Generator Protection 1

Generator protection

Presented by

Bipasha Jash (PEE)

NTPC

Classification Classificat ion of Generator Protection

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

Classification Classificat ion 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 EFFECT CONDITION

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 out Welding of core lamination

Differential prot 100% E/F prot/95% E/F Inter 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 relay U/F relay

Loss of field

Induction gen operation Absorb 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 TRIP ALL ELECTRICAL TRIPS TRIP TURBINE, GENERATOR, GT, UT 

CLASS-B TRIP MECHANICAL TRIPS AVOID 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 C TRIPS ONLY HV CB IN CASE OF GCB SCHEME

Typical Generator protection scheme

GENERATOR PROTECTIONS NTPC PRACTICES

DIFFERENTIAL PROTECTION UNIT 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 10A THIS MINIMIZES THE DAMAGE FIRST FAULT LESS CRITICAL NEEDS 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/F 95 % E/F

95 % STATOR E/F RESULTS IN VOLTAGE SHIFT OF GEN NEUTRAL W.R.T. GROUND DETECTED BY OVER VOLTAGE RELAY CONNECTED ACROSS GROUNDING RESISTOR PROTECTS APPROX 95% OF STATOR WDG TYP 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 3 RD 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 protection LOW 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 UNIT  14 mA /1 sec (current setting ) OR Trip : 10 Ohm/1 sec Alarm : 20 ohm/10 sec( for Imp measuring relay SETTINGS 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 DAMAGE SECOND E/F RESULTS IN A WDG SC OF ROTOR CAUSE MAGNETIC UNBALANCE/MECH FORCES /DAMAGE METHODS OF DETECTION POTENTIOMETER METHOD A CENTRE TAPED RESISTOR IS CONNECTED ACROSS THE MAIN FIELD WINDING THE CENTRE TAP IS CONNECTED TO EARTH THROUGH A VOLTAGE RELAY AN EARTH FAULT ON THE FIELD WINDING WILL PRODUCE VOLTAGE IN THE RELAY, MAXIMUM VOLTAGE OCCURRING FOR END FAULTS A 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 ZONE

LOW FREQUENCY INJECTION METHOD

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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 SYSTEM FOR 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 CONDITION DURING 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 SEC TRIP - 47.4 HZ 2 SEC SETTING NEED TO BE CO-ORDINATED WITH THE RESPECTIVE RLDC AND THE ISLANDING SCHEME SETTINGS

Negative Sequence protection NEGATIVE SEQUENCE PROTECTION FOR GENERATOR PROTECTS THE GENERATOR FROM EXCESSIVE HEATING IN THE ROTOR RESULTING FROM UNBALANCED STATOR CURRENTS CAUSED DUE TO ONE POLE OPEN IN LINE ONE POLE OPEN OF A CIRCUIT BREAKER CLOSE IN UNCLEARED UNBALANCED FAULTS

THE NEGATIVE SEQUENCE PROTECTION RELAYS SHALL BE SET TO THE NPS CAPABILITY OF THE MACHINE WHICH IS K = I22* T  TYP FOR 500 MW PERMISSIVE NEG SEQ CURRENT = 5 – 8 % OF STATOR CURRENT  PERMISSIVE I22* T = 5 – 10 SETTINGS ADOPTED FOR NTPC I2 = = 7.5 % I22*T = 8.3

Loss of Field Protection ACTS AS AN INDUCTION GENERATOR INDUCED 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

BIG M/C WITH GOOD AVR

SMALL MACHINES

LOSS OF FIELD RELAY

OUT OF STEP PROTECTION MACHINE RUNS OUT OF SYNCHRONISM WITH THE NETWORK CYCLIC 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 IMP DISTINGUISH 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 TURBINE THE 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 OPEN SET 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 BY OVER CURRENT  DISTANCE DISTANCE PREFFERED AS THE LINE IS PROVIDED WITH DISTANCE RELAYS SETTING SHOULD BE MADE TO COVER THE GT IMP AND THE LONGEST LINE IMP SETTING SHOULD TAKE CARE OF THE INFEED FROM OTHER GENERATORS CONNECTED TO THE SAME BUS ALSO TIME 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 DIFFERENTIAL BIASED DIFFERENTIAL 10 % BIAS SETTING (TO COVER TAP RANGE AND CT MISMATCH, IF ANY) TIME INSTANTANEOUS

BACK UP EARTH FAULT  DEF TIME OR IDMT RELAY 20 % WITH 2 SEC TIME DELAY TO BE COORDINATED WITH DISTANCE PROT ZONE 3

UT PROTECTION DIFFERENTIAL BIASED DIFF USED BIASED SETTING 10%

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

RESTRICTED E/F HIGH IMP DIFF SET TO 5%-7% IN HIGH IMP EARTHING

COMMONLY USED GEN/GEN TRFR RELAYS PROTECTI ALSTOM ON

ABB

HIGH IMP CAG 34 RADHA DIFF MICOM P343  REG 216

SIEMENS

7UM 62 

REMARK In case of duplicated diff, one low imp & one high imp preferred For trfr biased relay preferred

BIASED DIFF

MBCH MICOM P 633

RADSB RET 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  /

100% E/F

PVMM MICOM P343

GIX 

PG871 

95% E/F

VDG

BACK UP IMP

YCG15

admittance

REG  216 

RAKZB

7UE22 

Low frequency  7UM SERIES  injection type 

preferred over 3 rd harmonic principle

7UM SERIES

Open delta of gen sec VT 

7UM 516

Minimum impedance

PROTECT  ALSTOM ION

ABB

SIEMENS

Remarks

OVER FLUXING

GTTM

RATUB RALK

7RW

IDMT 

POLE SLIPPING

ZTO+YTG RXZF+RXPE M15

7UM 516 

IMPEDANCE IMP+ DIR O/C IMP+NO OF POWER  SWINGS 

ACC. BACK ENERG

CTIG 

INTER TURN

VDG

NEG PH SEQ

CTN

REF

CAG/FAG

ROTOR E/F

VDG

RAGUA

RARIB RADHD

7UM SERIES

O/C +CB AUX  CONTACT  CURRENT  ELEMENT+U/V

7UM SERIES

comp of open delta 0n gen term+ngt sec voltage

7UM SERIES

MEASUREMENT OF

7UM SERIES

HIGH IMP PREFFERED

7UR 22 7 UM SERIES

I2

Stator protections Type of fault

Protection

Channel

Recommendation

Short circuit

87 G1 87G2 87 GT 

1 2 1&2

Stator Earth Fault

64G1 64G2

1 2

Inter turn

95G

1 &2

unbalance

46G

1&2

Over load

51G

Alarm

<

Loss of excitation

40G1 40G2

1 2

>100 MW

Out of step

98G

1&2

Motoring

32 G1/2 / 37 G1/G2

1/2

O/V,O/F U/F

59/99 81G1/81G1

1 /2 1/2

System back up

21G

1&2

Accidental energisation

50GDM

1 &2

Rotor E/F

64F

1 &2

Generator Transformer/Unit Transformer Protections

Fault Short ckt (GT)

Device no

87T  87 HV 51 GT  Earth Fault(GT) 51 NGT  64GT(3 Ph GT) 64T  Short 87 UT  circuit(UT) 51UT  Earth Fault UT  51 NUT  64 UT 

Channel 1 2 2 1 1 1/2 1/2 1/2 1 2

Recommendation