure type and material),and elevation.


The interference levels measured with the HVDC system in operation must be within

the specified design limits and cause no degradation of low-level electric circuits.

5.5 Earth Electrode

Introduction

The earth electrodes for a bipole configured HVDC transmission system provide a

ground reference for the neutral bus at each converter station. The earth electrodes

are generally designed to permit the HVDC system to operate for a limited time

period in the monopolar ground return mode when one pole of the bipole is out of

service. Earth electrodes for monopole systems are designed to permit continuous

operation. Since earth return operation may interfere with third party communication

circuits and may cause corrosion of underground structures, the commissioning test

program should include coordination with third parties.





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Test Objectives

The objectives of the earth electrode test is to verify the design of the earth

electrodes and establish the maximum measured interference levels in various

operating modes. Design criteria for HVDC earth electrodes are project specific

and depend on the local soil conditions, anticipated operating modes, and

duration of earth return operation in the worst case scenario- Design criteria

typically considered include the following:

Current rating.

Temperature rise.

Thermal time constant.

Current distribution between electrode wells.

Current density at the electrode surface.

Earth resistivity design value.

Resistance of electrode to remote earth.

Electrode resistance.

Overload current capability.

Step and touch potential.

Preconditions

Before conducting the tests the following preconditions shall be fulfilled.

Earth electrode precommissioning tests including measurement of structure-

to-soil potential, current-in-structure, soil potential, soil resistivity, structure-

to-remote electrode-potential, and background interference level.

Test stations for cathodic protection and stray current measurements

established,

Coordination established with all participants, e.g., railroad operators,

telephone companies, municipalities, underground utility operators, electrode

supplier, and HVDC supplier.

Test coordination center and communication method established. Test instrumentation and data acquisition system available.





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Test Procedures

System tests for commissioning of the earth electrodes can be broadly classified

into the following separate groups:

low current tests with a direct current of 10-20 % of the rated direct current to

determine the basic earth electrode characteristics and identify the areas of

potential interference;

full rated tests of the earth electrodes with the direct current being increased

above the low current test level in incremental steps up to the full current

rating. Periodic cycling of the direct current and the magnitude of the direct

current incremental steps should be based upon a test schedule/plan.

Recording instruments need to be installed at all test points and areas of special

concern that are identified during the low current tests. The duration of the tests

at each direct current level must be appropriate for the thermal time constants of

concern and allow for the following test measurements to be taken.

Stray current magnitude caused by test current in affected structures.

Change in stray current flow caused by test current in affected structure. Change in potential caused by test current of structure-to soil.

Current distribution in each earth electrode well or electrode section.

Temperature rise versus test time to establish limits of full current test.

Induced voltage on communication circuits.

Stray currents in AC system transformer neutrals.

Structure-to-soil potential should be measured with the reference electrode

directly above buried structures or within one foot of above ground structures

such as railroads, and towers.





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The characteristics of the earth electrodes must be within the specified design

limits including temperature rise, voltage distribution, resistance, and uniformcurrent distribution among different electrode wells or electrode sections. The

results of the coordinated tests need to be analyzed and any unusual results

investigated and resolved between the affected parties.





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PART 6: OPERATION AND INTEGRATION TESTS

General

Introduction

In this part the operation and integration tests are described. These are the

system tests which seek to prove the response to change of configuration and

the transient behaviour of the AC and HVDC systems including recovery from

faults. Operation and integration tests are the final group of system tests before

the period of trial operation. Previous commissioning tests, including converter

tests and end to end tests have verified that the DC equipment being

commissioned functions correctly. The tests described are those recommended

only for two terminal and back to back HVDC systems. The following tests are

outlined:

6.1 Changes of DC configuration

6.2 Control performance6.3 Switching AC side filters and transformers

6.4 Loading tests

6.5 AC and DC system staged fault tests

6.6 Loss of telecommunications, auxiliaries, or redundant equipment





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Integration testing may also include all tests that would verify proper interaction

and coordination between the HVDC system under test and such other on-line

equipment as static VAR compensators, other HVDC systems, torsional

interactions, etc.


The following activities shall have been completed before the operation and

integration tests can proceed:

- Off-site tests including AC/DC simulator tests

- Terminal tests including converter unit tests and converter station tests

- End-to-end tests to verify sequences and inter-station coordination

- Steady state operation including verification of steady state performance and

interference effects

Detailed procedures and plans should be prepared. As the tests may involve

some disturbance or increased risk to the connected AC systems, the operators

of the systems should be consulted-

The connected AC systems shall be within their specified range of parameters

and be capable to deliver the active and reactive power for the HVDC system re-

quired for the particular test/ Important parameters include.

AC voltage and frequency

Short circuit capacity

AC system configuration


The purpose of the operation and integration tests is to verify the correct

operation of the HVDC system in combination with the associated AC systems,

within the limits given by the specification, under transient and change-over

conditions.





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Operation and integration tests are intended to verify the effects of changes in

DC main circuit configuration, control performance, switching and connection of

AC side equipment, AC and DC staged faults where required and the loss of

telecommunications and auxiliary functions.

6.1 Changes of DC Configuration

Introduction

Most HVDC long distance transmission schemes can be operated in several DCconfigurations such as monopolar earth electrode return, monopolar metallic

return, bipolar normal and in parallel operation. Each of these configurations can

contain one or more DC filters.

If series connected converter units per pole are used switching in and out of

these groups should be demonstrated. If the HVDC system uses polarity

reversal, this should be verified. Manoeuvres involving two or more terminals

can normally be performed with or without telecommunication.

Back-to-back stations do not require changes of DC configuration.

The test objectives, preconditions and test procedures are similar for the tests

under 6.1.1 through 6.1.3.

Regarding denomination of switches and their function refer to document CIGRE

WG 13/14.08: "Switching devices other than circuit breakers for HVDC systems",Part 1.

Test Objectives

Verify coordinated switching between converter terminals and for DC bus

reconfiguration and thus prevent switching conditions which can cause

severe stresses on the DC equipment.





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Switching of DC filters, if permitted, should not disturb the power flow of the

HVDG transmission more than specified.

Preconditions

Before conducting the tests the following preconditions must be fulfilled:

All sequences checked on the non-energized system

All AC and DC equipment energized

End-to-end tests completed

Test Procedures

During all tests AC and DC voltages and currents should be recorded. Outdoor

observations of sound and light phenomena should be made.

The timing of the various switching sequences should be verified. All switching

sequences should be performed step by step. If applicable, the sequences

should also be executed automatically. If two or more terminals are involvedtelecommunication will then be required.

Where appropriate, additional tests may be performed at lower than nominal

power and at minimum power.


The HVDC transmission should not be permanently disrupted by any change of

DC configuration. Currents and voltages should coincide with the results from

corresponding simulator tests and be in accordance with the specification.





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6.1.1 Tests from Monopolar Metallic Return Operation

From this configuration the following changes can be made:

- Connection and disconnection of DC filters- Transfer to monopolar earth electrode return

- Transfer to bipolar configuration

- Paralleling and deparalleling of another pole

Connection and disconnection of DC filters (if permitted):

- Record the DC filter harmonic current

- Operate at nominal power and disconnect one DC filter

- Connect the DC filter again

- Repeat the procedure for all DC filters

Additional test criteria:

- Rating of remaining filters should not be exceeded

- No burn marks should occur on the disconnects

Monopolar earth electrode return:

- Operate at nominal power

- Close the MRTB (Metallic Return Transfer Breaker)

- Open the GRTS (Ground Return Transfer Switch)

Bipolar operation:


- Close the MRTB

- Open the GRTS

- Close line pole disconnect of the non-operating pole

- Deblock non-operating pole

- Verify the current balance in both poles





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Paralleling/deparalleling:

- Operate one pole at nominal power

- Close paralleling disconnect of the non-operating pole

- Deblock the non-operating pole


6.1.2 Tests from Monopolar Earth Return Mode

Monopolar earth return may only be allowed during a limited period of time. From

this configuration the following changes can be made:- Transfer to monopolar metallic return

- Transfer to bipolar operation

Transfer to monopolar metallic return:


- Close the GRTS

- Open the MRTB

Transfer to bipolar operation:


- Close the line pole disconnect of the non-operating pole

- Deblock the non-operating pole


6.1.3 Tests from Bipolar Operation

From this specific configuration the following changes can be made:

- Connection/disconnection of DC filters

- Monopolar earth electrode return

- Monopolar metallic return





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Connection and disconnection of DC filters (if permitted):

- Record the DC filter harmonic current

- Operate at nominal power and disconnect one DC filter

- Connect the DC filter again

- Repeat the procedure for all DC filters

Additional test criteria:

- Rating of remaining filters should not be exceeded

- No burn marks should occur on the disconnects

- No adverse transient effects

Monopolar earth electrode return:

- Operate up to maximum specified power, for monopolar operation

- Reduce the current in one pole to a minimum but keep the bipole power

constant

- Block the pole with minimum power

- Open the line pole disconnect in the blocked pole

Monopolar metallic return:

- Operate up to maximum specified power

- Reduce the current in one pole to a minimum but keep the bipole power

constant

- Block the pole with minimum power

- Open the line pole disconnect in the blocked pole

- Close the GRTB

- Open the MRTB





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6.2 Control Performance General

In HVDC systems various control loops are utilized. This chapter describes the

recommended system tests to prove the steady state and dynamic performanceof the control system.

Test Objectives

The site tests are performed to fine-tune the response of each control loop

following a change in the reference value and to verify the control behaviour

during disturbances causing changes in the actual measured quantities (DCcurrent, DC voltage etc.) of the respective control loop.

The tests are designed to verify that the controllers will not interfere with each

other or cause disturbances to the AC and/or DC systems. Controllers

insensitivity to AC system disturbances should also be verified. The following

sections will describe the tests necessary for control performance verification

under:

- Step response

- Control mode transfer

- AC system interaction / control

- Commutation failure

Preconditions

Before conducting all the tests the following preconditions must be completed:

- Off-site testing of the control system

- Converter unit tests

- Start/stop sequences checked

- Protection sequences checked

- Communication system operational





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All the equipment from the DC system necessary for power transmission of one

pole must be available. The AC systems should be set up as close as possible to

the minimum design effective short circuit ratio.

Test Procedures

The procedure for each test will be described in each section separately. All

tests for optimization and verification of the control performance require a similar

recording and monitoring set-up. This monitoring and recording set-up is listed

below. In cases where additional or different set-ups are required this is outlined

in the individual sections. The control performance tests would have been

performed during the off-site tests and can be used as a reference for all site

tests. It is also recommended to use the same recording equipment, monitored

signals and test report sheets as were used during the off-site tests. The

variables which should be monitored, include:

- Current order fat the current controller input)

- Actual DC current (at the current controller)

- Output of current controller

- DC voltage

- Extinction angle at the extinction angle controller

- Output of extinction angle controller

- Final control voltage to tiring controls (alpha order)

- Identification of active controller

- AC busbar voltage (3 phases)

- DC power

- Force retard command

- Stabilizing (damping) control signals (if any)





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6.2.1 Step Response Introduction

This type of test will depend on the various control modes in the HVDC system.

There are different modes of operation for each terminal of an HVDC system.

The most common ones are:

- Constant current control

- Constant minimum extinction angle control

- Constant DC voltage control

- Current error control

- Constant power control

Some of these control modes are only valid for operation as a rectifier or an

inverter.

Based on the large variety of possible HVDC system controls it is difficult to

design one test to cover all possible control scenarios. Thus the tests will be

described for the most common control loops.

If we consider the case of an HVDC system where the rectifier is in constant

current control and the inverter is in constant minimum extinction angle control,

then the controllers to be optimized in this case are:

- Rectifier current controller

- Inverter extinction angle controller

- Inverter current controller (margin current control)

- Rectifier extinction angle controller- Constant power controller

- Voltage dependant current order limiter (VDCOL)

Test Objectives

The objective of a step response test is to optimize the individual controller

which is active in a specific mode of operation to achieve stability.





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Preconditions

Before conducting step response tests the following preconditions shall be

completed:- Off-site testing of the control system


- Start/stop sequences

- Protection sequences

- Communication system

Test Procedures

It has proved to be good practice to implement the control settings obtained from

the off-site tests on the simulator and then fine tune them during the step

response tests with the real HVDC system.

Step response tests are performed at all controllers beginning at the converter

level and continuing toward station and system level.





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Rectifier Current Controller

Several methods can be used to fine tune the current controller at the rectifier The

typical one is to apply a step change in the current order. The duration of theapplied step (T) should be long enough to allow the system to stabilize following

the change in current order. The level of the current order (lo) prior to the step

change should be chosen keeping in mind that during the application of a step-up

in the current order (I) no limits are encountered. The location of application of

the current order step should be as close as possible to the input of the current

controller. During the test the rectifier must always be in DC current control and

the current controller at the inverter should be prevented from interfering.

Inverter Extinction Angle Controller

In order to optimize this controller at the inverter the rectifier will be in its normal

mode of control and the inverter should be in the minimum constant extinction

angle control- A step change in the extinction angle reference at the controller

should be applied. The first step change, A) should be applied only in the

direction of increasing the reference to avoid commutation failures. The

magnitude of the step () should be such that reaching any extinction angle

limits is avoided.

After optimization of the controller parameters a second step change B) from

o + back to o should be applied.





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It should be noted that in the case where the controller has other inputs, whichwill affect the extinction angle, the process should also be repeated for these

inputs. For example if there is an input that will increase the extinction angle due

to a sudden increase in DC current or a sudden decrease in the AC voltage, then

these functions should be checked following the optimization with step changes

in the extinction angle.

Inverter Current Controller (Margin Current Controller)

During this test the rectifier has to be In constant firing angle control (alpha

minimum) and the inverter is in constant current mode of operation. A step

change in the current order at the input of the inverter current controller is

applied. The duration of the step (T) is long enough to achieve stable operation

following its application. Before applying the step, the magnitude of current (lo) is

such that no current order limits are encountered when a step l is applied.





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Rectifier Extinction Angle Controller

In order to fine tune the extinction angle controller at the rectifier, two situations

can be considered.

1. In the case of a transmission system where the direction of power is

always constant. One terminal is always a rectifier and the other is

always an inverter. Then the function of this controller will be limited to

the situation when force retard is applied. The response of the controller

should be checked during the application of force retard.

2. In the case of an HVDC system where power reversal is used the

optimization of the controller at the second terminal should be performed

during transmission in the reverse power direction.

Constant Power Controller

In order to optimize this controller a step change in the power order (P) is

applied. The duration of the applied step (T) should be long enough to allow

stable operation to be achieved. The level of the power order (P o) prior to the

step change should be chosen, keeping in mind that during the application of a

step up in power order P no limits are encountered. The step size of P should

be chosen such that the change in current order exceeds the current margin.

Additional tests with smaller P may be conducted to check for instability and

interaction with other control loops.





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All controller settings must be adjusted such that recovery times and design

criteria as determined in system studies and requested by the specifications are

not exceeded.

All control system parameters shall be similar to those obtained during the off-

site tests. Deviations between the real DC system and the off-site tests can

occur, but must be explained.

Optimization shall be such that the response of the DC system to the step

change will be in a manner that shows the least amount of overshoot and the

shortest settling time.

No instability shall occur during the step response tests.


Introduction

The basic control mode transfer can be divided into two general types:

- From constant power control mode to constant current control mode and

back.

- From constant extinction angle control mode or constant DC voltage control

mode to constant current control mode and back at the inverter.

Additional control modes may be applied for special applications. Their control

mode transfer needs to be tested with respect to the particular application. The

test criteria, however, are anticipated to be similar as for the basic control mode

transfer as discussed here after.

HVDC systems are typically operated in constant power order control mode.

Under certain conditions it will be necessary to transfer from constant power

control mode to constant current order control mode. At the pole control level it

is a typical configuration to operate the rectifier in constant current control and

the inverter in constant extinction angle control.





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It is common practice to have the inverter also equipped with a constant current

controller. During certain AC system conditions a transfer from constant

extinction angle control mode to constant current control mode and back will be

required at the inverter.

Test Objectives

Control mode transfer tests are performed to verify that the HVDC control

system will transfer from one control mode to another smoothly and without any

adverse effect on the power system.

Preconditions

Before conducting control mode transfer tests the following preconditions shall

be completed:

- Off-site testing of the control system


- Start/stop sequences

- Protection sequences

- Communication system

The step response tests will have been completed and the individual controllers

are optimized.

Test Procedures

For DC systems where the transfer from constant power order control mode to

constant current order control mode is a manual function, the DC system will be

started in constant power order control mode. The transfer to constant current

order control mode must only take place when the actual DC current and the

new DC current reference value match. The matching can be performed

manually or automatically.





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In some cases the transfer from constant power order control mode to constant

current order control mode is also activated automatically during any decline in

the AC bus voltage, in order to avoid power/voltage instability. If such a feature

is available this control mode transfer shall be checked. This can be performed

by simulating AC undervoltage or during AC system faults.

The control mode transfer from constant extinction angle control to constant

current control at the inverter can be checked by operating the rectifier at its

minimum firing angle and forcing the inverter into current control- A typical

means to perform this test is through the use of tap changer operations.

This test can be performed in constant current order control mode or in constant

power order control mode.


The control mode transfer is expected to be a smooth transition with stable

operation during and after the transfer from constant power control to constant

current control and back.

Step changes in power shall not occur during the control mode transfer.

The control mode transfer from constant extinction angle control to constant

current control at the inverter shall be stable.





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6.2.3 AC System Interaction / Control Introduction

The controllability of HVDC systems is an important advantage of this

technology. Additional DC controls also make it possible to operate HVDCsystems with weak AC systems. This feature can be valuable in improving the

dynamic performance of large AC systems.

To achieve these advantages, the control systems must perform appropriately

for various disturbances and system conditions. The control loops shall not

interact unfavourably with each other.

To obtain adequate control performance, operation at higher than nominalrectifier firing angles or higher than minimum inverter extinction angles may be

required.

Commonly specified AC system interaction/control functions are:

- Frequency control

- Reactive power regulation

- AC voltage control

- Temporary overvoltage control

- Damping of frequency or power oscillations

- Frequency or power dependent power changes

- Special controls during faults

Test Objectives

The objectives are to optimize the individual controller active in a specific mode

of operation to achieve stability with the actual HVDC transmission system.





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Preconditions

Before conducting the tests of AC system/interaction control functions the

following shall be completed:- Off-site testing of the control system


- Start/stop sequences checked

- Protection sequences checked

- Communication system operational

The step response tests will have been completed and the individual controlloops are optimized.

AN control mode transfer tests shall have been completed. The AC system

should be set up in a condition which refers to the specific AC system

interaction/controller.

Test Procedures

The test procedures for the AC system interaction controllers may differ for the

individual application and functions.

Where applicable it has proved to be good practice to implement the control

settings obtained from off site tests on the simulator and then fine tune them to

the real HVDC system.

Testing the control behaviour during disturbances shall be planned very

carefully. The execution involves the connected AC system and can have a

serious impact on it.





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AC system disturbances or HVDC operating mode changes to check the

interaction of the HVDC system with the AC system may be initiated by:

- Load rejection (simulated DC line faults)

- AC line switching

- Generator tripping

- Ramping of DC power

- Power reversal

- Transformer energizing

- Filter switching

- AC or DC line faults

- Modulation of generator excitation system

Some of the above mentioned initiating functions are also dealt with in 6.3

and 6.5.


All controller settings must be adjusted such that recovery times and design

criteria as determined in system studies and requested by the specifications are

not exceeded.

All control system parameters shall be similar to those obtained during the off-

site tests. Deviations between the results from site tests and the results from off-

site tests can occur, but shall be explained.

No instability shall occur during the AC interaction/control tests.

-





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6.2.4 Commutation Failures Introduction

Commutation failures on a DC system can be caused by either:

- AC system disturbances- Converter control malfunction

Commutation failures may occur only once (single commutation failure), or

during a number of consecutive periods (multiple commutation failure), or may

be persistent (persistent commutation failure). Commutation failures by

converter control malfunction will be simulated during the tests described in this

part. Trial operation provides a useful further test with regard to normal system

disturbances (rather than those simulated).

Test Objectives

To observe and confirm that the control system is stable during and after

commutation failures and valve misfires, and that recovery is achieved within the

prescribed time period.

In the case of a DC system where the DC power circuit is resonant at a

frequency close to the fundamental, the commutation failures may excite

oscillations on the DC side. The tests will demonstrate that the control system

will be able to damp such oscillation upon the removal of the excitation. In

addition the tests will demonstrate that these oscillations will not be magnified

due to the action of the control system.

To check the commutation failure protection and any valve overload protection

such as voltage dependent current limits.

In the case of HVDC systems with overhead lines or cables, the tests are

designed to ensure that the DC line or cable protection will not operate during

these disturbances.





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Preconditions

Before conducting commutation failure tests, the optimization of the control

system must be completed.

All equipment from the DC system for one pole necessary for power

transmission must be available.

In the case of long-distance transmission, the DC line or cable protection should

be with its final settings.

The telecommunications should be available in the case of long-distance

transmission.

The inverter AC system should be set up as close as possible to the minimum

design effective short-circuit ratio.

Test Procedures

The tests are described for one direction of power flow and should be repeated ifthe system is designed for power reversal.

The tests may be carried out at several current levels up to rated current,

In the case of long-distance transmission and series connected groups per pole,

the tests shall be performed for all groups in operation and, if permitted by the

plant design, also with reduced numbers of groups.

In addition to the monitoring points listed in the general section the following

signals will be monitored during the tests:

- The measured firing angle

- The firing pulses to the valves





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These monitoring points may vary from one system to another.

The valve commutation failure in inverter operation or a firing failure in rectifier

operation will be achieved by blocking the firing pulses to one valve. Cautionshould be observed that sometimes the block of one firing pulse in the control

system may initiate certain actions in the control and protection, which will create

abnormal situations, hence the location of blocking the pulse should be

considered carefully, depending on the particular valve design.

The duration of the applied commutation failure or firing failure respectively shall

be increased in steps:

- Duration shorter than the time required to initiate any protective action

- Duration long enough to activate any valve overload protection such as

voltage dependent current limiting

- Duration long enough to initiate the persistent commutation failure or firing

failure protection (tripping of the converter)

In the case of long-distance transmission this test should be performed with and

without telecommunications.


The test criteria depends on the duration of the commutation failure and the

respective station of the selective protection system.

- No protection system initiated:

The controls shall perform reliably and recover within the specified time. Any

oscillations must be damped.

- Initiation of valve overload protection:

The DC current shall be reduced by control action and the system should

recover following the end of the commutation failures.





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- Initiation of the persistent commutation failure protection:

The designed sequence of tripping the converter should be verified. DC

system and AC system components must not be affected.

6.3 Switching AC Filters and Transformers Introduction

The switching of AC system components such as AC filters and transformers can

create disturbances to the AC system. These disturbances are due to inrush

current, saturation of components as well as resonance conditions.

Test Objectives

The objectives are to confirm that switching will not have any adverse effect on

the operation of the DC system and that AC system voltage disturbances are

within the prescribed limits. Proper recovery from commutation failures, if any,

should be verified.

The HVDC control system should perform without instability during suchconditions and in cases where it is used to produce a damping effect to such

disturbances, its performance should be verified.

Preconditions

All control performance tests under 6.2 should have been performed.

The commutation failure protection is functional.

All AC and DC equipment for the pole under test are available.





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Test Procedures

The tests should be performed in those configurations with those levels of the

DC current per pole and the inverter and rectifier AC systems and with thoseswitching instants which according to system studies tend to produce the most

onerous over-voltage or resonance conditions.

The tests will vary depending on the HVDC system configuration, for example:

Bipolar or monopolar systems

Groups in series or single groups per pole (long-distance transmission)

Back-to-back systems

If the DC system is bipolar and with a single group per pole whether back-to-

back or long-distance, then the test should be performed by switching on and off

the converter transformer in one pole while the other pole is in operation. This

means that the test can be performed only in monopolar operation, unless there

is a transformer bank on site or close to the DC terminal of comparable rating

that can be switched on and off after suitable time intervals. The same testshould also be repeated for switching each type of AC filter. In some systems

where redundant AC filters are available this test can be performed in bipolar

operation.

If the DC system is bipolar with more than one valve group in series per pole

(long-distance transmission), the test should be performed by switching on and

off one converter transformer while the system is in bipolar operation, provided

that the plant design permits unbalanced numbers of groups per pole. The sametest can also be repeated for the switching of an AC filter.

The tests should be performed at both the rectifier and inverter stations.





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The monitoring includes:

- DC current

- DC voltage

- DC power

- AC system voltage - 3 phases

- AC filter currents

- Transformer primary currents

- Extinction angle


In all the switching tests the HVDC system should recover to stable operation.

Commutation failures may occur during the test. AC system overvoltages may

occur but must remain within the specified limits.

6.4 Loading Tests

Introduction

All components in an HVDC system are designed for a certain permissible

operating temperature. They are either cooled naturally or require forced cooling.

Special cooling systems are required for the main components like valves, trans-

formers and smoothing reactors. For availability reasons most of the cooling

systems have redundancy. The loading tests should include a complete

performance test of the cooling systems.

Overload capabilities depend on the loading capability of the main components

and the ambient temperature.

Loading tests are also known as heat run tests.

If any additional loss tests to those performed in the factory have been specified,

they may be performed during the loading tests.





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Test Objectives

The general test objectives are to verify the proper function of all cooling

systems with respect to cooling controls and redundancy requirements and toverify the loading capability within specified temperatures of all major HVDC

system components and the total system under rated load conditions and, as far

as possible, at overload conditions.

Preconditions

To perform loading tests the relevant part of the HVOC system must be ready foroperation.

The AC systems must be available for power transfer at levels as required for

the loading test.

The ambient temperatures should ideally be close to the maximum design

temperature. For testing maximum overload conditions ambient temperatures

should be as low as possible.

Test Procedures

The HVDC system has to be operated in a mode of operation allowing the HVDC

components to reach their maximum operating temperature. In most cases this

will be achieved at the maximum current level, at maximum ambient temperature

and with the cooling system redundancy not in use.

In some applications the HVDC components will have their maximum loading

and operating temperature at a different mode of operation than the maximum

current level (e.g. with firing angles close to 90 degrees electrical).





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The loading test must be performed with ample duration to ensure that all

components will have reached a steady state operating temperature. If maximum

ambient temperature is not available, the tests shall be performed at the existing

ambient temperature and the anticipated component temperatures at maximum

ambient temperature shall be calculated from design load curves.

The temperatures are monitored by means of temperature indicators or by

infrared scanning. Ambient temperature, room temperatures and cooling circuit

temperature will be recorded- All equipment is monitored by visual inspection.

For overload conditions the cooling systems may be operated with ail available

redundancy. Ambient conditions may support the cooling capacity.

After the loading tests are finished gas in oil analysis may be performed for the

major oil filled components.


Neither under normal operating conditions or under overload conditions shall

design temperature rise of any HVDC component be exceeded.

No hotspots should occur in the buswork and the connectors.

The cooling control shall control the cooling systems to dissipate all heat losses

specified ambient conditions

The power capability shall be in accordance with the specified loading curve.

Proper function of the various cooling control circuits and the redundancy

requirements should be verified.





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6.5 AC and DC System Staged Faults Introduction

The reaction of an HVDC system with the connected AC networks must be

known precisely in case of AC network faults so that appropriate control actionscan be applied. Normally, to maintain stability, the DC power flow shall be re-

established without undue delay when commutating voltages reappear after fault

clearance. Due to inrush currents the voltages may be severely distorted. This

may complicate the tasks of the control system and circulate unconventional

current waveforms in the protection relays of the AC system with the associated

risk of spurious protection operation.

The use of staged fault-testing to verify proper function of the HVDC system may

be a controversial issue. Verification by utilizing simulation or digital study tools

could be an alternative. If staged fault tests are conducted, they may require the

arrangement of particular system conditions which may imply high cost and

expose the affected equipment to high stress which may effect equipment life

time. However, since faults to the AC and DC systems are most likely to occur

during the life of a project, some power utilities recognize the merits of staged

fault testing to verify the AC and DC system protection and survivability.

Modern digital study and simulation tools are considered to be adequate and

precise and some utilities prefer to rely on such tools or HVDC simulators in

preference to performing staged fault tests. In such cases it will also be required

to verify the proper function and coordination of the control and protection

equipment. It should be also verified that the stress on all components in the AC

and DC system will not exceed their permissible limits. This pertains particularly

to the AC system faults.

Transient faults on the DC line are normally cleared by the combined action of

protection and converter control, which extinguishes the arc by temporary

blocking, or by force retarding the rectifier.





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In case of permanent fault on the DC side, depending on the design of the DC

line, an automatic sequence must localize the faulted section, isolate it and re-

establish if possible the initial power flow as rapidly as possible. Consequences

of DC system faults on the AC systems are generally less severe than those due

to AC system faults.

Test Objectives

The objective of these tests is to verify the ability of the combined AC/DC system

to react as specified during and immediately after fault clearance.

In case of AC system distant faults, that means the converters must continue to

operate without commutation failure or with a specified maximum number of

commutation failures. In case of close faults, the DC power flow must be re-

established within a specified time after fault clearance. In case of DC side

faults, the rated DC power, if possible (depending on the HVDC design), must be

re-established within specified time after clearance of the fault.

Preconditions

The following preconditions shall be fulfilled.

- All AC and DC equipment energized

- End-to-end tests completed

- Changes of DC configuration (6.1) completed

- Response of protection relays in the AC system checked with distortedwaveforms

- Control and protection performance tests (6.2) completed

- Switching of AC filters and transformers (6.3) completed)





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Test Procedures

For any type of staged fault the following pre-fault system conditions must be

specified and post-fault conditions estimated:- Telecommunication on or off

- Frequency of the AC systems (in case of an isolated system)

- AC system voltage before and after the fault

- Configuration of the AC system where staged faults are applied, before and

after the fault (lines in service to the nearest substations, close-by

generating units, loads)

- Short-circuit levels on both sides of the DC link

- Initial DC power flow

- HVDC system configuration including AC and DC filters

- Expected time duration of the fault

- Location of the fault

DC staged faults normally do not require special AC system configurations.

However, to maintain an acceptable level of system security during an AC

staged fault test due to risk of misoperation of some protection relays or

breakers, a special configuration of the AC system shall be generally adopted

with a very specific plan of generation. That may make this type of test highly

costly.

The safe monitoring of many signals is particularly important because this type

of test cannot be repeated many times because of its cost, the stresses on theequipment and for security reasons. For the staged fault tests it is thus highly

recommended to use redundant recorders.





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The following signals shall be recorded.

On the system side:

- AC voltages and currents on each phase- DC voltages on both poles

- AC currents in the converter feeders on each phase

- DC currents on each pole and on each DC neutral connection

- AC currents in filter banks

On the controls:

- Main sequencing signals (start-stop, blocking-deblocking, breaker openingand closing orders)

- Crucial protection signals (e.g. overvoltage protection, commutation failure

protection)

- Firing angle order

- Firing pulses on each 12 pulse converter unit

- Alpha and gamma angle measurements

- Current order

Some extra signals may be usefully recorded:

- One valve voltage on each converter unit (that may require specific dividers

to be provided)

- Timing signal allowing synchronization between recordings in the converter

stations and possibly in some generating stations in the vicinity.

For AC faults a limited number of signals need to be recorded on the remote

end, basically DC voltage and current, current order, and firing angle order.

The HVDC power level adopted depends on the AC system configuration

prepared for the test with its associated generation plan.





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Staged faults can be dangerous and a special effort should be made to ensure

safe fault throwing equipment and procedures are used.

For application of some faults (specifically in the station) it will be necessary totemporarily install fault throwing switches.

Type of tests:

- Distant phase-to-earth fault and distant 3 phase-to-earth fault: the particular

objective is to assess the sensitivity of the converters to commutation failure.

- Close phase-to-earth fault: the particular objective is to assess the ability of

controls to contain the 100 Hz or 120 Hz ripple on the DC voltage to an

acceptable level, and sometimes to assess the ability to recover rapidly in

case of automatic protective blocking during the fault

- Close 3 phase-to-earth fault: the particular objective is to assess the ability

to re-establish the power flow rapidly.

- AC busbar phase-to-earth fault: same objective as for the close phase-to-

earth fault plus the assessment of possible consequences of fault neutral

current circulation in the earthing grid. This test will also verity the

insensitivity of the controls towards such disturbances and the proper

function of the protection.

- DC line fault: the particular objective is to check the correct response of the DC

line protection and the automatic sequence of recovery. For close faults the ob-

jective is to check the insulation coordination in the DC switchyard. For distant

faults it is to check whether the line protection can differentiate between line

faults and station faults at the remote end. It should also be checked whether

there is any false operation of the DC protection of the sound pole.


- The HVDC system shall remain stable during and after the fault.

- No spurious operation of protection relays either in the DC system or in the

AC system should occur.

- Transient AC and DC voltages shall be contained within the envelope

determined by the insulation coordination study.





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- Speed of DC power recovery shall be within the specified time.

- A specified maximum number of commutation failures during a defined time

period shall not be exceeded.

Records should be compared with those obtained by the simulations normally

achieved before the staged faults on site. Differences in response should be

explained.

6.6 Loss of Telecom, Auxiliaries or Redundant Equipment

6.6.1 Loss of Telecommunications between Terminals

Introduction

As indicated in IEC 60919-1, different types of information may be transmitted

between HVDC substations, each type requiring specific performances:

a) Signals for control:

- Power order

- Current order

- Frequency control

- Damping control

- Force retard for the remote station

b) Operation orders:

- Change of control mode of operation

- Operation of switches

- Block/deblock

c) Status indications:

- Position of switches

- Number of converters in operation





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d) Measured values

e) Alarm signals

f) Voice communication

g) DC line fault location

The consequence of a loss of telecommunications is related to each of the

above types of information transmitted, furthermore the most critical types of

information are generally transmitted using redundant telecommunication

systems and the tests should address the loss of redundancy.

Most HVDC systems are designed such that loss of telecommunication typically

does not interrupt or significantly modify the existing level of power transmission.

Telecommunications between terminals are used to improve the operating

conditions, the transient performance or to provide some special control facilities

such as frequency or damping control.

The loss of telecommunication should initiate a predefined control sequence that

may end in freezing the power transfer to the pre-fault value.

The objectives and conditions of the loss of telecommunication tests should be

stated bearing in mind these general considerations.

Test Objectives

The general objective of the loss of telecommunication test is to check that when

loosing telecommunication during power transfer or during a start up sequence

the operation of the HVDC system and its effects on the AC system are

compliant with the specification. Message security from external influences such

as noise, etc. should be verified as well.





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Test Procedures

It is advisable to perform the tests under the following modes of operation:

a) During steady state operation:

Check the impact on:

- Power order/current order

- Transmitted power

b) During current or power order ramping

c) During start-up and shut-down sequences

The tests are done by disconnecting all end-to-end telecommunication channels

except for operator voice channels at a predetermined instant in time. Message

security should be verified by simulating external noise and/or other

disturbances.

Preconditions

Prior to the tests covered hereafter the proper operation of the

telecommunication system should have been tested during subsystems tests at

site. The function of the control loops and protection circuits should have been

tested with and without telecommunication. The ability of the system to shut-

down safely without telecommunication should have been tested during the end-

to-end tests (see part 4).

The topology of the link, the amount of power transmitted and the sequence of

operation existing when the loss of telecommunication occurs shall be

determined depending on the objective of each individual test.

- During steady state operation and ramping of power/current order: The

HVDC system should be transmitting power at a significant level and operate

under the appropriate control mode.

- During start-up and shut-down sequences: The detailed testing procedureshould be established depending on the specific control strategy.





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The loss of telecommunication tests is satisfactory if the HVDC system remains

stable and the effects on the AC systems remain within the specified limits.

No unplanned operation of protective relays should occur.

Start-up and shut-down sequences should terminate safely, with indications

showing the status at which the sequence was terminated.

6.6.2 Loss of Auxiliary Power Supplies Introduction

The auxiliary power supplies of an HVDC substation may be broken down into

three classes according to the level of reliability needed for the performance of

the HVDC system:

- General AC supply with no back-up

- AC supplies with automatic change-over to a back-up AC source

- Uninterruptible supplies including diesel back-up generators

Test Objectives

The tests should verify that short interruptions in the auxiliary supply during

change-over do not disturb the HVDC power transmission and safe and

controlled shut-down of the HVDC substation takes place in the event of a total

loss of all auxiliary supplies except the uninterruptible supply.

The tests should also verify safe shut-down of the HVDC system in case of

failure of the uninterruptible power supply (if any).





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Preconditions

Proper function of change-over systems such as automatic undervoltage transfer

should have been checked. Uninterruptible power supply functions, capacity andperformance should have been tested.

Test Procedures - Test Acceptance Criteria

For the AC supplies with automatic change-over, which are used to supply the

cooling fans and pumps and other critical equipment for significant power, tests

should be performed at no load and at full load. The tests should be performedseparately on each type of auxiliary subsystem as follows.

- Loss of the entire AC auxiliary power: the DC system should shut down

safely in a coordinated sequence. If a back-up generator group is provided, it

should start up within the specified time limit and supply the appropriate

load.

- Loss of auxiliary power with automatic change-over: a transfer to the second

AC source will be initiated and the time of change-over will be checked;special care should be given to the restart condition of induction motors.

Special care shall be given to ensure that cooling media flow is not

interrupted longer than acceptable to the relevant equipment.

- Loss of redundant supply of control, protection and communication

equipment: the control and protection equipment should continue to operate

as specified.

- Loss of the entire supply to the control equipment: the DC system shouldshut down safely. For such a catastrophic failure one would normally have to

rely on protective shut-down.





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6.6.3 Loss of Redundant Equipment Introduction

HVDC systems may contain various redundant control, protection, and

measuring equipment to enhance system reliability.

Test Objectives

Take-over of the sound redundant element in case of failure should be verified-

The HVDC system should continue to operate as specified.

Preconditions

Proper function of the control and protection system with all redundant elements

in place and functional should have been checked.

Test Procedures - Test Acceptance Criteria

The appropriate tests of this category depend greatly on the redundancy conceptused for the system design under test The tests should simulate the loss of

redundant elements in the failure modes which are the most probable on the one

hand and those which may cause the most onerous conditions for the system.

The tests are successful, if the system responds as designed.





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PART 7: TRIAL OPERATION

General

Introduction

Trial operation provides the opportunity to operate the HVDC system together

with the connected AC systems for an extended period of time. Operation shall

be as close as possible to real operating conditions.

This time period is a particular time of accurate observation of the completeHVDC system and all its components.

The HVDC system will be operated by the owners personnel. Additionally

training of manpower particularly in fault analysis and system performance can

take place.

Test Objective

The purpose of trial operation is to verify the performance and the availability of

the HVDC system during a specified period (typically 2 to 4 weeks) of operation

under real but within specified conditions.





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Preconditions

Before conducting trial operation the following preconditions shall be fulfilled.

- All equipment is available.- All operation and integration tests are completed.

- Operators sufficiently trained to conduct system operation.

- Power transfer schedules between the AC systems coordinated.

- The AC system configurations are within specified limits.

Test Procedure

During trial operation the HVDC system shall be utilized in such a manner as re-

quired by the original design and the owner's intended use.

Typically trial operation is performed for a period of 2 to 4 weeks. During this

period, operation shall be conducted by the owner's personnel and the

manufacturer's assistance shall be limited.

During trial operation accurate observation of the complete HVDC system and all

its components will be carried out.

Any abnormal or unexpected responses or incidents which effects the availability

of the system must be carefully analyzed to determine the cause.

For monitoring and recording purposes the station recorders are sufficient.

These can be complemented by additional recording equipment for the purpose

of collecting data for design study verification.

AC system configuration data shall be available. Should an HVDC system be

built in stages, trial operation could be allocated to the individual stages.





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The tests having no influence on the behaviour and safety of the HVDC system

(radio interference test etc.) can be performed during or after trial operation upon

mutual agreement


During trial operation no forced outages or disturbances due to malfunction of

any HVDC equipment shall occur.

If the need for modifications or adjustments arise during trial operation, action

shall be taken and depending on their extent, a portion of the period of trialoperation or the complete trial operation may have to be repeated.

The criteria for repetition of part or all of the test shall be agreed prior to the start

of trial operation.





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PART 8: SYSTEM TEST PLAN AND DOCUMENTATION

General

Introduction

This part describes the documentation including the system test plan required to

perform the system tests, consisting of:

8.1 Plant Documentation and Operating Manuals

8.2 System Study Reports and Technical Specifications8.3 Inspection and Test Plan

8.4 System Test Program

8.5 Test Procedures for each Test

8.6 Documentation of System Test Results

8.7 Deviation Reports





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8.1 Plant Documentation and Operating Manuals

Plant Documentation is required to provide sufficient up-to-date information on

all equipment installed and its function, location and interconnections.

Up-to-date settings of the control and protection equipment shall be provided.

This shall include the data acquisition system.

The operators should have had sufficient training to be able to execute all

switching and control operations under ail operating conditions with the guidance

of the Operating Manuals.

8.2 System Study Reports and Technical Specifications

System Study Reports shall provide the information needed to operate the

HVDC system together with the connected AC systems within the specified

limits. Particular System Study Reports required for this purpose are for

example:

- AC/DC system interactions- Reactive Power Control

- Protection Coordination

- Control coordination, changing of System configuration and switching

Equipment Specifications will be required to verify the individual equipment

ratings.

8.3 Inspection and Test Plan

The Inspection and Test Plan together with the System Test Program form the

System Test Plan. The Inspection and Test Plan identifies all inspections and

tests to be performed on:

- HVDC station equipment and DC line/cable/bus including the earth electrode

- HVDC control and protection- environmental considerations





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- AC/DC system interaction


This guide is structured in a logical sequence and can thus provide background

information for any specific project application.

Each Inspection and Test Plan should comprise the following as appropriate and

similar to this guide:

- Specific objectives per test

- Test procedures (reference to standard procedures)

- Test acceptance criteria

- Preconditions for the Test

- References to system studies on specifications

- References to off-site tests

- Special conditions

All inspections and tests to be performed shall be identified with a unique

number. This number should also be used as cross-reference on all standard

Test Procedures and for the Documentation of the Inspection and System Test

Results.

The Inspection and Test Plan shall also contain all required load levels and load

changes as well as the expected effective short-circuit level (ESCR).

Procedures for each test should be thoroughly discussed with all parties involved

before finalizing the test schedule.

All inspections and systems tests shall be mutually agreed among all parties

concerned well in advance off the actual tests to allow sufficient time for

scheduling. Contingency measures in case of any failure shall also be

discussed.





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8.4 System Test Program

The System Test Program schedules the system tests.

It coordinates the requirements with dispatch authorities and specifies:

- preconditions including environmental and AC systems configuration and

generation schedules;

- preconditions to be selected (control locations, control mode, control level

energy transfer mode);

- initial and final HVDC system configuration;

- energy direction;- power levels, duration and ramp rates.

The System Test Program shall identify the test team and the test leader for

each individual test, for each converter terminal, the load dispatch center,

affected substations or power stations, and test locations where faults or short

circuits are created-

The test schedule should have suitable hold points to check the status ofprotection, auxiliaries and fire protection.

Final permission from the operating authorities shall be requested immediately

before test execution.

Test duration should be planned with an adequate time margin to allow for

contingencies. As system tests have a high potential of causing major

disturbance on the HVDC and the connected AC systems considerations shallbe given to execution during off peak hours.

It is also preferable that the AC systems are configured to have sufficient loading

margin to sustain total HVDC load rejection.

In case of planning staged fault tests ample precaution and support by specialist

staff should be provided including studies showing the effect of faults on the AC

system.





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8.5 Test Procedure for Each Test

The test procedures describe for each individual test the instructions to be

followed by the test team.

The preconditions to be fulfilled and the references to system studies and

specifications or off-site tests are defined.

Test objectives, procedure and acceptance criteria specify the actual

performance of each specific test.

The test procedure also lists the test equipment to be used and the

measurements to be taken, and the exact measurement points. Cross-reference

to the documentation of system test results shall be identified by using a

numbering system. The same numbering system shall also reference the

application of the test procedure and the inspection and teat plan.

The test procedures may be one report, specifically written for the project or an

assortment of generic test descriptions compiled for the project, in case generic

test descriptions are used these should be packaged complete with an index.

8.6 Documentation of System Test Results

A commissioning report describing the results of each test series, together with

the test results, data acquisition traces and data captured by station recorders,

sequence of event recorders printout, alarm printout and other special results

shall form the documentation of System Test Results.

Forms used to sign off successful tests should differ from unsuccessful tests to

be recorded.





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A failed test should be recorded in a failure report which shall have the test

number description and the reason for failure noted. Upon correction of the

problem the manufacturer should briefly describe the modification on the same

failure report and indicate that the system is ready for a retest of the failed test.

All modifications shall be formally tracked according to the applicable quality

assurance procedures in all applicable parts of the documentation.

A detailed description of the modification should be provided in the deviation

report. For tracking the status of failed tests a database shall be kept indicating

the status (new, modification implemented, retested, passed, failed again) of

each test.

A record of equipment outages, malfunctions and diagnostic test results related

to the commissioning shall be kept.

Any special occurrence on the results of any test shall be highlighted by the test

team prior to submitting the commissioning report.

All system test results should be well indexed and referenced to the individual

tests.

8.7 Deviation Reports

The deviation report shall form part of the commissioning report with a detailed

description of modifications and reference to the specific tests and test results.

A discussion of the modifications carried out should be included for furtherreference.

____________





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Standards Survey

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Customer Service Centre (CSC)

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