Upgrading gen protect and grounding

Upgrading Generator

Protection and Grounding for

Industrial Generators-Why & How

Chuck MozinaBeckwith Electric Co., Inc.

Section I

Introduction

A major US manufacturer of :

– Digital multifunction generator and transformer protection

– Generator synchronizing and bus transfer equipment

– Voltage control devices for LTC transformer, regulators, and capacitor banks

– Packaged systems using Beckwith products

– Family-owned company founded by Mr. Bob Beckwith

– Formed May 1967 in Chicago, Illinois

– Moved in 1974 to Largo, Florida

– Additional Facility in October 1992

– Temporary Facility in October 1992(Result of tornado damage)

– New facility November 1993

ISO Registered

The InstructorChuck Mozina is Manager of Application Engineering for Protection and Protection Systems at Beckwith Electric Co. He is responsible for the application of Beckwith products and systems used in generator protection and intertie protection, synchronizing and bus transfer schemes.

Chuck is an active member of the IEEE Power System Relay Committee and is the past chairman of the Rotating Machinery Subcommittee. He is active in the IEEE IAS I&CPS committee which addresses industrial system protection. He is the U.S. representative to the CIGRE Study Committee 34 on System Protection and chairs a CIGRE working group on generator protection. He also chaired the IEEE task force which produced the tutorial “The Protection of Synchronous Generators,” which won the PSRC‘s 1995 Outstanding Working Group Award. Chuck is the 1993 recipient of the PSRC‘s Career Service Award.

Chuck has a Bachelor of Science in electrical engineering from Purdue University and has authored a number of papers and magazine articles on protective relaying. He has over 25 years of experience as a protective engineer at Centerior Energy, a major investor-owned utility in Cleveland, Ohio where he was the Manager of the System Protection Section. He is also a former instructor in the Graduate School of Electrical Engineering at Cleveland State University as well as a registered Engineer in the state of Ohio.

Relay Seminar

October 14 – 19, 2001Clearwater, Florida

Beckwith Electric has announced a date forthe next Relay Seminar, which will cover Generator, Transformer, and Interconnection Protection.

To sign up to be on the mailing list and receive seminar details, please visit:www.beckwithelectric.com/semreg.htm

Seminar OutlineDay 1 — AM

Generator Protection M-3425Fundamentals of Digital Protection

• Generator basics and generator grounding– Generator fundamentals– Generator grounding

• Traditional• IAS proposed hybrid scheme

– Industry standards– Function numbers

• Developing a protection upgrade program– Why upgrade at all– Improved sensitivity– New protection areas– Special protection applications– Basic upgrade options

• Use of digital technology to upgrade– Beckwith M-3425– Features– Software, oscillograph demonstration

• Questions

Section II

Fundamentals of Digital Protection

General Information on Generator Protection

Generator fundamentals

Industry standards

Function numbers

Functional breakdown

Fundamentals

Fundamental Section

Basic synchronous generators

Connections to the system

Generator grounding

C37 guidelines

Device numbers

Ib

Basic Synchronous Generator

Gen3-Ø

Electrical Output

DC Field Source

Ia

Ic

Prime Mover(Mechanical Input)

Synchronous Generator Types

Direct Connected Generator to Power

System

G

POWER SYSTEM

LOAD BUS

AUXILIARYLOAD

LOAD LOAD

Unit Connected Generator to Power

System

GAUXILIARY

LOAD

POWER SYSTEM

Low Impedance Grounding

* RESISTOROR

REACTOR

*

GENERATORWINDINGS

High Impedance Grounding

* RESISTOR*

GENERATORWINDINGS

Generator Short-Circuit Current

(Xd” Tdo”)

(Xd’ Tdo’)

(I)

Generator Terminal Fault Current

Key Industry Guides & Standards

Key Industry Guides & Standards

IEEE/ANSI

C-37.102-1995

C-37.101-1992

C-37.106-1992

Guide for ACGenerator Protection

Guide for Generator Ground Protection

Guide for Abnormal Frequency Protection for Power Generating Plant

Relay Function Numbers

21 Distance relay. Backup for system and generator zone phase faults.

24 Volts/Hz protection for the generator.27 Undervoltage protection for the generator.27TN Third-Harmonic Undervoltage.32 Reverse power relay. Anti-motoring

protection.40 Loss-of-Field protection.50BF Instantaneous overcurrent relay used as

current detector in a breaker failure scheme.

51N Time overcurrent relay. Backup for ground faults.

51V Voltage-controlled or voltage-restrained time overcurrent relay. Backup for system and generator phase faults.

59 Overvoltage protection.59N Voltage relay. Primary status ground fault

protection for a generator.

60FL Voltage balance relay. Detection of blown potential transformer fuses.

62B Breaker failure timer.64F Primary protection for rotor ground

faults.78 Loss of synchronism protection.81O/U Frequency relay. Both under frequency

and overfrequency protection.86 Hand-reset lockout auxiliary relay.87G Differential relay. Primary phase-fault

protection for the generator. 87GD Sensitive ground fault protection for the

generator.87T Differential relay. Primary protection for

the transformer. May be used to provide phase fault backup for the generator in some station arrangements.

87U Differential relay for overall unit and transformer.

Relay Function Numbers

M-3425Typical Connection Diagram

Standard Protective Functions

This function provides control for thefunction to which it points: it cannot beused independently.

Premium Protective Functions

NOTE: Some functions are mutuallyexclusive; see Instruction Book for details.

Utility System

52Unit

3

IN

52Gen

M-3425

59 2481 27

50BF-Ph

46

87

CT

VT

78 51T

+

-

64F

GeneratorField

60FL 40

27

51V 50 21 32 50

27

51NCT

R

M-3425

Low-impedance Groundingwith Overcurrent StatorGround Fault Protection

50N

87GD

59N27TN

32

27

R

M-3425

High-impedance Stator GroundFault with Third Harmonic 100%Ground Fault Protection

3CT

50DT

50BF-N

Section IIIUpgrading Generator Protection

Using Digital Technology

Upgrading Generator Protection Using

Digital Technology

Charles J. MozinaBeckwith Electric Company

Why Upgrade Generator Protection?

• Generators fail due to:+Abnormal operating conditions+Internal short circuits

• Proper generator protection can prevent many failures or minimize damage when failures occur.

• Cost of generator loss can be substantial+Added purchase power costs+Cost associated with impact on plant production+Companies have found preventing one failure can pay for entire upgrade program

• Insurance companies base premiums on generators being protected to the level recommended by IEEE C-37.103

• Less skilled operators make sustained operation outside of generator capability more likely. This warrants upgraded protection.

• The longer you wait, the older equipment gets, the more likely the failure

• Upgrading with digital technology can reduce future relay maintenance and provide data (oscillographs)toreduce outage time.

Areas of Protection Upgrade OnOlder Generators

– Improved sensitivity– New or additional protection areas– Special protection application

considerations

MULTIFUNCTION DIGITAL RELAYS

Improved Sensitivity

– Negative sequence (unbalanced current)

– Field ground 100% fault detection+Brush pull-off detection

– Dual-level loss-of field protection– Sensitive overexcitation protection


New or Additional Protection Areas

– Inadvertent generator energizing

– VT fuse-loss protection– Sequential tripping– Oscillographic monitoring



Special Protection Application Special Protection Application ConsiderationsConsiderations

Generator breaker failureGenerator breaker failureOver/under frequencyOver/under frequency

Improved Sensitivity

• Negative sequence (unbalanced current)

• Field ground fault detection+ Brush pull-off detection

• Dual-level loss of field protection

• Sensitive overexcitationprotection

Negative Sequence


Negative Sequence (46)

– Unbalanced phase currents create negative sequence current in generator stator,



– Negative sequence current interacts with normal positive sequence current to induce a double frequency current (120 Hz)

– Current (120 Hz) is induced into rotor causing surface heating


Currents in the Rotor Surface



Salient PoleWith connected amortisseur

10With non-connected amortisseur

5Cylindrical

Indirectly10Directly cooled - to 960 MVA

8961 to 1200 MVA

61200 to 1500 MVA

5

% StatorRating



– Generator has established short-time rating,



Two Types of Relays

1. ELECTROMECHANICAL– Sensitivity restricted to about

60% I2 of generator ratings– Fault backup provided– Generally insensitive to load

balances or open conductors



Two Types of Relays

1. DIGITAL AND STATIC–Protects generator down to

its continuous I2 rating–Can detect open conductor

conditions


Advanced Protection Functions

Field (Rotor) Ground Fault Protection (64F)

Insurance companies tell us this is the most frequent internal generator faultReview existing 64F voltage protection methodsDiscuss a new 64F injection method

Typical Generator Field Circuit

The first ground fault will:establish a ground reference making a second ground fault more likelyincrease stress to ground at other points in field winding

Ground #1

Typical Generator Field Circuit

The second ground fault will:short out part of field winding causing unit vibrationscause rotor heating from unbalanced currentscause arc damage at the points of fault

Ground #2

Ground #1

Field Ground Fault Protection

Detection Using a DC Source


Detection Using a Voltage Divider


Using Injection Voltage Signal


Real-Time Insulation Measurements

Field Insulation Real-Time Monitoring

Advanced Protection Functions

Brush lift-off detection (64B)– Brushes on older generators

are a maintenance headache for plant personnel

– When brushes should be replaced or re-adjusted is important diagnostic information

– If brushes open on an in-service generator they cause:

• arc damage to brush mounting structure

• eventual unit tripping by loss-of-field protection


Using Injection Voltage SignalBrush lift-off

Analyzer voltagereturn signal

Brush Lift-off Voltage Return Signal

Ground Brush Lift-off

When the ground brush lifts off the rotor

– Low resistance path to ground for stray rotor flux is removed

– Generator bearings carry stray shaft ground current

– Bearing will pit and will need to be replaced

Dual-LevelLoss-of-Field Protection

Older, Single-Zone Off-Set Mho Relay Loss-of-Field Characteristics


Loss-of-Field Using Two-Zone Off-Set Mho Method


SensitiveOverexcitation

Protection


Overexcitation /

Volts per Hertz (24)

GENERATORTRANSFORMER ≈EXCITATION

Voltage V

Freq. Hz

GENERATOR LIMITS (ANSI C 50.13)Full Load V/Hz = 1.05 puNo Load V/Hz = 1.05 pu

TRANSFORMER LIMITSFull Load V/Hz = 1.05 pu (HV Terminals)No Load V/Hz = 1.10 pu (HV Terminals)


Overexcitation/

Volts per Hertz (24)CAUSES OF V/HZ PROBLEMS

• Generator voltage regulator problems

- operating error during off-line manual regulator operation

- control failure

- loss of VT regulator supply voltage

- overexcitation when regulator is on–line

• System problems

- unit load rejection: full load, partial rejection

- power system islanding during major distrubances


Overexcitation/Volts per Hertz (24)

PHYSICAL INSIGHTS• As voltage rises above rating leakage flux increases

• Leakage flux induces current in transformer support structure causing rapid localized heating


Typical Relay Characteristics for Typical Relay Characteristics for DualDual--Level DefiniteLevel Definite--Time Time

V/Hz ProtectionV/Hz Protection


Optimum Optimum OverexcitationOverexcitationProtectionProtection


New or Additional Protection Areas

• Inadvertent generator energizing

• VT fuse-loss protection

• Sequential tripping

• Oscillographic monitoring


Inadvertent Generator Energizing


How Inadvertent Energizing Has Occurred

– Operating errors– Breaker head flashover– Control circuit malfunctions– Combination of above


Generator Response and Damage to Three-Phase Energizing

– Generator behaves as an induction motor

– Rotating flux induced into the generator rotor

– Resulting rotor current is forced into negative sequence path in rotor body


Inadvertent Energizing Equivalent Circuit



Inadvertent Energizing Function Logic Diagram


VT Fuse-Loss Protection


Application of Voltage Balance Relay Protection


Modern VT Fuse-Loss Detection


Sequential Tripping


Sequential Tripping Logic

– Used in steam turbine generators to prevent overspeed

– Recommended by manufacturers of steam turbine generators as a result of field experience

– This trip mode used only for boiler/reactor or turbine mechanical problems

– electrical protection should not trip through this mode



– STEP 1 Abnormal turbine/boiler/reactor condition is detected

– STEP 2 Turbine values are closed; generator allowed to briefly “motor” (i.e. take in power)

– STEP 3 A reverse power (32) relay in series with turbine valve positionswitches confirms all valves haveclosed

– STEP 4 Generator is separated from powersystem




OscillographicMonitoring


Benefits– Determine if relay and circuit breaker

operated properly– relay control problem – generator experience fault / abnormal

conditions– Speed generator return to service

– identify type of testing needed– provide data to generator manufacturer

– Gives relay engineer data to force unit off-line for inspection

– Uncovers unexpected problems: I.e. synchronizing

Oscillographic MonitoringOscillographicOscillographic MonitoringMonitoring


Digital Relay Oscillograph


Special ProtectionApplication

Considerations

Generator breaker failureOver/under frequency


Generator Breaker Failure


Typical Transmission Line Breaker Failure Functional Diagram


Functional Diagram of a Generator Breaker Failure Scheme


Under/Overfrequency(81U/81O)


Under/Overfrequency(81U/81O)

Underfrequency (81U)Generator Limits • Generator overloaded high strator

current

• Heating limits need to reduce output

•Overexcitation same as 24 Overvoltage (V/Hz)

•Turbine blade resonance

•All system generators are overloaded

•System load shedding via 81U to restore balance

•Need to coordinate tripping with system loadshedding.

Turbine Limits(Steam/GT)

System Problems


FIGURE 1 Generator Capability Versus Frequency

FIGURE 2 Generator Short-Term Thermal Capability


Under/Under/OverfrequencyOverfrequency(81U/81O)(81U/81O)

OverfrequencyOverfrequency (81O)(81O)

Generator LimitsTurbine Limits(Steam/GT)

System Problems

• None published

•Similar limits to underfrequency

•Overspeed trip at 110%

•More generation than load

•Governor actions to reduce MW output


FIGURE 3Steam Turbine Partial or Full-Load Operating

Limitations During Abnormal Frequency


ECAR Document # 3Generator Under Frequency

Tripping Requirements

0 minutesBelow 58.2 Hz

7.0 minutes58.5 - 58.2 Hz

30.0 minutes59.5 - 58.5 Hz

Unlimited60.0 – 59.5 Hz

Time DelayBefore Isolation

Frequency


FIGURE 4

Example of Trip Characteristics:

Overfrequency and Underfrequency


Use of Digital Technology to

Implement and Upgrade Program


Two Basic Design Options

– Retain Existing Protection– add additional functions to upgrade

to current standards

– Remove all existing protection– upgrade with all new protection


Dual-Relay Approach for Major Generators


Multifunction Relays Are The Right Choice, Either Way

– Panel space savings– Oscillographic capability– Communication (RS232 & RS485)– Low CT and VT burdens– Self diagnostic


Conclusions

– There are a number of serious protection shortcomings on generators with relays older than 20 years

– This paper attempted to bring those risks to the attention of utilities/generator owners


Conclusions

– Utilities/generator owners should address these risks through comprehensive upgrade programs to protect their generator investment

– Multifunction digital relays are an ideal, cost effective way to implement such a program

Section IVBeckwith M-3425Generator Relay

Function & Features

The World’s Leading Manufacturer of Digital Generator

Protection Proudly Presents the New

M-3425 Relay

The World’s Leading Manufacturer of Digital Generator

Protection Proudly Presents the New

M-3425 Relay

Digital Integrated Protection System® for Generators of All Sizes

What’s New from Beckwith in

Generator Protection

New Features

• Field Ground (64F)

• Out-of-Step(78)

• Split Phase Diff.. (50DT)

• Stator Thermal Overcurrent Protection (51T)

New FeaturesNew Features

•• Field Ground (64F)Field Ground (64F)

•• OutOut--ofof--Step(78)Step(78)

•• Split Phase Diff.. (50DT)Split Phase Diff.. (50DT)

•• Stator Thermal Overcurrent Protection Stator Thermal Overcurrent Protection (51T)(51T)

M-3420M-3420 M-3430M-3430

NewM-3425

NewM-3425

995 relays in service worldwide

489 relays in service worldwide

MM--34253425Typical Connection DiagramTypical Connection Diagram

Standard Protective Functions

This function provides control for thefunction to which it points: it cannot beused independently.

Premium Protective Functions

NOTE: Some functions are mutuallyexclusive; see Instruction Book for details.

Utility System

52Unit

3

IN

52Gen

M-3425

59 2481 27

50BF-Ph

46

87

CT

VT

78 51T

+

-

64F

GeneratorField

60FL 40

27

51V 50 21 32 50

27

51NCT

R

M-3425

Low-impedance Groundingwith Overcurrent StatorGround Fault Protection

50N

87GD

59N27TN

32

27

R

M-3425

High-impedance Stator GroundFault with Third Harmonic 100%Ground Fault Protection

3CT

50DT

50BF-N

Standard M-3425 Functions

• Overexcitation (V/Hz) protection (24)with both discrete time and inverse time curves

• 100% Stator Ground Fault Protection via third harmonic neutral undervoltage (27TN)

• Sensitive dual-setpoint reverse power, low forward power or over power detection, one of which can be used for sequential tripping (32)

• Dual-zone, offset-mho loss-of-field protection (40)• Sensitive negative sequence overcurrent

protection and alarm (46)• Generator Breaker Failure protection (50BF)• Inadvertant generator energizing protection

(50/27)• Definite Time Overcurrent (50DT) can be used for

split phase differential applications• Neutral inverse time overcurrent (51N) and

instantaneous overcurrent (50N) protection

Standard MStandard M--3425 Functions3425 Functions

ThreeThree--phase inverse time overcurrent phase inverse time overcurrent (51V) and instantaneous overcurrent (50) (51V) and instantaneous overcurrent (50) protectionprotectionPhase overvoltage (59) and undervoltage Phase overvoltage (59) and undervoltage (27) protection(27) protectionGenerator ground fault protection (59N)Generator ground fault protection (59N)VT fuseVT fuse--loss detection and blocking (60FL)loss detection and blocking (60FL)FourFour--step Over/Underfrequency (81) step Over/Underfrequency (81) protectionprotectionTwo step Rate of Change of Frequency Two step Rate of Change of Frequency (81R)(81R)Generator phase differential protection Generator phase differential protection (87) and ground differential (87GD) (87) and ground differential (87GD) protectionprotectionExternal Function allows external devices External Function allows external devices to trip through Mto trip through M--3425 Generator 3425 Generator Protection RelayProtection Relay

Premium MPremium M--3425 Functions3425 Functions

Field ground using advanced injection system (64F)

Stator thermal protection using positive sequence inverse overcurrent (51T)

Additional Standard M-3425 Functions• Eight programmable outputs and six

programmable inputs• Oscillography recording (170 cycles)• Time-stamped target storage for 24 events.• Metering of all measured parameters• Two RS-232C and one RS-485 communications

ports• M-3425 IPScom® Communications and Setting

Software• Includes Modbus and BECO2200 protocols• Standard 19" rack-mount design• Removable printed circuit board and power

supply• Both 50 and 60 Hz models available • Both 1 and 5 A rated CT inputs available • Additional trip inputs for externally connected

devices• IRIG-B time synchronization

MM--34253425External ConnectionsExternal Connections

ABC

50

LowImpedanceGrounding

52Gen

High Impedance Grounding

a b c

A B C

52b

2

M-3425 Three-Line Connection Diagram

M-3425OtherRelays

UTILITY SYSTEM

M-3425

Three VT Wye-WyeConnection

Three VT Wye-WyeConnection Ungrounded

Three VT Open-DeltaConnection

51

48 49

46 47 1011

OptionalField

GroundModule

58 59

56 57

54 55

M-3425

M-3425

52 53

M-3425

M-3425

M-3425

4544

42 43 40 41 38 39 42 43 40 41 38 39 42 43 40 41 38 39

Generator

OtherRelays

M-3425

Power Supply

Power Supply

(Optional)

ProgrammableGain Amplifier

MUX

Digital Signal Processor

(DSP) TMS 320C52

2K byte Dual-Ported

RAM

2-Line by 24-Character

Liquid Crystal Display

126K byte RAM

256K byte Flash-

Programmable ROM

Host Processor 10 MHz Zilog

64181

512 Byte EEPROM

8K byte RAM, Clock with

battery backup

MMI Module

(Optional)

Target Module

(Optional)

RS232 and RS485

Communi-cation ports

IRIG-B Time Code

input

Relay Outputs

Contact Inputs

Anti-Aliasing Low-Pass Filters (LPF)

Analog Multiplexer

VTs & CTs

Va

Vb

Vc

Vn

ia

ib

ic

iA

iB

iC

iN

32K X 16 RAM

Address/Data Bus

14-bit Analog-to-

Digital Converter

M-3425 Hardware Block Diagram