Discussion of Protection Relays | Electricity

Protection relay has a vital role in the generation system so that it needs special attention in its maintenance. The maintenance system carried out on relays usually consists of 2 types of maintenance, namely:

1. Preventive maintenance is a scheduled and planned maintenance action to anticipate problems that can cause damage and keep it normal during operation.

2. Predictive maintenance is a predictive maintenance action, in this case an evaluation of periodic maintenance. This detection can be evaluated from the indicators installed on the installation of a device.


There are two general functions of protection relays, namely:

give an alarm signal and disconnect a circuit to isolate a disturbance or abnormal condition, such as: overload, short circuit, and temperature rise.

quickly fix disturbed equipment to avoid further damage.


Protection relay classification:

  • electromagnetic attaraction is used in instantaneous relays. The types are: plunger type, balanced beam type, hinged armature type, polarized moving coil type.
  • electric magnetic induction (electromagnetic induction) is used in relays with a time delay (time delay relay). Type: induction type, induction cup type.
  • temperature / thermal relays (thermal relays) are used in the protection of motors, generators, and transformers. Type: bimetal strip type, rtd type, thermo couple type, and electronic
  • relay driven by gas, used in protection transformer. Type: Bucholz relay, sudden pressure relay.
  • static relays are used in the complex protective system of generators, motors. Type: electronic circuit.


Relay code:

  • 21, the distance relay functions to detect 2 phase or 3 phase faults in front of the generator to its range limit.
  • 25, synchronous check relay (synchro check relay) serves as a safety auxiliary generator to assist synchronization / parallel requirements.
  • 27, the under voltage relay detects the voltage drop to the allowable limit.
  • 32, the reverse power relay functions to detect reverse power so that it prevents the generator from working as a motor.
  • 37, under current relay
  • 40, the loss of excitation relay detects the loss of amplifier current in the rotor.
  • 46, the negative sequence relay functions to detect negative sequence currents caused by unbalanced loads from the allowable limits.
  • 49, Thermal relay
  • 50, instantaneous over current relay
  • 51, relay over current with time delay (time over current relay)
  • 59/81, over-excitation relay
  • 59, over voltage relay
  • 60, the balanced voltage relay (over voltage relay) detects the voltage loss from the voltage transformer to the AVR and to the relay
  • 62, time delay relay (time auxiliary relay)
  • 63, gas pressure relay
  • 64, ground fault relay (ground fault relay)
  • 81, frequency relay
  • 86, lock out relay functions to receive trip signals from protection relays and forward them to PMT, alarms, and other equipment and their locks.
  • 87, differential relay
  • 95, bucholz relay.

Incoming search terms:

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RPR reverse power relay theory

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