Differential relays generally fall within one of two broad categories:
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.
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.
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 example one-line diagram representation of the transformer differential protection from 1 is given in figure 2 below:
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.
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.
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.
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 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 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.
Reference: System Protection – Bill Brown, P.E., Square D Engineering Services