Generator Protection 1

(1)

Generator protection

Presented by

Bipasha Jash (PEE)

NTPC

(2)

Classificat

ion of

Generator Protection

Stator Protection

Generator

Abnormal

Operating

conditions

System back up

protections

Rotor protection

GT protections

UT protections

(3)

Classificat

ion of

Generator Protection

Stator Protection

Generator

Abnormal

Operating

conditions

System back up

protections

Rotor protection

GT protections

UT protections

(4)

Over view of type of fault Vs protection

FAULT/ABNORMAL FAULT/ABNORMAL CONDITION CONDITION EFFECT PROTECTION EFFECT PROTECTION Thermal over Thermal over loading loading

Over heating of stator winding/ Over heating of stator winding/ insulation failure

insulation failure

Thermo couples/ Thermo couples/ Over current relays Over current relays

External fault

External fault Unbalanced Unbalanced loading loading stress stress Over Over load/negative load/negative phasephase sequence relay sequence relay Stator fault Stator fault Ph to Ph/Ph to E Ph to Ph/Ph to E Inter turn Inter turn

Winding burn out Winding burn out

Welding of core lamination Welding of core lamination

Differential prot Differential prot

100% E/F prot/95% E/F 100% E/F prot/95% E/F Inter turn prot

Inter turn prot Rotor fault

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

Motoring LFPR/Rev powerLFPR/Rev power O/V,O/F,U.F

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

core failure of blades

O/V relay Volt/Hz relay O/V relay Volt/Hz relay U/F relay

U/F relay Loss of field

Loss of field Induction gen operationInduction gen operation Absorb MVAR from

Absorb MVAR from

system/damage to rotor wdg system/damage to rotor wdg

Loss of field Loss of field

(5)

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

.

(6)

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

(7)

(8)

Typical Generator

protection

(9)

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%

(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

(11)

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)

(12)

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.

(13)

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.

(14)

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

(15)

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.

(16)

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

(17)

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

(18)

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 I_{2 =} = 7.5 %

(19)

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

(20)

LOSS OF FIELD RELAY

BIG M/C WITH GOOD AVR

SMALL MACHINES

(21)

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

(22)

POLE SLIPPING

CHARACTERISTIC

(23)

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

(24)

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

(25)

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

(26)

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

(27)

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

(28)

COMMONLY USED GEN/GEN TRFR RELAYS

PROTECTI ON

ALSTOM ABB SIEMENS REMARK HIGH IMP DIFF CAG 34 MICOM P343 RADHA REG 216 7UM 62 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 /

admittance 100% E/F PVMM MICOM P343 PG871 GIX REG 216 7UE22 7UM 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

(29)

PROTECT ION

ALSTOM ABB SIEMENS Remarks

OVER

FLUXING GTTM RATUB_RALK 7RW IDMT

POLE

SLIPPING ZTO+YTG_M15 RXZF+RXPE

7UM 516 _IMPEDANCE

IMP+ DIR O/C

IMP+NO OF POWER SWINGS

ACC. BACK ENERG

CTIG RAGUA 7UM

SERIES O/C +CB AUX CONTACT CURRENT ELEMENT+U/V INTER TURN VDG 7UM SERIES

comp of open delta 0n gen term+ngt sec

voltage

NEG PH

SEQ CTN RARIB 7UM_SERIES MEASUREMENT OF

I

2

REF _{CAG/FAG RADHD} _7UM

SERIES HIGH IMP PREFFERED ROTOR E/F VDG 7UR 22 7 UM SERIES

(30)

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

(31)

Generator Transformer/Unit Transformer Protections

Fault Device no Channel Recommendation Short ckt (GT) 87T 87 HV 51 GT 1 2 2 Earth Fault(GT) 51 NGT 64GT(3 Ph GT) 64T 1 1 1/2 Short circuit(UT) 87 UT 51UT 1/2 1/2 Earth Fault UT 51 NUT

64 UT

1 2