IntroductionThe development of modern power systems has been reflected in the advances in transformer design. This has resulted in a wide range of transformers with sizes ranging from a few kVA to several hundred MVA being available for use in a wide variety of applications.The considerations for a transformer protection package vary with the application and importance of the transformers.

IntroductionSmall distribution transformers can be protected satisfactorily, from both technical and economic considerations, 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 system operation/stability and cost

IntroductionTransformer faults are generally classified into six categories:Winding and terminal faultsCore faultsTank and transformer accessory faultsOn-load tap changer faultsAbnormal operation conditionsSustained or uncleared external faults

Transformer faultsWinding faultA fault on transformer winding is controlled in magnitude by the following factor:Source impedanceNeutral earthing impedanceTransformer leakage reactanceFault voltageWinding connection

Transformer faultsStar-Connected Winding with Neutral Point 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

Transformer faults

Earth fault current in resistance-earth star winding

Transformer faults

Star-connected winding with neutral point earthed through an impedance

Transformer faultsStar-Connected winding with Neutral Point Earthed through an impedanceFor 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.

Star-Connected Winding With Neutral Point Solidly EarthedThe 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 result in the highest fault current.

Transformer faults

Earth fault current in solidly earthed star winding

Transformer faultsStar-Connected Winding With Neutral Point Solidly EarthedThe 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.

Transformer faultsDelta 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 that this value if the source or system earthing impedance is appreciable.

Transformer faultsDelta 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.

Transformer faultsPhase to Phase FaultFault between phases with in transformer are relative rare; if such a fault does occur it will give rise to a substantial current comparable to earth fault currents.

Transformer faultsInterturn faultsIn 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.

Transformer faultsInterturn faultsA 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.

Transformer faults

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

Transformer faultsCore faultsA conducting bridge across the laminated structures of the core can permit sufficient eddy-current to flow to cause serious overheating.The bolts that clamp the core together are always insulated to avoid this trouble. If any portion of the core insulation become defective, the resultant heating may reach a magnitude sufficient to damage the winding.The additional core loss, although causing severe local heating.

Transformer 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

Transformer faultsExternal Applied ConditionsSources of abnormal stress in a transformer are:OverloadSystem faultsOver voltageReduced system frequency

Transformer faultsOverloadOverload causes increased copper loss and a consequent temperature rise.System faultsSystem short circuits produce a relativelyintense rate of heating of the feedingtransformers, the copper loss increasing inproportion to square of the per unit fault current.

Transformer faults

Transformer reactance (%)Fault current (Multiple of ratingPermitted fault duration (seconds)42525202616.62714.22The 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

Transformer faultsOver voltageTransient surge voltagesTransient overvoltages arise from faults, switching and lightning disturbances and are liable to cause interturn faults.Power frequency overvoltagePower 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.

Transformer faultsReduced system frequency Reduction of system frequency has an effect with regard to flux density, similar to that of overvoltage. If follows that a transformer can operate with some degree of overvoltage with a corresponding increase in frequency, but operation must not be continued with a high voltage input and low frequency. Operation can not be sustained when the ratio of voltage to frequency with these quantities given values in per unit of their rated valued, exceeds unity by more than a small amount, for instance if V/f = 1.1

Transformer faultsMagnetizing inrush current The phenomenon of magnetizing inrush is a transient condition that occurs primarily when a transformer is energized. It is not a fault condition, and therefore transformer protection must remain stable during the inrush transient.

Magnetizing inrush current

Transformer faults

Magnetizing inrush currentUnder normal steady-state conditions the magnetizing current associated with the operation flux level is relative small

Transformer faults

Magnetizing inrush current However, if a transformer winding is energized at a voltage zero, with no remnant flux, the flux level during the first voltage cycle (2* normal flux) will result in core saturation and a high non-sinusoidal magnetizing current waveform

Transformer faults

Magnetizing inrush 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.

Transformer faults

Harmonic contentComponent typical valueDC 55%2nd 63%3rd 26.8%4th 5.1%5th 4.1%6th 3.7%7th 2.4%Magnetizing inrush current Typical inrush currents contain substantial amounts of second and third harmonics and diminishing amounts of higher order.

Transformer faults

Magnetizing inrush current This current is referred to as magnetizing inrush and may persist for several cycles.

Transformer ProtectionThe problems relating to transformers require some means of protection. In the table, summaries the problems and the possible form of protection that may be used.

Fault TypeProtection UsedPrimary winding Phase-phase faultDifferential; OvercurrentPrimary winding Phase-earth faultDifferential; OvercurrentSecondary winding Phase-phase faultDifferentialSecondary winding Phase-earth faultDifferential; Restricted Earth FaultInterturn FaultDifferential; BucholzCore FaultDifferential; BucholzTank FaultDifferential, Bucholz; Tank-EarthOverfluxingOverfluxingOverheatingThermal

Transformer ProtectionTransformer over current protectionFuses: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 magnetizing inrush currents drawn when power transformers are energized.

Transformer protection Transformer over current protection. Overcurrent 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 protection Restricted 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 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 the high impedance type or the biased low impedance type.

Transformer protection

Restricted earth fault protection For the high-impedance type, the residual current of three current transformer is balance against the output of current transformer in neutral conductor.

Transformer protection Restricted earth fault protection In the biased low-impedance version, the three phase current and neutral current become the bias input to a differential element. The system is operative for fault with in the region between current transformers, that is the fault on the star winding in question. The system remain stable for all fault outside this zone.

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

Transformer protection Differential protection The principle current transformer on the primary and secondary side are connected to form a circulating current system.

Transformer protection Differential protection In applying the principles of differential protection to transformer, 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.( filter of zero sequence currents) Correction for possible unbalance of single from CTs on either side of the winding . (ration correction ) The effect of magnetising inrush during initial energization. The possible occurrence of overfluxing.

Differential protection Phase correction correct operation of transformer differential protection requires that the transformer primary and secondary current, 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.

Differential protection

Phase correction

Differential protection Phase correction Electromechanical and static relay use appropriate Ct/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 al winding 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.

Differential protection Filtering of zero sequence current The differential protection will see zero sequence differential current for an external fault and if could incorrectly operate as a result. This is achieved by use of delta-connected line CTs or interposing CTs for older relays. For digital/numerical relays, the required filtering is applied in relay software.

Differential protection Ratio correction Correct operation of the differential element requires 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 are provided with ratio correction factors for each of CT inputs. The connection factors may be calculated automatically by the relay from knowledge of the line CT ratio and the transformer MVA rating.

Differential protection Bias setting Bias is applied to transformer differential protection for the same reason as any unit protection scheme to ensure stability for external fault while allowing sensitive setting to pick up internal faults. Some relay 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 setting while the third has larger bi as slope beginning well above rated current to cater for heavy through-fault condition.

Differential protection Bias setting

Differential protection Transformer with multiple winding The unit protection principle remains valid for a system having more than two connections, so a transformer with three or more winding can still be protected by the same application.

Differential protection

Transformer with multiple winding when the power transformer has only one of its three winding connected to a source of supply with the other two winding feeding load, a relay with only two sets of CT input can be used.

Differential protection

Transformer with multiple winding when more than one source of fault current infeed exists, these is a danger in the scheme of current circulating between the two paralleled set of CTs without producing any bias it is therefore important a relay is used with separate CT input for the two secondaries.

Differential Protection

Transformer with multiple winding when the third winding consists of a delta-connections brought out, the transformer may be regarded as a two winding transformer for protection purpose and protection .

Differential protection Stabilisation during magnetizing inrush condition The inrush current, although generally resemblingan in-zero fault current, differs greatly when thewaveform are compared. The difference in thewaveform 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.

Differential protection Stabilisation during mangnetising inrush condition The second harmonic is therefore anattractive basis for a stabilising bias againstinrush effect. The differential current is passedthrough a filter that extracts the secondharmonics.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.

Transformer protectionOver 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 currents circulating through a transmission system.

Transformer protectionOver fluxing protection Since 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 is exceeded. Geomagnetic disturbance may result in over fluxing without the v/f being exceeded. Some relays provide a 5th harmonic detection features, which can be used to detect such a condition, as levels of this harmonic rise under over fluxing conditions.

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 oil will release gas.

Buchholz protectionBuchholz protection is normally provided on all transformers fitted with a conservator. A typical buchholz relay will have two contacts. One is arranged to operate for low accumulation of gas, the other for bulk displacement of oil in the event of a heavy internal faults.

Buchholz protection

Buchholz protectionThe device will therefore for the following fault conditions, all of which are of low order of urgency.Hot spots on the core due to short circuit of lamination insulationCore bolt insulation failureFaulty jointsInterturn faults or other winding faults involving only lower power in feedsLoss of oil due to leakage

Buchholz 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 for All severe winding faults, either to earth or interphase Loss of oil if allowed to continue to a dangerous degree.

Neutral displacementAn earth fault occurring on the feeder connected to an unearthed transformer winding should be cleared by the feeder circuit, but if there is a source of supply on the other side of the transformer, the feeder may still be alive. The feeder will then be a local unearthed system, and if the earth fault continues in an arcing condition, dangerous over voltages may occur.

Transformer protection

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