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INSTITUTE OF ENERGY TECHONOLOGY

GROUNDING FOR OFFSHORE WIND FARM ELECTRICAL SYSTEM

GROUP WPS2 – 864,

SPRING SEMESTER 2010

i

Title: Grounding for Offshore Wind Farm Electrical System Semester: 8th Project period: Spring 2010 Supervisor: Zhe Chen Project group: WPS2-864 _____________________________________ [Lucia Pereiro Estévez] _____________________________________ [David Rodriguez Lamoso]

Copies: 4 Pages, total: 111 Appendix: 3 Supplements: 4 CD By signing this document, each member of the group confirms that all participated in the project work and thereby that all members are collectively liable for the content of the report.

SYNOPSIS:

The main objective of the project is to model and analyze different grounding philosophies for the infield cable system of an offshore wind farm. The grounding in wind farm is an important point to treat. It is important provide a correct grounding for the security of the personal that work in this place and protection of the equipment. The main problems have the wind farms are the single-line-ground faults (80%). In this project will be analyzed the system behavior with this disturbance using the selected grounding. It will be obtained the voltage and current in different points of the system. These obtained parameters will be analyzed and compared for obtain the best grounding.

iv

v

Abbreviations

AC Alternative current

DFIG Doubly fed inductor generator

HV High voltage

LV Low voltage

MHV Medium / high voltage

MV Medium voltage

WT Wind turbine

vi

vii

Nomenclature

XGO

RN

XN

ørotor

Estep50

Etouch50

Cs(hs,k)

ρs

ts

I’’K

If

Sf

Df

Cp

Rg

Em

Es

ρ

Km

Ks

Ki

Kii

Kh

XcoT

Xco

R

VLN

IR

Ico

IL

zero-sequence reactance of transformer

resistance of grounding resistor

reactance of grounding reactor

diameter of the rotor

step voltage for 50 kg of reference

touch voltage for 50 kg of reference

reduction factor for derating the nominal value of surface layer resistivity

wet resistivity of the surface

shock duration in seconds

short-circuit current

total zero sequence rms fault current (3I0)

split factor or current division factor

decrement factor

corrective projection factor

grid resistance in transformer 150/33 kV

mesh voltage

step voltage

soil resistivity in Ω.m

mesh voltage geometric correction factor

step voltage geometric correction factor

correction factor that into account the increase in current at the extremities

of the grid

corrective weighting factor that adjusts the effects of inner conductors on the

mesh

corrective weighting factor that emphasizes the effect of grid depth

distributed capacitive reactance of the line

distributed capacitive reactance per phase

total grounding resistance

system line to neutral voltage

current through the neutral resistance

capacitance charging current

inductive current through the coil

viii

Ir

XL

f

ZSC

Z0

ZL

Un

In

ZG

UG

SGcc

RG

XG

ZL1

ZL2

ZT1

UKT

U

Sn

XT1

RT1

ZL3

ZL4

ZT2

XT2

RT2

ZAS

Iras

Uras

Sras

ILR

RAS

XAS

resistive current

reactance of coil

frequency of the system

short circuit impedance

zero sequence impedance

line impedance

voltage in the neutral of the transformer

current in the neutral of the transformer

impedance of the grid

rated voltage in the connection point

initial short circuit apparent power of the external grid

resistance of the grid

reactance of the grid

impedance in line 1

impedance in line 2

impedance of the transformer 150/33 kV

short circuit voltage in percent value of the primary winding

phase to phase voltage

apparent power of the transformer

reactance of the transformer 150/33 kV

resistance of the transformer 150/33 kV

impedance in line 3

impedance in line 4

impedance of the three-windings transformer 33/0.69/3.3 kV

reactance of the three-windings transformer 33/0.69/3.3 kV

resistance of the three-windings transformer 33/0.69/3.3 kV

doubly-fed induction generator impedance

rated current of generator

rated voltage of generator

rated apparent power of generator

locked-rotor current

doubly-fed induction generator resistance

doubly-fed induction generator reactance

ix

Table of contents

1 INTRODUCTION ......................................................................................... 1

1.1 Background ........................................................................................... 1

1.2 Problem formulation .............................................................................. 2

1.3 Objective ............................................................................................... 2

1.4 Project limitation .................................................................................... 3

1.5 Report structure .................................................................................... 3

2 GROUNDING PHILOSOPHIES AND REQUIREMENTS ............................ 4

2.1 Introduction ........................................................................................... 4

2.2 Methods of systems neutral grounding.................................................. 5

2.2.1 Ungrounded .................................................................................... 5

2.2.2 Solid grounding ............................................................................... 6

2.2.3 Resistance grounding ..................................................................... 6

2.2.4 Ground fault neutralizer .................................................................. 7

2.3 Comparison of different grounding systems .......................................... 8

2.4 Obtaining the system neutral ................................................................. 9

3 MODELLING OF OFFSHORE WIND FARM ............................................ 13

4 STATION GROUNDING SYSTEM ............................................................ 19

4.1 Design ground grid substation ............................................................. 19

4.2 Design ground grid wind turbine .......................................................... 27

5 MODELLING OF DIFFERENT TYPES OF GROUNDING ........................ 29

5.1 Ungrounded ........................................................................................ 29

5.2 Solid grounding ................................................................................... 30

5.3 Low-resistance grounding ................................................................... 31

x

5.4 High resistance grounding ................................................................... 32

5.5 Resonant grounding (Petersen coil) .................................................... 34

6 ANALYSIS ................................................................................................. 37

6.1 Introduction to shorts circuits analysis ................................................. 37

6.2 Single-phase to ground short circuit .................................................... 38

6.3 Simulations with the chosen groundings ............................................. 40

6.3.1 Single-phase to ground short circuit on busbar 33A ..................... 41

6.3.2 Single-phase to ground short circuit on line 8 ............................... 68

6.4 Comparison of the results ................................................................... 93

7 ANALYSIS AND DISCUSSION ............................................................... 104

8 CONCLUSSIONS ................................................................................... 108

9 FUTURE WORK ..................................................................................... 110

10 REFERENCES ..................................................................................... 111

A. Short Circuit Current from DIgSILENT ......................................................... I

B. Equivalent Circuit of the wind farm .............................................................. V

C. Tables of Short Circuit Currents on busbar 33A and on line 8 ................... IX

xi

List of Figures

Figure 1-1 Global installed wind power capacity 08/09 MW ................... 1

Figure 2-1 Ungrounded. ..................................................................................... 5

Figure 2-2 Solid grounding ................................................................................. 6

Figure 2-3 Resistance grounded. ....................................................................... 7

Figure 2-4 Ground fault neutralizer ..................................................................... 8

Figure 2-5 Grounding in generators ................................................................. 10

Figure 2-6 Connection wye-delta ...................................................................... 10

Figure 2-7 Zigzag grounding transformers. ...................................................... 11

Figure 2-8 Wye-delta grounding transformers .................................................. 12

Figure 3-1 Model offshore wind farm, 2 string with 8 wind turbine each one .... 13

Figure 3-2 Distance between wind turbines. .................................................... 14

Figure 3-3 General scheme of DFIG ................................................................ 14

Figure 3-4 Transformation of mechanical energy to electrical energy in the wind

turbine. ............................................................................................................. 15

Figure 3-5 Pitch control .................................................................................... 16

Figure 3-6 Model wind turbine DFIG ................................................................ 17

Figure 4-1 Ground grid 60 x 60 m. ................................................................... 27

Figure 4-2 Ground grid 7 x 7 m with 2 rods. ..................................................... 28

Figure 5-1 Ungrounded .................................................................................... 30

Figure 5-2 Solid grounding ............................................................................... 31

Figure 5-3 Low-resistance grounding ............................................................... 32

Figure 5-4 High-resistance grounding. ............................................................. 33

Figure 5-5 Resonant grounding. ....................................................................... 35

Figure 6-1 Generic scheme of single-phase short circuit to earth .................... 39

Figure 6-2 Scheme impedance single-phase short circuit to earth ................... 39

Figure 6-3 Scheme with the measuring and simulation points ......................... 40

Figure 6-4 Waveform of fault current on busbar 33A. ........................... 42

Figure 6-5 Waveforms of voltage and current in HV side ................................. 42

Figure 6-6 Waveforms of voltage and current in LV side. ................................. 43

xii

Figure 6-7 Waveform of fault current on busbar 33A. ....................................... 44

Figure 6-8 Waveforms of voltage and current in HV side. ................................ 45

Figure 6-9 Waveforms of voltage and current in LV side. ................................. 46

Figure 6-10 Waveform of fault current on busbar 33A. ..................................... 46

Figure 6-11 Voltage and current in HV side...................................................... 47

Figure 6-12 Waveforms of voltage and current in LV side. ............................... 48

Figure 6-13 Waveform of Fault current on busbar 33A. ................................... 49

Figure 6-14 Waveform of voltage and current in HV side. ................................ 49


Figure 6-16 Waveform of fault current with solid grounding ............................. 51

Figure 6-17 Waveforms of voltage and current (solid grounding) ..................... 52


Figure 6-19 Fault current on busbar 33A. ......................................................... 54

Figure 6-20 Waveforms of voltage and current in HV side. .............................. 55









Figure 6-29 Voltage and current in HV side...................................................... 61








Figure 6-37 Waveform of fault current on busbar 33A. ......................... 66


xiii


Figure 6-40 Waveform of fault current in the line 8 ........................................... 68

Figure 6-41 Waveforms of voltage and current in HV side .............................. 69

Figure 6-42 Waveforms of voltage and current in LV side ................................ 70

Figure 6-43 Waveforms of fault current in line 8 ............................................... 71

Figure 6-44Waveforms of voltage and current in HV side ................................ 72


Figure 6-46 Waveforms fault current ................................................................ 73

Figure 6-47 Waveforms of voltage and current in HV side ............................... 74


Figure 6-49 Waveform of fault current in line 8................................................. 75



Figure 6-52 Waveforms of fault current ............................................................ 77


Figure 6-54 Waveforms of voltage and current in LV side ................... 79

Figure 6-55 Waveforms fault current ................................................................ 79



Figure 6-58 Waveform fault current .................................................................. 81

Figure 6-59 Waveforms of voltage an current in HV side ................................. 82

Figure 6-60 Waveform of voltage and current in LV side ................................. 83

Figure 6-61 Waveform of fault current in the line 8 ........................................... 83



Figure 6-64 Waveform of fault current .............................................................. 85






xiv







Figure 6-76 Fault currents with solid grounding (short circuit on busbar 33A) .. 93

Figure 6-77 Distribution of fault current. ........................................................... 94

Figure 6-78 Fault currents with low-resistance grounding (short circuit on

busbar 33A) ...................................................................................................... 94

Figure 6-79 Fault currents with Petersen coil (short circuit on busbar 33A) ..... 95

Figure 6-80 Fault currents with ungrounded (short circuit on busbar 33A) ....... 95

Figure 6-81 Fault current with solid grounding (short circuit on line 8). ............ 96

Figure 6-82 Fault currents with low-resistance grounding (short circuit on line 8).

......................................................................................................................... 97

Figure 6-83 Fault currents with Petersen coil (short circuit on line 8). .............. 97

Figure 6-84 Fault currents with ungrounded (short circuit on line 8). ................ 98

Figure 6-85 Faut currents with both sectionalizers open. ................................. 98

Figure 6-86 Fault currents with both sectionalizers open. ................................ 99

Figure 6-87 Fault currents with both sectionalizers open. ................................ 99

Figure 6-88 Fault currents with sectionalizer 33 closed, left, and with

sectionalizer 150 closed, right. ....................................................................... 100

Figure 6-89 Tendency of short circuit current for different situations of

sectionalizers .................................................................................................. 102

Figure 7-1 Estimate of short circuit current for different number of strings ..... 104


Figure 7-3 Step voltage in WT1 with different groundings .............................. 105

Figure 7-4 Touch voltage in WT1 with different groundings ........................... 106

Figure 7-5 Step voltage in WT1 with different groundings .............................. 106

Figure 7-6 Touch voltage in WT1 with different groundings ........................... 107


Figure 10-1 Equivalent circuit of wind farm......................................................... V

xv

List of Tables

Table 1 Advantages and disadvantages of grounding methods ......................... 9

Table 2 Parameters wind generator ................................................................. 15

Table 3 Parameters of the lines ........................................................................ 17

Table 4 Parameters offshore substation transformer ....................................... 18

Table 5 Three phase short circuit in HV with the sectionalizer 150 closed. ...... 21

Table 6 Single phase to ground in LV side with both sectionalizers open. ....... 21

Table 7 Single phase to ground in LV side with the sectionalizer 33 closed. ... 22

Table 8 Single phase to ground in LV side with the sectionalizer 150 closed. . 22

Table 9 The percents of short circuit types ....................................................... 38

Table 10 Effective values and peak values of the fault current (short circuit on

busbar 33A). ................................................................................................... 100

Table 11 Effective values and peak values of the fault current (short circuit on

line 8). ............................................................................................................ 101

Table 12 Voltages and currents in the neutral with short circuit on busbar 33A.

....................................................................................................................... 102

Table 13 Voltages and currents in the neutral with short circuit on line 8. ...... 103

xvi

1 INTRODUCTION

1.1 Background

As the demand for energy is constantly increasing, lot expert

looking for new forms to reduce dependence of fossil energy. Wind energy is

cheaper compared with solar or renewable energy. In the past years wind

power has grown more rapidly than other renewable energy in electricity

generation.[1]

Figure 1-1 Global installed wind power capacity 08/09 MW

The offshore wind farms

future. It is hoped that by the end of this decade

megawatt will be installed in the sea of Europe

production of the traditional thermal power station. The offshore wind farms are

a solution to solve problems like the noise, visual pollution and problems of land

dispute.

Offshore wind turbines are less obtrusive than turbines on land, as their

apparent size and noise is mitigated by distance. Because water has less

roughness surface than land (especially deeper water), the average wind speed

is usually considerably higher over

1.274

AFRICA & MIDDLE EAST

EUROPE

NORTH AMERICA

INTRODUCTION

As the demand for energy is constantly increasing, lot expert




Global installed wind power capacity 08/09 MW

The offshore wind farms will become an important source of energy in the near

future. It is hoped that by the end of this decade wind farms of

installed in the sea of Europe. This would be equivalent to the



ore wind turbines are less obtrusive than turbines on land, as their



is usually considerably higher over open sea.

865

38.909

76.152

1.274

38.478

2.221

AFRICA & MIDDLE EAST ASIA

LATIN AMERICA & CARIBBEAN

NORTH AMERICA PACIFIC REGION

1

As the demand for energy is constantly increasing, lot experts have been




Global installed wind power capacity 08/09 MW

become an important source of energy in the near

wind farms of thousand of

equivalent to the



ore wind turbines are less obtrusive than turbines on land, as their



LATIN AMERICA & CARIBBEAN

2

1.2 Problem formulation

This projects wants analyze and shows the different grounding configurations

for an offshore wind farm. Four different kind of grounding have been

considered: ungrounded, solid grounding, low resistance grounding and

Petersen coil.

The wind farm is formed of two strings of wind turbine each one. The model of

wind turbine is doubly fed inductor generator (DFIG).

For this purpose it has been decided to make the model of wind farm, using the

software DIgSILENT PowerFactory. DIgSILENT is a powerful and specialized

tool for simulating problems related to power systems. Different configurations

have to be analyzed when a single phase to ground short circuit in low voltage

side happens.

Different waveforms of voltage and current have been obtained for the

considered grounding and the behaviors in these waveforms have been

studied.

1.3 Objective

The main objective of the project is to model and analyze different grounding

philosophies for an offshore wind farm. DIgSILENT PowerFactory simulation

tool is used for simulations. The analysis compares and studies technical the

advantages and disadvantages to find the optimal grounding method.

The main goals of the project are summarized below:

• Study of grounding philosophies.

• Modeling of specific offshore wind farm.

• Modeling the different types of grounding philosophies/configurations.

• Analysis: Voltage, current and fault current.

• Choice the best grounding.

3

1.4 Project limitation

The range of work on this project could be much wider. Unfortunately there are

some limitations. The most important limitations of this project are described

below.

• DIgSILENT key license only can work for 100 nodes, for this reason we

simulated only 16 wind turbines.

• In the short circuit analysis only single phase to ground has been

considered in low voltage side.

• The lightning fault analysis has not been considered.

• Evaluate the cost and requirement equipment has not been considered.

1.5 Report structure

The present report consists of 9 chapters. In the first chapter a presentation of

the report is made. At the beginning an introduction to the subject is presented.

The problem formulation, the objective and the project limitation are also

presented. The purpose of the second chapter is to describe the different

groundings and compared them. The model of offshore wind farm is presented

in chapter 3. Here the different parts of wind farm are described. Chapter 4

presents the ground grind design of the system and method of calculate. In

chapter 5 the different groundings are modeled. The short circuit analysis is

studied in the chapter 6. An estimation of real wind farm is realized in chapter

7. Chapter 8 shows the conclusions of the analysis. Future works are

enumerated in chapter 9.

4

2 GROUNDING PHILOSOPHIES AND REQUIREMENTS

In this chapter, the different types of grounding and the reasons of using the

system are presented.

2.1 Introduction

The grounding has in several functions, which have in common the use of earth.

There are two types of grounding:

• Grounding of protection: to protect persons and equipment from

dangerous voltage.

• Grounding of system: connection between the earth and an electric

system. Usually is realized in the neutral points.

Below they are the reasons why the grounding is used:

• Security: protection of the persons and the equipments of high values of

voltage.

• Set the grounding network to the earth potential: to avoid dangerous

voltage due to the capacitive coupling (parasitic capacitances phase-

earth or capacitances between phases of systems at different voltage).

• Reduce the currents of earth fault: the connection of earth system

through an impedance to limit the fault currents in case earth faults.

• Reduce the overvoltages: the grounding can reduce the overvoltage by:

Transitional earth faults: the faults with arc generate overvoltages in

the healthy phases. These overvoltages are high in the isolated earth

system.

Increasing of the neutral potential: in an isolated system, an earth

fault causes that the neutral of the system has a voltage equal to the

phase voltage. The healthy phases increase in √3 times its voltage. If

the system is put to earth, the overvoltage will be lower if the

grounding is effective and the isolated level of the equipment can be

lower.

5

Manoeuvre transients and lightning: the grounding system, although

does not reduce the overvoltages from manoeuvre and lightning,

allows redistribute the voltage between the phases and reduce the

possibility of an isolated fault between phase and earth.

Simplify the location of the faults: a grounding system generates a

fault current which can be detected with easy for locating the fault

point.

2.2 Methods of systems neutral grounding

The grounding system can be classified according to the connections of neutral

of the earth system:

2.2.1 Ungrounded

The ungrounded system does not have an intentional connection of the neutral

to earth.

Really, the isolated systems are connected to earth trough of the capacitive

coupling between system conductors and ground

Figure 2-1 Ungrounded.

Advantage:

• It is not necessary invest in equipment for the grounding. But for the

protection system is necessary.

6

Disadvantages:

• High cost of isolating of equipments to earth. A fault causes that healthy

phases are increased √3 times their voltage.

• High possibilities of transient overvoltages by faults with arc, resonances

or other causes.

Its use is restricted to medium voltage systems. This grounding requires

systems of fault detection.

2.2.2 Solid grounding

The system with solid grounding has a direct connection of the neutral to earth.

Figure 2-2 Solid grounding

Advantages:

• Easy detection and localization of system earth faults.

• Limitation of the overvoltage by earth faults and transients by

manoeuvres and lightning.

Disadvantages:

• The earth faults are more energetic. The protections of high speed are

required for limiting the thermal and mechanical effects over equipments.

They are used in HV and MHV systems.

2.2.3 Resistance grounding

The system is connected to earth through resistance. The figure shows the

connection of the system.

7

Figure 2-3 Resistance grounded.

Depending of used value of the grounding resistance there are differentiated

two methods:

• High resistance grounding.

Advantages of high resistance grounding:

It is not necessary an instant trigger against to a first fault to

earth.

Decrease of the damage by thermal effects and electrodynamics.

Decrease of the transient overvoltages by manoeuvres and

lightning.

Disadvantages of high resistance grounding:

Behaviour similar to ungrounded. The healthy phases increase

√3 times its voltage.

• Low resistance grounding

The advantages and disadvantages of grounding with low resistance are similar

to solid grounding but with less harmful effects during the fault. This is because

it is decreased the earth current.

2.2.4 Ground fault neutralizer

The ground fault neutralizer system is to connect earth through a variable

reactance, such as Petersen coil.

8

Figure 2-4 Ground fault neutralizer

The induction coefficient of the coil is calculated to resonate with the earth

capacity of the system, so for an earth fault, the fault current decreases until

small resistive value.

Advantages:

• During earth fault, the current is very low and is in phase with the

voltage, so the faults with arc are extinct easily.

• An earth fault does not involve an instant trigger, then the continuity of

supply is better.

Disadvantages:

• The healthy phases are composite voltage during the fault. Similar to

ungrounded.

• The protection system is more complex.

This method is used in grounding of MV grid, especially in central Europe. [2][3]

2.3 Comparison of different grounding systems

In HV and MHV grids direct grounding systems are used for decrease the

solicitations by transient overvoltages and decrease the cost of the isolating.

In MV grids an optimum method of grounding does not exist. The choice of the

method is always a relation between costs of installation and operation.

In the following table the advantages and disadvantages of the different

methods of grounding are shown.

9

Table 1 Advantages and disadvantages of grounding methods

Grounding methods Advantages Disadvantages

Ungrounded Limited the earth fault

currents (less than 1%)

Caused overvoltages more

complex

Solid grounding The detection of earth

faults is easier

Caused high earth fault

currents

Resistance grounding (if it

is compared with solid

grounding)

Limited the earth fault

currents

Required protections more

complex

Reactance grounding (if it

is compared with

ungrounded)

Decreased the

overvoltages

Caused earth fault currents

higher

Ground-fault neutralizer Nearly zero fault current Required protections more

complex

2.4 Obtaining the system neutral

The normal form of obtaining a neutral point for grounding the system is use

transformer with wye-connected windings or the neutral of the generators. If it is

not possible, the grounding transformers or reactances are used.

The different forms of obtaining the system neutral are mentioned as follows:

• Grounding in generators

The neutrals of generators usually are connected to earth through an

impedance to decrease the single phase fault current (most common fault),

either directly or through single phase transformer.

10

Figure 2-5 Grounding in generators

• Grounding in transformers

They following may be used the neutral points of transformers with connection

wye-delta and the neutrals of the autotransformers and transformers wye-wye

with tertiary of compensation in delta.

An example of transformer with wye-delta connection is the following.

Figure 2-6 Connection wye-delta

Usually, the transformers with connection wye-wye are not used except in some

cases of high-resistance or resonant grounding.

• Grounding transformers

11

Grounding transformers can be used to obtain a neutral. The two types of

grounding transformers more used are:

• Zigzag grounding transformers

It is seen in the following scheme the internal connection of the transformer.

Figure 2-7 Zigzag grounding transformers.

When there is not fault in the system, a small magnetizing current flows in the

transformer winding. This is because the impedance of the transformer to

balanced three-phases voltages is high. Instead, the impedance of the

transformer to zero-sequence voltage is low so that the ground-fault current can

be high. The transformer divides the ground-fault current into three equal

components. These components flow in the three windings of the transformer.

• Wye-delta grounding transformers.

12

An example of wye-delta grounding transformers can be the one shown

below.

R IG

GROUND

FAULT

TO UNGROUNDED

3 PHASE VOLTAGE

SOURCE

IG

Figure 2-8 Wye-delta grounding transformers

This type of configuration is used for effective grounding or to accomplish

resistance-type grounding of an existing ungrounded system. To provide a path

for the zero-sequence current, the delta connection must be closed and the

delta voltage rating is fixed for any standard value.

In the figure it is shown a resistor between the primary neutral and ground, this

resistor limits the ground-fault current to a level satisfying for resistance-

grounded systems. [2]

13

3 MODELLING OF OFFSHORE WIND FARM

The next figure 3-1 shows the offshore wind farm model.The model was done

with DIgSILENT PowerFactory software. [8]

Transformer 150/33 kV

DFIG

Offshore

Substation

Cable

Nexan

Cable

JDR

Sectionalizer 33

Sectionalizer 150

Figure 3-1 Model offshore wind farm, 2 str ing with 8 wind turbine each one

14

This model is consisting for 16 wind turbines of 5MW each, due to software

lomitation on the nodes. The distance between wind turbines is of 882 m, which

was calculated trough the next relation, ∅ × 7, where Ø is the rotor

diameter. 126 × 7 = 882 .

Figure 3-2 Distance between wind turbines.

For the wind turbine has been selected a variable speed wind turbine with a

doubly fed induction generator (DFIG) and blade pitch control.

Figure 3-3 General scheme of DFIG

This system consists in a gearbox and an asynchronous generator whose stator

is connecting directly to grid and whose rotor is connecting via two frequency

converters to grid.

The figure 3-4 shows the transformation of mechanical energy to electrical

energy in the wind turbine.

15

Figure 3-4 Transformation of mechanical energy to electrical energy in the wind turbine.

These wind turbines are more efficient then the wind turbines connected to the

grid directly. It is due to they could be run at variable speed. Also the indirect

connection to the grid can control the reactive power to improve power quality

for the electrical grid.

The disadvantages are the increase of price due to use a more complex control

system and the power electronic.

The next table 2 shows the wind turbine characteristics.

Table 2 Parameters wind generator

Electrical & Mechanical parameter

Rated power 5 MW

Rated voltage 0.95 kV

Number of poles pairs 3

Frequency 50 Hz

Stator resistance 0.00298989 p.u

Stator reactance 0.125 p.u

Rotor resistance 0.004 p.u

Rotor reactance 0.05 p.u

16

The applied control is the pitch control. The power control by pitch variation of

the blades is a mechanical process, then the reaction time of the change pitch

mechanism is a important point in the design of the turbine.

The generator slip begins to increase when is near of the rated power of the

turbine. There are two control power strategies depending on the generated

power is over or below the normal operating regime:

When the wind is strong, the obtained power is higher than rated power,

and then the slip increases and the rotor rotates faster. This occurs until

the pitch change mechanism of the blades takes over the problem,

guiding the blades and obtaining less wind power.

If the opposite happens, the wind suddenly drops, the control checks

several times per second the generated power, and how to get as much

power as possible, the pitch of the blades is changed.

The mechanism of pitch change is often with hydraulic motors or with

continuous machine accommodated in the nacelle.

Figure 3-5 Pitch control

When the wind reaches a speed over 15 m/s, the control regulates the pitch, to

obtain less power wind. Then, the obtained power is constant as shown in the

figure.

The figure 3-6 shows the model of DFIG:

Figure

The wind turbines are connected through the cable

cable from offshore substation to land is

Parameters

Longitud km

Rated Voltage kV

Rated Current (in

ground) kA

Nominal Freq uency Hz

System Type

Parameters per L

Resistance R’ Ohm/km

Reactance X’ Ohm/km

ws the model of DFIG:

Figure 3-6 Model wind turbine DFIG

wind turbines are connected through the cable JDR 36kV 3x500 mm

cable from offshore substation to land is NEXANS TKVA 245 kV 3x1x400 mm

Table 3 Parameters of the lines

Nexans TKVA 245 kV JDR 36 kV

50 1.264 / 0.882

245

0.72 0.812

50

AC

Parameters per L ength 1,2 Sequence

0.09 0.0506

0.15 0.1072

17

JDR 36kV 3x500 mm2. The

NEXANS TKVA 245 kV 3x1x400 mm2.

JDR 36 kV

1.264 / 0.882

36

0.812

50

AC

0.0506

0.1072

18

Susceptance B’ uS/km 40.8407 87.02212

Parameters per Length Zero Sequence

Resistance R0’ Ohm/km 0 0.2829

Reactance X0’ Ohm/km 0 0.0971

Susceptance B0’ uS/km 36.76189 87.02212

Max End Temperature ºC 90 80

Another place very important in the last figure 3-1 is the offshore substation.

This substation are consisting in two transformer with the next characteristics.

Table 4 Parameters offshore substation transformer

Rated Power 160 MVA

Nominal frequency 50 Hz

Rated Voltage

HV-Side 150 kV

LV-Side 33 KV

Positive Sequence Impedance

Short -Circuit Voltage uk 12 %

SHC-Volatge (Re(uk))ukr 0.28125 %

Zero Sequence. Short citcuit voltage

Absolu te uk0 1.6%

The connection of the transformer to earth is YNzn0. The resistance in the

transformer neutral in LV side will be changed to study the different grounding.

19

4 STATION GROUNDING SYSTEM

The grounding has two main objectives:

• Protecting personnel from injury and damages. These connections are

made to parts of the system that are not usually energized but may

become energized due to an abnormal condition.

• To provide means to carry electric currents into the earth under normal

and fault conditions without exceeding any equipment limits on continuity

of service.

These two objectives are obtained taking into account the following design

objectives.

• Provide a low-impedance ground fault current return path in order to

activate the protection and clear the ground fault as soon as possible.

• Limit to safe levels, the voltages on station and accessible equipment in

normal operations and during transitions electrical.

• Minimize electrical noise interference in control and instrumentation

systems.

• Minimize the effect of lightning strikes on personnel, equipment and

structures.

4.1 Design ground grid substation

Before start the calculations, it is important to mention that it has been followed

the guidelines given in the IEEE Guide for Safety in AC Substation

Grounding.[4]

The touch voltage and step voltage are important factors to design and insure a

safe design. The touch voltage and step voltage should be below the maximum

values, which are calculated with the next expressions [4]:

20

= [1000 + 6ℎ, ] 0.116#$ (01)

%&' = [1000 + 1.5ℎ, ] 0.116#$

(02)

Where

1000 is the body resistance (in Ω)

1.5 is the resistance of two feet in parallel

6 is the resistance of two feet in series

Cs(hs,K) is 1 if there is no protective surface layer

ρs is the wet resistivity of the surface

ts is the shock duration (in s)

0.166 is a constant based on body weight of 50 kg

In this project was used the next parameters:

ρs =24.4 Ω.m because the substation are on the seabed, and the sand resistivity

is the previously given one. This value was retrieved from IEEE Guide for

Safety in AC Substation Grounding for gravel (type and size unknown) and

wetted with salt water.

ts= 0.115 s

= [1000 + 6 × 1 × 24.4] 0.116√0.115 = 392.14 + (03)

%&' = [1000 + 1.5 × 1 × 24.4] 0.116√0.115 = 354.585 + (04)

The next step to calculate the short circuit current, this value was obtained by

software DIgSILENT PowerFactory, simulating the worst short circuit. Different

short circuits were simulated in HV side and in the LV side. Fault impedance

was considered zero as the resistance of neutral transformer, and the short

21

circuit durations is 0.115 seconds. The next tables provides the short circuit

currents obtained from DIgSILENT PowerFactory. The short circuit current

generated on the fault on HV side busbar is 150A and the short circuit current

generated on the LV side busbar is 33 A.

Table 5 Three phase short circuit in HV with the sectionalizer 150 closed.

Single phase to ground and three phase short circuit were simulated and it was

checked that the worst fault in HV side is the three phase short circuit with the

sectionalizer 150 closed, its values can be seen in the table 6.

It was also checked that the worst short circuits have been the single phase to

ground generates in LV side, like it is showed in the next tables.

Table 6 Single phase to ground in LV side with both sectionalizers open.

22

Table 7 Single phase to ground in LV side with the sectionalizer 33 closed.

Table 8 Single phase to ground in LV side with the sectionalizer 150 closed.

It has been possible to see that the worst fault is the single phase to ground

with the sectionalizer 33 closed generated on the busbar 33A which causes a

short circuit current of 27.49 kA. But to calculate the ground grid of the

substation the worst short circuit is the single phase to ground on the busbar

33A with the sectionalizer 150 closed. The short circuit current on the busbar

33A has a value of 20.60 kA, and the short circuit current in the transformer is

23

16.690 kA as it is shown in the table 9, this last value is used to calculate the

ground grid. Three phase short circuit has been calculated in the low voltage

side and the results have been put in the annex A.

I’’K= 16.690 kA

This parameter also can be calculated by the next expression [4]:

,-" = ,/ × 0/ × 1/ × (05)

Where

If is the total zero sequence rms fault current (3I0)

Sf is the split factor or current division factor

Df is the decrement factor

Cp is the corrective projection factor

The grid resistance to remote earth can be calculated by[4]

23 = 02 ×45617 + 1√208 ×

461 + 1

1 + ℎ × 9208 :;

:<; (06)

Where

A is the area of the grid (in m2)

L is Lc+Lr for grids with few or no ground rods, and also for grids with ground rods predominantly around the perimeter

Lc in the total length of grid conductor (in m)

Lr is the total length of ground rods (in m)

h is the burial depth of the grid (in m)

SR is the resistivity of the soil (in Ω.m)

The parameters used in this project are the next:

24

A 3600 m2 (60 m wide by 60 m long)

Lc 2520 m (length of grid conductor x number of grid conductor 60x21x2=2520 m)

Lr 0 m (rods was not used)

L 2520 m (L=Lc+Lr)

h 0.5 m

SR 24.4 Ω.m

23 = 24.2 ×456 12520 + 1√20 × 3600 ×

461 + 1

1 + 0.5 × 9 203600:;

:<; = 0.1882

≈ 0.19 Ω

(07)

The next step to obtain the design grid is to calculate the mesh and step

voltages, theses voltages can be calculated with the next equations[4]:

? = ,′′-?@7& + 1.157 (08)

= ,′′-@7 (09)

Where

ρ is the soil resistivity (in Ω.m)

Km is the mesh voltage geometric correction factor

Ks is the step voltage geometric correction factor

Ki is the correction factor that into account the increase in current at the extremities of the grid

The coefficients Km, Ks and Ki can be obtained by the next expressions[4]:

= 1A B 12ℎ + 11 + ℎ + 11 1 − 0.5DEFG (10)

? = 12A Hln K 1F16ℎL + 1 + 2ℎF81L − ℎ4LM + @@' NO 8A2O − 1P (11)

25

@ = 0.656 + 0.172O (12)

Where

D is the spacing between parallel conductors (in m)

d is the diameter of the grid conductor (in m)

h is the depth of the grid (in m)

n is the number of parallel conductors in one direction

Kii is the corrective weighting factor that adjusts the effects of inner conductors on the mesh

Kh is the corrective weighting factor that emphasizes the effect of grid depth

And these parameters can be calculated by[4]:

@@ = 12OF DQ (13)

' = 91 + ℎ ℎQ (14)

Where

ho 1m (reference depth of grid)

n 21

D 3 m

d 0.02 m

h 0.5 m

@@ = 12 × 21F FRQ = 0.7

(15)

' = 91 + 0.5 1Q ≈ 1.225 (16)

@ = 0.656 + 0.172 × 21 = 4.268 (17)

26

Once calculated the above variables, now it can be calculated Km and Ks

? = 12A Hln K 3F16 × 0.5 × 0.02 + 3 + 2 × 0.05F8 × 3 × 0.02 − 0.54 × 0.02M+ 0.71.225 NO 8A2 × 21 − 1P = 0.45

(18)

= 1A B 12 × 0.5 + 13 + 0.5 + 13 1 − 0.5FREFG = 0.4092 (19)

and with this values it can be calculated Em and Es

? = FS.S×RTTU×.S×S.FTVFFWR.R× = 311.035 V (20)

= 24.4 × 16690 × 0.4092 × 4.2682520 = 282.27 + (21)

Now it can be checked that Em<Etouch50 (311.035<354.58) and Es<Estep50

(285.27<392.14) therefore our design is good. In the next figure 4-1 is possible

to look the grid design.[5] [6]

27

Figure 4-1 Ground grid 60 x 60 m.

4.2 Design ground grid wind turbine

The ground grid in the wind turbine transformer is calculated using the same

expressions that in the last case but with the next values. The worst fault has

been calculated when the three phase short circuit happens on busbar 33WT1.

I’’k 477 A (this current was obtained with DIgSILENT PowerFactory).

A 49 m2

Lc 42 m (length of grid conductor x number of grid conductor 8x6=48 m)

Lr 4 m (2 rods x 2 m each)

L 46m

n 3

D 3.5 m

d 0.02 m

28

h 0.5 m

Rg 1.9 Ω

Using the previous parameters it can be calculate Em and Es.

= 209.78 + (22)

= 131.52 + (23)

Now it is possible compare the Em with Etouch and Es with Estep50.

< $Z[\ℎ50 (24)

< $]^50 (25)

Therefore this model is correct. The next figure 4-2 the ground grid model can

be seen.

Figure 4-2 Ground grid 7 x 7 m with 2 rods.

29

5 MODELLING OF DIFFERENT TYPES OF

GROUNDING

This chapter is focused in computing the different parameters used in the

analyzed grounding types in this project. These grounding types were

mentioned in the chapter 2.

The grounding types used in this project will be the followings:

• Ungrounded.

• Solid grounding.

• Low-resistance grounding.

• High resistance grounding.

• Resonant grounding (Petersen coil).

The grounding types are analyzed separately below. The grounding location is

the substation transformer (33/150 KV).

5.1 Ungrounded

In this case, there is not an intentional grounding connection of transformer. The

ungrounded system is in reality a capacitance grounding system like the figure

5-1 shows.

30

Figure 5-1 Ungrounded

In the previous figure, it is shown the distributed capacitive reactance to ground,

Xco, which is assumed to be balanced. To obtain the value, it is analyzed with

the DIgSILENT model.

• R = 2.083 _` (positive-sequence capacity)

Then the distributed capacitive reactance is:

a&b = 2.083 _`

This value is the total distributed capacitive reactance of the line, so if this value

is divided per three to obtain the distributed capacitive reactance per phase. [2]

a& = 0.694 _`

5.2 Solid grounding

The solid grounding is obtained when there is a connection between the neutral

of a transformer or generator and ground. This connection is direct, without any

intentional intervening impedance.

The next figure 5-2 shows the solid grounding in the transformer of the high

voltage side.

31

Figure 5-2 Solid grounding

In this grounding type only the grounded resistance is considered, that is to say,

the resistance of ground grid calculated in the chapter 4. Then, the existing

resistance between the neutral of transformer and ground is: [2]

23 = 0.19 Ω

5.3 Low-resistance grounding

Low-resistance grounding is designed to obtain a ground-fault current between

100 A and 1000 A. To limit this current, it is used a neutral resistor which is

calculated with the next formula [2]:

2c = +dc,e (26)

Where:

• +dc is the system line to neutral voltage. In the model, the voltage in the

low side is 33 KV but it is line to line voltage, then:

+dc = 33√3 = 19.05 + (27)

• ,e is the desired ground-fault current. It is chosen 400 A for be a typical

value. ,e = 400 8

32

Using the equation 26, it is calculated the neutral resistance:

2c = 19.05 · 10g400 = 47.62 Ω (28)

To obtain the total ground resistance, it is should add the neutral resistance and

the ground grid resistance. The latter was calculated in the chapter 4.

2b = 2c + 23 = 47.62 + 0.19 = 47.81 Ω (29)

When this resistance is used in the model, the ground-fault current is 0.44 KA, it

is checked that the current is limited between the values desired.

The figure 5-3 shows the low-resistance grounding in the transformer in the high

voltage side:

AB

C

N

G

LNN I

VR =

GROUND

FAULT IG

IR

Figure 5-3 Low-resistance grounding

In the figure 5-3:

• IR is the current through the neutral resistance

• IG is the ground-fault current before mentioned.

5.4 High resistance grounding

This grounding type is similar to low-resistance grounding but in this case the

resistance has a high ohmic value. The resistance is calculated to limit the

33

current Ir, current through of the neutral resistance. The value of Ir should be

equal or slightly greater than the total capacitance charging current, 3Ico.

The figure 5-4 shows the high-resistance grounding in the transformer of the

high voltage side. It also shows the current Ir mentioned before.

G

LNN I

VR

·3≤

Figure 5-4 High-resistance grounding.

The calculated resistance should be checked because usually when there is a

line-to-ground fault with a fault current greater than 10 A, this grounding type

should be avoided. The reason is the potential damage caused by an arcing

current greater than 10 A in a confined space.

So, it will be calculated the neutral resistance with the capacitance charging

current obtained of the model and after it will be checked if the ground-fault

current is lower than 10 A.

The formula [2], used to calculate the neutral resistance, shown in the figure 5-

4, is:

2c ≤ +dc3 · ,& (30)

Where:

• +dc is the line to neutral voltage.

34

• ,& is the capacitance charging current.

Replacing the values obtained from the DIgSILENT software into the last

formula:

2c ≤33 · 10g√340 = 476.31 Ω

(31)

If a neutral resistance of 476.31 ohm is chosen and it is adding the ground grid

resistance calculated in the chapter 5, the total resistance for introduce in the

model is obtained as.

2b = 2c + 23 = 476.31 + 0.19 = 476.50 Ω (32)

This total resistance is introduced in the model and the ground fault current is

calculated.

,e = 0.06 i8

It is can say that it is not correct use this grounding type in the wind farm model

because the value is greater than 10 A and their use can be dangerous.

5.5 Resonant grounding (Petersen coil)

The resonant grounding systems are constituted by a variable reactance which

is connected between the neutral of substation transformer and ground. This

reactance is called also 'Petersen Coil', XL. The more important characteristic is

that during the ground faults, the inductive current of the reactance eliminates

the capacitive fault current produced by the grid. So, the current that flows for

the fault point is decreased to a small resistive current, Ir.

35

Figure 5-5 Resonant grounding.

The figure 5-5 shows the distribution of the fault current and the resistance “r”

represents the reactor losses. The capacitive fault current is annulated for the

inductive current of the coil as the next equation shows: [2]

,e = ,d + , + 3,& ≈ , (33)

This type grounding represents a great advantage to the appearance of single-

line-ground fault in the lines, which are the most common faults. Supposing the

ground fault is in an insulator flashover, it may be self-extinguishing. This

method permits network operation during long time in these fault conditions, it

permits to decrease the transient triggers of the protections. This means an

improvement in the service and the decreasing of the maintenance in the

switches.

For correct operation of this system, the Petersen coil should be correctly tuned,

so the distributed capacitance of the grid (Xco) could be compensated by the

inductance of the coil. Due to continuous variations in the grid, the resonant of

the grounding systems need a tuning and control system that control them

dynamically.[2]

36

For obtaining the reactance value, XL, the formula following is used [7]:

ad = 13j (34)

Where:

• j = 2Ak, where k is the frequency of the system.

• is the distributed capacitance of the system or Xco, with a value

obtained of the model. = F.Vg lmg = 0.694 _` .

So the reactance value will be:

ad = 13 · 2 · A · 50 · 0.694 · 10ET = 1,528.86 Ω (35)

37

6 ANALYSIS

This chapter will present the short circuit calculations. The model analysis is

focused on the grounding methods selected in the chapter 5. With each

grounding type, the voltage and current graphics will be obtained. The different

of grounding types are compared and analyzed.

6.1 Introduction to shorts circuits analysis

The short circuit analysis is based on the calculation and determination of the

magnitudes of the fault currents and the contributions of each element to the

fault. These characteristics permit the breakers design and the adaption of the

protection mechanisms.

The short circuit current of the system, permits establish the characteristics of

the protection elements that should remove the fault current, then it is

necessary make the calculation for all system voltage levels.

From electric point of view, a short circuit is the accidental or unintentional

connection, through a resistance o impedance of low value, of two or more

points of a circuit that is working at normal conditions and different voltages. A

short circuit generates surges in the system currents, it can damage the

equipment.

The values of short circuit current to be considered are:

• The short circuit maximum current.

• The short circuit minimum current.

The short circuit maximum current is calculated for design the protection

equipment, protection adjustments and design of grounding.

In electric systems can produce different fault types, these are:

• Three-phase short circuit.

• Single-phase short circuit to earth.

• Two-phase short circuit with or without earth contact.

38

The single-phase faults to earth can generate fault currents whose value can be

higher than the three-phase fault current. However, this more frequent happens

in the transmission or distribution systems in medium voltage, usually when the

fault appears near of the substation. The single-phase fault current seldom has

a value higher than the three-phase fault current.

The percentages of the short circuit types in a system are:

Table 9 The percents of short circuit types

SHORT CIRCUIT TYPES IMPACT (%)

Single-phase short circuit to earth 80

Two-phase short circuit 15

Three-phase short circuit 5

However for simplify, this project is focused in the single-phase short circuit to

earth.

6.2 Single-phase to ground short circuit

The single-phase short circuit is responsible for the greatest number of short

circuits in the system (80% of the short circuits are single phase). This short

circuit causes short circuit currents that depend on the fault impedance and the

connection to earth of the transformers in the line.

This is the short circuit more frequent and violent, appearing more frequently in

solid grounding systems or through impedances with low value.

The computation is important due to the high currents and the connection to

earth. This permits to calculate the leaks to earth, the touch voltage or step

voltage to assess the interferences that these currents can cause.

39

The single-phase to ground short circuit is unbalanced and presents energy

losses, so it is necessary use the three sequences grid (positive, negative and

zero) for its calculation.

Figure 6-1 Generic scheme of single-phase short circuit to earth

Figure 6-2 Scheme impedance single-phase short circuit to earth

The short circuit current between a phase and earth has a value[9]:

,n = o √3Qpqr + p (36)

Where:

• o is the line to line voltage.

• pqr is the short circuit impedance, being the positive and negative

sequence.

• p is the equivalent impedance of return from the earth or zero

sequence impedance.

40

The equivalent circuit has been developed in more detail in the annex B.

6.3 Simulations with the chosen groundings

The short circuits have been realized in two different points of the system. One

is in the busbar 33A and the other point is on line 8, between WT7-WT8. A

single phase to ground short circuit has been calculated and the voltage and

current for the high and low voltage side have been obtained. The measuring

and simulation points (M1, M2) are showed in the figure 6-3.

Figure 6-3 Scheme with the measuring and simulation points

The two short circuits will be analyzed separately. The results with the short

circuit on busbar 33A and on the line 8 will be obtained, these will be analyzed

and compared.

When the short circuit is simulated, the following conditions were chosen,

impedance fault is zero, the neutral cables resistance is zero. The simulation

time is 200 ms and the fault is cleaned in 115 ms as it was write in the chapter

4. As it was written in previous chapters the solid grounding, low resistance and

41

Petersen coil configuration are analyzed in the following simulations. The

parameters used in the software are as follows:

Solid grounding : Rg=0.19 ohms

Low resistance : RN=47.81 ohms

Petersen coil: Rg=0.19 ohms and XL=1528.86 ohms

Below, the behaviour of the system is analyzed against a single-phase to

ground short circuit on busbar 33A using the different groundings of the chapter

4. The system is analyzed in three different situations:

• Both sectionalizers open

In this case the two strings of generators are connected to the grid

through the transformer and the connection between them does not

exist.

• The sectionalizer 33 is closed and the sectionalizer 150 is open

Now the sectionalizer 33 is closed and the two strings of generators are

joined as seen in the figure 6-3.

• The sectionalizer 150 closed and the sectionalizer 33 open.

This is the opposite case, the closed sectionalizer is the 150. The two

strings are connected after of the transformers.

The analysis is divided in these three states. In each situation the results with

the different groundings will be obtained and they will be discussed.

6.3.1 Single-phase to ground short circuit on busba r 33A

BOTH SECTIONALIZERS OPEN

• Solid grounding

In the first place, the fault current is obtained when the system has a solid grounding.

42

Figure 6-4 Waveform of fault current on busbar 33A.

The fault current reaches a value around 20 kA when the fault starts and then

its value decreases to 16.5 kA approximately.

The graphics of the voltage and current in the high voltage side (point M1) are

the following:

Figure 6-5 Waveforms of voltage and current in HV side

43

The voltage suffers a transient period, decreasing its value, while the fault is

kept until the time 0.115 seconds, in this moment the fault is cleared and the

voltage comes back to normal level. The current also suffers a transient period

but its value increases and reaches a value around 2 kA in the fault phase.

Theses transient periods are due to the high fault current which affects the high

voltage side.

In the other measurement point (line 1, LV side) the graphics of the voltage and

current are the following:

Figure 6-6 Waveforms of voltage and current in LV side.

In the low voltage side, the voltage in the fault phase has a value almost zero

but with variations, the phase C decreases its voltage until a value of 21.50 kV

approximately and the voltage in the phase B decreases until a value around

12.5 kV. When the fault is cleared the three phases suffers a transient period

and after they come back to normal level. The current when the fault appears

44

suffers a peak value of 3.15 kA in the fault phase. After the fault current goes

decreasing and the phases B and C goes increasing. When the fault is cleared

the current in the three phases starts to decrease until reach to its normal level.

It can be observed that the current in the phase C increases more than in the

fault phase, this may be due to the generator control.

• Low-resistance grounding

The obtained fault current with this grounding is the following:


When the fault appears, the fault phase suffers a peak current reaching a value

of 1.9 kA approximately. The peak current disappears very fast and the fault

current keeps a value around 0.65 kA while it is not cleared. The fault current

value is quite low if it is compared with the solid grounding.

To follow, they are shown the graphics of the voltage and current in the point

M1 (HV side).

45

Figure 6-8 Waveforms of voltage and current in HV side.

These graphics show that the voltage almost does not suffer any variation, it is

kept practically in its normal level. So, the current suffers a small transient

period while the fault is kept but practically without importance, returning to

normal level when the fault is cleared, this is due to the low fault current which

does not affect to the high voltage side.

Now, the graphics in the point M2 are obtained like the figure 6-9 shows.

46


It can be observed that the voltage in the fault phase falls to zero and the

phases B and C increase √3 time their values until 49 kV approximately in the

moment that the fault is produced. When the fault is cleared, the three phases

come back to the normal level. The current in the fault phase suffers a peak in

the moment that the fault appears but during a short time period, after the

current increases its value to 1.0 kA approximately in the fault phase while the

fault continues. The phases B and C are not influenced by the fault.

• Petersen coil

To continue, it is analyzed the system with Petersen coil. The following graphic

shows the fault current when it is used this grounding type.


47

The fault current reaches a value around 1.45 kA when the fault is produced

and as time progresses, the current is decreased quite fast until a value around

0.03 kA. If it is compared this fault current with the two previous fault currents, it

can be said that this has a low value and its behaviour is good.

The voltage and current in the point M1 (HV side) are shown in the figure 6-11.

Figure 6-11 Voltage and current in HV side.

It can be said that both voltage and current do not suffer practically any

alteration. The fault has almost no influence on the phases.

The graphics of the voltage and current are shown in the next figure.

48


In the figure 6-12 it can be observed that the voltage, when the fault is

produced, falls to zero in the fault phase and increases √3 times its value in the

healthy phases until a value around 50 kV. When the fault is cleared the voltage

in three phases is unbalanced. This is due to the behaviour of the wind turbine

control.

In contrast, the current in the fault phase in t=0 seconds has a peak value of

1.75 kA approximately and a transient period during a short time period, after

the current comes back to the normal level.

• Ungrounded

The fault current with this grounding is shown in the figure 6-13.

49

Figure 6-13 Waveform of Fault current on busbar 33A.

In this case, the current reaches a value around 1.4 kA and it decreases with

the time. Its behaviour is very similar to the Petersen coil due to the

capacitances of the line.

The voltage and current in the point M1 (HV side) are in the next figure 6-14.

Figure 6-14 Waveform of voltage and current in HV side.

50

Its voltage and current is practically the same that the Petersen coil and its

values match.

In the point M2 (LV side), the obtained current and voltage are the following.


The voltage, in the moment that the fault appears, falls to zero in the fault phase

and in the other two phases suffers a short transient period with a overvoltage,

after the voltage has a value of √3 times its initial values, around 46 kV. When

the fault is cleared the DIgSILENT software losses the reference point, but the

peak to peak voltage is the same before the fault. The current presents a peak

value and after it comes back the normal current.

51

SECTIONALIZER 33 CLOSED

• Solid grounding

Below, it is obtained the waveform of fault current when it is used the solid

grounding.

Figure 6-16 Waveform of fault current with solid grounding

The fault current reaches a value around 32 kA when the fault starts and it goes

decreasing with the time until a value around 19 kA approximately. This current

has a value higher than the obtained when both sectionalizers are open. This is

due to the closing of the sectionalizer, as for this path the current flows until the

busbar 33A.

In the figure 6-17 are shown the voltage and current in the point M1 (HV side).

52

Figure 6-17 Waveforms of voltage and current (solid grounding)

When the fault starts, the three voltages suffer a transient period decreasing

their values until a value around 40 kV in the fault phase. In the moment that the

fault is cleared (t=0.115 sec), the three phases suffer again a transient period.

The current on the contrary, when the fault appears, increases its value in the

three phases. The fault phase reaches a value around 1.5 kA and when the

fault is cleared, the current suffers a transient period due to voltage fluctuations,

when the voltage is steady the current recoups its normal values.

The figure 6-18 shows the current and voltage in the other measurement point,

M2.

53


The voltage of the fault phase, like in the case with both sectionalizers open,

takes a value almost zero while the fault is kept. The voltage in the others

phases decreases until that the fault is cleared, then every phases suffer a

transient period and come back to normal level.

The fault current presents a peak value of 3.10 kA when the fault happens and

after the current goes decreasing. The current increases its value in the other

two phases but also suffers a transient period when the fault is cleared. The

behaviour is very similar to the case with both sectionalizers open.


The fault current when the sectionalizer 33 is open is quite higher than when the

both sectionalizers are open.

54

Figure 6-19 Fault current on busbar 33A.

In this case, the current reaches a value around 3.8 kA when the fault appears,

this is because when the sectionalizer is closed, the fault current is the sum of

the current coming from the transformer and from the other string of wind

turbines through of the sectionalizer 33. After of the peak current, the current

decreases until a value around 1 kA and this value is kept while the fault exists.

The obtained voltage and current in the point M1 (high voltage side) are shown

to following.

55


It can be said that the voltage is not affected by the fault and the current only

suffers a small transient period while the fault exists. This behaviour is similar to

the case with both sectionalizers open. The measurement point M1 is not

influenced by the closing of the sectionalizer.

To continue, the voltage and current in the other measurement point, M2, are

shown in the figure 6-21.


56

In this point the voltage and current do not vary if it is compared with the

situation where the both sectionalizers are open.

• Petersen coil

In this case, the peak current is higher than when both sectionalizers are open,

practically 2 times 1.5 kA. In the figure 6-22 is shown the fault current for this

case.


The fault current in this situation reaches a value around 2.8 kA when the fault

appears. Like in the case with both sectionalizers open, the fault decreases with

the time to a value of 0.05 kA.

The voltage and current in the point M1 are shown to following.

57

Figure 6-23 Waveforms of voltage and current in HV side. The obtained results are the same that when the both sectionalizers are open,

the voltage and current in this point are not influenced by the closing of

sectionalizer 33A.

In the same way that in the measurement point M1, the obtained voltage and

current in the point M2 (LV side) are very similar to that obtained with both

sectionalizers open like the figure 6-24 shows.


58

These results show that the current and voltage in the point M2 are not

influenced by the closing of sectionalizer 33A.

• Ungrounded

The fault current in this situation is shown in the figure 6-25.


When the fault appears, the fault current reaches a peak value of 2.9 kA

approximately, with the time the current goes decreasingly until a value of 0.080

kA. This behaviour of the current is very similar to the Petersen coil and if it is

compared with the case with both sectionalizers open, it can be seen that the

fault current also is high.

The obtained voltage and current in the point M1 (HV side) are shown in figure

8-26.

59


The obtained results are very similar to both sectionalizers open case and also

to the results of Petersen coil. Both voltage and current are almost not

influenced by the fault.

In the measurement point M2 (LV side), the obtained results are shown in the

figure 6-27.


60

The voltage and current in this point is the same as that in the case with both

sectionalizers open. This measurement point is not affected by the current flows

through sectionalizer 33.

SECTIONALIZER 150 CLOSED

• Solid grounding

The fault current, when the sectionalizer 150 is closed, is the following.


In this situation, the fault current reaches a value around 24 kA, this current is

higher than when both sectionalizers are open and lower than when the

sectionalizer 33A is closed. The current also decreases with the time until a

value of 16 kA approximately.

Below, the graphics of the voltage and the current in the measurement point M1

are shown.

61

Figure 6-29 Voltage and current in HV side.

If it is compared the voltage in this case, figure 6-29, with the case which has

both sectionalizers open, figure 6-5, it can be seen that the voltage behaviour is

very similar. In contrast, the current behaviour changes. In this case the current

increases but not as much as in previous situations of the sectionalizers.

Now the graphics show the obtained current and voltage in the point M1.

62


In this situation the current and voltage behaviour is the same that in the case of

both sectionalizer open and very similar to the case with sectionalizer 33

closed. It is due to that the current from the sectionalizer 33 does not flow for

the point M2.


The obtained fault current for this case is the following.


When the sectionalizer 150 is closed the fault current almost does not vary

against the case with both sectionalizers open. It reaches a value around 1.95

kA in the moment that the fault appears and after decreases until a value of 0.6

kA approximately.

63

The following figures show the voltage and current in the point M1 (HV side).


The graphics of the figure 6-32 show that the behaviour of the current and the

voltage is the same as that in the case with both sectionalizers open and in the

case with sectionalizer 33 closed.

The measured voltage and current in the point M1 are shown in the figure 6-33.

64


In the point M2 happens the same happens as that in the point M1. The current

and voltage coincide with the case with both sectionalizers open and with

sectionalizer 33 closed .

• Petersen coil

The fault current in this case where the Petersen coil is used reaches a value of

around 1.4 kA in the moment that the fault appears.


The fault current is practically the same as that in the case with both

sectionalizers open and its variation also.

The voltage and current in the measurement point M1 are represented in the

figure 6-35.

65


The obtained values are very similar with the result with both sectionalizers

open and with sectionalizer 33 closed. As the currents, the voltages almost are

not affected by the fault. Their behaviour is ideal.

The currents and voltages in the measurement point M2 are obtained, like that

the figure 6-36 shows.

66

Figure 6-36 Waveforms of voltage and current in LV side. The obtained results are the same that in the case with both sectionalizers open

and with sectionalizer 33 closed. The current and the voltage are not affected

by the fault. This behaviour is ideal.

• Ungrounded

Finally, it is obtained the fault current with the ungrounded system. The next

figure 6-37 shows the waveform of this current.


The current value in the moment that the fault appears is the same as that in

the case with both sectionalizers open. Also coincide with the fault current in

Petersen coil.

The graphics of the voltage and current in the point M1 (HV side) are shown in

the figure 6-38.

67


The obtained results coincide with the obtained in the other three situations of

the sectionalizers.

The results in the other measurement point, M2, are shown to following.

68


The same happen in the point M2, LV side, because this measurement point is

not affected by the current flows through of both sectionalizers.

6.3.2 Single-phase to ground short circuit on line 8

BOTH SECTIONALIZERS OPEN

• Solid Grounding

It is started with the analysis of the fault current in the line 8, the graphic is

9showed in the next figure 6-40.

Figure 6-40 Waveform of fault current in the line 8

In the previous figure 6-40 it is possible to look at that the current almost

reaches a value of 15 kA and it keeps this value while the fault is not cleaned.

69

The next figure 6-41 shows the waveforms voltage in high voltage side which

are measured in the point M1, it can be seen that the voltage in the phase A

and B are decreased but the phase C is increased, while the fault happens. It

can also be seen that the voltage comes back to the normal voltage after the

fault.


The waveform of current in HV side measured in point M1 shows that the

current in the phase A is increased and it reaches a superior value of 1 kA and

the other phases increase but to a lesser extent like it is seen in the last figure

6-41.

This behaviour is due to the high value of the fault current which influence in the

voltage and the current in the high voltage side.

If the current is compared with fault current case on busbar 33A it is possible to

see that the current is low in this case, this is normal due to the line resistance.

70

Now the voltage and the current in low voltage side are analysed in the next

graphics.

During the fault the voltage in the phase C has a little increment, but the other

phases are decreased their values at half voltage, and after the fault is cleaned

and the voltage come back to normal value.

Figure 6-42 Waveforms of voltage and current in LV side

The current obviously in the phase A is increased its values at 10 kA during the

fault. The phase B increases but in a little value and C is decreased in their

values.

• Low Resistance

As in the solid grounding the fault current is showed in the figure 6-43. It is

possible to see the current and it is seen that it reaches a peak value of 1.5 kA

in the phase A when the fault happens, but this value is lower than the fault

current in the solid grounding case. It is seen that after of peak current, it keeps

a constant value during the fault.

71

Figure 6-43 Waveforms of fault current in line 8

The next figure shows that the value of voltage in the high voltage side does not

vary, this is a good behavior. If it is remembered the solid grounding case the

voltage in high voltage side varied its nominal values during the fault. Therefore

this configuration is better with regard the voltage in the solid grounding. The

current neither varies its values too much with regard its normal values.

72

Figure 6-44Waveforms of voltage and current in HV side

In low voltage side the voltage is increased, its values in the phases B and C √3

times their values reaching the values of 50 kV and the voltage in the phase A

is lower.


73

On the other hand the current in the phase B and C decreased their normal

values a little and the current in the phase A is decreased below value of 0.5 kA

and it is kept constant during the fault, this problem was analyzed in detail and it

was checked that this is due to the wind turbine controller.

• Petersen Coil

The fault current in this case almost reaches a value of 1.5 kA in a peak current,

but these values are being decreased in the time, to reach extremely low

values, as shown in the next figure 6-46.

Figure 6-46 Waveforms fault current

The next figure shows that the waveforms of voltage and current in high voltage

side do not vary their values during the fault. This configuration is better with

regard to the current than in the low resistance configuration. There are not

differences between the voltage in the low resistance case and the Petersen

coil case.

74


In the low voltage side the voltage in the phases A and B is increased to almost

50 kV like the low resistance case but the difference is that after the fault is

cleaned the voltage is unbalance. This characteristic was analyzed and it was

checked that the voltage unbalance is due to the behavior of wind turbine

controller. This problem and the transients in the beginning did not exist with the

low resistance configuration.

75


The current too has transients during the short time when the fault happens,

after the current keeps the normal values, in this case the current has better

characteristics that the current in the low resistance case.

• Ungrounded

The next waveform is similar to the waveform of fault current with the

configuration Petersen coil, peak values are practically the same too.

Figure 6-49 Waveform of fault current in line 8

The figure 6-50 shows that with this configuration the waveforms of voltage and

current are not affected by the short circuit in the line 8.

76


In the low voltage side the voltage is affected, the phase A is practically zero

and the other phases increase their values almost to 50 kA, when the fault is

cleared the DIgSILENT software losses the reference point, but the peak topeak

current is the same before the fault happens.By other hand the current suffers

some little transients when the fault happens, but it rapidly gets its normal

values during the fault.

77


SECTIONALIZERS 33 CLOSED

• Solid Grounding

The fault current is similar to fault current in solid grounding case with the

sectionalizers open, it is obvious that the fault current in this case is higher than

the last case due to the fault current is the sum of fault current from transformer

and from second string of wind turbines.

Figure 6-52 Waveforms of fault current

The voltage in high voltage side is practically the same that in the solid

grounding with the sectionalizers open case. In the current there are

differences, it is possible to see that the current in the phases A and B are

78

increased and the phase C is decreased, but if this waveform of current are

compared with the waveform of current in the case with both sectionalizers

open, it is possible to see that the current increase a low value than with both

sectionalizers open, this is normal as the current from high voltage side is low.


In low voltage side the waveforms of voltage is the same that the waveforms of

voltage in the case with both sectionalizers open. The waveforms of current are

higher than the case both sectionalizers open, as in this case the waveform of

phase A above the 10 kA.

79


• Low Resistance

In figure 6-55 it is possible to see that the peak current in the fault is higher than

the peak current in the case with both sectionalizers open, in this case the fault

current reaches the 2 kA, after the peak current the short circuit current has a

constant values during the fault until that the fault is cleaned.

Figure 6-55 Waveforms fault current

The waveforms of voltage and current in high voltage side are the same as the

waveforms of voltage and current with both sectionalizers open. It is possible to

see that the voltage and current almost do not vary.

80


The waveform of voltage in this case and the case with both sectionalizers open

is practically the same, but the waveform of current in this case is low in the

phase A than in the case with both sectionalizers open and also the phase A is

unbalance, this problem did not exist when the case with both sectionalizers

open was studied. As mentioned the phase A has a low value than the other

phases due to the wind turbine controller.

81


• Petersen Coil

If comparing the fault current waveform in figure 6-58 with fault current when

the sectionalizers are open, it is possible to see that the waveform in figure 6-58

is higher than the other case. This is obvious as the fault current is the sum the

two fault currents one coming from the transformer and other coming from the

other string of wind turbines.

Figure 6-58 Waveform fault current

The waveform in figure 6-59 has the same values as the case with both

sectionalizers open. Therefore it can be seen that the waveform of voltage and

current are not affected with the changes in the sectionalizer 33.

82

Figure 6-59 Waveforms of voltage an current in HV side

The waveforms of voltage and current in the low voltage side are the same like

the case with both sectionalizers open. The only difference is that the next

waveforms have more transients when the fault happens.

83

Figure 6-60 Waveform of voltage and current in LV side

• Ungrounded

This waveform has the same peak values that the last case, but the current

during the fault is higher than in the last case like the next figure 6-61 shows.

Figure 6-61 Waveform of fault current in the line 8

As when the sectionalizers are closed the waveform of voltage and current in

high voltage side are not affected by the short circuit in the line 8.

84


The below waveform shows the voltage and current has a similar behavior than

the case with the sectionalizers open, the only difference is that the current has

a peak current greater than the previous case.

85


SECTIONALIZERS 150 CLOSED

• Solid Grounding

The fault current waveform in this case and the case with both sectionalizers

open are very similar, the fault current in this case is a little more high.

Figure 6-64 Waveform of fault current

The waveforms in high voltage side of figure 8-66 are the same that the case

with both sectionalizers open, the phases A and B decreased and the phase C

is increased.

86


Practically the voltage waveform in this case and the case with sectionalizer 33

closed is the same, the only difference is current value is a little smaller than

the case with sectionalizer 33 closed which reaches to 0.5 kA. When both

sectionaliters are open this current is higher.

87


The waveform of current in phase A in this case is smaller than in the case with

the sectionalizer 33 closed which reaches around 12.5 kA during the fault, also

it is possible to see that the waveform of current during the fault are

unbalanced.

• Low Resistance

The next figure 6-67 shows fault current waveform and if it is compared with the

fault current when both sectionalizers are opens it is possible to see that the

waveform is practically the same.


88

The next waveforms are the same to the different situations of the sectionalizers

(both sectionalizers open, sectionalizer 33 closed).


The next waveform of voltage and current were seen previously when the case

with both sectionalizer are opened.

89


• Petersen Coil

The figure 6-70 shows the waveform of fault current and this current is the same

that when the configuration Petersen coil was analyzed with both sectionalizer

open.


The waveforms of voltage and current have their normal values during the fault,

therefore the fault does not affect to the voltage and current like in the low

resistance case.

90


The next graphics are similar to the other cases studied with different situations

of sectionalizers for the Petersen coil configuration

91


• Ungrounded

With the next figure 6-73 has been checked that the fault current is the same regardless the position of the sectionalizers.


The same thing happens with the waveforms of voltage and current in high

voltage side and in the low voltage side. The waveform of current in low voltage

side has a peak value lower than the seccionalizer 33 is closed.

92



93

6.4 Comparison of the results

In the first place, the fault currents have been compared in the different

situations which have been raised in the chapter 6.3.

• Both sectionalizers open.

• Sectionalizer 33 closed.

• Sectionalizer 150 closed.

Below, the analysis has been performed when the fault appears on busbar 33A.

The different groundings have been analyzed in these situations in the same

graphic. The comparison has been started with the solid grounding.

Figure 6-76 Fault currents with solid grounding (short circuit on busbar 33A)

The figure 6-76 shows that the closing of the sectionalizer 33 produces an

increasing of the fault current. Instead, the closing of the sectionalizer 150 does

not produce any variation practically with regard to the case with both

sectionalizers open.

This increasing is due that the fault current has a component more, that is to

say, the fault current is now the sum of the current from the string of the wind

turbines, the current from the transformer and the current from the other string

of wind turbines like the figure 8-78 shows.

94

Figure 6-77 Distribution of fault current.

The same happens with the rest of groundings. The figures 6-78, 6-79 and 6-80

present the fault currents in the three situations of the low-resistance grounding,

Petersen coil and ungrounded respectively.

Figure 6-78 Fault currents with low-resistance grounding (short circuit on busbar 33A)

95

Figure 6-79 Fault currents with Petersen coil (short circuit on busbar 33A)

Figure 6-80 Fault currents with ungrounded (short circuit on busbar 33A)

It can be seen that the behaviour of the fault current in the cases with Petersen

coil and ungrounded is very similar. This is due that the fault current in the

ungrounded system goes decreasing its value due the capacitances of the line

and in the case of Petersen coil, the inductive current of the coil cancels the

capacitive fault current, so the intensity flows by the fault point is decreased to a

small component resistive.

96

In the other case, when the fault appears on the line 8, the system behaves of

similar form but the values of the fault current change as the short circuit

happens in another part of the wind farm.

The figure 6-81 shows the fault current in the three situations if the solid

grounding has been used.

Figure 6-81 Fault current with solid grounding (short circuit on line 8).

The fault current increases when the sectionalizer 33 is closed, it reaches a

value of 14.93 kA, instead, when both sectionalizers are open the fault current

reaches a value of 12.46 kA. This last value is very similar to the obtained with

the sectionalizer 150 closed, 13.01 kA.

The other three types of grounding have been presented in the next figures

(low-resistance grounding, Petersen coil and ungrounded).

97

Figure 6-82 Fault currents with low-resistance grounding (short circuit on line 8).

Figure 6-83 Fault currents with Petersen coil (short circuit on line 8).

98

Figure 6-84 Fault currents with ungrounded (short circuit on line 8).

The previous figures show that the fault current also increases when the

sectionalizer 33 is closed in every groundings. This is due to the current flows

by the sectionalizer 33.

In the second place a comparison has been realized between the different

grounding types with each position of the sectionalizers.

To begin, the fault currents have been obtained when both sectionalizers are

open and the fault appears on busbar 33A.

Figure 6-85 Faut currents with both sectionalizers open.

99

It is possible see that the fault current with solid grounding is more high than in

the other grounding three types.

With the other two situations, sectionalizer 33 closed or sectionalizer 150

closed, happen the same. The fault current with the solid grounding is more

high.

Figure 6-86 Fault currents with both sectionalizers open.

The graphic 6-86 presents the different fault currents with the sectionalizer 33

closed to the left and with the sectionalizer 150 closed to the right.

When the fault appears in the line 8 and both sectionalizers are open, the fault

current comes back to have the higher value with the solid grounding.

Figure 6-87 Fault currents with both sectionalizers open.

100

In the other two situations, sectionalizer 33 closed or sectionalizer 150 closed,

happen the same like the figure 6-88 shows.

Figure 6-88 Fault currents with sectionalizer 33 closed, left, and with sectionalizer 150 closed,

right.

The next table presents the effective values of the fault current and its peak

values when the fault appears on busbar 33A.

Table 10 Effective values and peak values of the fault current (short circuit on busbar 33A).

CONNECTIONS Sectionalizer Maximum peak

fault current (kA)

Fault current (kA)

Solid grounding

Both open 24.057 18.07

33 closed 37.128 26.85

150 closed 28.183 19.85

Low -resistance grounding

Both open 1.91 0.41

33 closed 3.819 0.81

150 closed 1.966 0.41

Petersen Coil

Both open 1.511 0.02

33 closed 3.025 0.05

150 closed 1.505 0.02

101

Ungrounded


33 closed 3.030 0.08

150 closed 1.508 0.04

As the table shows, the solid grounding is the worst case as it presents a high

value of fault current.

The next figure presents the effective values and peak values of the fault

current when the fault appears on the line 8.

Table 11 Effective values and peak values of the fault current (short circuit on line 8).

CONNECTIONS Sectionalizer Maximum peak fault

current (kA)

Fault current

(kA)

Solid grounding

Both open 13.787 10.29

33 closed 16.287 12.13

150 closed 14.407 10.75

Low -resistance grounding


33 closed 2.081 0.79

150 closed 1.594 0.40

Petersen Coil


33 closed 1.599 0.05

150 closed 1.434 0.03

Ungrounded


33 closed 1.599 0.08

150 closed 1.434 0.04

The conclusion is the same that in the previous case. The solid grounding has a

bad behaviour for the system.

102

To summarize, the next graphic shows the tendency of short circuit current and

the peak fault current, for the different situations of sectionalizer.

Figure 6-89 Tendency of short circuit current for different situations of sectionalizers

In last place, it is presented a table where have been compared the currents

and voltages in the neutral of the transformer with the different groundings,

except with the ungrounded as it does not have neutral connected to earth.

The table 16 shows the obtained values in each situation when the fault

appears on busbar 33A.

Table 12 Voltages and currents in the neutral with short circuit on busbar 33A.

Fault in Busbar 33A Sectionalizers Open Sectionalizer

33 Closed

Sectionalizer

150 Closed

Solid

Grounding

Un 3.4336 kV 2.5511 kV 3.7718 kV

In 18.0718 kA 13.4270 kA 19.8515 kA

Low

Resistance

Un 19.3264 kV 19.3284 kV 19.3440 kV

In 0.4092kA 0.4043kA 0.4046 kA

Petersen Coil Un 19.3720 kV 19.3815 kV 19.3699 kV

In 0.0127kA 0.0127 kA 0.01277 kA

0

5

10

15

20

25

30

35

40

Bo

th o

pe

n

33

clo

sed

15

0 c

lose

d

Bo

th o

pe

n

33

clo

sed

15

0 c

lose

d

Bo

th o

pe

n

33

clo

sed

15

0 c

lose

d

Bo

th o

pe

n

33

clo

sed

15

0 c

lose

d

Solid

grounding

Low-

resistance

grounding

Petersen Coil Ungrounded

Short circuit in busbar 33A

Maximum peak fault current

(kA)

Short circuit in busbar 33A

Fault current (kA)

Short circuit in line 8 Maximum

peak fault current (kA)

Short circuit in line 8 Fault

current (kA)

103

The results have been compared and it is possible to see that the solid

grounding has the higher current in the neutral, reaching a value of 19.85 kA

when the sectionalizer 150 is closed while the low-resistance grounding

reaches a value of 0.4046 and in the Petersen coil a value of 0.0127 kA.

In the other hand, the solid grounding has the lower voltage with the

sectionalizer 150 closed (the worst case) with a value of 3.77 kV. The low-

resistance grounding and the Petersen coil have a value very similar, 19.34 kV

and 19.37 kV respectively.

The table 17 shows the voltages and currents in the neutral of the transformer

when the fault appears in the line 8, between WT7 and WT8.

Table 13 Voltages and currents in the neutral with short circuit on line 8.

Fault in line 8 Sectionalizers Open Sectionalizer

33 Closed

Sectionalizer

150 Closed

Solid

Grounding

Un 1.9562 kV 1.1522 kV 2.0422 kV

In 10.2960 kA 6.0641 kA 10.7485 kA

Low

Resistance

Un 19.1723 kV 18.8581 kV 19.188 kV

In 0.4010 kA 0.3944 kA 0.4013kA

Petersen Coil Un 19.5279 kV 19.5533 kV 19.5260 kV

In 0.0128kA 0.0128 kA 0.0128 kA

It is possible to see that the solid grounding has the higher current and the

lower voltage with regard to the other two grounding types. The same happens

when the fault appears on busbar 33A.

104

7 ANALYSIS AND DISCUSSION

The analyzed model is not the real system, due to the limitation of nodes in the

DIgSILENT software. The following points show an estimate of the short circuit

currents for the real system which has four strings with 8 wind turbines in each

busbar 33.

The studied model has only one string with 8 wind turbines in each busbar 33

as it is possible to see in the figure 3-1. The next estimate has been carried out

in the case of four strings when the fault happens on busbar 33A.

Figure 7-1 Estimate of short circuit current for different number of str ings

The figure 7-1 shows the increment of short circuit current in the busbar 33.

With each string the short circuit current increases 3.34 kA reaching a value of

26.58 kA for four strings.

The figure 7-2 presents the estimate of short circuit current in the transformer. It

is possible to see that the increase is of 0.35 kA with each string. When the

busbar has four strings the short circuit current is 14.62 kA. This could influence

in the design of ground grid realized in the chapter 4.

The touch voltage and step voltage have been obtained for this case and the

results are:

Vstep=247.26 V

Vtouch=272.45 V

1 2 3 4

Short Circuit current on

Busbar 33A (kA)16,56 19,9 23,24 26,58

0

5

10

15

20

25

30

kA

Short Circuit current on Busbar 33A

Nº Strings in busbar

105

These values are lower than the reference values Vstep=392.14 V and

Vtouch=354.58 V.

Figure 7-2 Estimate of short circuit current for different number of str ings

The touch voltage and step voltage have been checked for the different

grounding in the WT1, when the short circuit happens on busbar 33A.

Figure 7-3 Step voltage in WT1 with different groundings

1 2 3 4

Short Circuit current in

transformer 160 MVA-A (kA)13,57 13,92 14,27 14,62

0

2

4

6

8

10

12

14

16k

AShort Circuit current in transformer 160 MVA-A


106

Figure 7-4 Touch voltage in WT1 with different groundings

The figures 7-3 and 7-4 show that the touch voltage and step voltage do not

reach the reference touch voltage and step voltage. The short circuit currents

used to calculate these voltages are presented in the annex C.

When the fault happens on line 8 short circuit current is lower than the short

circuit current in the busbar 33A, so the touch voltage and step voltage also are

below of reference touch voltage and step voltage.

Figure 7-5 Step voltage in WT1 with different groundings

107

1 2 3 4

Short Circuit current in

transformer of WT1 (kA)0,3901 0,3917 0,3933 0,3949

0,387

0,389

0,391

0,393

0,395

0,397

kA

Short Circuit current in transformer of WT1


Figure 7-6 Touch voltage in WT1 with different groundings

The previous cases have been analyzed for one string in each busbar 33. An

estimate will be done for the case of four strings below . It is possible to see that

the increase is of 0.0016 kA with each string, these current was calculated

when the short circuit happens on busbar 33A. The obtained currents are lower

than the short circuit currents on busbar 33WT1, which is the worst case, as it

was calculated in the chapter 4. Therefore, the touch voltage and step voltage

are lower than the reference touch voltage and step voltage.

Figure 7-7 Estimate of short circuit current for different number of strings

108

8 CONCLUSSIONS

As it has been checked in the previous chapter 6, there are two extreme cases

of grounding.

• Solid grounding system presents a fault current very high with regard to

the others grounding.

• Ungrounded system has a short circuit to ground current of capacitive

nature, which can produce a re-ignition of single-phase to ground fault.

The figures 6-27 (when the fault appears on busbar 33A and the

sectionalizer 33 is closed) and 6-63 (when the fault appears on line 8 and

the sectionalizer 33 is closed) show that in this grounding type an

overvoltage appears after the fault is cleared. This behaviour can be

harmful for the system.

For these reasons, these two groundings are not selected for the issue of this

project.

In the other hand, Petersen coil and low-resistance grounding have been

analyzed.

Petersen coil presents a good behaviour in every situations that have been

analyzed. This grounding also presents a low fault current and does not

produce overvoltages. But this method has some disadvantages.

• Risk of Ferro-resonance.

• More complex.

• More expensive.

These disadvantages make that it may not be recommendable for the wind

farm.

Finally, low-resistance grounding also presents a good behaviour against the

single-line to ground short circuit. Its maximum fault current (with sectionalizer

33 closed) is of 3.819 kA when the fault appears on busbar 33A (the worst

case) and not presents important overvoltages.

109

To summarize, the best compromise for reducing earth fault currents and

transient overvoltages has been obtained with low-resistance grounding.

110

9 FUTURE WORK

Some additional tasks could be performed:

• The short circuit analysis could be studied for three phases short circuit

or two phase short circuit.

• Analyze the short circuits in wind farm with ring topology.

• Study of different turbine types are also interesting (SCIG,...).

• Economical study about the different groundings.

111

10 REFERENCES

[1] Justin Wilkes and Jacopo Moccia, “Wind in power 2009 European

stadistics”, February 2010 [Online]. Available:

http://www.ewea.org/fileadmin/ewea_documents/documents/statistics/10

0401_General_Stats_2009.pdf. Accessed : May 15, 2010.

[2] Donald W.Zipse and Gene Strycula, “System grounding”, IEEE Std. 142-

2007. Grounding of industrial and Commercial power systems. 2007.

[3] Douglas C. Dawson, “IEEE Guide for the Application of Neutral

Grounding in Electrical Utility Systems-Part I:Introduction” in IEEE Std

C62.92.1-2000. Guide for the application of neutral grounding in

electrical utility systems.2000.

[4] John E. May, John P. Riganati and Sava I. Sherr, IEEE Std 80-1986

Guide for safety in AC Substation Grounding. 1985

[5] E.G. “AL” Kiener, Donal C. Loughry and Andrew G. Salem, IEEE Std.

665-1995.Guide for Generating Station Grounding.1995.

[6] A.P.Sakis Meliopoulos, Power system Grounding and Transitients, USA:

Marcel Dekker,1988.

[7] B.M. Weedy and B.J. Cory,Electric Power Systems, Fourth edition,UK,

Wiley.1998.

[8] DIgSILENT PowerFactory, DIgSILENT PowerFactory Version 13.1,

2004.

[9] B. de Metz-Nublat, F.Duma and C. Poulain, “Cahier technique nº 158”,

Calculation of Short Circuit Currents.2005.

112

I

A. Short Circuit Current from DIgSILENT The next tables have been obtained from DIgSILENT PowerFactory, and it is showed the different short circuit (three phase short circuit and single phase to ground short circuit) in high voltage side in the busbar 150A and low voltage side in the busbar 33A with different situations of the sectionalizers.

High Voltage Side :

II

III

Low Voltage Side:

IV

V

B. Equivalent Circuit of the wind farm The equivalent circuit of the wind farm is presented in the figure:

Figure 10-1 Equivalent circuit of wind farm.

The values determination of the different impedances are shown as follows the

equations and explanations from are used for computing [9].

• External grid

For obtaining this impedance, the follow formula is used.

pe = oeF0e&& (01)

Where oe is the rated voltage of the connection point, 0e&& is the initial short

circuit apparent power of the external grid.

Substituting the available values:

pe = 150F1000 = 22.5 Ω (02)

Like as showed from [34], stut ≈ 0.1 at 150 KV and:

VI

aepe = v1 − w2epe xF (03)

If the values are substituted

aepe = 0.995 (04)

Then, it is possible to suppose that:

ae ≈ pe (05)

The obtained impedance will be:

pe = 2e + ae = 2.25 + 22.5y Ω (06)

• The line impedances of high voltage

The impedance value depend on the cable type used. In this case, the

impedance is obtained from the DIgSILENT software where a cable NEXANS

400 mm2 is used. The impedance is:

pdR = pdF = 9 + 15y Ω (07)

• The transformer impedance 150/33 KV

To calculate the transformer impedance of high voltage side the following

equation[9] is used:

pbR = onb100 · oF0D (08)

Where onb is the short circuit voltage in percentage value of the primary

winding, U is the phase to phase voltage and 0D is the apparent power of the

transformer. The values of these parameters are: onb = 12% o = 33 + 0D = 160 |+8

Substituting the values of the transformer:

pbR = 0.816 Ω

VII

From [34], the resistance and reactance of the transformer are:

pbR ≈ abR (09)

2bR = 0.2abR (10)

Substituting the values, the value of transformer impedance is obtained:

pbR = 0.163 + 0.816y Ω (11)

• The line Impedances of the low voltage side

The obtained line impedance is between busbar 33 A or 33 B and the first wind

turbine, figure 8-1.

This impedance depends on the used cable, in this case JDR 500 mm2 and the

length of the cable. The impedance is obtained from DIgSILENT software.

pdg = 0.486 + 0.393y Ω

In second place the impedance between wind generators is obtained. The cable

is the same but the length is different.

pdS = 0.339 + 0.275y Ω

• Transformer impedance of three-windings 33/0.69/3.3 KV

The calculation is the same as the transformer of the high voltage side. The

same equation is used.

pbF = onb100 · oF0D (12)

The values in this case are: onb = 5.36 % o = 33 + 0D = 5.6 |+8

Substituting the values in the equation, the transformer impedance is: pbF = 10.42 Ω

From [9], the resistance and reactance of transformer are obtained:

VIII

abF ≈ pbF (13)

2bF = 0.2abF (14)

The transformer impedance is: pbF = 2.08 + 10.42y Ω

• The doubly-fed induction generator impedance

To obtain the doubly-fed induction generator impedance, the following

expression [9] is used:

pq = 1,ds ,~Q · o~√3 ,~ = 1,ds ,~Q · o~F0~ (15)

Where ,~, o~ and 0~ are respectively rated current, voltage and apparent

power of the generator, and ,ds is the locked-rotor current.

These parameters are obtained from the DIgSILENT software and the values

are: ,ds ,~Q = 7

o~ = 3.3 + 0~ = 5891.47 +8

Substituting the values into the equation, the impedance is obtained: Z = 0.264 Ω

That the relation between the resistance and reactance is:

2qaq = 0.43 (16)

And like the reactance may be comparable to impedance, aq ≈ pq, the

doubly-fed induction generator impedance will be: pq = 0.1135 + 0.264y Ω

IX

C. Tables of Short Circuit Currents on busbar 33A a nd on line 8

The step touch voltage are shown in the next figure for the different short circuit

currents, when the fault happens in the busbar 33A and on line 8.

Short Circuit on

Busbar 33A

Fault Current

(kA) Vs (V) Vt (V)

SOLID GROUNDING

BOTH SECTONALIZERS

OPEN 0,3901 107,49 171,49

SECTIONALIZER 33

CLOSED 0,3924 108,154 172,5

SECTIONALIZER 150

CLOSED 0,3921 108,072 172,37

LOW RESISTANCE

BOTH SECTONALIZERS

OPEN 0,0916 25,247 40,268

SECTIONALIZER 33

CLOSED 0,0951 26,211 41,806

SECTIONALIZER 150

CLOSED 0,0908 25,02 39,91

PETERSENCOIL

BOTH SECTONALIZERS

OPEN 0,0827 22,794 36,355

SECTIONALIZER 33

CLOSED 0,0827 22,794 36,355

SECTIONALIZER 150

CLOSED 0,0827 22,794 36,355

UNGROUNDED

BOTH SECTONALIZERS

OPEN 0,0828 22,82 36,399

SECTIONALIZER 33

CLOSED 0,0828 22,82 36,399

SECTIONALIZER 150

CLOSED 0,0827 22,794 36,355

X

Short Circuit on line 8 Fault Current

(kA)

Vs (V) Vt (V)

SOLID GROUNDING BOTH SECTONALIZERS

OPEN

0,2654 73,15 116,672

SECTIONALIZER 33

CLOSED

0,2378 65,543 104,538

SECTIONALIZER 150

CLOSED

0,2599 71,6345 114,25

LOW RESISTANCE BOTH SECTONALIZERS

OPEN

0,0916 25,2471 40,268

SECTIONALIZER 33

CLOSED

0,0954 26,294 41,938

SECTIONALIZER 150

CLOSED

0,0909 25,054 39,96

PETERSENCOIL BOTH SECTONALIZERS

OPEN

0,0827 22,794 36,355

SECTIONALIZER 33

CLOSED

0,0827 22,794 36,355

SECTIONALIZER 150

CLOSED

0,0827 22,794 36,355

UNGROUNDED BOTH SECTONALIZERS

OPEN

0,0828 22,82 36,3995

SECTIONALIZER 33

CLOSED

0,0828 22,82 36,3995

SECTIONALIZER 150

CLOSED

0,0827 22,794 36,355

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