transformer protection2009 9-2

7/28/2019 Transformer Protection2009 9-2

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Introduction The development of modern power systems has

been reflected in the advances in transformerdesign. This has resulted in a wide range oftransformers with sizes ranging from a few kVA toseveral hundred MVA being available for use in a

wide variety of applications.

The considerations for a transformer protectionpackage vary with the application and importanceof the transformers.


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Introduction Small distribution transformers can be protected

satisfactorily, from both technical and economicconsiderations, by the use of fuse or over current relay.

This result in time-delayed protection. However, time-delayed fault clearance is unacceptable

on larger power transformers, due to systemoperation/stability and cost


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Introduction Transformer faults are generally classified into six

categories: Winding and terminal faults

Core faults

Tank and transformer accessory faults

On-load tap changer faults

Abnormal operation conditions

Sustained or uncleared external faults


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Transformer faultsWinding fault

A fault on transformer winding is controlled inmagnitude by the following factor:

Source impedance

Neutral earthing impedance

Transformer leakage reactance

Fault voltage

Winding connection


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Transformer faults Star-Connected Winding with Neutral Point Earthed

through an impedance.

The winding earth fault current depends on the earthingimpedance value and is also proportional to the distanceof the fault from neutral point, since the fault voltage

will be directly proportional to this distance


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Transformer faults

Earth fault current in resistance-earth star winding


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Transformer faults

Earth fault current in solidly earthed starwinding


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Transformer faultsPhase to Phase Fault

Fault between phases with in transformerare relatively rare; if such a fault doesoccur it will give rise to a substantialcurrent comparable to earth fault

currents.


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Transformer faults Interturn faults

In low voltage transformers, Interturn insulationbreakdown is unlikely to occur unless the mechanicalforce on winding due to external short circuits hascaused insulation degradation, or insulating oil hascaused contaminated by moisture.

In high voltage transformers, connected to an

overhead transmission system will be subjected to steepfronted impulse voltages, arising from lightning strikes,faults and switching operations, caused Interturnisolation breakdown.


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Transformer faults Interturn faults

A short circuit of a few turns of winding will give riseto a heavy fault current in the short-circuited loop, butthe terminal current will be small, because of high ratioof transformer between the whole winding and theshort-circuited turns.


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Transformer faults Core faults

A conducting bridge across the laminated structuresof the core can permit sufficient eddy-current to flow tocause serious overheating.

The bolts that clamp the core together are alwaysinsulated to avoid this trouble. If any portion of the coreinsulation become defective, the resultant heating may

reach a magnitude sufficient to damage the winding.

The additional core loss, although causing severe localheating.


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Transformer faultsTank faults

Loss of oil through tank leaks willultimately produce a dangerous condition,either because of a reduction in windinginsulation or because of overheating on

load due to the loss of cooling


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Transformer faults External Applied Conditions

Sources of abnormal stress in a

transformer are: Overload

System faults

Over voltage

Reduced system frequency


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Transformer faults Overload

Overload 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 inproportion to square of the per unit fault current.


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Transformer faults

Transformer

reactance (%)

Fault current

(Multiple of rating

Permitted fault

duration

(seconds)

4 25 25 20 2

6 16.6 2

7 14.2 2

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


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Transformer faults Over voltage

Transient surge voltages Transient overvoltages arise from faults, switching

and lightning disturbances and are liable to causeinterturn faults.

Power frequency overvoltagePower frequency overvoltage causes both an increase in

stress on the insulation and a proportionate increase inthe working flux, this lead to a rapid temperature rise inthe bolts, destroying their insulation if the conditioncontinues.


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Transformer faults Reduced system frequency

Reduction of system frequency has an effect with regardto flux density, similar to that of overvoltage.

If follows that a transformer can operate with somedegree of overvoltage with a corresponding increase infrequency, but operation must not be continued with ahigh voltage input and low frequency.

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


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Transformer faults Magnetizing inrush current

The phenomenon of magnetizing inrush is a

transient condition that occurs primarily when atransformer is energized.

It is not a fault condition, and therefore transformerprotection must remain stable during the inrush

transient.


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Magnetizing inrush When a transformer is first energized, a transient

magnetizing or exciting inrush current may flow. Thisinrush current, which appears as an internal fault tothe differentially connected relays, may reach

instantaneous peaks of 8 to 30 times those for fullload.


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The factors controlling the duration and

magnitude of the magnetizing inrush The factors controlling the duration and magnitude of

the magnetizing inrush are:

Size and location of the transformer bank

Size of the power system Resistance in the power system from the source to the

transformer bank

Type of iron used in the transformer core and itssaturation density

Prior history, or residual flux level, of the bank

How the bank is energized


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Magnetizing inrush current


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Transformer faults


Under normal steady-state conditions the magnetizing currentassociated with the operation flux level is relative small


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Transformer faults

Magnetizing inrush currentHowever, if a transformer winding is energized at a voltage zero, with

no remnant flux, the flux level during the first voltage cycle (2* normalflux) will result in core saturation and a high non-sinusoidalmagnetizing current waveform

T f f lt


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Transformer faults


The energizing conditions that result in an offset current produce awaveform that is asymmetrical. Such a wave typically contains botheven and odd harmonics.


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Transformer faults

Harmonic content

Componenttypical value

DC55%

2nd63%

3rd26.8%

4th5.1%

5th4.1%

6th3.7%

7th2.4%


Typical inrush currents contain substantial amounts of second and third

harmonics and diminishing amounts of higher order.


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Transformer faults


This current is referred to as magnetizing inrush and maypersist for several cycles.


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Transformer Protection

The problems relatingto transformersrequire some means

of protection. In thetable, summaries theproblems and thepossible form of

protection that maybe used.

Fault Type Protection Used

Primary winding

Phase-phase fault

Differential;

Overcurrent

Primary winding

Phase-earth fault

Differential;

Overcurrent

Secondary windingPhase-phase fault

Differential

Secondary winding

Phase-earth fault

Differential;

Restricted Earth

Fault

Interturn Fault Differential; Bucholz

Core Fault Differential; Bucholz

Tank Fault Differential,

Bucholz; Tank-Earth

Overfluxing Overfluxing

Overheating Thermal


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Transformer Protection Transformer over current protection

Fuses: Fuses commonly protect small distributiontransformers typically up to ratings of 1 MVA at

distribution voltages.The fuse must have a rating well above the maximumtransformer load current in order to withstand the shortduration overloads that may occur. Also, the fuses must

withstand the magnetizing inrush currents drawn whenpower transformers are energized.


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Transformer protection Transformer over current protection.

Overcurrent relays: overcurrent relays are also usedon larger transformers provided withstand circuitbreaker control.

The time delay characteristic should be chosen todiscriminate with circuit protection on the secondary

side.


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Transformer protection Restricted earth fault protection

This is particularly the case for a star-connectedwinding with an impedance-earthed neutral,because of faults in the winding produce very littlecurrent in primary winding, making faultdetection by primary current measurementdifficult.

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


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Transformer protection

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

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


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Transformer protection Restricted earth fault protection

In the biased low-impedance version, the three

phase current and neutral current become the biasinput to a differential element.

The system is operative for fault with in theregion between current transformers, that is the

fault on the star winding in question. The systemremain stable for all fault outside this zone.


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Transformer protection Differential protection

A differential system can be arranged to cover the complete transformer.


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The principle current transformer on the primary and secondary side

are connected to form a circulating current system.


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In applying the principles of differential protection totransformer, a variety of consideration have to be taken to

account.Correction for possible phase shift across the transformer winding(phase correction)

The effect of the Varity of earthing and winding arrangement.( filterof zero sequence currents)

Correction for possible unbalance of single from CTs on either sideof the winding . (ration correction )

The effect of magnetising inrush during initial energization.

The possible occurrence of overfluxing.


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Differential protection Phase correctioncorrect operation of transformer differential

protection requires that the transformer primaryand secondary current, are measured by the relay,are in phase.

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

If left uncorrected, this phase difference wouldlead to the relay seeing through current as anunbalanced fault current, and result in relayoperation.


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Differential protection

Phase correction


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Differential protection Phase correction

Electromechanical and static relay use appropriateCT/ICT connections to ensure that the primary and

secondary current applied to the relay are in phase.For digital and numerical relay, it is common to use star-

connected line CTs on all winding the transformer andcompensate for the winding phase shift in software.

Depending on relay design, the only data required in suchcircumstances may be the transformer vector groupdesignation. Phase compensation is then performedautomatically.


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Differential protection Filtering of zero sequence current

The differential protection will see zero sequence

differential current for an external fault and ifcould incorrectly operate as a result.

This is achieved by use of delta-connected lineCTs or interposing CTs for older relays. For

digital/numerical relays, the required filtering isapplied in relay software.


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Differential protection Ratio correction

Correct operation of the differential elementrequires that current in the differential element

balance under load and through fault conditions.As the primary and secondary line CTs ration

may not exactly match the transformer areprovided with ratio correction factors for each ofCT inputs.

The connection factors may be calculatedautomatically by the relay from knowledge of theline CT ratio and the transformer MVA rating.


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Differential protection Bias setting

Bias is applied to transformer differential protection forthe same reason as any unit protection scheme to ensure

stability for external fault while allowing sensitive setting topick up internal faults.

Some relay use a bias characteristic with three sections.The first section is set higher than the transformermagnetising current. The second section is set to allow for

off-nominal tap setting while the third has larger bi asslope beginning well above rated current to cater for heavythrough-fault condition.


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Bias setting


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Differential protection Transformer with multiple winding

The unit protection principle remains valid for asystem having more than two connections, so atransformer with three or more winding can still beprotected by the same application.


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Transformer with multiple winding

when the power transformer has only one of its three windingconnected to a source of supply with the other two winding feedingload, a relay with only two sets of CT input can be used.


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when more than one source of fault current in feed exists, these is adanger in the scheme of current circulating between the two paralleledset of CTs without producing any bias it is therefore important a relayis used with separate CT input for the two secondaries.


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Differential Protection


when the third winding consists of a delta-connections brought out, thetransformer may be regarded as a two winding transformer for protectionpurpose and protection .


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Differential protection Stabilisation during magnetizing inrush conditionThe inrush current, although generally resembling

an in-zero fault current, differs greatly when thewaveform are compared. The difference in the

waveform can be used to distinguish between theconditions.

Normal fault current do not contain second or othereven harmonics.

The output of a CT that is energized into steady state saturation

will contain odd harmonics but even harmonics.


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Differential protection Stabilisation during mangnetising inrush

conditionThe second harmonic is therefore an

attractive basis for a stabilising bias againstinrush effect. The differential current is passed

through a filter that extracts the second

harmonics.

This component is then applied to produce a restrainingquantity sufficient to overcome the operating tendencydue to the whole of the inrush current that flows in theoperating circuit.


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Transformer protection Over fluxing protection

Over fluxing arises principally from the following

system conditions. High system voltage

Low system frequency

Geomagnetic disturbances

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


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Transformer protection Over fluxing protectionSince momentary system disturbance can cause

transient over fluxing that is not dangerous time delay

tripping is required.The protection is initiated if a defined V/f threshold isexceeded.

Geomagnetic disturbance may result in over fluxingwithout the v/f being exceeded. Some relays provide a 5th

harmonic detection features, which can be used to detectsuch a condition, as levels of this harmonic rise under overfluxing conditions.


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Oil 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 composition of the oilwill release gas.


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Buchholz protection Buchholz protection is normally provided on all

transformers fitted with a conservator.

A typical buchholz relay will have two contacts. One isarranged to operate for low accumulation of gas, theother for bulk displacement of oil in the event of aheavy internal faults.


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Buchholz protection


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Buchholz protection The device will therefore see the following fault

conditions, all of which are of low order ofurgency.

Hot spots on the core due to short circuit oflamination insulation

Core bolt insulation failure

Faulty joints

Inter turn faults or other winding faults involvingonly lower power in feeds

Loss of oil due to leakage


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Buchholz protectionWhen a major winding fault occurs, this causes a surgeof oil which displaces the lower float and thus causeisolation of transformer.

This action will take place for All severe winding faults, either to earth or interphase

Loss of oil if allowed to continue to a dangerousdegree.


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BUCHHOLZ

alarms for Local winding overheating -alarm

Local core overheating (short circuited

laminations) Bad contacts or joints

Partial discharge

Broken down core bolt insulation


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BUCHHOLZ

trips for Detection of loss of or low oil due to

1.Leaky pipe joints

2.Tank faults

3. Contraction of oil under low

temperatures and light load

major internal faults (inter-turn faults or

faults involving earth) which result in oil

surges to the conservator.


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Over temperature Generally regarded as overload protection also deals

with failure of or interference with pumps and fans orshutting of valves to pumps

Winding hot spot temperature is the main issue, butboth oil and winding temperature are usuallymeasured and used to:

initiate an alarm

trip circuit breakers

control fans and pumps


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Over temperature Two temperatures must be monitored:

Winding temperature(WTI) -(short

thermal ) this can rise rapidly, without

much of an increase in oil temperature

Oil temperature (OTI) -(long thermal )

this can rise slowly to a critical point

without an unacceptable winding

temperature increase


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Typical alarm and trip levelswinding alarm - 90C to 110C

winding trip - 110C to 135C

oil alarm - 80C to 95C

oil trip - 95C to 115C


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OVERCURRENT & EARTH FAULT

PROTECTION RELAYSUsed in transformers up to approximately

50MVA

For 10MVA transformers provides mainprotection

For 50MVA transformers provides backup

protection onlyCommon at voltages up to about 66kV


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Over current (O/C) ProtectionAn over current relay sees phase currents

and hence all types of fault

Over current relay settings must be above transformer emergency overload as with

fuses, this determines the fundamental

limit to their sensitivity


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Over current (O/C) ProtectionA suitable margin should also be allowed in the

current setting for:

growth in load -always relay reset ratio -optional

cold load pick-up -optional (often a relay

feature)

transformer taps -optional


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Over current (O/C) ProtectionAn instantaneous O/C element can

usually be used to provide very fast

clearance for faults close to the HV terminal

Must be set such that LV faults are not

seen -discrimination


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Earth Fault (E/F) ProtectionAn earth fault (E/F) relay sees either

transformer neutral or residual (sum of

three phases) current, depending on CTlocationhence sees earth faults only.

E/F relays can be set well below load 10% of

load typical.


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Interposing current transformers


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transformer protection2009 9-2

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