Frequency Protection Relay - ANSI Standards

Updated: Feb 26

The protection relay is a backbone of an electrical network. The protection relays are used for protection of the electrical equipment and power system. It is desirable that protection relay must trip the breaker in order to isolate the faulty section before any catastrophic damage takes place.The protection relay has ANSI standard device number tells about the features of the protection relay. The frequency protection relay has following ANSI codes.

ANSI codes of frequency relay

ANSI Standards(Protection)- Index


Frequency Protection Functions

  • ANSI 81H - Overfrequency

  • ANSI 81 L - Underfrequency

  • ANSI 81 R - Rate of change of frequency( df/dt)

Frequency Protection Functions


ANSI 81H - Overfrequency


Electrical equipment give reliable operation if operated at its rated frequency. Aberration in the frequency above or below of the rated frequency may cause equipment failure. Therefore it is important to monitor and control the frequency. The normal operational frequency in india is 50+/- 5%. The equipment operates reliably if the frequency in the range of 48.5 to 51.5 Hz. The over frequency relay (81H) measures the frequency and if the frequency is more than the set upper frequency limit, relay commands output to trip the breaker.


ANSI 81L - Underfrequency


Under frequency relay measures the frequency and outputs contact if frequency lowers from the set lower limit. The load shedding or total cut off of the supply can be done depending on the system protection requirement. Under frequency relays are generally used to shed certain portion of load automatically whenever the system frequently drops to lower set frequency limit causing threat to the stability of the power system.


ANSI 81R - Rate of Change of Frequency ( ROCOF Relay)


The df/dt relay monitors the rate of change of frequency and it outputs the tripping contact to circuit breaker to switch off the load.Frequency drops when load increase above the generation capacity and frequency increase when generation capacity is more than the demand.


When load increase on the grid,the frequency drops which shutdown the one of the generator connected to the grid and if the frequency is still below the lower limit another generator trips and thus the total power generation may affected. To avoid this type of situation, the df/dt relay senses the rate of change of frequency and if the rate of lowering frequency with respect to time is high, then certain part of the load is automatically shed to maintain the frequency within range.


Related Post

  1. Working Principle of Rate of Change of frequency Relay(81R)

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