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Commonly used terms in the Electrical industry.
auxiliary relay
a relay employed in power system protection schemes that does not directly sense fault presence and location. Typical auxiliary relays include lockout relays, reclosing relays, and circuit breaker anti-pump relays.
bus differential relay
a differential relay specifically designed to protect high power buses with multiple inputs.
computer relay
a protective relay that digitizes the current and/or voltage signals and uses a microprocessor to condition the digitized signal and implement the operating logic.
See
digital relay
differential relay
a differential relay is a protective relay that measures current going into a device from all sources by means of a network of paralleled current transformers. Ideally, the operational current is zero for normal conditions, and rises to a high value (proportional to fault current) when a fault comes on inside the differential zone.
Differential relays are commonly applied in bus protection, transformer protection, generator protection, and large motor protection.
directional overcurrent relay
an over-current relay that operates only for overcurrents flowing in the tripping direction. Direction sensing is typically done with respect to a voltage or current signal, which is not affected by fault location.
directional power relay
a protective relay that operates for power flow in a given direction. Applications are in cases where normal power flow is in one direction, including anti-motoring protection on a turbine-generator and fault backfeed protection on parallel step-down transformers.
electromechanical relay
a protective relay that uses electrical, magnetic, and mechanical circuits to implement the operating logic.
frequency relay
a protective relay which monitors the frequency of the electric power system.
generator differential relay
a generator differential relay is a differential relay specifically designed for protection of electric power generators. Variations include allowances for split-phase winding machines.
impedance relay
a protective relay that senses the operational impedance at a location, i.e., the ratio of voltage to current at any given time. During fault conditions on the protected line, the impedance relay will sense the impedance (distance in ohms) between the location of the relay and the fault.
Typical impedance relay characteristics are mho and reactance. Impedance relays are widely used in sensing phase faults on transmission lines. Ground impedance relays are available that measure the distance to a single phase to ground fault using a modified technique.
See
distance relay
instantaneous overcurrent relay
an overcurrent relay that operates with no intentional delay following sensing of a power frequency overcurrent, i.e., a current above its set point.
lockout relay
an auxiliary relay which is operated by protective relay(s) that in turn opens the appropriate circuit breakers or other fault clearing devices. The lockout relay will remain in the trip position until manually reset, and is used in protective zones where temporary faults are unusual and the potential for equipment damage is high.
loss-of-field relay
a protection relay used to trip a synchronous generator when the excitation system is lost. Loss of excitation causes the generator to run as an induction generator drawing reactive power from the system. This can cause severe system voltage reductions and damage to stator due to excessive heating.
master control relay (MCR)
used in programmable logic controllers to secure entire programs, or just certain rungs of a program. An MCR will override any timer condition, whether it be time-on or time-off, and place all contacts in the program to a safe position whenever conditions warrant.
negative sequence overcurrent relay
a protective relay that senses and operates on negative sequence overcurrent. Typical applications include the sensing of unbalanced faults and the protection of synchronous and induction machines from rotor overheating.
out-of-step relay
a protective relay that senses that a synchronous generator has pulled out of step, and is operating at a frequency different than the system frequency.
overcurrent relay
a protective relay that operates when fed a current larger than its minimum pick-up value.
overload relay
a device designed to detect and interrupt motor overload conditions.
Motor overload relays may be actuated by thermal (temperature), magnetic (current), or electronic (voltage and current) sensors.
overvoltage relay
a protective relay that operates on overvoltage.
phase comparison relay
a phase comparison relay is a protective relay used on transmission lines which operates by comparing phase angles of signals generated at opposite ends of the line. They employ a dedicated communications channel to make the comparison. The signals compared are typically corresponding phase currents or sequence currents.