transformer proection

TATA CONSULTING ENGINEERS SECTION: TITLE

TCE. M6-EL-PI-G-TF-6508 PROTECTION OF TRANSFORMERS SHEET ( ) OF (iii)

REV.NO RO R1 ISSUE

INITIALS SIGN INITIALS SIGN INITIALS SIGN INITIALS SIGN

PPD.BY PVD Sd/- DB Sd/-

CKD.BY CPS Sd/- SJH Sd/- R1

APP.BY DKB Sd/- UAK Sd/-

DATE 1982-10-30 1999-01-22

i

FORM NO. 020R2

PROTECTION OF TRANSFORMERS

TATA CONSULTING ENGINEERS73/1, ST. MARK’S ROAD

BANGALORE 560 001

FLOPPY NO : TCE.M6-EL-FP-DOC-005FILE NAME : M6-6508.DOC

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TATA CONSULTING ENGINEERSSECTION:CONTENTS

TCE.M6-EL-PI-G-TF-6508 PROTECTION OF TRANSFORMERS SHEET ( ) OF (iii)ii

FORM NO. 120 R1

CONTENTS

CLAUSE NO. DESCRIPTION SHEET NO.

1.0 SCOPE 1

2.0 RECOMMENDED PRACTICE 1

3.0 DISCUSSIONS 18

TABLES

TABLE-1 PROTECTION OF TWO WINDINGTRANSFORMERS OF 500 TO 5000 KVARATING

2

TABLE-2 PROTECTION OF TWO WINDINGTRANSFORMERS OF 5000 TO 10,000 KVARATING 6

TABLE-3 PROTECTION OF TWO WINDINGTRANSFORMERS OF MORE THAN 10,000 KVARATING 10

TABLE-4 PROTECTION OF THREE WINDINGTRANSFORMERS 14

TABLE-5 PROTECTION OF AUTO TRANSFORMERS 16

TABLE-6 PROTECTION OF EARTHING TRANSFORMERS 18

APPENDIX

APPENDIX-1 TRANSFORMERS OF 500 KVA AND BELOWPROVIDED WITH FUSES ON H.V. SIDE

24

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TATA CONSULTING ENGINEERSSECTION:REV. STAT.

TCE.M6-EL-PI-G-TF-6508 PROTECTION OF TRANSFORMERS SHEET ( ) OF (iii)iii

FORM NO. 120 R1

REVISION STATUS SHEET

REV NO. DATE DESCRIPTION

R1 1998-12-15 DOC. GENERALLY REVISED

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TATA CONSULTING ENGINEERS SECTION:WRITE -UP.

TCE.M6-EL-PI-G-TF-6508 PROTECTION OF TRANSFORMERS SHEET OF 241

FORM NO. 120 R1

1.0 SCOPE

This guide gives recommended protections for two winding or threewinding transformers, auto transformers and earthing transformerswhich are controlled by circuit breakers and protected by relays.Protection of transformers provided with fuses on HV side iscovered separately in Appendix-1.

2.0 RECOMMENDED PRACTICE

Transformers are classified into the following groups for thepurpose of defining their protection requirements (which are givenin the tables to follow)

2.1 Two winding transformers

(a) Transformers of rating 500 to 5000 kVA – Table-1

(b) Transformers of rating 5000 to 10000 kVA – Table-2

(c) Transformers of rating 10,000 kVA and above – Table-3

2.2 Three winding transformers of all ratings – Table-4.Generally such transformers do not have ratings below 10,000 kVAin practical applications.

2.3 Auto-transformers of all ratings – Table-5.These are generally used as interconnecting transformers betweentwo system voltages such as 220 kV to 110 kV.

2.4 Earthing transformers of all ratings – Table-6Ratings of earthing transformers are decided by desired groundfault level of the system to which they are connected.

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FORM NO. 120 R1

TABLE – 1

PROTECTION OF TWO WINDING TRANSFORMERS OF500 TO 5000 kVA RATING

SR.NO.

DESCRIPTION DEVICENO.

DETAILS SETTINGRANGE

REMARKS

1.1 Instantaneoushigh set overcurrentprotection forphase faults onHV side (sourceside)

‘50’ Instantaneousover currentrelay shouldbe provided onall threephases, o/crelay shouldhave lowtransient over-reach of lessthan 5% andhigh drop offto pick up ratio

Range 500to 2000%

In case of low ratio CTs –say 200/1A & less in a highfault level system of say 40kA, (i.e. CT sec. side faultcurrent ≥200 XIs) a separateprotection CT core withhigher ratio to be used for‘50’ to avoid CT saturationAlternatively inst. overcurrent relays may beconnected directly to themain CT whose ratio isselected with due regard tosensitivity to terminal faults.Time delayed over currentrelays may be connected tosec. side of an aux. CT ofsuitable ratio & knee point.The pry of aux. CT will be inservices with theinstantaneous overcurrentrelays

1.2 Backupovercurrentprotection forphase faults onHV side

‘51’ Relay shouldbe provided onall threephases. Relayshould havevery inversenormal timecharacteristics

Range 50to 200%

Though very inverse timecharacteristic is preferred,for proper coordination,normal inverse curve mayhave to be used if the samehas been used on the LVside incomer.In genral the characteristicof the relays should beselected to give fast &selective tripp-ing throughproper gradation. As far aspossible use relays withsimilar characteristic on bothsides of the transformers

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FORM NO. 120 R1

TABLE – 1 (cont’d)

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REMARKS

1.3 Ground faultprotection onHV side

‘50N’ Applicable only fordelta connectedwinding on HVsideInstantaneousovercurrent relayconnected inresidual modeshould haveseries stabilisingresistor to ensurestability formagnetisinginrush currentsand LV throughfault currents. Forhigh impedanceearthed systemthe relay shall bewith adjustabletime delay of 01. –1 sec. and shallbe connected tocore balance C.T.

Range 20-80% foreffectivelyearthedsystem.10-40% forsystem withground faultcurrentlimited toratedcurrent /impedanceearthedsystem

Consider 3 phase faulton secondary terminalsof transformer forselection of stabilisingresistor

1.4 Backup groundfault protectionon HV side

‘51N’ Applicable for starconnected HVwinding. Normalinverse timecharacteristic relayconnected inresidual mode isrecommended

Range 20-80% foreffectivelyearthedsystem and10-40% forsystem withground faultcurrentlimited toratedcurrent

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REMARKS

1.5 Backup overcurrentprotection onLV side forphase faults

‘51’ Protection shouldbe provided on allthree phases. Veryinverse timecharacteristicsrecommended forco-ordination withrelay protected out-going feeders andextremely inversetime characteristicsto be used whenco-ordination withfuse protectedfeeders is required

50-200% See discussions foruse of breaker seriestrips in place of theserelays in case of 415Vsystems

1.6 Backup groundfault protectionfor LV systemswith neutraldirectly earthed

‘51N’ Protection to beconnected to CT inthe transformerneutral connection.Normal inverse timecharacteristic relayis recommended.This protection isrequired evenwhere all theoutgoing feedersare fuse protected.

20-80%Neutral CTratio shallbe same asthat ofphase CTs

See discussion for415V systems whereoutgoing feeders areprovided with fusesand LV circuit breakeris provided with seriestrip device

1.7 Standby groundfault protectionfor LV systemwith neutralearthed throughlow resistances

‘515N’ Relay to beconnected toneutral CT. Longinverse timecharacteristic relayis recommended.(Relay 51N abovemay either bedeleted orconnected toresidual connectionof phase CTs if Ig <IR

10-40%CT ratiosame asphase CTsif Ig ≥ IratedIn othercasesselect CTratio basedon groundfault currentvalueRange 10to 40%

Relay time settingshould be co-ordinatedwith short time rating ofgrounding resistance

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REMARKS

1.8 Gas protection ‘63G’ Two stages buchholzrelay with one alarmstage and onetripping stage. This issupplied along withthe transformer.Each stage willactuate an auxiliaryrelay with four pairsof self reset contacts(Alarm) or Handreset contacts (Trip)

For transformersprotected by fuse, tripstage without anyswitching devicecannot be used astripping device is notavailable. Only alarmstage may be used.

1.9 Transformer oiltemperature trip

‘OTI’ Oil temperatureindicators are usuallyprovided with twocontacts, one foralarm on “Oiltemperature high”and other for trip on“Oil temperature veryhigh”

These are optionalitems depending ontransfomer rating andapplications

1.10 Transformerwindingtemperature trip

WTI Winding temp.indicators areprovided with twostages, one for alarmon ‘winding temp.high’ and the otherfor trip on ‘windingtemperature veryhigh’

- DO -

1.11 Oil pressure trip PRD Pressure reliefdevice supplied withthe transformer shallbe provided withauxiliary relay withfour pairs of handreset contacts fortripping

This will be providedfor transformers above2500 kVA.

NOTE: 1. Items 1.8, 1.9 & 1.11 are not applicable for dry type transformers.

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FORM NO. 120 R1

TABLE – 2

PROTECTION OF TWO WINDING TRANSFORMERS OF5000 TO 10,000 kVA RATING

SR.NO.



REMARKS

2.1 Differentialprotection

‘87T’ Relay should beconnected on all threephases. Relay shouldincorporate

(a) Adjustablepercentage biasingfor through faultstability and tapchangingconditions.

(b) Harmonic restraint /

zero current sensingto provide stabilityagainst magnetisinginrush currents.

(c) Auxiliary CTs on

both sides for ratiomatching andcompensation ofmismatch of lineCTs

Referdiscussionsregardingharmonicrestraints formagnetic inrushcurrents.

2.2 Instantaneousshort circuitprotection forphase faults onHV side

‘50’ Same as item 1.1 Range 500-2000%

Same as in 1.1

2.3 Backup overcurrentprotection forphase faults onHV side

51 Same as item 1.2 Range 50-200%

Same as in 1.1

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REMARKS

2.4 Earth faultprotection onHV side

50N Applicable only for deltaconnected HV.Protection same as item1.3

20-80% foreffectivelyearthedsystems and10-40% forlow resistanceearthedsystem

2.5 Backup earthfault protectionon HV side

51N Applicable only for starconnected HV side.Same as item 1.4

20-80% foreffectivelyearthedsystem and10-40% forsystem withlow resistanceearthing

2.6 Backupovercurrentprotection onLV side forphase faults

51 Same as item 1.5 50-200%

2.7 Backup groundfault protectionfor LV side

51N Relay to be connectedto neutral CT. CT ratiosame as phase CTswhen Ig ≥IR

20-80% foreffectivelyearthedsystem and10-40% forsystem withlow resistanceearthing

2.8 Restricted earthfault protectionof LV sidewhen LVneutral isgroundedthrough lowresistance

‘64’ CT ratio for phase andneutral CTs should besame

10-40%range, Stand.values ofstabilisingresistor are50 ohm for 5A CT and 200ohm for 1 ACT

Designstabilisingresistance valueon the basis of 3phase throughfault. RCTshould be keptlow to limit valueof Vk &stabilisingresistor.Recommendedsetting is 10%.

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FORM NO. 120 R1


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REMARKS

2.9 Standby earthfault protectionfor LV sidewhen neutral isgroundedthrough lowresistance

‘515N’ Relay connected toneutral CT. Longinverse timecharacteristic relay isrecommended. (Relay‘51N’ in 2.8 aboveshould be connected toresidual connection ofphase CTs in this case)

CT ratio sameas phase CTsif Ig ≥ IR. Inother casesselect CTratio basedon designedground faultcurrent value.Range 10-40%

Relay timesetting shouldbe co-ordinatedwith short timerating ofgroundingresistance

2.10 Gas protection 63G Same as 1.82.11 Winding

temperature trip WTI Same as item 1.102.12 Transformer oil

temperaturehigh

DTI Same as item 1.9

2.13 Oil pressure trip PRD Same as item 1.11

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FORM NO. 120 R1

TABLE – 3

PROTECTION OF TWO WINDING TRANSFORMERS OF RATINGMORE THAN 10,000 KVA

SR.NO.



REMARKS


‘87T’ Relay should beconnected on all threephases. Relay shallbe high speed typeincorporating

(a) 2nd & 5th harmonicrestraint / zerocurrent sensingfor magnetisinginrush currents

(b) Adjustable

percentage biasfor through faultstability

(c) Auxiliary CT on

both sides forratio matchingandcompensation forline CT mismatch

(d) High set

instantaneousunit in differentialcircuit

3.2 Instantaneousshort circuitprotection forphase faults onHV side

‘50’ Same as item 1.1 Range500-2000%

(i) Thisprotection shall notprovided forgenerator transfor-mers

(ii) Same as in 1.1

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FORM NO. 120 R1


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REMARKS



3.4 Instantaneousground faultprotection on HVside

‘50N’ Applicable only whenHV is delta connectedSame as item 1.3

Range 20-80% foreffectivelyearthedsystemand 10-40% forsystemwithgroundfaultcurrentslimited toratedcurrent/impedanceearthedsystems


51N Applicable for starconnected HVwindingSame as item 1.4

Range 20-80% foreffectivelyearthedsystemand 10-40% forsystemswith limitedgroundfaultcurrent

3.6 Backup phaseovercurrentprotection forphase faults onLV side


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FORM NO. 120 R1


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REMARKS

3.7 Backupovercurrentprotection forground fault onLV side

‘51N’ Same as item 2.7 Range 20-80% foreffectivelyearthedsystemand 10-40% forsystemwith lowresistanceearthing

3.8 Restricted earthfault protectionof LV side whenLV neutral isgroundedthrough lowresistance

‘64’ Same as item 2.8 10-40%range

3.9 Standby earthfault protectionof LV side whenLV neutral isgroundedthrough lowresistance

‘51SN’ Same as item 2.9 Relay time setting tobe co-ordinated withshort time rating ofresistor

3.10 Gas protection 63G Same as 1.83.11 Winding

temperature tripWTI Same as item 1.10

3.12 Transformer oiltemperature trip

DTI Same as item 1.9


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FORM NO. 120 R1

TABLE – 4

PROTECTION OF THREE WINDING TRANSFORMERS

SR.NO.



REMARKS


87T Relay should beconnected to allthree phases.Relay shall havebias from all thethree windings. Iftertiary is notloaded and broughtout, then the biasshall be taken fromHV & LV sidesonly. Otherrequirements sameas 3.1

4.2 Short circuitprotection withinstantaneouso/c relays on HVside (sourceside)




4.4 Instantaneousground faultprotection on HVside

‘50N’ Applicable only fordelta connectedHV windingSame as item 1.3

Range 20-80% foreffectivelyearthedsystem and10-40% forsystems withrestrictedground faultcurrent

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FORM NO. 120 R1


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REMARKS


51N Applicable for starconnected HVwindingSame as item 1.4

Range 20-80%for effectivelyearthed systemand 10-40% forsystems withlimited groundfault current

4.6 Backup o/cprotection on LVside, for phasefaults

‘51’ Same as item 1.5 Range 50-200% Both LV windingsshall haveseparateprotections

4.7 Backup o/cprotections forground fault onLV side

‘51N’ Same as item 2.7 Range 20-80% or10-40%depending onavailable faultcurrent

- do -

4.8 Restricted E/Fprotection of LVside when LV isgroundedthrough aresistance

‘64’Same as item 2.8 10-40% range

- do -

4.9 Standby E/Fprotection whenLV is groundedthrough aresistance

‘51SN’ Same as item 2.9 - do -

4.10 Gas protections 63G Two stageBuchholzprotection is part oftransformerSame as 1.8

4.11 Windingtemperature trip WTI Same as item 1.10

4.12 Oil temperaturetrip OTI Same as item 1.9


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FORM NO. 120 R1

TABLE – 5

PROTECTION OF AUTO-TRANSFORMERS

SR.NO.



REMARKS


‘87T’ Relay to be connectedon all three phases.High speed, harmonicrestraint, percentagebias differential relaysare to be used.

If tertiary winding isbrought out forloading, additionalbias from this sidealso to be provided

5.2 Restricted E/Fprotection ‘64’

Alt:1 7CT method –See fig.1All CT ratios shouldbe identical

Alt-2 9CT method –See fig.2All CT ratios shouldbe equal. One CT isrequired on eachphase at neutral endof common winding.This alternative alsocovers phase faultsinside transformers

Singlephaseinstantaneous currentoperatedrelay.Range 20-80%

Three phaseinstantaneous currentoperatedrelayRange 20-80%

Specify valueof stabilisingresistor takingRCT intoaccount

5.3 Backup overcurrentprotection for phasefaults (Directional)on HV side

67/51-50

Protection shall beprovided on allphases.Directionalisation isrequired as powerflow can be bothways. Additional highset instantaneous unitcan be provided toclear internal faults

3 relays –one perphase.Range 50-200% forinversecurrentelement and500-2000%for high setunit

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REMARKS

5.4 Backup o/cprotection forground faults(Directional) on HVside

67/51N Directionalisation isrequired as currentflow can be in bothdirections.Adopt potentialpolarisation

Directionalo/c relayRange 20-80%

5.5 Backup o/cprotection for phasefaults on LV side(Directional)

67/51-50

Similar to 5.3 aboveDirectionalo/c relay - 3relaysRange 50-200% forinverse timeo/c & 500-2000% forhigh set unit

5.6 Backup o/cprotection forground faults(Directional)

67/51N Same as item 5.4Range 20-80%

5.7 Overload relay (foralarm) for HTwinding

50/OLTo be connected toHT line CTs tomonitor HT windingcurrent

Definitetime o/crelay.Range 50-200%Time 1-10sec.High DO/Puratio

5.8 Overload relay (foralarm) for commonwinding

50/OLTo be connected toCTs at the neutralend of phase winding

- do -

5.9 Gas protection63G

Two stage Buchholzprotection is part oftransformerSame as 1.8

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REMARKS

5.10 Windingtemperature trip WTI Same as item 1.10

5.11 Oil temperature trip OTI Same as item 1.95.12 Oil pressure trip PRD Same as item 1.115.13 For very large

transformer (above100 MVA) compre-hensive numericalprotection in onegroup &conventionalprotection in anothergroup shall beprovided

5.14 Over fluxingprotection (for gridtransformers)

5.15 In cases wheretrans-formers arecontinuouslyoperated at full load,and in addition, arelikely to besubjected to shorttime overloads,(which may raise thewinding temp. tomore than 100°Ctemp. timeintegrators shouldbe provided toenable estimation ofdegree of aging.

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FORM NO. 120 R1

TABLE – 6

PROTECTION OF EARTHING TRANSFORMERS

SR.NO.



REMARKS

6.1 Instantaneouso/c protection

‘50’ Relay should beprovided on all threephases of theearthing transformer

Instantaneouso/c relayRange 20-80%

The CTs shouldbe connected inDelta

6.2 Gas protections ‘63G’ Two stage gasprotection is a part oftransformer.Same as item 1.8

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FORM NO. 120 R1

3.0 DISCUSSIONS

3.1 Transformers below 500 kVA are generally used for distributionsystems and are usually protected by fuses. Hence this range ofpower rating is not included in the recommendations. Appendix-1gives such a typical system for general guidance. For particularapplications where H.V. circuit breakers are provided table I maybe followed. LV side of the transformers may have circuit breakersor fuses. In case of fuses, LV backup overcurrent and ground faultprotections are not applicable.

3.2 Ground fault protections to be provided for the transformers dependon the type of earthing of the systems to which the transformer isconnected and also the way in which transformer neutrals aregrounded. Following types of system earthing are possible.

(a) Effective earthing

(b) System neutral earthed through a resistance to limit the faultcurrent to a value equal to the rating of the transformer. Insome cases this limit may be further reduced to provide onlyadequate current for ground fault relaying.

(c) System neutral earthed through resistance loaded distributiontransformer to limit ground fault current to very small values.

Earthing discussed in (c) above is usually adopted in powerstation for unit and station transformer earthing. In this casetwo types of relaying are possible.

(i) To provide ground fault alarm by a voltage relayconnected across the resistor of grounding transformersecondary. No further discriminative ground fault relayingis done on outgoing feeders.

(ii) To provide sensitive core balance CTs on the outgoingfeeders in addition to the voltage relay in (i) above andensure time discrimination on the voltage relay. Thisscheme is adopted in TCE power station practice. Theground fault current available depends on the totalcapacitance in the connected system. Core balance CTsare specified to give a primary sensitivity of 5 to 10 A. Insuch systems the outgoing transformer feeders should be

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FORM NO. 120 R1

provided with core balance CTs and instantaneous overcurrent relays for ground fault protection in place of‘50N’ and ‘51N’ relays. A small time delay of 50 – 100min is desirable.

3.3 Most of the indigenous make differential relays do not incorporateratio matching taps in the relay. Hence separate auxiliary CTsshould be provided wherever adopted relays do not have this built-in facility. Even with built-in ratio matching feature in therelayauxiliar CTs are required for vector correction.

3.4 Protections recommended in tables 1 to 4 are on the basis that thesource is connected to HV side of transformer only. Most of theapplications fall under this category and hence the generalisation.If particular applications have additional source on the LV side alsothen backup over current protection for phase faults on the LV sideshould be made directional over current protection. Additionaldirectional ground fault protection should be added and connectedto residual connections of the phase CTs.

3.5 When two transformers are operated in parallel, and they are notprovided with differential protection, directional over current phaseand ground fault relaying mentioned in 3.4 should be provided forselective isolation of faulty transformer (internal fault).

3.6 Transformers of smaller rating usually have percentage impedancevalue of about 5%. The current for secondary three phase fault inthis case will be 20 times rated current. The instantaneous phaseover current relay on HV side should be set atleast 1.5 times thisvalue. Since the maximum setting is 2000%, CT ratio will have tobe increased to reach this setting. Phase over current elementshave 50-200% range thus permitting the raised CT ratio.

3.7 A common high speed, hand reset trip relay should be used forsimultaneous tripping of both HV and LV breakers whenever theprotections operate.

3.8 Buchholz trip should be routed through a shunt connected auxiliaryrelay with hand reset contacts. The auxiliary relays shall providenecessary flag indication.

3.9 Winding temperature, alarm and trip, oil temperature alarm & tripnormally form part of transformer and are covered here for the sake

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FORM NO. 120 R1

of completeness. These trips also should be routed throughauxiliary relays to obtain flag indication / remote indication.

3.10 In case of transformers feeding to 415 V auxiliary supply system,the LV side may be provided with time delayed electromagneticreleases for short circuit protection and thermal overload relays inplace of CT connected time over current protections. In suchsystems where secondary neutral is directly grounded, no separateground fault relaying is envisaged. Under the presumption that thephase connected releases will operate for ground faults also, sincefault current is not limited by neutral resistances. Even with all fuseprotected O/G feeders it is desirable to have E/F relay 51N.

(a) For transformers with solidly earthed neutral residuallyconnected earth fault relay shall be provided.

(b) For transformers with resistance earthed neutral to restrictearth fault current to about rated current of the transformer,residually connected earth fault relay or earth fault relayconnected to transformer neutral CT or core balance CT shallbe provided.

(c) For transformers with resistance earthed neutral to restrictearth fault current to a very low value (5 A or 10 A).

Earth fault relay connected directly across the NGR or acrossthe loading resistor on the secondary of the earthingtransformer shall be providwed . Relay may be wired only foralarm, but in cases where transformer feeder extensivecabling, each outgoing feeder should be protected additionallyby an earth fault relay connected to a core balance CT. In thiscase the earth fault relay of each feeder is wired to trip thefeeder.

All the earth fault relays should be either inverse time ordefinite time relays and where connected to trip are to begraded with earth fault relays or fuses provided for theoutgoing feeders.

(d) For Delta connected transformers either an instantaneousresidually connected earth fault relay with a suitablestabilising resistor or a time delayed earth fault relay shall beprovided. This relay need not be graded with earth fault relaysor fuses on the outgoing feeders but should have adequate

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FORM NO. 120 R1

time delay to prevent its operation during magnetising inrushcurrent.

3.7 (a) Differential & restricted earth fault protection for star connectedwindings can be combined for transformer upto 63 MVA or 132kV. If earthing transformers are provided these can be includedin the REF scheme. Earthing transformer can also be coveredby the differential protection.

(b) Differential & restricted earth fault protection should beseparately provided for transformer of rating greater than 63MVA or for all 220 kV / 400 kV transformers. REF protectionuses bushing CTs while differential protection uses transformeryard CTs. Double wound transformer should have REFprotection for both the windings.

(c) Large auto transformer should be provided with REF protectionusing nine (9) CTs.

transformer proection

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