GENERATOR PROTECTIONBySubhash ThakurPE-Electsthakur@ntpceoc.co.in

Gen Stator Thermal Protection Field Thermal Protection Gen stator fault Protection Gen rotor field Protection Gen abnormal operating conditions System backup Protection Power transformer Protection

Generator Protection

Generator ProtectionStator Thermal protectionThermal protection for the generator stator core and windings Generator overloadWinding Temperature Over currentFailure of cooling systemsRTDs ThermocoupleFlow and pressure sensorLocalized hot spots caused by core lamination insulation failures or by localized or rapidly developing winding failuresGenerator Core monitor

Generator ProtectionTurbine-generator short time thermal capability for balanced three-phase loading

Generator ProtectionGenerator Field Thermal protection

Thermal ProtectionDirect rotor Body temperature measurement not possible Core Monitor may detect overheatingProtection for field over excitationIDMT/ Definite Time Excitation limiters

Generator Protection

Generator field short time thermal capability

Generator Protection Requirement

Generator faults are considered to be serious since they may cause severe and costly damage to insulation, windings, and the core may also produce severe mechanical torsional shock to shafts and couplings.

Fault current may continue to flow for many seconds even after the generator is tripped, because of trapped flux within the machine, thereby increasing the amount of fault damage.

As a consequence, for faults in or near the generator that produce high magnitudes of short-circuit currents, some form of high-speed protection is normally used to trip and shut down the machine as quickly as possible in order to minimize damage.

Stator fault ProtectionHigh Speed Differential protectionWill detect Phase to Phase Faults, Double phase faults involving earthSingle phase to Earth will not be detected due to limited earth fault current available.

Two types of high-speed differential relays are commonly used for stator phase fault detection:High-impedance differentialBiased differential

High Impedance Differential RelayUse two sets of identical dedicated CTs.PS class CT with stringent parameters to be usedThis scheme has higher sensitivity than the percentage differential relay.Through fault stability achieved by using stabilising resistors in the relay circuit.

High Impedance Differential Relay

Stabilizing resistor calculation : Vs = If (Rct+2Rl) If - Maximum through fault current in the system (converted to sec side) Rct- Secondary resistance of the CT Rl lead resistance of the sec connection (typ 8.73 ohms per km for 2.5 sq mm cu cable) Rs = Vs/Is (VA/Is*Is)

Typical setting 5- 10% of rated current.

Biased Type Diff Relay

Less stringent CT parameters. CTs can be shared with other protections.Through fault stability achieved through biasing.CT mismatch (typ of the order of 1:5 ) can be accommodated.More suitable for numerical integrated protection systems as the CTs can be shared for many functions.Modern numerical relays have flexible settings for Id, b (point of slope change) and the slopes.

Biased Differential protectionTypical bias setting: 10% of rated current.

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 Genertaor terminal open delta voltage. Balance during external 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.

Typical setting

Definite time type relays: minimum setting with 1 sec delay.

INTERTURN PROTECTION

Inter turn protection

Split Phase ProtectionVoltgaVoltage Based

Generator Grounding PracticesIt is common practice to ground all types of generators through some form of external impedancelimit the mechanical stresses and fault damage in the machine, to limit transient voltages during faults, and to provide a means for detecting ground faults within the machine.Typical Grounding practicesUngroundedSolid GroundingHigh-impedance groundingLow-resistance groundingReactance groundingGrounding-transformer grounding

Generator Grounding PracticesUngroundedPhase to ground fault current limitedGenerators are not often operated ungrounded as it may produce high transient over-voltages during faults and makes the fault location difficult to determine.

Solid GroundingSolid grounding of a generator neutral is not generally used since this practice may result in high mechanical stresses and excessive fault damage in the machine.

Generator Grounding PracticesHigh Impedance GroundingHigh resistance groundingThe high-resistance grounding method utilizes a resistor connected across the secondary of the distribution transformer to limit the maximum ground fault current. For a single-phase-to-ground fault at the machine terminals, the primary fault current will be limited to a value in the range of about 3 A to 25 A.Ground fault neutralizer groundingThe ground fault neutralizer grounding method utilizes a secondary tunable reactor to limit the maximum ground fault current.Low resistance groundingIn this method, a resistor is connected directly between the generator neutral and ground. For a single-phase-to-ground fault at its terminals the primary fault current will be limited to a value in the range of about 200 A up to 150% of rated full-load current. Resistor cost and size usually preclude the use of resistors.

Stator Earth Fault Protection

E/F current is typically limited to 5-10A to minimizes the damage to laminations.First earth fault is less critical but needs clearance asIt may develop into a ph to ph fault .Second fault will result in very high current.Two types of coverage:100 % winding95 % winding

Any fault involving earth results shift of Neutral voltage.

This shift can be detected by measuring the Voltage across Grounding Resistor Or from the generator terminal Open Delta voltage.Typical coverage 95% Of Stator Winding.

Typical Setting:5% with 1 Sec TD

95 % Stator Earth Fault

100 % Stator E/F Protection

Third Harmonic PrincipleRelay responds to the reduction of the 3rd Harmonic Component For a Stator Phase-to-ground fault at or near the Generator Neutral, there will be an increase in third Harmonic Voltage at The Generator Terminals, which Will Cause Relay Operation.

100% SEF based on third harmonics measurements

100% SEF based on third harmonics measurements

Disadvantages

Due to design variations, certain generating units may not produce sufficient third harmonic voltages.

This method does not protect the machine during stand still conditions.

100% stator earth fault protection (Low freq. injection principle) Detects the ground faults by injecting a low frequency signal (say 20 hz) at the neutral earthing transformer and monitor the earth current in the winding.

SEF USING INJECTION PRINCIPLE TYPICAL CONNECTIONTypical settings for 500 MW unitTrip : 1 KOhm / 1 secAlarm : 10 Kohm /10 sec

Rotor Earth Fault ProtectionEffectsFirst rotor E/F does not cause immediate damageSecond E/F results in short circuit of rotor winding.Causes magnetic unbalance/mechanical forces

MeasureLow frequency injection method Modern rotor earth fault protection relay operates on the principle of low frequency injection into the field winding via capacitors.Corresponding current or resistance during E/F is sensed

Typical setting for a 500 mw Generator Alarm 25 k ohm time = 10 sec Trip 5 k ohm time = 1 sec

Rotor E/F Using Low frequency injection method

Rotor E/F Using Low frequency injection method

Negative sequence protection

Causes of negative squence currentone pole open in lineUnbalanced loadsUnbalanced system faults

Induces double frequency rotor current in the rotor surface thereby leading to high and dangerous temperatures in a short span of time.

Negative sequence protection relays shall be set to the NPS withstand capability of the machine which is given byk = i22x t

Typical for 500 mw Permissible neg seq current = 5 8 % of stator currentpermissive i22x t = 5 10

settings adopted for ntpc i2 = = 7.5 % i22xt = 8

Negative sequence protection

Loss of field protection

Loss of field protectionActs 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 moves in to the forth quadrant of x-y plane

Method of detection: Impedance measurement with Under Voltage

Some relays are set in the admittance plane matching with the capability curve of the machine.

Trip characteristics of loss of field protection

Trip characteristics of loss of field protection

Trip characteristics of loss of field protection

Generator Capability CurveRELAY LINE

Out of step protectionMachine runs out of synchronism with the networkCyclic variation of rotor angle Current increases. Results in the winding stress It may also damage the auxiliaries of the affected unit

Method of detection Variations in impedance measured at Gen TerminalDistinguish between the recoverable swing and the irrecoverable swingblinders and a supervisory mho element, Trips the machine when imp is inside the mho and cross the blinders with the specified time.Minimum impedance (multiple zone) + counting no of swings

Out of step protection settings

Typical Over Fluxing Withstand Capability

Accidental back energisation

Cause

Flash over of the generator breakerIncorrect closing of the generator breaker

Effects

Cause operation as an induction motorDamage machine and turbineThe rapid heating iron paths near the rotor surface due to stator induced current.

Over current + CB auxiliary contacts checks for the current when the gen breaker contacts are openset below the rated current(90%) o/c and u/v measurements

Setting - o/c 1.2 times & u/v 70%

Accidental Back Energisation

Reverse /Low forward power Protection

Low forward and reverse power inter lockTo allow entrapped steam in the turbine to be utilized to avoid damage of the turbine blade.

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 -30 sec)

Power setting typ 0.5 % of rated power

O/V & U/F protection

Typical settings of a 3 stage o/v relay is as follows Alarm 110 % 2 sec Trip 120 % 1 sec 140 % instantaneous

Abnormal Frequency protection

Typical setting: U/FO/FAlarm - 48.5hz 5 sec 51 hz 1 secTrip - 47.4 hz 2 sec

For uncleared system fault

The backup protection is time delayed to coordinate with the zone 3 setting of lines Detected byover current impedanceImpedance type preferred as the line is provided with distance relaysSetting should be made to cover the GT imp and the longest line impedance.Setting should take care of the infeed from other generators connected to the same bus also.Time setting 1.5 2 secBackup impedance protection

Over view of type of fault Vs protection

FAULT/ABNML EFFECTPROTECTIONThermal over loadingOver heating of stator wdg / insulation failureThermo couples/Over current relaysExternal fault Unbalanced loading stressOver load/negative phase sequence relay, Backup Impedance/ Earth FaultStator faults Winding burn outShorting of of core laminationDifferential protection100% E/F prot/95% E/FInter turn protectionRotor faultDamage to shaft/bearingTwo stage rotor E/F protectionMotoringDamage to turbine bladesLFPR/Rev power Inter lock

O/V,O/F,U.FInsulation failure,Heating of core failure of bladesO/V relay Volt/Hz relayU/F relayLoss of fieldInduction gen operationAbsorb MVAR from system/damage to rotor wdgLoss of field

COMMONLY USED GEN/GEN TRFR RELAYS

PROTECTIONALSTOM/AREVAABBSIEMENSREMARKHIGH IMP DIFFCAG 34MICOM P343RADHAREG 2167UM SERIESIn case of duplicated diff, one low imp & one high imp preferredFor trfr biased relay preferredBIASED DIFFMBCHMICOM P 633RADSBRET 316 7 UT

POWER RELAYSRXPEPPX7 UM SERIESDirectional power relays LOSS OF FIELDYCGFRAGPC(DIR O/C+U/V)7UM SERIESImpedance / admittance

100% E/FPVMMMICOM P343PG871GIX REG 2167UE227UM SERIESLow frequency injection type preferred over 3 rd harmonic principle95% E/FVDG7UM SERIESOpen delta of gen sec VTBACK UP IMPYCG15MICOM SERIESRAKZBREG 7UM 516Minimum impedance

PROTECTIONALSTOMABBSIEMENSRemarksOVER FLUXINGGTTMRATUBRALK7RWIDMTPOLE SLIPPINGZTO+YTGM15RXZF+RXPE7UM 516IMPEDANCEIMP+ DIR O/CIMP+NO OF POWER SWINGS ACC. BACK ENERGCTIG RAGUA7UM SERIESO/C +CB AUX CONTACTCURRENT ELEMENT+U/VINTERTURNVDGMICOM REG7UM SERIEScomp of open delta 0n gen term+ngt sec voltageNEG PH SEQCTNRARIB7UM SERIESMEASUREMENT OF I2REFCAG/FAGRADHD7UM SERIESHIGH IMP PREFFEREDROTOR E/FVDGMICOM SERIESREG SERIES7UR 227 UM SERIES

Type of faultProtectionChannelRecommendationShort circuit87 G187G287 GT121 OR 2Stator Earth Fault64G164G212Inter turn95G1 OR 2unbalance46G1 OR 2Over load51GAlarmLoss of excitation40G140G212Out of step98G1 OR 2>100 MW

Motoring32 G1/2 / 37 G1/G21 / 2O/V,O/FU/F59/9981G1/81G11 /21/2System back up21G1 & 2Accidental energisation50GDM1 &2Rotor E/F64F1 OR 2

Generator Transformer ProtectionDifferentialbiased differential 20 % bias setting (to cover tap range and ct mismatch if any)time: instantaneous

Back up earth faultDefinite time or IDMT relay30 % with 2 sec time delayTo be coordinated with distance prot zone 3

UT PROTECTIONDifferential Biased differential used biased setting 20%

Back up over current 2-3 times the full load current Delay of 1 sec to take care of any large motor starting case

Restricted E/F High impedance Set to 5%-10% in high impedance earthingBackup E/F Set to 30% rated current with delay of 1 sec

Other ProtectionsOverall Differential Protection (87GT) - Covers generator, GT & UT

GT overhang differential Protection (87HV) - Protects GT HV wdg & overhang portion between GT bushing and switchyard.

Typical Generator protection scheme Typical Gen Prot SLD

GCB SCHEMENON GCB SCHEME

TRIP LOGIC OF GENERATOR PROTECTIONTwo independent channels with independent CT/VT inputs/ DC supply/ Trip relays

Class A Trip (Urgent Trips)All electrical tripIssues instantaneous Trip toTurbine , Excitation, Generator EHV CBs,UT LV CBsIn GCB Scheme Class A1 and A2Class A1 Issues instantaneous Trip toTurbine , Excitation, Generator EHV CBs,GCB, UT LV CBsClass A2 Issues instantaneous Trip toTurbine , Excitation, GCBClass-B Trip (Non-urgent Trips)Turbine Trips, GT and UT OTI/WTI tripsIssues delayed Trip to (After Low Forward Power timer)In Non-GCB scheme-Excitation, Generator CBs,UT LV CBsIn GCB scheme, only GCB and field are tripped, UT remains charged through GT.Class C TripTrips HV CB only.

CLASS OF TRIP

BREAKERS TO BE TRIPPED UNDER VARIOUS CLASSES OF TRIPPING

GCB SCHEME

(additional LV CB between Gen and GT)

NON GCB SCHEME

Class A

A1: GCB,HVCB,UT LV CB, FIELD, TURBINE

(All the system tripped)

A2 : GCB, FIELD, TURBINE

(Generator circuit tripped & Auxiliaries charged from the grid through GT&UT)

HVCB,UT LV CB, FIELD, TURBINE

(All the system tripped)

Class B

GCB,FIELD BREAKER

Initiated by Turbine trip & Low Forward /reverse power, to release the trapped steam. Generator circuit breaker tripped & Auxiliaries charged from the grid through GT&UT)

HVCB,UT LV CB, FIELD BREAKER.

Class C

HVCB

(Generator under House load )

HVCB

(Generator under House load )

RELAY GROUPING

SL

NO

PROTECTION FUNCTION

CLASS OF TRIP

Preferred grouping of protection

NON GCB

GCB

1.

Generator Differential Protection, (87 G)

(DUPLICATED IN CASE OF GCB SCHEME)

A

A2

87 G and 87 GT shall be on two different channels of protection.

2.

Overall Differential Protection (87GT).

A

A1

3.

Generator Transformer Differential protection (87 T)

A

A1

87 T shall be in a different channel than 87 GT

4.

Over hang differential protection(87 HV)

A

A1

87 HV shall be in a different channel than 87T

5.

Stator Earth Fault Protection covering 100% of winding based on low frequency injection principle.(64G1).

A

A2

64 G1 and 64 G2 shall be on two different channels of protection.

6.

Stator Standby Earth Fault Protection covering 95% of winding (64 G2)

A

A2

7.

Inter-turn Fault Protection (95G1),

A

A2

8.

Duplicated Loss of field protection (40G1/2 ).

A

A2

40G1 and 40G2 shall be on two different channels of protection.

9.

Back up Impedance Protection, 3 pole (21G)

A

A1

21 G and 51 NGT be in different channels

10.

Backup Earth Fault Protection on Generator Transformer HV neutral (51NGT)

A

A1

11.

Negative Sequence Current Protection, (46G)

A

A2

1.

Duplicated Low-Forward Power / reverse power Interlock for steam turbine generator (37 /32G1 & 37/32 G2), each having two stages,

a) short time delayed interlocked with turbine trip

b) long time delayed independent of turbine trip.

B

A

B

A2

37/32 G1 and 37/32 G2 shall be in two different channels of protection

2.

Two Stage Rotor Earth Fault Protection based on injection principle.(64F).

A

A2

3.

Definite Time Delayed Over-Voltage Protection (59G)

A

A2

Over Flux function (99) shall be in a different channel than O/V and U/F functions

4.

Generator Under Frequency Protection (81G) with df/dt elements.

C

C

5.

i) Over Fluxing Protection (99 T) for Generator Transformer

ii) Over fluxing protection for

Generator (99 G)( only

incase of GCB scheme)

A

-----

A1

A2

6.

Accidental Back Energisation protection (50GDM) on two principles

a) based on U/V and O/C

b) based on CB status and O/C

A

A1

50 GDM based on the two principle shall be on two different channels.

7.

Instantaneous and time delayed Over Current protection to be used on HV side of excitation transformer.

A

A2

8.

Generator Pole slipping protection(98G)

A

A2

1.

Unit Transformer Differential Protection, 3 pole (87UT)

A

A1

87 UT & 51 NUT can be in one channel and 64 UT LV & 51UT shall be in another channel.

2.

Unit Transformer LV back-up earth fault protection . ( 51NUT).

A

A1

3.

Unit Transformer LV REF (64 UT LV)

A

A1

4.

Unit transformer back-up over current protection (51UT).

A

A1

5.

Gen Transformer OTI/WTI trip

Turbine Trip

Turbine Trip

After turbine trip other breakers are tripped through class B

6.

Gen Transformer Buchholtz, PRD /other mechanical Protections

A

A1

7.

Unit Transformer OTI/WTI trip

UT LV CB Trip & signal for change over of unit board.

UT LV CB Trip & signal for change over of unit board.

8.

Unit Transformer Buchholtz, PRD /other mechanical Protections

A

A1

9.

64 GT (For GT LV wdg & UT HV wdg)

A1

10.

EHV CB/GCB LBB

A

A1

11.

EHV BB PROTN

A

A1

ADDITIONAL control/protection interlocks realized through GRP

SL.NO

INITIATION

ACTION

1

GT FIRE PROTECTION

TRIP CLASS A AND DISCONNECT POWER SUPPLY TO GT MB

2

UT FIRE PROTECTION

TRIP CLASS A AND DISCONNECT POWER SUPPLY TO UT MB

3

GT TAP CHANGER OPERATED & HVCB CLOSED

TRIP CLASS A

4

HV CB /FCB CLOSED

START GT COOLER

5

GT COOLER SUPPLY TOTAL FAILURE

TRIP CLASS A AFTER TIME DELAY

6

AVR SERIOUS TROUBLE

TRIP CLASS A

Numerical integrated generator protection systemsMany functions in the same relayTakes multiple CT/VT inputs.Minimum of 2 nos to be used.All the prot functions are to be divided in to 2 groups .Built in DR(fast scan)/SOE functionsSelf supervisionCommunicableHas programmable logic gates which simplifies the auxiliary circuits.COMMON RELAYS ARE REG series OF ABB7UM SERIES OF SIEMENSMICOM SERIES OF AREVA.

GENERATOR DISTURBANCE RECORDERRecord the graphic form of instantaneous values of power system variablesFast scan (1-5 khz) and slow scan (5/10 hz) featuresSufficient analogue/digital inputs.Triggering from digital inputs and threshold/rate of change of analogue values.Adequate memoryGood frequency responseIndividual acquisition units and commom evaluation unit for a station

Thank YouFor Your Time