Over Frequency Protection Working Principle -81O

Over Frequency Protection:

Over frequency protection or over speed protection is used to protect the generator from over speeding of generator’s rotor, reduce the eddy current losses as the frequency increases and protect the winding against v/f over fluxing protection.

Over frequency relay operation
60 Hz Over frequency relay operation

Principle of Over frequency protection:

Normally, Generator is an energy conversion device, which converts mechanical energy into electrical energy and the mechanical energy is given by the steam turbine or diesel engine or gas turbine etc. here the input fuel may be changed but the mechanical energy is given to the alternator/generator by the manner of rotation (revolution). The revolution is directly proportional to the frequency of the alternator/generator. Under Normal condition the frequency is stable around 50Hz or 60Hz. The turbo-generator rotates at rated speed under normal condition. During abnormal condition, increasing the speed in the turbine leads to elevated frequency in the generator. The machine operated at higher speeds at which the rotor material no longer contain the centrifugal forces imposed on them resulting in serious damage to the turbine-generator set.


The generator can tolerate moderate over frequency operation provided voltage is within an acceptable limit. The abnormal over frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller or severe grid disturbance or sudden load through off.

Note: Sudden load through off leads to over speed of the turbine.

Over frequency relay setting:

Over frequency consist of two stage of operation such as stage 1 and stage 2. For 50Hz generator First stage setting will be 51.5 Hz and second stage will be 52.5 Hz. Here the first stage tripping command will be given to the 110kV grid circuit breaker. Second stage will be given to the generator circuit breaker.

Over frequency relay ANSI code: 81O – Over Frequency

Relays acted :

  • Flag operation at 81O – Over Frequency
  • 86 M grid master trip acted for stage-1, tripping of 110kV grid breaker
  • 86 M Gen Mater trip acted for stage -2 if the speed of the turbine is not controlled.
  • Indication at Annunciation Panel.

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