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Home / Technical Articles / Power transformer protection relaying (overcurrent, restricted earth fault & differential)

Transformer protection methods

The problems relating to transformer temperature rise above an assumed maximum ambient temperature require some means of protection. Let’s summarize the problems and the possible forms of transformer protection that may be used.

Power transformer protection methods & relay schemes
Power transformer protection methods & relay schemes

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

It is normal for a modern relay to provide all of the required protection functions in a single package, in contrast to electromechanical types that would require several relays complete with interconnections and higher overall CT burdens.

Table 1 – Transformer fault types/protection methods

No.Fault TypeProtection used
1.Primary winding Phase-Phase faultDifferential; Overcurrent
2.Primary winding Phase-Earth faultDifferential; Overcurrent
3.Secondary winding Phase-Phase faultDifferential
4.Secondary winding Phase-Earth faultDifferential; Restricted Earth Fault (REF)
5.Interturn FaultDifferential; Buchholz
6.Core FaultDifferential; Buchholz
7.Oil tank FaultDifferential; Buchholz; Tank Earth

The following sections in this article provide more detail on the individual protection methods. Note that combined differential and REF, overfluxing, tank-earth and oil/gas protection are described in the next part of this article.


  1. Transformer overcurrent protection
    1. Fuses
    2. Overcurrent relays
  2. Restricted earth fault protection (REF)
  3. Differential protection
    1. Basic considerations
    2. Line current transformer primary ratings
    3. Phase correction
    4. Filtering of zero sequence currents
    5. Ratio correction
    6. Bias setting
    7. Transformers with multiple windings
  4. How to preserve differential protection during magnetizing inrush conditions
    1. Time delay
    2. Harmonic restraint
    3. Inrush detection blocking – Gap detection technique

1. Transformer overcurrent protection

Fuses may adequately protect small transformers, but larger ones require overcurrent protection using a relay and CB, as fuses do not have the required fault breaking capacity.

1.1 Fuses

Fuses commonly protect small distribution transformers typically up to ratings of 1MVA at distribution voltages. In many cases no circuit breaker is provided, making fuse protection the only available means of automatic isolation.

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

High Rupturing Capacity (HRC) fuses, although very fast in operation with large fault currents, are extremely slow with currents of less than three times their rated value. It follows that such fuses will do little to protect the transformer, serving only to protect the system by disconnecting a faulty transformer after the fault has reached an advanced stage.

Table 1 shows typical ratings of fuses for use with 11kV transformers.

Table 2 – Typical fuse ratings for use with distribution transformers

Transformer RatingFuse
kVAFull Load Current (A)Rated Current (A)Operating Time at 3 × Rating (s)

This table should be taken only as a typical example. Considerable differences exist in the time characteristics of different types of HRC fuses. Furthermore grading with protection on the secondary side has not been considered.

HRC fuses
Figure 1 – HRC fuses

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1.2 Overcurrent relays

With the advent of ring main units incorporating SF6 circuit breakers and isolators, protection of distribution transformers can now be provided by overcurrent trips.

For example, tripping controlled by time limit fuses connected across the secondary windings of in-built current transformers) or by relays connected to current transformers located on the transformer primary side.

Overcurrent relays are also used on larger transformers provided with standard circuit breaker control.

Improvement in protection is obtained in two ways: The excessive delays of the HRC fuse for lower fault currents are avoided and an earth-fault tripping element is provided in addition to the overcurrent feature. The time delay characteristic should be chosen to discriminate with circuit protection on the secondary side.

A high-set instantaneous relay element is often provided, the current setting being chosen to avoid operation for a secondary short circuit.

This enables high-speed clearance of primary terminal short circuits.

Overcurrent relay arrangement with CT’s, including 50/51N
Figure 1 – Overcurrent relay arrangement with CT’s, including 50/51N

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2. Restricted earth fault protection (REF)

Conventional earth fault protection using overcurrent elements fails to provide adequate protection for transformer windings. This applies particularly to a star-connected winding with an impedance-earthed neutral.

The degree of protection is very much improved by the application of restricted earth fault protection (or REF protection). This is a unit protection scheme for one winding of the transformer. It can be a high impedance type as shown in Figure 4 or a biased low-impedance type.

For the high-impedance type, the residual current of three line current transformers is balanced against the output of a current transformer in the neutral conductor. In the biased low-impedance version, the three phase currents and the neutral current become the bias inputs to a differential element.

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

Restricted earth fault protection for a star winding
Figure 3 – Restricted earth fault protection for a star winding

The gain in protection performance comes not only from using an instantaneous relay with a low setting, but also because the whole fault current is measured, not merely the transformed component in the HV primary winding (if the star winding is a secondary winding).

Hence, although the prospective current level decreases as fault positions progressively nearer the neutral end of the winding are considered, the square law which controls the primary line current is not applicable, and with a low effective setting, a large percentage of the winding can be covered.

Restricted earth fault protection is often applied even when the neutral is solidly earthed. Since fault current then remains at a high value even to the last turn of the winding (Figure 4), virtually complete cover for earth faults is obtained.

This is an improvement compared with the performance of systems that do not measure the neutral conductor current.

Earth fault current in resistance-earthed star winding
Figure 4 – Earth fault current in resistance-earthed star winding

Earth fault protection applied to a delta-connected or unearthed star winding is inherently restricted, since no zero sequence components can be transmitted through the transformer to the other windings.

Both windings of a transformer can be protected separately with restricted earth fault protection, thereby providing high-speed protection against earth faults for the whole transformer with relatively simple equipment.

A high impedance relay is used, giving fast operation and phase fault stability.

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3. Differential protection

The restricted earth fault schemes described above depend entirely on the Kirchhoff principle that the sum of the currents flowing into a conducting network is zero.

A differential system can be arranged to cover the complete transformer. This is possible because of the high efficiency of transformer operation, and the close equivalence of ampere-turns developed on the primary and secondary windings.

Figure 5 shows the principle. Current transformers on the primary and secondary sides are connected to form a circulating current system.

Principle of transformer differential protection
Figure 5 – Principle of transformer differential protection

3.1 Basic considerations for transformer differential protection

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

These include:

  1. Correction for possible phase shift across the transformer windings (phase correction)
  2. The effects of the variety of earthing and winding arrangements (filtering of zero sequence currents)
  3. Correction for possible unbalance of signals from current transformers on either side of the windings (ratio correction)
  4. The effect of magnetizing inrush during initial energization
  5. The possible occurrence of overfluxing
In traditional transformer differential schemes, the requirements for phase and ratio correction were met by the application of external interposing current transformers (ICTs), as a secondary replica of the main winding connections, or by a delta connection of the main CTs to provide phase correction only.

Digital numerical relays implement ratio and phase correction in the relay software instead, thus enabling most combinations of transformer winding arrangements to be catered for, irrespective of the winding connections of the primary CTs.

This avoids the additional space and cost requirements of hardware interposing CTs.

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3.2 Line current transformer primary ratings

Line current transformers have primary ratings selected to be approximately equal to the rated currents of the transformer windings to which they are applied. Primary ratings will usually be limited to those of available standard ratio CTs.

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3.3 Phase Correction

Correct operation of transformer differential protection requires that the transformer primary and secondary currents, as measured by the relay, are in phase. If the transformer is connected delta/star, as shown in Figure 6, balanced three-phase through current suffers a phase change of 30°.

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

Differential protection for two-winding delta/star transformer
Figure 6 – Differential protection for two-winding delta/star transformer

Electromechanical and static relays use appropriate CT/ICT connections to ensure that the primary and secondary currents applied to the relay are in phase.

For digital and numerical relays, it is common to use starconnected line CTs 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.

Caution is required if such a relay is used to replace an existing electromechanical or static relay, as the primary and secondary line CTs may not have the same winding configuration.

Phase compensation and associated relay data entry requires more detailed consideration in such circumstances.

Rarely, the available phase compensation facilities cannot accommodate the transformer winding connection, and in such cases interposing CTs must be used.

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3.4 Filtering of zero sequence currents

It is essential to provide some form of zero sequence filtering where a transformer winding can pass zero sequence current to an external earth fault. 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 CTs or interposing CTs for older relays, and hence the winding connection of the line and/or interposing CTs must take this into account, in addition to any phase compensation necessary.

For digital and numerical relays, the required filtering is applied in the relay software. Table 3 summarizes the phase compensation and zero sequence filtering requirements.

Table 3 – CT connections for power transformers of various vector groups

Transformer ConnectionTransformer ShiftClock Face VectorPhase Compensation RequiredHV Zero Sequence FilteringLV Zero Sequence Filtering
YyHYzH(H/12) × 360°Hour ‘H’−(H/12) × 360°YesYes

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3.5 Ratio correction

Correct 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 ratios may not exactly match the transformer rated winding currents, digital/numerical relays are provided with ratio correction factors for each of the CT inputs.

The correction factors may be calculated automatically by the relay from knowledge of the line CT ratios and the transformer MVA rating. However, if interposing CTs are used, ratio correction may not be such an easy task and may need to take into account a factor of √3 if delta-connected CTs or ICTs are involved.

If the transformer is fitted with a tap changer, line CT ratios and correction factors are normally chosen to achieve current balance at the mid tap of the transformer.

It is necessary to ensure that current mismatch due to off-nominal tap operation will not cause spurious operation.

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3.6 Bias setting

Bias 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.

The situation is slightly complicated if a tap changer is present.

With line CT/ICT ratios and correction factors set to achieve current balance at nominal tap, an off-nominal tap may be seen by the differential protection as an internal fault. By selecting the minimum bias to be greater than sum of the maximum tap of the transformer and possible CT errors, mal-operation due to this cause is avoided.

Some relays use a bias characteristic with three sections, as shown in Figure 7.

The first section is set higher than the transformer magnetizing 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 conditions.

Typical bias characteristic
Figure 7 – Typical bias characteristic

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3.7 Transformers with multiple windings

The 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 application of the above principles.

When the power transformer has only one of its three windings connected to a source of supply, with the other two windings feeding loads, a relay with only two sets of CT inputs can be used, connected as shown in Figure 8(a). The separate load currents are summated in the CT secondary circuits, and will balance with the infeed current on the supply side.

Schemes shown are single phase for the sake of simplicity.

Differential protection arrangements for three-winding transformer (one power source)
Figure 8a – Differential protection arrangements for three-winding transformer (one power source)

When more than one source of fault current infeed exists, there is a danger in the scheme of Figure 8(a) of current circulating between the two paralleled sets of current transformers without producing any bias.

It is therefore important a relay is used with separate CT inputs for the two secondaries – Figure 8(b).

Differential protection arrangements for three-winding transformer (three power sources)
Figure 8b – Differential protection arrangements for three-winding transformer (three power sources)

When the third winding consists of a delta-connected tertiary with no connections brought out, the transformer may be regarded as a two winding transformer for protection purposes and protected as shown in Figure 8(c).

Differential protection arrangements for three-winding transformer with unloaded delta tertiary
Figure 8c – Differential protection arrangements for three-winding transformer with unloaded delta tertiary

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4. How to preserve differential protection during magnetizing inrush conditions?

The magnetizing inrush phenomenon produces current input to the energized winding which has no equivalent on the other windings. The whole of the inrush current appears, therefore, as unbalance and the differential protection is unable to distinguish it from current due to an internal fault.

The bias setting is not effective and an increase in the protection setting to a value that would avoid operation would make the protection of little value.

Methods of delaying, restraining or blocking of the differential element must therefore be used to prevent mal-operation of the protection.

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4.1 Time delay

Since the phenomenon is transient, stability can be maintained by providing a small time delay. However, because this time delay also delays operation of the relay in the event of a fault occurring at switch-on, the method is no longer used.

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4.2 Harmonic restraint

The 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.

As stated before, the inrush current contains all harmonic orders, but these are not all equally suitable for providing bias.

In practice, only the second harmonic is used. This component is present in all inrush waveforms. It is typical of waveforms in which successive half period portions do not repeat with reversal of polarity but in which mirror-image symmetry can be found about certain ordinates.

The proportion of second harmonic varies somewhat with the degree of saturation of the core, but is always present as long as the uni-directional component of flux exists. The amount varies according to factors in the transformer design.

Normal fault currents do not contain second or other even harmonics, nor do distorted currents flowing in saturated iron cored coils under steady state conditions.
The output current of a current transformer that is energized into steady state saturation will contain odd harmonics but not even harmonics.

However, should the current transformer be saturated by the transient component of the fault current, the resulting saturation is not symmetrical and even harmonics are introduced into the output current. This can have the advantage of improving the through fault stability performance of a differential relay.

The second harmonic is therefore an attractive basis for a stabilizing bias against inrush effects, but care must be taken to ensure that the current transformers are sufficiently large so that the harmonics produced by transient saturation do not delay normal operation of the relay. The differential current is passed through a filter that extracts the second harmonic.

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.

By this means a sensitive and high-speed system can be obtained.

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4.3 Inrush Detection Blocking – Gap Detection Technique

Another feature that characterizes an inrush current can be seen from Figure 9 where the two waveforms (c) and (d) have periods in the cycle where the current is zero.

The minimum duration of this zero period is theoretically one quarter of the cycle and is easily detected by a simple timer T1 that is set to 1/4f seconds.

Transformer magnetizing inrush
Figure 9 – Transformer magnetizing inrush

Figure 10 shows the circuit in block diagram form. Timer T1 produces an output only if the current is zero for a time exceeding 1/4f seconds. It is reset when the instantaneous value of the differential current exceeds the setting reference.

Block diagram to show waveform gap-detecting principle
Figure 10 – Block diagram to show waveform gap-detecting principle

As the zero in the inrush current occurs towards the end of the cycle, it is necessary to delay operation of the differential relay by 1/f seconds to ensure that the zero condition can be detected if present. This is achieved by using a second timer T2 that is held reset by an output from timer T1.

When no current is flowing for a time exceeding 1/4f seconds, timer T2 is held reset and the differential relay that may be controlled by these timers is blocked. When a differential current exceeding the setting of the relay flows, timer T1 is reset and timer T2 times out to give a trip signal in 1/f seconds. If the differential current is characteristic of transformer inrush then timer T2 will be reset on each cycle and the trip signal is blocked.

Some numerical relays may use a combination of the harmonic restraint and gap detection techniques for magnetizing inrush detection.

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Keep reading the next part:

Power transformer protection relaying (combined differential / REF, overfluxing, tank-earth and oil / gas)

  1. Combined differential and restricted earth fault schemes
    1. Application when an earthing transformer is connected within the protected zone
  2. Overfluxing protection
  3. Tank-earth protection
  4. Oil and gas protection devices
    1. Oil pressure relief devices
    2. Sudden pressure rise relay
    3. Buchholz protection

Source // Network protection and automation guide by (ex) Alstom Grid, now General Electric

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Alosa Zephaniah iyaka
    Jun 10, 2021

    I need a PDF on ” performance analysis of earth faults protection scheme of power transformer at high voltage substation “

  2. Daniel.B
    Mar 01, 2021

    Hi Ed, i must admit, your website is very educational and informative for any level of electrical engineer professional.
    Do you offer ETAP or Amtech Powernet course on your website and how much does it cost please?
    Many thanks

  3. Mohammed
    Oct 23, 2020

    As we are energized 310 mva transformer which is connected to the generator, during back Energization of transformer we are experiencing trip from ref protection, we are unable to do tf stability, so all the secondary wire was correctly connected, but we we don’t know why transformer is tripling for ref, kindly advice your best opinion to Over come this issue

  4. Abu Hashim
    Jul 17, 2020

    We have 5 nos. 2000KVA, 11KV/415V transformer where 1 transformer will work as spare and feeding to 4 different LV section with the REF protection. If we are going to connect one spare transformer to either of LV section need to be protect by REF relay.
    Can one MV breaker have more than one REF realy trip signal?
    Appreciate your reply.

    Jun 27, 2020

    Thanks in advance

  6. Ajay Raj
    May 31, 2020

    We are having 500MVA 400/220kv ICT ( 3 units of 167MVA ) . There are 3 nos. of REF relays, each one for unit. Make ER model 4B3 ( Electromagnetic ).
    Now we have ABB make REU615 relay for those 3 relays. So plz guide how to connect and all propable aspects

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