Elect Protection

7/31/2019 Elect Protection

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ELECTRICAL PROTECTION(PMI Workshop)

ENGINEERING

Knowledge Management System

ELECTRICAL ENGG.


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WORKSHOP ON ELECTRICAL PROTECTION

@ PMI

TODAYS TOPIC

(1) PROTECTION OF AUX POWER SUPPLY SYSTEM

(2) PROTECTION OF GENERATOR


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WORKSHOP ON ELECTRICAL PROTECTION @ PMI

AUXILIARY POWER SUPPLY SYSTEMIt is the Electric power supply systemrequired to run the auxiliary equipment ofa power plant smoothly, reliably andeconomically. In case of interruption itshould be possible to restore in fastestpossible time.

The auxiliary power system is bestdescribed by the Key SLD.

KSLD.pdf/SLD.pdf


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ELEMENTS OF Aux Power Sup System ARE Transformers

Motors

Switchgears

Connectors like Bus-duct/Cables DC System

All these above elements are to be protectedagainst faults and abnormal operating

conditions. Therefore the other element is Protection system


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TRANSFORMER PROTECTION

Transformer faults are of 5 categories

Winding & terminal faults(70% of all faults)

Core faults Tank faults

Abnormal operating conditions

Un-cleared external faults


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SEVERAL CASES OF FAULTS

Earth fault with star connected transformerhaving NGR (Neutral Grounding Trans/Resitr.)

In this case Fault Current depends on(i) NGR value

(ii) Distance of fault from the neutral

Primary side relay can not detect if the fault is at< 30% distance away from neutral.

It would be necessary to provide a 64R LV


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100%

100%Distance From Neutral

Earth

FaultCurrentPrimary Current

Secondary Fault Current

10%

30%


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Earth fault with star connected transformerhaving Neutral solidly grounded

In this case Fault Current depends on

(i) Transformer Impedence only; NGR value is zero

(ii) Distance of fault from the neutral

This is the case for LT Transformers where thesystem is solidly earthed. Heavy earth fault currentflows in the secondary and primary current is alsosubstantial.


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20

100%Distance From Neutral

PerUnit

EarthFaultCurrent Primary Current

Secondary Current

10

15

5


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Phase to Phase fault

This case shall be like the previous slide. There isno additional resistance to restrict phase to phasefault so heavy fault current flows in the secondaryand primary.


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Inter-turn fault

This is not very likely phenomenon , unless thetransformer is subjected to several Short-Circuit

stress.For transformers connected to O/H line directly,very steep-front high voltage may cause inter-turninsulation failure & inter-turn fault.

Shorted turns shall have very high current butprimary relay may not see the fault as very high turn

ratio shall be applicable for the shorted turns.


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100

25%

Percent Of Winding turns Shorted

Per Unit

FaultCurrent

Primary input Current

Fault Current in shorted turns

60

80

40

20

0 5 10 15 20

10

8

6

4

2

Per Unit

PrimaryCurrent


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Core fault

Any conducting bridge across laminated structure ordefective core insulation may allow eddy current toflow across. This will generate local heating anddegeneration of transformer oil and further gasgeneration.

Electrical relays shall not be able to detect thisother than the gas operated relays.


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Tank fault

This fault results in

loss of cooling oil

over heating of core & winding

deterioration of insulation

Electrical relays shall not be able to detect this

unless it develops into an electrical fault.


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EXTERNAL FAULTSOver load

Transformer is designed based on temperature risecriteria. Any over load is viewed in terms of theheating caused by the over load.

Over load can be allowed considering the initialthermal status of the transformer. IEC 60354 givesthe guide for transformer overloading.

The time constant of forced cooled transformers is

less as compared to that of the natural cooled ones.


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EXTERNAL FAULTSSystem Fault

Current fed by a transformer is responsible forheating of its winding. Heat generated isproportional to Sq of the current.

For a transformer having 4% impedance may feedup to 25 times its rated current. Which means 625times heat will be generated as compared to its fullload operation.

Normally transformers are specified to stand thefault for 2 secs. Over current relays are provided to

clear the fault before it reaches the limit.


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EXTERNAL FAULTSOver voltage

There could be two types of over voltage

Transient O/V

Power frequency O/VPower frequency over voltage causes stress

in insulation, increase in the value of Fluxassociated with excessive increase ofmagnetizing current.


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EXTERNAL FAULTSReduced system frequency

Like power frequency over voltage, itincrease the value of Flux associated withexcessive increase of magnetizing current.

Reduction in frequency associated withincrease in voltage would be very harmful tothe transformer.

For bigger transformer V/f protection isprovided.


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OVER HEATING OF TRANSFORMERRating of a transformer is based on a specificvalue of allowable temperature.

At low temperature over loading may beallowed as per IEC60354 guidelines.

Sustained over load ,if it takes thetemperature of the winding 10 deg C abovepermissible temperature, the life of insulationwill be eaten away by about a half of original.

WTI/OTI is used for alarm as well as tripping

of the HV side breaker. LV side is isolated byinter-tripping of LV breaker.


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SUMMARY OF FAULTS AND PROTECTION(i) Primary winding Ph-Ph 87,50

(ii) Primary winding Ph-E 87,50

(iii) Secondary winding Ph-Ph 87

(iv) Secondary winding Ph-E 87,64R(v) Inter-turn 87,63

(vi) Tank Fault 87,63

(vii) Core Fault 87,63


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SUMMARY OF FAULTS AND PROTECTION

(i) Primary winding Ph-Ph 87,50(ii) Primary winding Ph-E 87,50

(iii)Seccondary winding Ph-Ph 87

(iv) Seccondary winding Ph-E 87,64R

(v) Inter-turn 87,63

(vi) Tank Fault 87,63(vii) Core Fault 87,63

(viii) Over Fluxing 99

(ix) Over Loading/ Design margin,WTI(Alarm/Trip)Heating OTI (Alarm), Cooler Alarm

(x) Gas generation 63(xi) Fire Protection Trip and Alarm


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PROTECTIONS FOR Stn TRANSFORMER

(i) 87 Internal Ph-Ph / Ph-Earth(ii) 51 Ph-Ph / Ph-Earth (Covers up to 11kv

System also. So def time for co-ordination)

(iii) 64R HV HV Winding Ph-Earth

(iv) 64R LV LV Winding Ph-Earth

(v) 51N LV Winding Ph-Earth (upto 1kV)

(vi) 63

(vii) WTI Alarm/Trip

(viii) OTI Alarm/Trip

(ix) Cooler/OLTC Alarm(x) Fire Protection Alarm/Trip


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PROTECTIONS FOR UNIT TRANSFORMER

(i) 87 Internal Ph-Ph / Ph-Earth(ii) 51 Ph-Ph / Ph-Earth (Covers up to 11kv

System also. So def time for co-ordination)

(iii) 64G1/64G2 HV Winding Ph-Earth

(iv) 64R LV LV Winding Ph-Earth

(v) 51N LV Winding Ph-Earth (upto 1kV)

(vi) 63

(vii) WTI Alarm/Trip

(viii) OTI Alarm/Trip

(ix) Cooler/OLTC Alarm(x) Fire Protection Alarm/Trip


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PROTECTIONS FOR LT TRANSFORMER

(i) 87 (>5MVA) Internal Ph-Ph / Ph-Earth--OR--

50 (


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MOTORS PROTECTIONS

Causes for Motor failure

External

Unbalanced Supply

Under Voltage

Single Phasing

Reverse Phase Sequence


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MOTORS PROTECTIONS

Causes for Motor failure

Internal

Bearing Failure

Winding Fault

Over Load


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POSSIBLE FAULTS & ITS PROTECTION

Thermal (Over Load) protection

Majority of winding failure are caused byover loading on either unbalanced supplyvoltage or by single phasing.

Excessive heating leads to windinginsulation failure and further electricalFaults.

Sustained over load of a few percentagemay result in premature aging of the motor

insulation.


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Start/Stall Protection

Motor draws around 6 times current duringstarting. But it is not a constant value. Thequantum of current reduces as it reaches itsrated speed.

So to distinguish starting from stalling, it is

required to measure the time also alongwith the inrush of current.

Some times it is found that starting time ismore than safe stalling time. Then itbecomes difficult to provide the stalling

protection.


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Stalling Protection when tSTARTING < tSTALLINGIt is very simple to have this protection byfollowing current and time settings:

tSTARTING < tSETTING < tSTALLING

and

IFULL-LOAD< ISETTING < I STARTING


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Stalling Protection when tSTARTING > tSTALLINGOver and above time & current sensing a speedswitch signal is also required to sense that themotor has taken-off. Here as motor speeds up,current reduces and cooling increases and motor

remains safe even after tSTARTING exceeds tSTALLINGThe following current and time settings:

tSTARTING< tSETTING< tSTALLING and IFULL-LOAD< ISETTING < I STARTINGwhen ANDED with the speed switch contactgives desired stalling protection. The speed switchsetting is normally kept at 20%.


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SHOULD A MOTOR STALL WHILE RUNNING

The motor current will increase from rated valueto its starting current value. Presently thissituation is protected by 51I,alarmed by 50Aand RTD tripping through control system.

However with numerical relays it is possible tocapture the conditions of Motor has startedand current is nominal which signifies motorrunning. Now if current again reaches itsstarting time value , stalling situation can be

detected.


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However, this protection shall operate in theevent of restarting of motors after a voltage dip( EHV Fault nearby or Auto C/O).

The difference could be only the duration ofhigh current flow which will be less ascompared to the situation at the time of starting.


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Short circuit Protection

Motor short circuit protection is provided formotor terminal flash-over or major windingfaults.

Motor instantaneous over current protection(50) is provided for this. However 50 settingis kept at 800% of rated current to take careof motor starting.


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Earth Fault Protection

Any fault due to any cause, in statorwinding will result into an earth fault.

For LT motors, where system is solidly

earthed, earth fault relay in the residualcircuit with 20% setting serves the purpose.

HT motors where system is earthed throughNGR, sensitive earth fault relay with CBCTis provided.


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Under Voltage Protection

Motor may stall under severe under voltagecausing serious damage to it.

Time delayed under voltage trip is providedhaving system voltage falling below 80% fora duration of 1000 mS.


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RTD Temperature Detection

This gives actual direct measurement ofmotor temperature. Severe over load/development of hot spot is detected byRTD.

RTDs are provided in all HT motors whichare embedded during manufacturing.

RTDs are fed into the control system whichfurther sends trip signal/ generates alarm.


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Protection against excessive no. of Starts

Starting should be blocked if the permittednumber of starts are exceeded.

It is possible to build a thermal replica of the

motor in a numerical relay which will allownumber of starts considering actual thermalstatus of the motor.


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Summary Of Motor Protection

HT Motors87 For >2000MW Ph-Ph & Ph-Erth(Intnl)

50x3 For tSTALLINGU/V For under voltage protection


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LT Motors (Breaker Controlled)50x3 For Ph-Ph & Ph-Erth51x2 For Over Load (IDMT)

50Ax1 For Alarm

50N1 For Earth FaultU/V For under voltage protection


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LT Motors (Contactor Controlled)

Fuse For Ph-Ph & Ph-Erth

49 For Over Load (IDMT)U/V Contactors drop @ V


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OUR NEXT TOPIC

GENERATOR PROTECTION


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GEN. PROTN. PHILOSOPHY

The numerical relays constituting the protectionfor each Main Generator Circuit, is configuredinto two independent Numerical ProtectionSystemseach fed from a separate DC supply,

At least one numerical protection system shall bealways available to detect and operate for anytype of fault in the Generator Circuit, undercondition of failure of the other numericalprotection system AND/OR on failure of theassociated DC supply systems of other numericalprotection system.


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AC/DC SEGREGATION


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CLASSIFICATION OF GENERATORPROTECTIONS

INTERNAL FAULTi) PHASE TO PHASE FAULTS PROTECTION

ALL DIFFERENTIAL PROTECTIONS

II) PHASE TO EARTH PROTECTION FOR STATOR WINDING64G1, 64G2, 64R, 64IT

EXTERNAL FAULT

21G, 51NGT, 46G,

ABNORMAL OPERATING CONDITIONS

98, 50GDM, LFPR/RP, 59, 99


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PROTECTION FUNCTION CLASSOF TRIP

Remark on grouping ofprotection

Generator Differential Protection, 3 pole (87 G) havingoperating time of 25 milli sec. or lower at five timesthe current rating.

A

Overall Differential Protection (87GT). A

87 G and 87 GT shall be on twodifferent channels of protection.

Generator Transformer Differential protection(87 T)

A 87 T shall be in a differentchannel than 87 GT

Over hang differential protection(87 HV) A 87 HV shall be in a differe ntchannel than 87T

Stator Earth Fault Protection covering 100% of winding(64G1), operating on low frequency signal injectionprinciple suitable for continuous monitoring of statorinsulation even during machine shut down.

A

Stator Standby Earth Fault Protection covering 95% ofwinding (trip) (64 G2) with adjustable time delay.

A

64 G1 and 64 G2 shall be ontwo different channels ofprotection.

Inter-turn Fault Protection (95G1), throughcomparison of zero sequence voltage on generatorphase and neutral side.

A

Duplicated Loss of field protection (40G1/2 ). A 40G1 and 40G2 shall be on twodifferent channels of protection.

Back up Impedance Protection, 3 pole (21G) alongwith suitable timer for Co -ordination with lineprotections

A

Backup Earth Fault Protection on GeneratorTransformer HV neutral (51NGT)

A

21 G and 51 NGT be indifferent channels

Negative Sequence Current Protection, alarm and I 22t

element for trip (46G) matching with the machinecharacteristics.

A

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

a) short time delayed interlocked with turbinetrip

b) long time delayed independent ofturbine trip.

B

A

37/32 G1 and 37/32 G2 shall bein two different channels ofprotection

Two Stage Rotor Earth Fault Protection (alarm & trip)

operating on principle of continuously monitoring rotorinsulation value even during machine shut downperiod (64F).

A

CHANNEL WISE GROUPING AND TRIPPING LOGIC


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Definite Time Delayed Over -Voltage Protection(59G) for alarm and trip.

A

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

C

Over Fluxing Protection (99) for Generator /Generator Transformer having inverse time

characteristics suitable for Generator /GeneratorTransformer over fluxing capability.

A

Over Flux function (99) shallbe in a different channel thanO/V and U/F functions

Accidental Back Energisation protection foraccidental closu re/flashover of EHV breaker orEHV disconnecting switch (50GDM)

1. Based on Under voltage and Over currentMeasurement

2. Over current relay enabled/disabled byHV CB aux contact and drop out/pick uptimers.

A

A

50 GDM based on th e twoprinciple shall be on twodifferent channels.

Instantaneous and time delayed Over Currentprotection to be used on HV side of excitationtransformer.

A

Generator Pole slipping protection A

Generator under voltage relay for interlocks

Unit Transformer Differential Protection, 3 pole(87UT)

A

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

A

Unit Transformer LV REF (64 UT LV) A

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

A

87 UT & 51 NUT can be in onechannel and 64 UT LV & 51UTshall be in another channel.

Gen Transformer OTI/WTI trip Turbine Trip After turbine trip through classB other breakers are tripped.GT WTI/OTI IN DIFFCHANNELS

Gen Transformer Buchholtz, PRD /othermechanical Protections

A GT BUCH & PRD IN DIFFCHANNELS

Unit Transformer OTI/WTI trip UT LV CB Trip& signal forchange over ofunit board.

UT WTI & OTI IN DIFFCHANNELS

Unit Transformer Buchholtz, PRD /othermechanical Protections

A UT BUCH & PRD IN DIFFCHANNELS

CLASS A : TO TRIP HVCB,TURBINE,FIELD, UT LV CBCLASS B : TO TRIP HVCB,FIELD,UT LV CB : CLASS C : TO TRIP HV CB

CHANNEL WISE GROUPING AND TRIP LOGIC


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TypicGeneprotesche


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GRP NETWORKING The numerical relays constituting the protectionsystem for all the Generator Circuits shall be wired

to a central operator work station, located in CentralControl Room .

From the operators work station, it shall be possible to extract all the alarms,

events and fault data captured by numerical relays change / upload relay Settings for various numerical relays.

Sharing of the data with switchyard SAS throughSubstation Level LAN shall also be possible.

The communication protocol IEC 61850 is preferred

for Generator Circuit LAN.


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THANKS

FORPATIENT HEARING

!


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Elect Protection

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