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Applications and Characteristics Of Differential Relays (ANSI 87)

Applications and Characteristics Of Differential Relays ANSI 87 (on photo: VAMP 265 Generator, transformer and motor diferentialprotection relay)

Differential relays categoriesDifferential relays generally fall within one of two broad categories:

1. Current-differential and2. High-impedance differential

Current-differential relaysCurrent-differential relays are typically used to protect large transformers, generators, and motors. For these devicesdetection of low-level winding-to-ground faults is essential to avoid equipment damage. Current differential relaystypically 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 phaseis 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 thanas 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-wyetransformer connection produces a phase shift, the secondary CTs are connected in delta to counteract this phaseshift 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. thesecondary (it is impossible to match the primary and secondary CTs due to different current levels) are taken intoaccount 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 therelay operating windings during normal conditions, the relays are equipped with taps to internally adjust the currentlevels for comparison.

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

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 andthe CTs may be connected the same on the primary and secondary sides of the transformer regardless of thetransformer winding connections.

The manufacturers literature for a given relay make and model must be consulted when planning the CTconnections.

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 avariable-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-stateelectronic or microprocessor-based relays these characteristics are adjustable. For transformers relays with anadditional harmonic restraint are available. Harmonic restraint restrains the relay when certain harmonics, normallythe 2nd and 5th, are present.

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

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

Figure 2 Transformer differential relay applicationfrom figure 1 in one-line diagram format

An example one-line diagram representation of the transformer differential protection from 1 is given infigure 2 below:

Note that the secondary protective device is shown as a low voltagepower circuit breaker. It is important that the protective devices on bothsides of the transformer be capable of fault-interrupting duty andsuitable for relay tripping.

In figure 2 a lockout relay is used to trip both the primary andsecondary overcurrent devices. The lockout relay is designated 86Tsince it is used for transformer tripping, and the differential relay isdenoted 87T since it is protecting the transformer. The wye and deltaCT 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/orovercurrent 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 thissection), it is especially important in the application of differential relaysbecause the zone of protection is strictly defined by theCT locations.

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

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

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High-impedance differential relaysThe other major category of differential relays, high-impedance differential relays, use a different principlefor operation. A high-impedance differential relay has a high-impedance operating element, across which thevoltage is measured.

CTs are connected such that during normal load or external fault conditions the current through the impedance isessentially zero. But, for a fault inside the differential zone of protection, the current through the high-impedanceinput 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 thatthe relay only has one set of input terminals, without restraint windings. This means that any number of CTs may beconnected to the relay as needed to extend zone of protection, so long as the CT currents sum to zero during normalconditions.

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

Figure 3 High-impedance differential relay concept

the voltage across the input during a fault from damaging the input.

High-impedance differential relays are typically used forbus protection.

Bus protection is an application that demands many sets ofCTs be connected to the relays. It is also an application thatdemands that that relay be able to operate with unequal CTperformance, since external fault magnitudes can be quitelarge. The highimpedance differential relay meets bothrequirements.

Figure 4 shows the application of bus differential relays toa 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 thelarge number of circuit breakers to be tripped. Note that allcircuit breakers attached to the protected busses are equippedwith differential CTs and are tripped by that busses respective86 relay.

The 87 relays are denoted 87B since they are protecting busses. The same applies for the 86B relays. Note alsothat 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 istypically applied to protect bussing, where fault magnitudes are typically high, the additional sensitivity is not required.

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

Applications and Characteristics Of Differential Relays (ANSI 87)Differential relays categoriesCurrent-differential relaysHigh-impedance differential relays