Applications and characteristics of differential relays (ANSI 87)

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Differential relays categories

Differential relays generally fall within one of two broad categories:

  1. Current-differential and
  2. High-impedance differential
Applications and characteristics of differential relays (ANSI 87)
Applications and characteristics of differential relays (ANSI 87) – on photo: Micom protection relay

Current-differential relays

Current-differential relays are typically used to protect large transformers, generators, and motors. For these devices detection of low-level winding-to-ground faults is essential to avoid equipment damage. Current differential relays typically are equipped with restraint windings to which the CT inputs are to be connected.

For electromechanical 87 current differential relays, the current through the restraint windings for each phase is summed and the sum is directed through an operating winding. The current through the operating winding must be above a certain percentage (typically 15%-50%) of the current through the restraint windings for the relay to operate.

For solid-state electronic or microprocessor-based 87 relays the operating windings exist in logic only rather than as physical windings.

A typical application of current-differential relays for protection of a transformer is shown in figure 1 below. In figure 1, the restraint windings are labeled as “R” and the operating windings are labeled as “O.” Because the delta-wye transformer connection produces a phase shift, the secondary CT’s are connected in delta to counteract this phase shift for the connections to the relays.

Under normal conditions the operating windings will carry no current.

For a large external fault on the load side of the transformer, differences in CT performance in the primary vs. the secondary (it is impossible to match the primary and secondary CT’s due to different current levels) are taken into account by the proper percentage differential setting.

Because the CT ratios in the primary vs. secondary will not always be able to match the current magnitudes in the relay operating windings during normal conditions, the relays are equipped with taps to internally adjust the current levels for comparison.

The specific connections in this example apply to a delta primary/wye secondary transformer or transformer bank only. The connections for other winding arrangement will vary, in order to properly cancel the phase shift.

Typical application of current-differential relays for delta-wye transformer protection
Figure 1 – Typical application of current-differential relays for delta-wye transformer protection

For many solid-state electronic and microprocessor-based relays, the phase shift is made internally in the relay and the CT’s may be connected the same on the primary and secondary sides of the transformer regardless of the transformer winding connections.

The manufacturer’s literature for a given relay make and model must be consulted when planning the CT connections.

Percentage-differential characteristics are available as fixed-percentage or variable percentage. The difference is that a fixed-percentage relay exhibits a constant percentage restraint, and for a variable-percentage relay the percentage restraint increases as the restraint current increases.

For an electromechanical relay, the percentage characteristic must be specified for each relay; for solid-state electronic or microprocessor-based relays these characteristics are adjustable. For transformers relays with an additional harmonic restraint are available. Harmonic restraint restrains the relay when certain harmonics, normally the 2nd and 5th, are present.

These harmonics are characteristic of transformer inrush and without harmonic restraint the transformer inrush may cause the relay to operate.

An important concept in the application of differential relays is that the relay typically trips fault interrupting devices on both sides of the transformer. This is due to the fact that motors and generators on the secondary side of the protected device will contribute to the fault current produced due to an internal fault in the device.

An example one-line diagram representation of the transformer differential protection from 1 is given in figure 2 below:
Transformer differential relay application from figure 1 in one-line diagram format
Figure 2 – Transformer differential relay application from figure 1 in one-line diagram format

Note that the secondary protective device is shown as a low voltage power circuit breaker. It is important that the protective devices on both sides of the transformer be capable of fault-interrupting duty and suitable for relay tripping.

In figure 2 a lockout relay is used to trip both the primary and secondary overcurrent devices. The lockout relay is designated 86T since it is used for transformer tripping, and the differential relay is denoted 87T since it is protecting the transformer. The wye and delta CT connections are also noted.

An important concept in protective relaying is the zone of protection. A zone of protection is the area that a given protective relay and/or overcurrent device(s) are to protect.

While the zone of protection concept applies to any type of protection (note the term zone selective interlockingas described earlier in this section), it is especially important in the application of differential relays because the zone of protection is strictly defined by the CT locations.

In figure 2 the zone of protection for the 87T relay is shown by the dashed-line box around the transformer. For faults within the zone of protection, the currents in the CT’s will not sum to zero at the relay operating windings and the relays will operate.

Outside the zone of protection the operating winding currents should sum to zero (or be low enough that the percentage restraint is not exceeded), and therefore the relays will not operate.

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High-impedance differential relays

The other major category of differential relays, high-impedance differential relays, use a different principle for operation. A high-impedance differential relay has a high-impedance operating element, across which the voltage is measured.

CT’s are connected such that during normal load or external fault conditions the current through the impedance is essentially zero. But, for a fault inside the differential zone of protection, the current through the high-impedance input is non-zero and causes a rapid rise in the voltage across the input, resulting in relay operation.

A simplified schematic of a high-impedance differential relay is shown in figure 3 to illustrate the concept. Note that the relay only has one set of input terminals, without restraint windings. This means that any number of CT’s may be connected to the relay as needed to extend zone of protection, so long as the CT currents sum to zero during normal conditions.

Also note that a voltage-limiting MOV connected across the high-impedance input is shown. This is to keep the voltage across the input during a fault from damaging the input.

High-impedance differential relay concept
Figure 3 – High-impedance differential relay concept

High-impedance differential relays are typically used for bus protection.

Bus protection is an application that demands many sets of CT’s be connected to the relays. It is also an application that demands that that relay be able to operate with unequal CT performance, since external fault magnitudes can be quite large. The highimpedance differential relay meets both requirements.

Figure 4 shows the application of bus differential relays to a primary-selective system.

Note that in figure 4 the zones of protection for Bus #1 and Bus #2 overlap. Here the 86 relay is extremely useful due to the large number of circuit breakers to be tripped. Note that all circuit breakers attached to the protected busses are equipped with differential CT’s and are tripped by that busses’ respective 86 relay.

The 87 relays are denoted 87B since they are protecting busses. The same applies for the 86B relays. Note also that the protective zones overlap; this is typical practice to insure that all parts of the bus work are protected.

The high-impedance differential relay is typically set in terms of voltage across the input.

The voltage setting is typically set so that if one CT is fully saturated and the others are not the relay will not operate. By its nature, the high-impedance differential relay is less sensitive than the current-differential relay, but since it is typically applied to protect bussing, where fault magnitudes are typically high, the additional sensitivity is not required.

High-impedance differential relaying applied to a primary-selective system
Figure 4 – High-impedance differential relaying applied to a primary-selective system

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Reference: System Protection – Bill Brown, P.E., Square D Engineering Services

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About Author


Edvard Csanyi

Edvard - 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 fascilities. Professional in AutoCAD programming. Present on


  1. Rodolfo P Moreno
    Aug 21, 2017

    Very enhancing knowledge..good information for reference

  2. Rodolfo ponteno moreno
    Aug 21, 2017

    The knowledge you shared about the differential relay protection is very enhancing

  3. nitin
    May 04, 2015

    Good.. But please detail more with practical examples.. Thank you

    Jan 02, 2015

    Nice to gather knowledge

    Nov 17, 2014

    Dear Edvard, you are really doing a great service to the electrical engineering fraternity by disseminating knowledge on verity of EE topics. Really hats of to you!!

  6. uday kumar yadav
    Oct 31, 2014

    nice, helped me by giving such type of new concept

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