Over Voltage Protection Working Principle 59

Over Voltage protection 59:

Over voltage protection is used to protect the synchronous Generator/transformer/alternator form high voltage. Generally, overvoltage occurs due to sudden load through off, elevated grid synchronized voltage, AVR malfunctioning, power transformer taps changer failure, lightning strike on the transmission line, turbine over speed etc. the power system must be isolated when the system voltage high. Seviour overvoltage causes the winding or electrical insulation failure, over fluxing (u/f), transformer’s core saturation etc. The over voltage protection can be considered as a backup to the Volts-per-Hertz protection (Over fluxing).

Over Voltage protection Working Principle Voltage peak
Over Voltage protection Working Principle Voltage peak


The overvoltage protection consists of two stage operation. Stage 1 trip command will be given to the 110kV grid circuit breaker and stage 2 trip command will be given to synchronous generator’s circuit breaker.

ANSI Code for Overvoltage protection: 59

Relays acted :

  • 59 relay’s Flag operation at Protection panel.
  • 86M grid Acting of Master relay
  • 86H generator Opted when the turbine reaches over speed
  • Indication at Annunciation Panel.


Raise the generator voltage slowly with manual mode in AVR and keep generator voltage within the limits of normal voltage. If it is unable to control the generator voltage, trip the field breaker and inform to the maintenance staff for rectification of the AVR.

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