Under Frequency Protection Working Principle – 87U

Under Frequency Protection Working Principle:

Under Frequency Protection is used to protect the transformer/generator/alternator when the frequency drops below the operating frequency. It is a backup protection for over fluxing (V/F) protection. Under frequency occurs due to turbine low speed, AVR failure, diode failure, grid frequency fluctuation etc. The generator can tolerate moderate under frequency operation provided voltage is within an acceptable limit. Under frequency causes over fluxing in the generator/transformer, speed drops in the motors, output voltage dropping etc. The abnormal under frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller.

The generator under frequency protection consists of two stage tripping. Stage 1 trip command is given to grid circuit breaker and stage 2 trip command is given to generator circuit breaker.

Under frequency protection
Under frequency protection Make: ABB


Under Frequency relay setting:

Stage1: 48.5 % of the rated frequency & trip command to grid circuit breaker.

Stage2: 47.5 % of the rated frequency, trip command to Generator circuit breaker.

Learn More:   Rotor Earth Fault Relay 64R Working Function

ANSI Code for Under frequency relay: 81U

Relays acted :

  • 81U Flag operation at Protection panel.
  • 86M Master trip opted –Tripping of 110kV circuit breaker at stage-1
  • 86M Master trip opted –Tripping of generator circuit breaker at stage-2
  • Indication at Annunciation Panel



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