Generator protection

Presented by

Bipasha Jash (PEE)

NTPC

Classification of Generator Protection

Stator Protection

Generator Abnormal Operating conditions System back up protections Rotor protection GT protections UT protections

Over view of type of fault Vs protection

FAULT/ABNORMAL CONDITION

EFFECT PROTECTION

Thermal over loading

Over heating of stator winding/insulation failure

Thermo couples/Over current relays

External fault Unbalanced loading stress Over load/negative phase sequence relay

Stator fault Ph to Ph/Ph to E Inter turn

Winding burn outWelding of core lamination

Differential prot100% E/F prot/95% E/FInter turn prot

Rotor fault Damage to shaft/bearing 2 stage rotor E/F protection

Motoring LFPR/Rev power

O/V,O/F,U.F Insulation failure,Heating of core failure of blades

O/V relay Volt/Hz relayU/F relay

Loss of field Induction gen operationAbsorb MVAR from system/damage to rotor wdg

Loss of field

Generator Connection Methods

The protection scheme depends on the type of generator connection A)    Unit scheme: In this scheme no switchgear is provided between the generator and generator transformer, which are treated as a unit; a unit transformer is tapped off the interconnection for supply of power to auxiliary plant.B)    Generator circuit breaker scheme:In this scheme a generator circuit breaker is provided between the generator and generator transformer.

TRIP LOGIC OF GENERATOR PROTECTION

TWO INDEPENDENT CHANNELS WITH INDEPENDENT CT/VT INPUTS/DC SUPPLY/TRIP RELAY

CLASS A TRIPALL ELECTRICAL TRIPSTRIP TURBINE, GENERATOR, GT, UT

CLASS-B TRIPMECHANICAL TRIPSAVOID OVER SPEEDING OF TURBINE DUE TO STEAM ENTRAPPED IN TURBINE. TURBINE TRIP SIGNAL IS GIVEN FIRST AND THE ACTIVE POWER, SENSED BY THE LOW FORWARD RELAY (32G) GIVES THE TRIP SIGNAL TO THE UNIT BREAKER & FIELD BREAKER AFTER A TIME DELAY

Class CTRIPS ONLY HV CB IN CASE OF GCB SCHEME

Typical Generator protection scheme

GENERATOR PROTECTIONSNTPC PRACTICES

DIFFERENTIAL PROTECTIONUNIT TYPE PROTECTION

INSTANTANEOUS IN OPERATION.

COVERS THE STATOR WDG FOR PHASE TO PHASE FAULTS.

SINGLE PHASE TO EARTH FAULTS ARE NOT COVERED IN HIGH IMP EARTHING SYSTEMS (EARTH CURRENT IS LOW)

DUPLICATED DIFF USED FOR GCB SCHEME

TYPICAL SETTING: HIGH IMP TYPE: 5% BIASED 10%

STATOR EARTH FAULT PROTECTION

E/F CURRENT IS LIMITTED TO 10ATHIS MINIMIZES THE DAMAGEFIRST FAULT LESS CRITICALNEEDS CLEARANCE AS

IT MAY DEVELOP INTO A PH TO PH FAULT SECOND FAULT WILL RESULT IN VERY HIGH CURRENT

FIRE MAY RESULT FROM EARTH FAULT.TWO TYPES:

100 % E/F95 % E/F

95 % STATOR E/FRESULTS IN VOLTAGE SHIFT OF GEN NEUTRAL W.R.T. GROUNDDETECTED BY OVER VOLTAGE RELAY CONNECTED ACROSS GROUNDING RESISTORPROTECTS APPROX 95% OF STATOR WDGTYP SETTING:

5V TIME DELAY OF MORE THAN 1 SEC(GEN IDMT RELAY WITH PMS 5.4 AND TMS 1 IS USED)

100% Stator E/F Protection

THIRD HARMONIC PRINCIPLE

A RELAY WHICH RESPONDS TO THE REDUCTION OF THE 3RD HARMONIC COMPONENT A STATOR PHASE-TO-GROUND FAULT OCCURS AT OR NEAR THE GENERATOR NEUTRAL, THERE WILL BE AN INCREASE IN THIRD HARMONIC VOLTAGE AT THE GENERATOR TERMINALS, WHICH WILL CAUSE RELAY OPERATION.

DISADVANTAGES

DUE TO DESIGN VARIATIONS, CERTAIN GENERATING UNITS MAY NOT PRODUCE SUFFICIENT THIRD HARMONIC VOLTAGES THIS METHOD DOES NOT PROTECT THE M/C DURING STAND STILL CONDITIONS.

100 % Stator E/F protectionLOW FREQUENCY INJECTION PRINCIPLE

THIS SCHEME DETECTS THE GROUND FAULTS BY INJECTING A LOW FREQUENCY SIGNAL (SAY 20 HZ)AT THE NEUTRAL EARTHING TRFR OR LINE CONNECTED EARTHING TRFR AND MONITOR EITHER THE EARTH CURRENT IN THE WDG(INCREASES DURING A FAULT) OR THE STATOR EARTH RESISTANCE (REDUCES DURING FAULT)

PROVIDES PROTECTION EVEN DURING SHUT DOWN

SETTINGS TYPICAL FOR 500 MW UNIT14 mA /1 sec (current setting )

ORTrip : 10 Ohm/1 secAlarm : 20 ohm/10 sec( for Imp measuring relaySETTINGS ARE TO BE FINALISED DURING COMMISSIONING TEST IN LINE WITH INSTRUCTIONS OF RELAY CATALOGUE.

Rotor Earth Fault Protection

FIRST ROTOR E/F DOES NOT CAUSE IMMEDIATE DAMAGESECOND E/F RESULTS IN A WDG SC OF ROTOR CAUSE MAGNETIC UNBALANCE/MECH FORCES /DAMAGEMETHODS OF DETECTION

POTENTIOMETER METHODA CENTRE TAPED RESISTOR IS CONNECTED ACROSS THE MAIN FIELD WINDING THE CENTRE TAP IS CONNECTED TO EARTH THROUGH A VOLTAGE RELAYAN EARTH FAULT ON THE FIELD WINDING WILL PRODUCE VOLTAGE IN THE RELAY, MAXIMUM VOLTAGE OCCURRING FOR END FAULTSA BLIND SPOT EXISTS AT THE TAPPING POINT, TO AVOID THIS , THE TAPPING POINT IS VARIED WITH A PUSH BUTTON OR SWITCH , AND IS TESTED PERIODICALLY TO DETECT BLIND ZONESETTING IS 5% OF FIELD VOLTAGE

LOW FREQUENCY INJECTION METHOD

MODERN ROTOR EARTH FAULT PROTECTION RELAY OPERATES ON THE PRINCIPLE OF LOW FREQUENCY INJECTION INTO THE FIELD WINDING VIA CAPACITORS.

IF AN EARTH FAULT OCCURS, THE CORRESPONDING CURRENT OR RESISTANCE IS DETECTED

TYP SETTING (500 MW)

ALARM 40 K OHM TIME = 10 SEC

TRIP 5 K OHM TIME = 1 SEC

ACTUAL VALUES OF SETTING SHALL BE DECIDED AT SITE DURING COMMISSIONING TO ACCERTAIN THE HEALTHY VALUE OF THE PARTICULAR M/C.

INTER TURN PROTECTION

CURRENT BASED SYSTEMFOR GENERATORS WITH SPLIT NEUTRALS WITH ALL SIX TERMINALS BROUGHT OUT ON NEUTRAL SIDE DELAYED LOW-SET O/C RELAY WHICH SENSES THE CURRENT IN THE CONNECTION BETWEEN THE NEUTRALS OF THE STATOR WINDINGS

VOLTAGE BASED SYSTEM RELAY COMPARES THE NEUTRAL NGT SEC VOLTAGE AND GEN TERMINAL OPEN DELTA VOLTAGE

BALANCE DURING E/F OR NORMAL CONDITIONDURING INTER TURN FAULT OPEN DELTA VOLTAGE WILL BE DEVELOPED AND NGT SEC VOLTAGE WILL BE ZERO, RESULTING IN A DIFFERENTIAL VOLTAGE WHICH MAKES THE RELAY OPERATE

IDMT RELAYS WITH 5.4 PMS AND 1 TMS ARE ADOPTED.

O/V PROTECTION

TYP SETTINGS OF A 3 STAGE O/V RELAY IS AS FOLLOWS

ALARM 110 % 2 SEC TRIP 120 % 1 SEC

140 % INSTANTANEOUS

U/F O/F PROTECTION

TYPICAL SETTING:ALARM - 47.8HZ 1 SECTRIP - 47.4 HZ 2 SECSETTING NEED TO BE CO-ORDINATED WITH THE RESPECTIVE

RLDC AND THE ISLANDING SCHEME SETTINGS

Negative Sequence protectionNEGATIVE SEQUENCE PROTECTION FOR GENERATOR PROTECTS THE GENERATOR FROM EXCESSIVE HEATING IN THE ROTOR RESULTING FROM UNBALANCED STATOR CURRENTSCAUSED DUE TO

ONE POLE OPEN IN LINEONE POLE OPEN OF A CIRCUIT BREAKERCLOSE IN UNCLEARED UNBALANCED FAULTS

THE NEGATIVE SEQUENCE PROTECTION RELAYS SHALL BE SET TO THE NPS CAPABILITY OF THE MACHINE WHICH IS K = I2

2* T

TYP FOR 500 MW PERMISSIVE NEG SEQ CURRENT = 5 – 8 % OF STATOR

CURRENTPERMISSIVE I2

2* T = 5 – 10

SETTINGS ADOPTED FOR NTPCI2 = = 7.5 %

I22*T = 8.3

Loss of Field Protection

ACTS AS AN INDUCTION GENERATORINDUCED EDDY CURRENTS IN THE FIELD WINDING, ROTOR BODY, WEDGES AND RETAINING RINGS MW FLOW IN TO THE SYSTEM/MVAR FLOWS IN TO THE MACHINE.THE APPARENT IMP TRAVELS TO THE FORTH QUADRANT OF X-Y PLANE

METHODS OF DETECTION: UNDER VOLTAGE + O/C

MINIMUM IMPEDANCE WITH U/V

SOME RELAYS ARE SET IN THE ADMITTANCE PLANE MATCHING WITH THE CAPABILITY CURVE OF THE MACHINE

LOSS OF FIELD RELAY

BIG M/C WITH GOOD AVR

SMALL MACHINES

OUT OF STEP PROTECTIONMACHINE RUNS OUT OF SYNCHRONISM WITH THE NETWORKCYCLIC VARIATION OF ROTOR ANGLE CURRENT INCREASES HEAVILY FREQUENCY DEPEND ON THE RATE OF SLIP RESULT IN THE WINDING STRESS IT MAY ALSO DAMAGE THE AUXILIARIES OF THE AFFECTED UNIT DETECTED BY SENSING THE VARIATIONS IN IMPDISTINGUISH BETWEEN THE RECOVERABLE SWING AND THE IRRECOVERABLE SWING

TWO BLINDERS(IMPEDANCE) + A SUPERVISORY MHO ELEMENT, TRIP WHEN IMP IS INSIDE THE MHO AND CROSED THE BLINDERS

MINIMUM IMPEDANCE (MULTIPLE ZONE) + COUNTING NO. OF SWINGS

POLE SLIPPING CHARACTERISTIC

ACCIDENTAL BACK ENERGISATION

CAUSE OPERATION AS AN INDUCTION MOTOR

DAMAGE MACHINE AND TURBINETHE RAPID HEATING IRON PATHS NEAR THE ROTOR SURFACE DUE TO STATOR INDUCED CURRENT.

DETECTED BY OVER CURRENT + CB AUXILIARY CONTACTS

CHECKS FOR THE CURRENT WHEN THE GEN BREAKER CONTACTS ARE OPENSET BELOW THE RATED CURRENT (50 –70%)

O/C AND U/V MEASUREMENTS

Backup Impedance protection FOR UNCLEARED SYSTEM FAULT

THE BACKUP PROTECTION IS TIME DELAYED TO COORDINATE WITH THE ZONE 3 SETTING OF LINES .

DETECTED BYOVER CURRENT DISTANCEDISTANCE PREFFERED AS THE LINE IS PROVIDED WITH DISTANCE RELAYS

SETTING SHOULD BE MADE TO COVER THE GT IMP AND THE LONGEST LINE IMPSETTING SHOULD TAKE CARE OF THE INFEED FROM OTHER GENERATORS CONNECTED TO THE SAME BUS ALSOTIME SETTING 1.5 –2 SEC

LOW FORWARD AND REVERSE POWER INTERLOCK

To protect the machine from motoring action

Trip under class B after a short time delay in case the turbine is already tripped ( typ set at 2 sec)

Trip under class A, after a long time delay if turbine is not tripped (typically set at 10 sec)

Power setting typ 0.5 % of rated power

GEN TRANSFORMER PROTECTION

DIFFERENTIALBIASED DIFFERENTIAL

10 % BIAS SETTING (TO COVER TAP RANGE AND CT MISMATCH, IF ANY)

TIME INSTANTANEOUS

BACK UP EARTH FAULTDEF TIME OR IDMT RELAY20 % WITH 2 SEC TIME DELAY

TO BE COORDINATED WITH DISTANCE PROT ZONE 3

UT PROTECTIONDIFFERENTIAL

BIASED DIFF USEDBIASED SETTING 10%

BACK UP OVER CURRENT 2-3 TIMES THE FULL LOAD CURRENT DELAY 0.9 SECTAKE CARE OF ANY LARGE MOTOR STARTING CASE

RESTRICTED E/FHIGH IMP DIFFSET TO 5%-7% IN HIGH IMP EARTHING

COMMONLY USED GEN/GEN TRFR RELAYSPROTECTION

ALSTOM ABB SIEMENS REMARK

HIGH IMP DIFF

CAG 34MICOM P343

RADHAREG 216

7UM 62In case of duplicated diff, one low imp & one high imp preferredFor trfr biased relay preferred

BIASED DIFF

MBCHMICOM P 633

RADSBRET 316

7 UT

POWER RELAYS

RXPE PPX 7 UM SERIES Directional power relays

LOSS OF FIELD

RAGPC(DIR O/C+U/V)

YCGF 7UM SERIES Impedance / admittance

100% E/F PVMMMICOM P343PG871

GIX REG 216

7UE227UM SERIES

Low frequency injection type preferred over 3 rd harmonic principle

95% E/F VDG 7UM SERIES Open delta of gen sec VT

BACK UP IMP

YCG15 RAKZB 7UM 516 Minimum impedance

PROTECTION

ALSTOM ABB SIEMENS Remarks

OVER FLUXING

GTTM RATUBRALK

7RW IDMT

POLE SLIPPING

ZTO+YTGM15

RXZF+RXPE 7UM 516 IMPEDANCEIMP+ DIR O/CIMP+NO OF POWER SWINGS

ACC. BACK ENERG

CTIG RAGUA 7UM SERIES O/C +CB AUX CONTACTCURRENT ELEMENT+U/V

INTERTURN

VDG 7UM SERIES comp of open delta 0n gen term+ngt sec voltage

NEG PH SEQ

CTN RARIB 7UM SERIES MEASUREMENT OF

I2REF CAG/FAG RADHD 7UM SERIES HIGH IMP

PREFFERED

ROTOR E/F

VDG 7UR 227 UM SERIES

Type of fault Protection Channel Recommendation

Short circuit 87 G187G287 GT

121&2

Stator Earth Fault 64G164G2

12

Inter turn 95G 1 &2

unbalance 46G 1&2

Over load 51G Alarm <

Loss of excitation 40G140G2

12 >100 MW

Out of step 98G 1&2

Motoring 32 G1/2 / 37 G1/G2 1 / 2

O/V,O/FU/F

59/9981G1/81G1

1 /21/2

System back up 21G 1 & 2

Accidental energisation

50GDM 1 &2

Rotor E/F 64F 1 &2

Stator protections

Generator Transformer/Unit Transformer Protections

Fault Device no Channel

Recommendation

Short ckt(GT)

87T87 HV51 GT

122

Earth Fault(GT)

51 NGT64GT(3 Ph GT)64T

111/2

Short circuit(UT)

87 UT51UT

1/21/2

Earth Fault UT 51 NUT64 UT

12

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