Transformer Protection

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Transformer ProtectionTransformer Protection

IntroductionIntroduction

The development of modern power systems The development of modern power systems has been reflected in the advances in has been reflected in the advances in transformer design. This has result in a wide transformer design. This has result in a wide range of transformers with sizes ranging from a range of transformers with sizes ranging from a few few kVAkVA to several hundred MVA being available to several hundred MVA being available for use in a wide for use in a wide varityvarity of applications.of applications.

The considerations for a transformer protection The considerations for a transformer protection package vary with the application and package vary with the application and importance of the transformers.importance of the transformers.

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IntroductionIntroductionSmall distribution transformers can be Small distribution transformers can be

protected satisfactorily, from both technical and protected satisfactorily, from both technical and economic considerations, by thee use of fuse or economic considerations, by thee use of fuse or overcurrentovercurrent relays. This result in timerelays. This result in time--delayed delayed protection.protection.

However, timeHowever, time--delayed fault clearance is delayed fault clearance is unacceptable on larger power transformers, due unacceptable on larger power transformers, due to system operation/stability and cost.to system operation/stability and cost.

IntroductionIntroduction

Transformer faults are generally clasTransformer faults are generally classsified in to ified in to five categories:five categories:

Winding and terminal faultsWinding and terminal faultsCore faultsCore faultsTank and transformer accessory faultsTank and transformer accessory faultsOnOn--load tap changer faultsload tap changer faultsAbnormal operation conditionsAbnormal operation conditionsSustained or Sustained or uncleareduncleared external faultsexternal faults

Winding and terminal CoreTank and accessoriesOLTC

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Transformer faultsTransformer faults

Winding faultWinding faultA fault on transformer winding is controlled in A fault on transformer winding is controlled in

magnitude by the following factor:magnitude by the following factor:Source impedanceSource impedanceNeutral Neutral earthingearthing impedanceimpedanceTransformer leakage reactanceTransformer leakage reactanceFault voltageFault voltageWinding connectionWinding connection


StarStar--Connected Winding with Neutral Connected Winding with Neutral Point Earthed through an impedancePoint Earthed through an impedance

The winding earth fault current depends on the earthing impedance value and is also proportional to the distance of the fault from neutral point, since the fault voltage will be directly proportional to this distance.

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I primary

X

Earth fault current in resistance-earth star winding

IF as multiple of IFL

.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

Delta side

Star side

X p.u.

1.00.90.80.70.60.50.40.30.20.1

- Star-connected winding with neutral point earthed through an impedance


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StarStar--Connected Winding with Neutral Point Connected Winding with Neutral Point Earthed through an impedanceEarthed through an impedance

For fault on transformer secondary winding, the corresponding primary current will depend on the transformation ratio between the primary winding and short-circuited secondary turns.

Faults in the lower third of the winding produce very little current in the primary winding, making fault detection by primary current measurement difficult.


StarStar--Connected Winding With Neutral Point Connected Winding With Neutral Point Solidly EarthedSolidly Earthed

The fault current is controlled mainly by the leakage reactance of the winding, which varies in a complex manner with position of the fault.

For faults close to the neutral end of winding the reactance is very low, and results in the highest fault current.

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Earth fault current in solidly earthed star winding

15

10

5

0

Current ( per unit )

Primary current

Fault current

10 20 30 40 50 60 70 80 90 100Distance of fault from neutral (percentage of winding)


StarStar--Connected Winding With Neutral Point Connected Winding With Neutral Point Solidly EarthedSolidly Earthed

The primary winding fault current is determined by the variable transformation ratio; as the secondary fault current magnitude stays high throughout the winding, the primary fault current is large for most points along the winding.

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Delta connected windingDelta connected winding

No part of a delta-connected winding operates with a voltage to earth of less than 50% of the phase voltage, and the impedance of a delta winding is particularly high to fault currents flowing to a centrally placed fault on one leg.

The earth fault current may be no more than the rated current, or even less than this value if the source or system earthing impedance is appreciable.


Delta connected windingDelta connected windingThe current will flow to the fault from

each side to the two half windings, and will be divided between two phases of the system. The individual phase currents may therefore be relatively low, resulting in difficulties in providing protection.

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Phase to Phase FaultPhase to Phase FaultFault between phases with in Fault between phases with in

transformer are relative rare; if such a fault transformer are relative rare; if such a fault does occur if will give rise to a substantial does occur if will give rise to a substantial current comparable to earth fault currents.current comparable to earth fault currents.


InterturnInterturn faultsfaultsIn low voltage transformers, interturn

insulation breakdown is unlikely to occur unless the mechanical force on winding due to external short circuits has caused insulation degradation, or insulating oil has caused contaminated by moisture.

In high voltage transformers, connected to an overhead transmission system will be subjected to steep fronted impulse voltages, arising from lightning strikes, faults and switching operations, caused interturn isolation breakdown.

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InterturnInterturn faultsfaultsA short circuit of a few turns of winding

will give rise to a heavy fault current in the short-circuited loop, but the terminal current will be small, because of high ratio of transformer between the whole winding and the short-circuited turns.

TransformerTransformer faultsfaults

Shows the corresponding data for a typical Shows the corresponding data for a typical transformer 3.25% impedance with the shorttransformer 3.25% impedance with the short--circuited circuited turns symmetrically located in the centre of the windingturns symmetrically located in the centre of the winding

Prim

ary

curr

ent

(mul

tiple

s of

rate

d cu

rren

t)

Primary input current

Fault current in short circuited turns

Turns short circuited (% of winding)

Faul

t cur

rent

(m

ultip

les

of ra

ted

curr

ent)

100

80

70

60

40

05 10 15 20

2

6

4

8

10

25

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Core faultsCore faultsA conducting bridge across the laminated A conducting bridge across the laminated

structures of the core can permit sufficient eddystructures of the core can permit sufficient eddy--current to flow to cause serious overheating.current to flow to cause serious overheating.

The bolts that clamp the core together are The bolts that clamp the core together are always insulated to avoid this trouble. If any always insulated to avoid this trouble. If any portion of the core insulation become defective, portion of the core insulation become defective, the resultant heating may reach a magnitude the resultant heating may reach a magnitude sufficient to damage the winding.sufficient to damage the winding.

The additional core loss, although causing The additional core loss, although causing severe local heating.severe local heating.


Tank faultsTank faultsLoss of oil through tank leaks will

ultimately produce a dangerous condition, either because of a reduction in winding insulation or because of overheating on load due to the loss of cooling.

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Externally Applied ConditionsExternally Applied ConditionsSources of abnormal stress in a transformer

areOverloadSystem faultsOver voltageReduced system frequency


OverloadOverloadOverload causes increased ‘copper loss’ and

a consequent temperature rise.System faults

System short circuits produce a relatively intense rate of heating of the feeding transformers, the copper loss increasing in proportion to square of the per unit fault current.

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The typical duration of external short-circuits that a transformer can sustain without damage if the current is limited only by the self-reactance is shown in table.

2214.214.277

2216.616.666

22202055

22252544

Permitted fault durationPermitted fault duration(seconds)(seconds)

Fault currentFault current(Multiple of rating)(Multiple of rating)

Transformer reactanceTransformer reactance(%)(%)


Over voltageOver voltageTransient surge voltagesTransient surge voltages

Transient overvoltages arise from faults, switching and lightning disturbances and are liable to cause interturn faults.Power frequency Power frequency overvoltageovervoltage

Power frequency overvoltage causes both an increase in stress on the insulation and a proportionate increase in the working flux, this lead to a rapid temperature rise in the bolts, destroying their insulation if the condition continues.

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Reduced system frequencyReduced system frequencyReduction of system frequency has an effect with Reduction of system frequency has an effect with

regard to flux density, similar to that of regard to flux density, similar to that of overvoltageovervoltage..

If follows that a transformer can operate with some If follows that a transformer can operate with some degree of degree of overvoltageovervoltage with a corresponding increase in with a corresponding increase in frequency, but operation must not be continued with a frequency, but operation must not be continued with a high voltage input and low frequency.high voltage input and low frequency.

Operation can not be sustained when the ratio of Operation can not be sustained when the ratio of voltage to frequency with these quantities given values in voltage to frequency with these quantities given values in per unit of their rated valued, exceeds unity by more than per unit of their rated valued, exceeds unity by more than a small amount, for instance if V/f = 1.1a small amount, for instance if V/f = 1.1


MagnetisingMagnetising inrush currentinrush currentThe phenomenon of The phenomenon of magnetisingmagnetising inrush inrush

is a transient condition that occurs is a transient condition that occurs primarily when a transformer is energized. primarily when a transformer is energized.

It is not a fault condition, and therefore It is not a fault condition, and therefore transformer protection must remain stable transformer protection must remain stable during the inrush transient.during the inrush transient.

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MagnetisingMagnetising inrush currentinrush current

Magnetizing current

Flux

Normal peak flux

Flux

and

Vol

tage

Time

Transient flux 80% residual at switching

Transient flux no residual at switching

Steady state flux

Typical Typical magnetisingmagnetising characteristiccharacteristic Steady and maximum offset fluxesSteady and maximum offset fluxes


MagnetisingMagnetising inrush current inrush current Under normal steady-state conditions the

magnetising current associated with the operation flux level is relative small.

Flux

and

Vol

tage

Time



Steady state flux

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MagnetisingMagnetising inrush currentinrush currentHowever, if a transformer winding is energized at a

voltage zero, with no remanent flux, the flux level during the first voltage cycle (2* normal flux) will result in core saturation and a high non-sinusoidal magnetising current waveform.

Flux

and

Vol

tage

Time



Steady state flux


MagnetisingMagnetising inrush currentinrush currentThe energizing conditions that result in an offset

current produce a waveform that is asymmetrical. Such a wave typically contains both even and odd harmonics.

Zero axis

Slow decrement

Typical inrush currentTypical inrush current

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MagnetisingMagnetising inrush currentinrush currentTypical inrush currents contain substantial amounts of second

and third harmonics and diminishing amounts of higher order.

Zero axis

Slow decrement


77thth

2.4%2.4%66thth

3.7%3.7%55thth

4.1%4.1%44thth

5.1%5.1%33rdrd

26.8%26.8%22ndnd

63%63%DCDC

55%55%Component Component typical valuetypical value

Harmonic contentHarmonic content


Zero axis

Slow decrement


MagnetisingMagnetising inrush currentinrush currentThis current is referred to as magnetising

inrush and may persist for several cycles.

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Transformer ProtectionTransformer ProtectionThe problems relating to transformers require some means of protThe problems relating to transformers require some means of protection.ection.

In the table, summaries the problems and the possibIn the table, summaries the problems and the possible form of le form of protection that may be used.protection that may be used.

ThermalThermalOverheatingOverheatingOverfluxingOverfluxingOverfluxingOverfluxing

Differential; Buchholz; TankDifferential; Buchholz; Tank--EarthEarthTank FaultTank FaultDifferential; BuchholzDifferential; BuchholzCore FaultCore FaultDifferential; BuchholzDifferential; BuchholzInterturnInterturn FaultFault

Differential; Restricted Earth FaultDifferential; Restricted Earth FaultSecondary winding PhaseSecondary winding Phase--earth faultearth faultDifferentialDifferentialSecondary winding PhaseSecondary winding Phase--phase faultphase fault

Differential; Differential; OvercurrentOvercurrentPrimary winding PhasePrimary winding Phase--earth faultearth faultDifferential; Differential; OvercurrentOvercurrentPrimary winding PhasePrimary winding Phase--phase faultphase fault

Protection UsedProtection UsedFault TypeFault Type


Transformer over current protectionTransformer over current protectionFuses: Fuses: Fuses commonly protect small distribution transformers typically up to ratings of 1 MVA at distribution voltages.

The fuse must have a rating well above the maximum transformer load current in order to withstand the short duration overloads that may occur. Also, the fuses must withstand the magnetisinginrush currents drawn when power transformers are energized.

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Transformer over current protectionTransformer over current protectionOvercurrentOvercurrent relays: relays: overcurrent relays are also used on larger transformers provided withstand circuit breaker control.

The time delay characteristic should be chosen to discriminate with circuit protection on the secondary side.

Transformer ProtectionTransformer ProtectionRestricted earth fault protectionRestricted earth fault protection

This is particularly the case for a star-connected winding with an impedance-earthed neutral, because of faults in the winding produce very little current in the primary winding, making fault detection by primary current measurement difficult.

This is a unit protection scheme for one winding of the transformer. If can be of the high impedance type or of the biased low impedance type.

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Restricted earth fault protection Restricted earth fault protection For the high-impedance type, the residual current of

three current transformers is balance against the output of current transformer in neutral conductor.

High Impedance relay

I >

Transformer ProtectionTransformer ProtectionRestricted earth fault protectionRestricted earth fault protection

In the biased low-impedance version, the three phase currents and neutral current become the bias inputs to a differential element.

The system is operative for faults with in the region between current transformers, that is, for faults on the star winding in question. The system remain stable for all faults outside this zone.

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Differential protectionDifferential protectionA differential system can be arranged to cover the

complete transformer.

Id >


Differential protectionDifferential protectionThe principle, current transformers on the primary and

secondary sides are connected to form a circulating current system.

Id >

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Transformer ProtectionTransformer ProtectionDifferential protectionDifferential protection

In applying the principles of differential protection to transformers, a variety of considerations have to be taken to account.

Correction for possible phase shift across the transformer winding. (phase correction)The effects of the variety of earthing and winding arrangements. (filter of zero sequence currents)Correction for possible unbalance of signals from CT’s on either side of the winding. (ratio correction)The effect of magnetising inrush during initial energization. The possible occurrence of overfluxing.

Differential protectionDifferential protection

Phase correctionPhase correctionCorrect operation of transformer differential

protection requires that the transformer primary and secondary currents, are measured by the relay, are in phase.

If the transformer is connected delta/star, balance three-phase through current suffers a phase change of 30 degree.

If left uncorrected, this phase difference would lead to the relay seeing through current as an unbalanced fault current, and result in relay operation.

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Phase correctionPhase correction

Id > Id > Id >

Differential protection for two-winding delta/star transformer


Phase correctionPhase correctionElectromechanical and static relays use appropriate

CT/ICT connections to ensure that the primary and secondary current applied to the relay are in phase.

For digital and numerical relays, it is common to use star-connected line CT’s on all windings of the transformer and compensate for the winding phase shift in software.

Depending on relay design, the only data required in such circumstances may be the transformer vector group designation. Phase compensation is then performed automatically.

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Filtering of zero sequence currentFiltering of zero sequence currentThe differential protection will see zero

sequence differential current for an external fault and if could incorrectly operate as a result.

This is to ensure that out-of-zone earth faults are not seen by the transformer protection as an in-zone-fault.

This is achieved by use of delta-connected line CT’s or interposing CT’s for older relays. For digital/numerical relays, the required filtering is applied in relay software.


Ratio correctionRatio correctionCorrect operation of the differential element

requires that currents in the differential element balance under load and through fault conditions.

As the primary and secondary line CT’s ratios may not exactly match the transformer rated winding currents, digital/numerical relays are provided with ratio connection factors for each of CT inputs.

The connection factors may be calculated automatically by the relay from knowledge of the line CT ratios and the transformer MVA rating.

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Bias settingBias settingBias is applied to transformer differential protection for

the same reasons as any unit protection scheme to ensure stability for external faults while allowing sensitive settings to pick up internal faults.

Some relays use a bias characteristic with three sections. The first section is set higher than the transformer magnetising current. The second section is set to allow for off-nominal tap settings, while the third has a larger bias slope beginning well above rated current to cater for heavy through-fault condition.

Differential protectionDifferential protectionBias settingBias setting

0 1 2 3 4 5 6

2

3

1 30% slope

70% sl

ope

Operate

Restrain

Effective bias (*In)

Diff

eren

tial c

urre

nt (*

Id)

Setting range

(0.1-0.5Id)

Typical bias characteristicTypical bias characteristic

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Transformer with multiple windingTransformer with multiple windingThe unit protection principle remains valid

for a system having more than two connections, so a transformer with three or more windings can still be protected by the same application.


Transformer with multiple windingTransformer with multiple windingWhen the power transformer has only one of its

three winding connected to a source of supply with the other two winding feeding loads, a relay with only two sets of CT inputs can be used.

Id >

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Transformer with multiple windingTransformer with multiple windingWhen more than one source of fault current infeed exists,

These is a danger in the scheme of current circulating between the two paralleled sets of CT’s without producing any bias it is therefore important a relay is used with separate CT input for the two secondaries.

Id >


Transformer with multiple windingTransformer with multiple windingWhen the third winding consists of a delta-connected tertiary

with no connections brought out, the transformer may be regardedas a two winding transformer for protection purpose and protected.

Id >

∆

YY

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StabilisationStabilisation during during magnetisingmagnetising inrush conditioninrush conditionThe inrush current, although generally

resembling an in-zone fault current, differs greatly when the waveforms are compared. The difference in the waveforms can be used to distinguish between the conditions.

Normal fault currents do not contain second or other even harmonics.

The output of a CT that is energized into steady state saturation will contain odd harmonics but not even harmonics.


StabilisationStabilisation during during magnetisingmagnetising inrush conditioninrush conditionThe second harmonic is therefore an

attractive basis for a stabilising bias against inrush effects. The differential current is passed through a filter that extracts the second harmonics.

This component is then applied to produce a restraining quantity sufficient to overcome the operating tendency due to the whole of the inrush current that flows in the operating circuit.

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OverfluxingOverfluxing protectionprotectionOverfluxing arises principally from the

following system conditions.High system voltageLow system frequencyGeomagnetic disturbances

The latter result in low frequency earth currents circulating through a transmission system.

Transformer ProtectionTransformer ProtectionOverfluxingOverfluxing protectionprotection

Since momentary system disturbances can cause transient overfluxing that is not dangerous time delay tripping is required.

The protection is initiated if a defined V/f threshold is exceeded.

Geomagnetic disturbance may result in overfluxing without the V/f threshold being exceeded. Some relays provide a 5th harmonic detection feature, which can be used to detect such a condition, as levels of this harmonic rise under overfluxing conditions.

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Oil and gas deviceOil and gas deviceAll faults below oil on an oil-immersed

transformer result in localised heating and breakdown of the oil; some degree of arcing will always take place in a winding fault and the resulting decomposition of the oil will release gases.


Buchholz protectionBuchholz protectionBuchholz protection is normally provided on

all transformers fitted with a conservator. A typical Buchholz relay will have two sets of

contacts. One is arranged to operate for slow accumulations of gas, the other for bulk displacement of oil in the event of a heavy internal fault.

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Buchholz protectionBuchholz protectionConservator

Transformer

76mm typical

Buchholz relay mounting arrangement

Transformer ProtectionTransformer ProtectionBuchholz protectionBuchholz protection

The device will therefore give an alarm for following fault conditions, all of which are of a low order of urgency.

Hot spots on the core due to short circuit of Hot spots on the core due to short circuit of lamination insulation.lamination insulation.Core bolt insulation failureCore bolt insulation failureFaulty jointsFaulty jointsInterturnInterturn faults or other winding faults involving faults or other winding faults involving only lower power only lower power infeedsinfeedsLoss of oil due to leakageLoss of oil due to leakage

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Buchholz protectionBuchholz protectionWhen a major winding fault occurs, this causes a

surge of oil, which displaces the lower float and thus cause isolation of transformer.

This action will take place forAll severe winding faults, either to earth or interphase.Loss of oil if allowed to continue to a dangerous degree.


Neutral displacementNeutral displacementAn earth fault occurring on the feeder

connected to an unearthed transformer winding should be cleared by the feeder circuit, but if there is also a source of supply on the other side of the transformer, the feeder may be still live.

The feeder will then be a local unearthed system, and if the earth fault continues in an arcing condition, dangerous overvoltages may occur.

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Neutral displacementNeutral displacementA voltage relay is energized from the brokenA voltage relay is energized from the broken--delta connected secondary delta connected secondary

winding of a voltage transformer on the high voltage line, and rwinding of a voltage transformer on the high voltage line, and receives an input eceives an input proportion to the zero sequence voltage of the line, that is, toproportion to the zero sequence voltage of the line, that is, to any displacement of the any displacement of the neutral point.neutral point.

Residual voltage relayUrsd


Neutral displacementNeutral displacementThe relay normally ,receives zero voltage but, in the presence The relay normally ,receives zero voltage but, in the presence

of an earth fault, the brokenof an earth fault, the broken--delta voltage will rise to three times the delta voltage will rise to three times the phase voltage.phase voltage.

Residual voltage relayUrsd

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Example of transformer protection settingExample of transformer protection setting

Id >

∆ YPrimary CT’s Secondary CT’s

Primary ICT’s Secondary ICT’sUnit protection relay

A deltaA delta--star Dyn1, 25 MVA 115/22 kV transformer with the star Dyn1, 25 MVA 115/22 kV transformer with the differential relay without phase and ratio compensation softwaredifferential relay without phase and ratio compensation softwareimplemented.implemented.

Transformer ProtectionTransformer ProtectionExample of transformer protection settingExample of transformer protection setting

Phase compensationPhase compensation

Id >

∆ Y


0 -30

Yy0

Yy00

Yy0

0Yd11

For simplicity, the CTFor simplicity, the CT’’s on the primary and secondary windings of the s on the primary and secondary windings of the transformer are connected in star.transformer are connected in star.

Selection of Yy0 connection for the primary side Selection of Yy0 connection for the primary side ICTICT’’ss and Yd11 (+30 ) for the and Yd11 (+30 ) for the secondary side secondary side ICTICT’’ss provides the required phase shift and the zeroprovides the required phase shift and the zero--sequence trap sequence trap on the secondary side.on the secondary side.

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Ratio correctionRatio correction

Id >

∆ Y

Primary CT’s

Yy0 300/5

Secondary CT’s

Yy0 1200/5


Yy0 Yd11


Ratio correctionRatio correctionHigh side full load current on secondary of main High side full load current on secondary of main CTCT’’s is s is

25 MVA

3 * 115 kV

5

300*

= 2.0919 Amp.

Select full load current via ICT’s (Yy0) to relay nearest 3 Amp.

So, full load current to relay is

So, select turn ratio of ICT’s Yy0 = 62/42 Turns

= 2.0919 6242*

= 3.0881 Amp.

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Ratio correctionRatio correctionLow side full load current on secondary of main Low side full load current on secondary of main CTCT’’s iss is

25 MVA

3 * 22 kV

5

1200*

= 2.7337 Amp.

Select full load current via ICT’s (Yd11) to relay nearest 3 Amp.

So, full load current to relay is

So, select turn ratio of ICT’s Yd11 = 31/48 Turns

= 2.7337 3148*

= 3.0579 Amp.

3*


Bias settingBias setting

Id >

∆ Y

Primary CT’s

Yy0 300/5

Secondary CT’s

Yy0 1200/5


Yy0 62/42 T Yd11 31/48 T

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Transformer ProtectionTransformer ProtectionBias settingBias setting

% Mismatch of full load current between two side of % Mismatch of full load current between two side of transformer istransformer is

3.0881 – 3.05793.0881 + 3.0579

2

* 100 %

= 0.98064 %

% of CT% of CT’’s error approximately 10 %s error approximately 10 %

% of on load tap change of transformer approximate 10 %% of on load tap change of transformer approximate 10 %

% of Total mismatch = 20.98 % of 3.073 Amp% of Total mismatch = 20.98 % of 3.073 AmpIf relay rated 5 Amp so =If relay rated 5 Amp so = 12.897 % of In.12.897 % of In.

A current setting of 20% of rated relay current is A current setting of 20% of rated relay current is recommended. The most relay have a dual slope bias recommended. The most relay have a dual slope bias characteristic with fixed bias slope setting about 20% up to ratcharacteristic with fixed bias slope setting about 20% up to rated ed current and about 80% above that level.current and about 80% above that level.

Transformer Protection

Documents

transformer winding

primary winding fault

transformer secondary

primary fault current

primary earth fault

highest fault current

distance of fault

fault detection