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A GO3-5005 EMarch 1988

Power Transformer ProtectionApplication Guide

AuthorR Nylen

Senior application engineer

)

ABB Relays Power transformer protection AGO3-5005 E

List of contents

)

1. INTRODUCTION

2. CONDITlONS LEADING TO FAUL TS2.1 Insulation breakdown

2.2 Aging of insulation

2.3 Overheating due to overexcitation

2.4 Oil contamination and leakage

2.5 Reduced cooling

3. FAULT CURRENT3.1 Ground faults in a solidly grounded

star-connected secondary winding

3.2 Ground faults in a high impedancegrounded star-connectedsecondary winding

3.3 Ground faults in a delta-connected

winding3.4 Turn-to-turn faults

3.5 Phase-to-phase faults

4. PROTECTIVE RELA VS4.1 General

4.2 Differential relays

4.2.1 General

4.2.2 Differential relays for fully insulatedtransformers

4.2.3 Differential relays for auto-transformers

4.3 Overcurrent protection and

impedance relays4.3.1 Time-overcurrent relays

4.3.2 Under-impedance relays

4.3.3 Distance relays

4.4 Grou~d fault protection

4.4.1 General

4.4.2 Low irrnpedance residualovercurrent relay

4.4.3 Harmonic restraint relay

4.4.4 High impedance restricted relay

4.4.5 Low impedance restricted relay

4.4.6 Tank protection

4.4.7 Residual voltage relay

4.5 Overexcitation protection

4.6 Flashover and ground faultprotections for low voltage systems

4.6.1 Systems without rectifiers orfrequency converters

4.6.2 Systems with rectifiers andfrequency converters withoutpulse'width-modulation

4.6.3 Systems with rectifiers and pulse-width-modulated frequencyconverters

5. MONITORS5.1 Gener,al

5.2 Gas d~tector relay

5.3 Temperature monitoring

5.4 PresslUre relay for on-load tap-

changsrs5.5 PresslUre relief valve

5.6 Oil level monitor

5.7 Silica gel dehydrating breather

6. SUMMARY

7. REFERENCES

List of illustrations Fig. 1 Fig.11

Fig.2 Fig.12

Fig. 3 Fig.13

Fig.4

Fig. 5

Fig.14

Fig.15Fig.6

Fig.7

Fig.8 Fig.16

Fig.17

Fig.18Fig.9

Distanlce relay used as atransfQrmer protection.

Connection of ground faultoverclilrrent relays.

Conn~ction of arestricted groundfault relay for a Y-connected

winding.

Connection of arestricted groundfault relay for a D-connectedwinding and agroundingtransf<i>rmer.

Transformer differential andrestricted ground fault relays on thesame CT cores.

Flash0ver relay.

Grou1d fault relay.

Grou~d fault and flashover relay.

Fig.10

Permissible short timeoverexitation.

Ground fault current in a solidlygrounded Y-connected winding.

Ground fault current in a highimpedance grounded Y-connected

winding.

Theoretical inrush currentlm.

Recorded inrush current for a 60MV A transformer 140/40/6.6 kV.connected YNyd.

Operating time.

Through faun restraint.

Magnetizing current atoverexitation.

Transformer differential relay forautotransformer.

Differential relay RADHA or RADSGfor autotransformer.

ASS Relays Power transformer protection AGO3-5005 E

1.INTRODUCTION To prevent faults and to minimize the damage

in case ot a fault, transformers are equippedwith both protective relays and monitors. Thechoice af protective equipment variesdepending, on transformer size, voltage level,etc.

ASEA RELA VS is the largest manutacturer ofprotective relays in the world, leading thedevelopm~nt of relays with microprocessors.The relays are built up in a modularizedplug-in system called COMBIFLEX@, a sys-tem characterized by great flexibility and re-Iiability. Tl11e relays in the COMBIFLEX sys-tem have l:iJeen optimized with respect to theirquaiity, dimension and Gast.

All COMBIFLEX relays can be tested by atest system called COMBITEST by plugging atest-handla inta a built-in test switch. Bythis it is possible to carry out a sate and easyinjection test of a relay. The road currentthrough a relay in service can be measuredby an ammeter connected to a current mea-suring plug with a built-in overvoltage pro-tection. If the am meter circuit is open bymistake the plug will be short-circuited bythe overvoltage protection. By this the currentin a CT -cirrcuit can be measured without anyrisk of getling an open CT -circuit. A veryimportant safety feature.

A power transformer is a very valuable andvitallink in a power transmission system. Highreliability of the transformer is thereforeessentiai to avoid disturbances in trans-mission of power.

A high quaiity transformer properly designedand supplied with suitable protective relaysand monitors is very reliable. Less than onefault in 100 transformer years can be

expected.

When a fault occurs in a transformer, thedamage is normal ly severe. The transformerhas to be transported to a workshop andrepaired, which takes considerable time. Tooperate a power transmission system with atransformer out of service is always difficult.Frequently, the impact of a transformer fault ismore serious than a transmission line outage.

The operation and maintenance of atransformer can be a contributory cause of afault. If a transformer is operated at too hightemperature, too high voltage, or exposed toan excessive number of high current externalfaults etc, the insulation can weaken to thepoint of breakdown.

On-road tap-changers should be checkedand maintained according to the operatinginstructions to prevent any faults. A fault in atap-changer with a separate housing cancause too high a pressure in the housing. Apressure relay can be used to trip the circuitbreakers at a certain set pressure, seepoint 5.4

2.CONDITlONSLEADING TO FAULTS

2.2Aging of insulation

Aging or deterioration of insulation is afunction of time and temperature. The part ofthe windin~ which is operated at the highesttemperature (hot-spot) will ordinarilyundergo the greatest deterioration and getsthe shorte$t length of life. However, it is notpossible tO! accurately predict the length of lifeas a functibn of temperature and time underconstant Gontrolled conditions, much lessunder widely varying service conditions.

In case a transformer gets too hot, Improvethe cooling if possible or reduce the load inorder to avoid accelerated aging of theinsulation. A temporary moderate over-temperature can be allowed as it takes aconsiderablle time to age the insulation.

2.1

Insulation breakdown

A breakdown of the insulation results in shortcircuits or ground faults, frequently causingsevere damage to the windings and thetransformer core. Furthermore, a high gaspressure may develop, damaging the trans-former tank.

Breakdown of the insulation between windingsor between windings and the core can becaused by:

.aging of insulation due to overtempe-rature during long time.

.contaminated oil

.corona discharges in the insulation

.transient overvoltages due to thunder-storms or switching in the network

.current forces on the windings due toexternal faults with high current.

A flash-over between the primary and se-condary windings usually results in a break-down of the insulation between the secondarywinding and ground.


2.4

Oil contaminatipn and leakage

The oil in a transformer constitutes an elect-rically insulating medium and also a coolingmedium. The service reliability of an oil-immersed transformer therefore depends to agreat extent on the quaiity of the oil. The oilshould fulfill the l1equirements of lEG 296.

The dielectric sttength of the oil is the mostimportant propert y of the oi!. If the dielectricstrength of the <Dil is reduced by water andimpurities etc, a breakdown of the insulationcan occur. Testing of the dielectric strength ofthe oil is normally conducted on site to get aquick check of the purity of the oil.

The oillevel must be monitored, a breakdownof the insulation occurs if the oil level gets tolow.

Oil immersed transformers with an oilconservator should therefore be provided withboth a silica gel breather and an oil levelmonitor.

2.5

Reduced cooling

Forced cooling systems must be supervised.and an alarm sh all be given if the coolingsystem stops. The oil temperature can thenbe watched and appropriate action takenbefore the transformer becomes overheated.

2.3

IOverheating due to overexcitation

According to lEG 76-1 guidelines, trans-i formers shall be capable of delivering ratedi currents at an applied voltage equal to 105%lot the rated voltage. Transformers may be!specified for operation at a voltage up to110% of rated voltage.

When a transformer is operated at too highvoltage or at too low frequency, the

Itransformer core gets overexcited. The flux isthen forced through surrounding steel partssuch as the sheet metal of the tank and otherlunlaminated parts of the transformer. Theseiparts are heated up in an unacceptable wayand the transformer can be damaged. As atransformer loaded with rated current canwithstand on ly 105% of rated voltagecontinuously, the transformer has to bedisconnected if the voltage is too high or thefrequency too low. According to IEEE generalguide for permissible short-time over-excitation of power transformers, see tig 1,transformers can only with stand overex-citation a short time.

pspecially turbo-generator transformer unitscan be exposed to overvoltage and under-frequency conditions. They should beprovided with an overexcitation relay opera-ting when the ratio between voltage andrrequency (V/Hz) gets too high.

To get a correct representation of the flux, theoverexcitation relays must be connected to apotential transformer, measuring the voltage?f an untapped transformer winding.

-1l-lLL IEEE' GEN'ER~L I G~;D~ ~ ~OR PE~MISSI~LEI JJ

SHORT-TIME OVEREXCITATION OFPOWER TRANSFORMERS

11.5

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135

130

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.4 .5 1.0 2 3 4 5 618~10Time in minutes:

20 30 40506010

IFig. 1 Permisslble short-time overexitation

AGO3-5005 E

Power transformer protectionABB Relays

3.FAUL T CURRENTS

It can also be seen that the primary currentfor aground fault between O .40% from theneutral is below 1,5 x In. Therefore in thiscase an overcurrent relay on the primary sidecan not detect ground faults located O -40%from the neutral point as it has to be set about1.5 x In due to the load current.

3.1Ground faults in a solid ly grounded star-connected secondary winding

The magnitude of fault current is mainlycontrolled by the reactance and the voltagebetween the point of the fault and ground. Thereactance decreases rapid ly for faults ap-proaching the neutral. The fault current cantherefore be higher for a fault close to theneutral than for a fault at the middle of the

winding.

Fig. 2 is valid for one type of transformerdesign. For this transformer the fault currentis higher for a fault close to the neutral thanfor a fault between 10 -60 % from theneutral.

Ground fault current in a solidly grounded star-connected windingFig.2

3.2Ground faults in a high impedance grounded star-connected secondary winding

proportional to the square of the shortcircuitedfraction of the winding. See fig 3.The fault current is controlled by the

grounding impedance and the position of thefault. The primary current is approximately

Ground fault current in high a impedance grounded star-connected windingFig. 3


The current in the short-circuited turns maybe 50-100 times the rated current. Thatmeans local overheating, arcing, decom-position of oil and release of gas. A gasdetector relay therefore detects a turn-to-turn fault. A rate-of-rise pressure relay mayalso detect the fault.

3.5

Phase-to-phase faults

Short-circuits between the phases will giverise to substantiai fault currents only limitedby the source impedance and the leakageimpedance of the transformer.

3.3

Ground faults in a delta-connectedwinding

The magnitude of the ground-fault currentdepends on the grounding of the powersystem.

The fault impedance of a delta-connectedwinding is highest for faults at the midpoint ofthe winding and can be expected to be 25-50%. based on the transformer rating.

The fault current is equally divided betweentwo phases for a fault at the midpoint. Thefault currents in the phases may therefore beequal to or less than the rated current whenthe source impedance is appreciable. Theserelatively low phase currents must be con-sidered when evaluating the performance of aprotection scheme.

For faults close to one end of the winding thefault current approaches the fault current for aphase-to-ground fault.

3.4

Turn-to-turn faults

A direct metallic contact or flashover between

conductors with in the same physical windingis called a turn-to-turn fault. The current

forces caused by high fault currents through atransformer during system faults, can crush orshave off the insulation and develop a turn-to-turn fault. This is particularly a risk forrelatively small and aged transformers in

powerful systems.

A turn-to-turn fault can also be caused by

steep fronted surge voltages or corona

discharges.-

A turn-to-turn fault short-circuits a small

part of the winding. This part behaves as anautoconnected winding of ils own with verylarge turns ratio to the remaining part of thewinding. An extremely high fault current istherefore transformed inta the short-circuitedloop. The resulting unbalanced current forcescan rip apart or crush the loop.

If a turn-to-turn fault develops, the da-

maged area has sometimes lost a burned-away volume of copper as large as a fist. Thewhole winding is sprayed with copper beadsand soot. The repair of the transformer willtherefore be extensive.

Turn-to-turn faults between a few turns aredifficult to detect by current measuring relaysas the terminal current increases very little.

The fault current at the terminals increaseswhen the fault spreads and more turns are

short-circuited. The fault current is equal tothe rated current when 2-4% of the turns are

short-circuited.


4.PROTECTIVE RELA VS

4.2

Differential relays

4.1

General

When a fault occurs in a transformer, thedamage is proportional to the fault time. Thetransformer should therefore be disconnectedas fast as possible from the network. Fastreliable protective relays are therefore usedfor detection of faults.

Monitors can also detect faults and they cansense abnormal conditions which may de-velop inta a fault, see section 5.

The size of the transformer and the voltagelevel have an influence on the extent andchoice of protective equipment. Monitorsprevent faults and protective relays limit thedamage in case of a fault. The Gast for theprotective equipment is marginal compared tothe total cost and the cost involved in case ofa transformer fault.

There are often different opinions about theextent of transformer protection. However, it ismore or less normal that transformers with anojl conservator are furnished with the following

equipment:

Transformers larger than 5 MVA

.Gas detector relay (Buchholz relay)

.Overload protection (thermal relays ortemperature monitoring systems)

.Overcurrent protection

.Ground fault protection

.Differential protection

.Pressure relay for tap-changer com-partment

.Oil level monitor

Transformers smaller than 5 MVA

.Gas detector relay (Buchholz relay)

.Overload protection

.Overcurrent protection

.Ground fault protection

4.2.1

General

A transformer differential relay campares the

current ted to the transformer with the currentleaving the transformer. Auxiliary transformersfor correction of phase shift in the powertransformer and for ratio corrections areneeded. For transformers with a ta p-changer, the ratio of the auxiliary currenttransformers should be calculated forbalanced currents when the tap-changer isin the middle position.

The protective zone of a differential relayincludes faults in the transformer and faults onthe buses or cables between the currenttransformer and the power transformer. Adifferential relay has therefore alargerprotective zone than a gas detector relay.When bushing CTs are used for thedifferential relay. the protective zones can beconsidered to be equal.

Fast operation of the relay is obtained whenthe differential current through the relay islarger than the setting of the relay.

A transformer differential relay must be ableto cope with the following conditions:

Magnetizing inrush current

The inrush currents develop when atransformer is switched on to a power system.Similar inrush currents can occur when thevoltage is returning to normal after a line fault.

The shape, magnitude and duration of theinrush current depend on the following

characteristic factors:

.The size of the power transformer

.The source impedance

.The magnetic properties of the core

material

.The remanence of the core

.The moment when the transformer isswitched in

Transformers which may be exposed to

overvoltage:

.Overexcitation protection should also beincluded.

In addition to the protective relays andmonitors. tri p units and alarm systems are

required.


The magnitude of the in rush current istherefore dependent on the moment when thetransformer is switched in.

Fig.4 Theoretical inrush current Im

The inrush current has a large dc componentand is also rich in harmonics. Thefundamental frequency and the second har-monic are the basic frequencies. The currentis more or less present in all three phasesand also in the neutral, see fig 5. The inrushcurrent in the neutral is spread out in theother grounded neutrals of the power systemaccording to the distribution of the zero-

sequenceimpedances.

The inrush current can appear in all threephases and in a grounded neutral. Themagnitude of the current is always different inall three phases as weil as in the neutral. Inpower transformers with oriented core steel,the magnitude can be 5-10 times the ratedcurrent when switching is done on the outerwinding (usually the high-voltage side) of thetransformer and 10-20 times the ratedcurrent when switching the inner winding(usually the low-voltage side).

The magnetizing inrush current can get themagnitude and shape shown in fig 4. Themaximum inrush current develops if themoment of switching occurs at the zerocrossing of the voltage and when the new fluxfrom the inrush current gets the samedirection as the already present magnetic fluxin the core. The two fluxes are added and thesaturation limit of the core can be exceeded.The magnetizing inrush current then increas-es to a value permitted by the impedance ofthe power system and the residual impedanceof the transformer. The probability that themaxi-mum inrush current should occur islow, but a considerable inrush current isobtained, say, one time out of 5-6 times of

switching.

When the new flux at switch-in of thetransformer gels the opposite direction of thealready present flux, there will be nosaturation of the core and the magnetizinginrush current will be small.

IR

Is

IT

Fig.5 Recorded inrush current for a 60 MVA transformer 140/40/6,6 kV, connected y Nyd.


Interna I faults

For faults inside the protective zone of therelay, a current proportional to the faultcurrent occurs in the differential circuit andthe relay operates.

The operating time of transformer differentialrelay type RADSB is shown in fig 6. For afault current 5 times the operating current, therestrained operation time is 27 ms.

The relay is also provided with an unre-strained operation circuit to speed up theoperation for a high fault current. Threesettings are available: 8, 13 and 20 timesrated current. The current setting for unre-strained operation has to be set above themax inrush current when the transformer isenergized. At a fault current 10 times the setoperating current, the relay operates in 8 ms.

Recommended setting for unrestrainedoperation:

The damping of the inrush current dependson the total resistance of the source network.The duration in a powerful system is usually afew seconds.

In cases when a transformer is switched inparalIei with another energized transformer. acorresponding inrush current can develop inthe energized transformer when the directcurrent from the switched-in transformer issaturating its core. The inrush current in theparailei transformer is shifted 180 degrees.The damping of the combined inrush currentwill the n be less than normal and the inrushcurrent may be traced as long as severalminutes.

The shape of the inrush current for a delta-connected transformer will not be the same asfor a Y-connected transformer. The reason isthat the phase current in a delta connectedtransformer is developed by currents fromwindings on two limbs.

The differential relay type RADSB is providedwith a MAGNETIZING INRUSH RESTRAINTbased on the 2nd harmonic content of theinrush current. Any unwanted operation of therelay due to the inrush current is therebyprevented.

Normal service

During normal service a small differentialcurrent flows through the differential relay.The current is due to the excitation current ofthe power transformer. ratio errors in thecurrent transformers and the position of thetap-changer. if provided.

A power transformer with a tap-changer inthe end position gives a differential current of10-20% of road current depending on theregulating range of the tap-changer.Therefore, the mismatch due to the tap-changer in an end position determines thesetting of the differential relay. Asetting 15%higher than the mismatch is usual.

1) The transformers are assumed to bestep-down transformers with power flowfrom the high voltage system to the lowvoltage system.

A setling of 20 x In is required when verylarge through-fault currents can saturate theCTs and cause a large differential current.This can, for example, be the case when thebus is included in the protective zone of thedifferential relay or when one and a halfbreaker arrangement is used.

Opera tingtime in ms

1.5 2 3 5 10 15 20 30

Ditt current in multiples of set operatin~ current Id

Operating timeFig.6


RADSB is therefore provided with a through-fault restraint circuit which makes the relayoperate for a certain percentage differentialcurrent related to the current through thetransformer -The differential relay is thereforealso called a PERCENTAGE RESTRAINTdifferential relay-

The through-fault restraint of RADSB isshown in tig 7. The curves show that for athrough current 10 times the rated current In.the relay operates for a differential current ~ 6times In-

External faults

For faults outside the protective zone of therelay a relatively large differential current canoccur due to the position of the tap-changerand differences between the CTs. With thetap-changer in an end position, the voltagecould be 20% off from the voltage at themiddle position. If then a short-circuit current10 times the rated current occurs, a diffe-rential current of twice the rated current isobtained.

The differential relay should not operate forthis differential current. The relay type

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Restraint through current in multiples of rated current In

Fig.7 Through (ault restraint

Overexcitation

When a transformer gets overexcited, theexcitation current increases dramatically andthe transformer may get damaged if thiscondition is sustained, see section 2.3. For anovervoltage of 20%. the excitation current canincrease about 10 times normal excitationcurrent. For a higher overvoltage. the ex-citation current can increase above the pick-up level of a differential relay unrestrained for..excitation current.

An overexcited transformer is not atransformer fault. It is an abnormal networkcondition for which a differential relay should,not operate. Operation of a differential relay!indicates a transformer fault. Investigation of a!transformer shall therefore always be doneiafter operation of a differential relay. If the'relay has operated during an overexcitationcondition of the transformer. valuable time forinvestigation of the transformer would then belost before the transformer can be put back inoperation again.

An analysis of the current during anoverexcitation condition shows a pronounced5th harmonic component. A typical examplefor a modern transformer is shown in fig 8This can be utilized to identify anovermagnetizing condition. The differentialrelay type RAOSB is therefore provided with a5th HARMONIC RESTRAINT to prevent the

18

14

10

6

2

ASS

Relays Power transformer protection AGO3-5005 E

4.2.2Differential relays for fully insulatedtransformers

When applying a differential relay to a Yyconnected power transformer, it maysometimes be possible to choose the mainCTs on both the high and low voltage side ofthe transformer with ratios making thesecondary currents equal at both sides.However, to prevent any operation for externalground faults, one set of auxiliary CTs with adelta connected tertiary winding or two sets ofauxiliary CTs connected Yd are required.

When the power transformer is connected Yd,auxiliary CTs are always required at least onone side of the transformer for balancing ofthe currents and for correction of the phaseangle between the currents.

With auxiliary CTs on only one side of apower transformer, a differential currentoccurs if the auxiliary CTs saturate during aheavy through fault. It is therefore re-commended to use auxiliary CTs on allwindinc;js of the power transformer in order toget the same time to saturation on all inputsto the differential relay.

The connection of the auxiliary CTs dependson the connection of the power transformer,see instruction RK 625-100E.

4.2.3

Differential protection for aula.transformers

An auto-transformer as weil as a tullyinsulated transformer can be protected by thetransformer differential relay type RADSB. Thedelta winding can be connected to thenetwork as in tig. 9 or not connected.lf notconnected to the network, CTs are notneeded tor the connection of RADSB. In bothcases, the delta winding is protected as weilas the main winding. The sensitivity otRADSB can be set between 20-50% of ratedcurrent and an operating time as shown in fig.6 is obtained.

By applying CTs in the neutral point ot themain winding, a faster and more sensitiverelay type RADHA or RADSG can be used toprotect the main winding, see tig. 10. Therelays operate tor phase-to-phase taultsand tor phase-to-ground faults in the mainwinding. Faults in the delta-connectedwinding can not be detected by RADHA orRADSG. A transformer differntial relayprotecting both winding is therefore also used.

RADHA is a high impedance differential relay.RADHA requires dedicated CT cores and allCTs 'must have the same ratio. No turnscorrections can be allowed on any CT and noauxiliary CTs tor ratio correction can be used.The saturation voltage of all CTs must be atleast twice the selected operating voltage otRADHA.

A sensitivity of about 5% can normally beobtained. The operating time is 15-20 ms.

The ultra-high-speed relay type RADSGcan be used instead of RAD HA. Comparedwith RADHA, no dedicated CT cores arerequired and different ratios ot the main CTscan be matched by using auxiliary CTs.

A sensitivity of about 4% and an operatingtime of 8-13 ms can be obtained.

ABE3 Relays Power transformer protection AGO3-5005 E

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Fig.9 Transformer differential felar for autotransformer

Fig. 10 Differential re/aV RADHA or RADSG for autotransformer


4.3

Overcurrent protection and impedancerelays

4.3.2

Under-impedance relays

Overcurrent relays are not always suitable asback-up relays for system transformersconnecting two networks or in networks with alarge difference between maximum andminimum short-circuit fault MVA.

The back-up protection must be able to seea fault at any one of the voltage systems andit must also be able to operate for theminimum short-circuit MV A. In such cases,an under-impedance relay type RAKZB canbe used. The reach is independent of themagnitude of the short-circuit current. Theversion of RAKZB measuring the current IR -IT, IS -IR and JT -IS should be used to getthe same reach for two- and three-phasefaults. The relay is not suitable for groundfaults.

4.3.3

Distance relays

Distance relays are sometimes used insteadof differential relays as the main transformerprotection.

Distance relays instead of non-directionalunder-impedance relays are also used as aback-up for the transformer differential relayand at the same time they can act as aprimaryor back-up relay for the buses.

The static distance relay RAZOA is a suitablerelay for this purpose. The direction ofmeasurement of any one of the threeimpedance zones can be reversed. The reachand directions of the zones, see tig 11, can beset as folIows: Zon e 1 reaches 70-80% intathe transformer, zone 2 is reversed andcovers bus 2 and zone 3 reaches through thetransformer and covers bus 1.

4.3.1

Time-overcurrent relays

Time-overcurrent relays are used on allfeeding circuits to a power transformer. Theirfunction is to back up the differential pro-tection and the protective relays on the roadside of the transformer. The overcurrentrelays perlorm as a primary short-circuitprotection if no differential protection is used.Instead of overcurrent relays, underim-pedance or distance relays are required whenthere is a large difference between themaximum and minimum short-circuit faultMVA.

Time-overcurrent relays with an in-stantaneous element for high-fault currentsare normal ly used in each phase. The time-overcurrent relay is normal ly set for operationat about 150% of the rated current of thetransformer. The time delay must be longenough to avoid tripping due to themagnetizing inrush current when the trans-former is energized. Time selectivity mustalso be attained between the relays on theprimary and secondary side of thetransformer.

The instantaneous element has to be setabout 25% above the maximum through-fault current and above the maximum inrushcurrent. With this setting, instantaneoustripping is only obtained for severe faults onthe feeding side of the transformer. Therecan, for example, be faults on the transformerwinding close to the bushing, faults in thebushing or on the circuits between the CTsand the transformer.

The relay operates delayed for faults on theremaining parts of the windings and for faultson the load side of the transformer if the faultcurrent and the duration exceed the setting ofthe relay.

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Bus 1 Bus 2

F;g. 11 Distance re/ar used as a transformer protection

ABE3 Relays Power transformer protection AGO3-5005 E

4.4Ground fault protection

4.4.1General

Power transformers with impedance-grounded or solidly grounded neutral, can beequipped with different types of ground faultrelays to protect the grounded winding.

Low-impedance residual overcurrent relaysor harmonic restraint overcurrent relays canbe connected according to A or B in tig 12.

When the transformer neutral in tig 12 issolidlyor effectively grounded and the trans-former is ted from either side H or side Lo afault at F1 or F2 is detected by a relay atpoint A. The relay at point B mayaiso operatedepending on the distribution of the zero-sequence impedance in the network. A fault atF3 is detected by the relays at point A and B.

Consider the transformer ted from either sideH or side L and that the transformer neutral isimpedance-grounded. With only one point inthe network grounded. a fault at F1 or F2 isdetected by a relay at point A. Fault F3 isdetected by the relays at point A and B.

These types of overcurrent relays musttherefore be delayed. or else they will operatefor faults which should be taken care of byother ground fault relays in the network.

The relays also have a back up functionregarding the ground fault protection of thelines. They are also a slow back up fortransformer differential relays in solidlygrounded networks.

H::::(

I

Impedanceor solidly

groundedneutral

Fig. 12 Connection of ground fautt overcurrent retays

Arestricted ground fault relay of the currentdifferential type can only operate for faultsinside the protective zone, see tig 13 and 14.

The relay is sensitive and reliable and a highspeed of operation is obtained.

3 S~

Fig. 13 Connection of arestricted groundfau/t re/ar for a Y-connected

winding.

Fig. 14 Connection of arestricted groundfau/t re/ar for a D-corinectedwinding and agroundingtransformer.


4.4.2Low-impedance residual overcurrentrelay

This type of ground fault relay can beconnected either to a current transformer inthe neutral or to residually connected phasecurrent transformers, see fig 12.

The relay should be released by a residualvoltage relay to prevent operation due tosaturation of any CT during a short-circuit ordue to a magnetiz.ing inrush current.

The re/av can operate for ground faults in thenetwork and also for magnetizing inrushcurrent containing a zero sequence com-ponent. The relay must therefore be delayedIonger than the duration of the inrush currentor Ionger than the delay of other ground faultrelays in the network.

4.4.3

Harmonic restraint overcurrent relay typeRAISA

An overcurrent relay with a second-harmonicrestraint is stable for the inrush current. Thetime setting of this relay is therefore in-dependent of the duration of the inrushcurrent. The delay can be chosen only withregard to other ground fault relays in thenetwork.

The basic version can be delayed by anindependent time-delay relay or an inversetime-delayed relay.

4.4.4High-impedance restricted relay

A high-impedance restricted ground faultrelay type RADHD (differential ground relay)will provide a sensitive and high speedrestraint protection. The relay is used insolidly grounded networks. It can also beused in impedance grounded networks with afault current above the sensitivity of the relay.see fig 13 and 14.

The current from residually connected lineCTs is balanced against the current from aCTin the grounded neutral. For an internat fault.the currents from the CTs have oppositedirection and a high voltage occurs across ahigh-impedance relay. The saturation vol-tage of the CTs should be at least twice theoperation voltage Us of the relay.

For an externat fault. the current circulatesbetween the current transformers. The relay isthen stab le for all external faults even if one ofthe CTs should be saturated.

The CTs in the phases and in the neutralshall, if possible, be dedicated for the relayand they must have exactly the same turnsratio. No turns correction can be accepted.

Relay type RADHD provides a more sensitiveprotection than the transformer differentialrelay but is not a back-up for a transformerdifferential relay which protects both the highand low voltage windings.

The relay may have to share the CT coreswith the differential relay. see fig 15.

Fig. 15 Transformer differential and restricted gro und fault relays on the same GT cores.

The combination of the relays on the sameCT cores should be avoided when on ly thewinding protected by the high impedancerelay is supplied from the network. Due to theimpedance of the restricted ground-faultrelay. the differential relay might not getenough current for operation for a ph ase-to-ground fault.

A non-linear resistor should be connected inparailei with the high impedance relay closeto the connection point of the CTs. Theresistor reduces the high peak voltage whichcan be developed during an internai fault. Theinterconnected secondary circuits of the CTshave to be grounded at only one point.

Each y -connected winding of a transformercan be protected with a separate high-impedance restricted ground fault relay, seefig 13. Delta-connected windings can also beprotected and the grounding transformer canbe located inside the protected zone. seefigure 14.

With RADHD. a sensitivity of about 10% ofrated current can normally be obtained. Theoperating time of the relay is approximately20 mg.

During a fault. a relatively high transientvoltage occurs across the relay circuit of ahigh-impedance relay. This voltage can beconsiderably reduced if the moderate high-impedance relay type RADSG is used insteadof RADHD. The relay type RADSG requiresno dedicated CT cores and auxiliary CTs canbe used for matching the ratios of the CTsand for reducing the transient voltage.

With RADSG. a sensitivity of about 4% andan operating time of 8 -13 ms can be ob-tained.

4.4.6Tank protection

The tank protection is aground fault pro-tection with a limited use in same countries.The transformer tank is insulated from theground. About 10 ohm insulation resistance issufficient. The tank is connected to groundthrough a current transformer. An instant-aneous overcurrent relay is connected to thecurrent transformer.

The relay operates for ground faults inside thetank and for flashovers on the bushings.

No path for the ground fault current exceptthrough the current transformer can beallowed. Therefore, the following must betaken care of:

.All cable sheaths must be insulated fromthe transformer tank.

.All pipes to heat-exchangers etc must beinsulated.

.Fan motors and motors in a tap-changermust be insulated from the tank to preventaground fault in a motor from activatingthe tank protection.

.Instead of insulating cable sheaths andpipes, all cables and pipes can be broughtthrough the cable current transformerlocated on the connection between tankand ground.

.Mistakes like leaving a metat bar etcleaning against the transformer tank can bemade.

.Hazardous potentials to personnel wo r-king in the vicinity of the transformer canoccur at aground fault

The tank protection is seldom used due tothe se points.4.4.5

Low-impedance restricted relay

It is sometimes possible to use a low-impedance current relay as agrounddifferential relay instead of a high im-pedance relay. The current transformer in theneutral can then have a different ratio from thephase GTs since an auxiliary GT can be usedfor ratio correction.

If a phase GT gets saturated during a short-circuit, the relay mayoperate. The relayshould therefore be released by a residual

voltage relay.

4.4.7Residual voltage relay

A residual voltage relay connected to potentialtransformers connected in broken deltameasures the neutral displacement for anyground fault in the network. The relay is aback-up for other ground fault relays andmust therefore have the long est delay amongthe ground fault relays.

Normal voltage setting of the relay in highimpedance grounded networks is 10 -40%of the phase voltage.

In solidlyor effectively grounded networks, aresidual voltage relay can be used as aback-up ground fault relay. The relay can beset to operate in cage the grounding of thenetwork is lost or reduced to such an extentthat the residual current relays do not getenough current for operation.

AGO3-5005 E

ASS Relays Power transformer protection

voltage and gives an alarm in case of aground fault.

4.5

Overexcitation protection

Overexcited transformers may becomeoverheated and damaged, see point 2.3 andtig 1. A V/Hz overexcitation relay is thereforeneeded for transformers which may beoperated at a too high voltage or at a too lowfrequency. Especially generator transformerunits can be overexcited during accelerationor deceleration of the turbine. The actual ratiobetween generator voltage and frequencysh all not be allowed to exceed 1.1 times theratio of rated voltage and frequency of thetransformer.

A V/Hz relay provides improved measurementof overexcitation as compared to onlymeasuring the voltage. The inverse-timeoperation characteristic of RATUB corres-ponds closely to fig 1. This relay thereforeensures maximum usage of a transformerduring system disturbances, causing anovervoltage or an underfrequency condition. Fig. 16 F/ashover re/ar.

4.6

Flashaver and ground fault protectionsfor low voltage systems.

4.6.2Systems with rectifiers and frequencyconverters with out pulse-width-modu-lation.

When rectitiers or static trequency convertersare connected to a low voltage system, avoltage transformer in the neutral point cannot be used. The voltage transformer can besaturated by dc and damaged.

In such cages the voltage transtormer mustbe replaced with a neutral point equipmentwith a voltage dependent resistor, which limitsthe voltage in cage of aflashover.

Aground tault in the relay circuit will not bedetected during normal operation. It will bedetected by a miniature circuit breaker (MCB)in the relay circuit if aground tault occurs inthe low voltage network. A resistor limits thecurrent through the MCB. See tig. 17.

If the dc-link voltage ot the static trequencyconverters is regulated and the maximumoutput frequency is 65 Hz, a relay of typeRAEUB is recommended as aground taultrelay. See tig. 17.

The RAEUB relay can also be used in sys-tems without any neutral point,See BO3-2712E.

111

Trip

1

..lo. T rip orAlarm

~

4.6.1

Systems with out rectifiers or frequencyconverters.

If a fault between the primary and secondarywinding in a transformer occurs. the voltagelevel of the secondary network can be ex-posed to the voltage level of the primarywinding. This can cause extensive damage tothe low voltage network.

AfIashover protection consisting of aninstantaneous voltage relay type RXEG 21 orRXEL 24 connected to a voltage transformerbetween the neutral point of the low voltagesystem and ground can be used to trip thecircuit breakers. The voltage relay is normallyset at 1 .5-2 times the phase voltage and nodelay of the relay is allowed.

No rectifiers or frequency converters areallowed to be connected to the low voltagesystem. The voltage transformer will get sa-turated by dc from the rectifiers or convertersand damaged if a ground-fault occurs on the

dc-system.

This type of flashover protection can only beused if the ground-fault current on theprimary side of the power transformer islimited to about 25 A. The condition has to befulfilled to be able to use a voltage-dependent resistor connected in parailei withthe primary side of the voltage transformer inthe grounded neutral. The resistor is used tolimit the voltage across the relay to max 2 kVin order to protect the low voltage system andthe relay against excessive overvoltage. Seetig 16.

A delayed residual voltage relay with RXEG21 or RXEL 24 can be connected in paralIeiwith the flashover relay The relay is normallyset for operation at 20-40% of the phase

Fig. Ground fau/t re/av


The neutral point equipment has a voltagedependent resistor in series with a spark-gap, a resistor and a miniature circuit breaker(MCB) in the relay circuit. See tig. 18.

The RAERA relay can also be used in sys-tems without any neutral point.See B03-2711 E.

4.6.3Systems with rectifiers and pulse-width-modulated frequency converters.

The RAEUB relay or other neutral point vol-tage relays can not be used if the frequencyconverters are pulse-width-modulated(PWM). The PWM frequency convertersgenerate harmonic voltages. which at certainfrequencies may be higher than thefundamental harmonic voltage in the neutralpoint at aground fault.

For a low voltage network with PWM fre-quency converters. a relay of type RAERAwith both ground fault and flashover functionsis used. See tig. 18.

The RAERA relay will cape with the con-ditions typically found on systems havingPWM frequency converters. The relay is alsoapplied on systems with ac/dc-convertersand low frequency induction stirrers. For op-eration at aground fault. RAERA injects a di-rect voltage inta the system and measuresthe dlrect current which occurs during aground fault.

Fig. 18 Ground fau/t and f/ashover re/ar.

5

MONITORS

oil which occurs when a serious faultsudden ly occurs in the transformer. Thedevice actuates a contact normal ly connectedfor tripping the circuit breakers for thetransformer. The operating time of the tripcontact depends on the size of thetransformer, the magnitude and location of thefault. The operating time can therefore varvbetween 0,1-0,3 s.

If the gas detector has operated, the gasshould be investigated. The following in-dications could be used for a preliminaryevaluation of the cause of gas.

Air collected in the gas detector usuallyoriginates from air bubbles trapped in thetransformer when the transformer was filledwith oil. In such cases, alarm signals causedby escaping air do not usually continue forany length of time.

5.1

General

Monitors are very valuable. They can detectfaults and abnormal service conditions whichmay develop into a fault.

For example, the gas released in thetransformer oil can be monitored by a gasdetector. Small amounts of gas from a slowlydeveloping faults can be detected before anyprotective relay can detect the fault. In case ofa serious fault the gas detector trips thecircuit breakers. The gas detector is then aback-up for the protective relays.

The extent of monitors on a transformerdepend mainly on the size of the transformerand the voltage level.

5.2Gas detector relay

During a fault in an oil-immersed trans-former, arcing willoccur, releasing gas fromdecomposition of the ojl. The gas passesthrough the oi! pipe to the conservator andcan therefore be detected by a gas detectorrelay.

Gas detector relays have an alarm and a tripdevice. Gas is collected in the alarm device,and when there is enough gas, an alarmcontact is closed. The gas detector relaydetects small amounts of gas developed overa long time. A slowly developing fault cantherefore be detected before it becomes moreserious.

The trip device responds to the high flow of

AGO3-5005 E

ASS Relays Power transformer protection

Bimetal relays are not suitable as overloadprotection as their time constant ~ and thetime constant of a transformer do notcorrespond.

5.4Pressure relay for on-load tap changers

If a fault causing gas development occurs in atap-changer compartment. a pressure relaycan be used to trip the circuit breakers beforeexcessive pressure occurs.

The pressure relay delivered by ASEA has asetting range of 30-150 kPa and an operat-ing time of 10-15 ms. The relay is preset ata function-value valid for the type of tapch anger used.

5.5

Pressure relief valve

A flashover or a short-circuit in an oil-filledtransformer is usually accompanied by over-pressure in the tank. By providing thetransformer with a pressure relief valve. theoverpressure can be limited to a magnitudeharm less to the tank.

The valve delivered by ASEA opens for apressure of 120 :t 12 kP A within ap-proximately 2 ms. The valve closes auto-matically when the overpressure is released.

5.6Oil level monitor

Monitoring of the ojl level is specified fortransformers with oil conservators. The in-dicator shows the oil level and also has twocontacts for alarm of the max and min ojllevel.

5.7

Silica gel dehydrating breather

The silica type breather is used for drying theair drawn into the oil conservator when thedrop in load and temperature cause s the oil tocontract. The silica gel in the breather is ableto ab sorb moisture to the extent of 20% of itsown weight. When the silica gel getssaturated with moisture, the color changesfrom blue to pale pink and it has to be

replaced.

5.3

Temperature monitoring

Overtemperature in a transformer may occurbecause of overioading or loss of coolingcapacity (forced-cooling units).

The largest contribution to the heat capacityof an oil-filled transformer comes from theoil. The time constant with which the oiltemperature responds to a change of loadingis several hours, which means that during adaily road cycle, the temporary loading mayweil exceed rated power without thetransformer reaching maximum allowabletemperature. According to standards, short-time overloads up to 1.5 times the rated loadcan be allowed.

Therefore, overcurrent relays can not be usedfor overload monitoring. They have to be setabove predicted short time overioad.

Temperature and overload monitoring of oil-filled transformers is carried out withindicating thermostats ("contact thermo-meters") which are standard accessories on atransformer. There are two conventionaifunctions, called "oil thermometer", and"winding thermometer" (or winding tem-perature indicator).

The oil thermometer type TITG 54 has aliquid thermometer bulb in a pocket at the topof the transformer, connected through acapillary tube to an indicating, bellow-typeinstrument with a set of adjustable contacts.The instrument is compensated for changesin ambient temperature and essentially feelsthe top oil temperature in the transformer.

The winding thermometer type TITG 64 has athermometer bulb measuring the top oiltemperature and the instrument is providedwith a heater element fed from the roadcurrent which introduces a bias to the reading.This bias is set by a rheostat according to theheat run test result so that it corresponds tothe temperature difference of the windingabove its surrounding air. The indication istherefore Glaser to the winding hot spottemperature, and the bias function has a timeconstant similar to that of the winding-over-oil temperature difference (a coup le ofminutes). The measuring system is com-pensated for changes in ambient temperature.

Depending on the climate, a suitable earlyalarm is set for high oil temperature in atemperate or arctic zone typically at 75 oG.An emergency tripcontact is set at 100-110oG. Additional contacts may be used forthermostatic controI of cooling pumps andfans as the loading and ambient temperaturevaries.

Winding thermometer signal and trip contactsare set correspondingly higher. The windingthermometer type TITG 64 provides forsettings up to 160 oG.

Static thermal relay type RXVE 4 with built-in compensation for the temperature of thecooling media can be used. Only onetemperature leve I can be set on one relay.

6

SUMMARY The protective equipment discussed isengineered to limit the damage and systemdisturbance caused by faults which can occurin a transformer.

The choice of protective equipment dependson the size and the connection of atransformer, voltage level, power systemgrounding and the protective relays of thepower network. The power companies alsohave different opinions about the extent andchoice of pr:otective equipment for atransformer. No general recommendationscan therefore be made.

7

REF=ERENCES COMBIFLEX Mounting and connection hardware 803-9302 E

COMBITEST Test system COMBITEST 803-9510 E

RADHA High-impedance three-phase differential relay 803-6011 E

RADHD 803-5013 EHigh-impedance. single phase, restricted ground-fault relay

RADSB Transformer differential relay 803-5012 E

RADSG Ultrahigh-speed generator differential relay B03-4011 E

RAISA 803-2311 EHarmonically restrained overcurrent or ground-fault relay

RAKZB Three-phase impedance relay 803-3213 E

RATUB V/Hz overexcitation relay for transformers 803-5011 E

RAZOA Three-zone, phase and ground distance relayfor transmission lines. B03-7012 E

RXEG 21 603.2534 EInstantaneous ac over- and undervoltage relays

AXEL 24 803-2513 EElectromechanical instantaneous ac and dc overvoltage relays

RXVE 4 Thermal overcurrent relay assemblies (RXVE 41, 42, 43, 45) 803-2270 E

RAERA 803-2711 EGround fault and flashover relay

RAEUB 803-2712 ENeutral point voltage relay for systems with current converters

ABB Relays AB, 5-721 71 Västerås, 5wedenTel. -j-4621321300,Telefax +4621-146918Telex 407 20 abbva s

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