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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
Magnetizing inrush current
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
Magnetizing inrush current
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%
Magnetizing inrush current
Typical inrush currents contain substantial amounts of second and third
harmonics and diminishing amounts of higher order.
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Transformer faults
Magnetizing inrush current
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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Transformer protection Differential protection
The principle current transformer on the primary and secondary side
are connected to form a circulating current system.
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Transformer protection Differential protection
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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Differential protection
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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Differential protection
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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Differential protection
Transformer with multiple winding
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
Transformer with multiple winding
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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Interposing current transformers
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Interposing current transformers
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Transformer protection
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Transformer protection
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