ANSI codes of frequency relay are 81H, 81L, 81 R. The 81H, 81L, 81 R are frequency relay acronyms for over-frequency, under-frequency, and rate of change of frequency.
The protection relay is a very important device for the protection of the electrical system. The various types of protection relays like over-current, overload, distance protection, frequency protection, reverse power, etc, have the ANSI protection code. The protection relay ensures that the faulty section isolates immediately at the time of the fault. Thus, protection relays are the backbone of the electrical [power network.
Every protection relay has ANSI standard device number. The ANSI code shows the type of relay & its function. We will confine our discussion to the frequency relay in this post.
ANSI Standards Code of Frequency Protection Functions
The followings are the ANSI standard codes of frequency relay.
- Frequency Relay ANSI Code 81H – Overfrequency
- Standard ANSI Code 81 L – Underfrequency
- ANSI Code 81 R – Rate of change of frequency( df/dt)
ANSI Code 81H – Overfrequency
The electrical equipment has its design frequency for which equipment intend to run. Any deviation in the frequency from its rated frequency deteriorates the efficiency of the equipment. As a result of variation in the frequency, the problem of overfluxing or underfluxing beget.
Generally equipment is designed for 50 Hz operation in India and 60Hz in the USA. The frequency tolerance is in the range of 50 Hz +/- 5 %. The 50 Hz rated capacity equipment work in the range of 48.5 to 51.5 Hz. The frequency if rises above 48.5 Hz, the under-frequency element actuates. The relay continuously measures the system frequency and gives a trip signal to the breaker when the frequency is above the set point.
ANSI Code of Frequency Relay 81L – Underfrequency
The frequency protection relay measures the frequency and outputs contact when the frequency drops below the lower setpoint of frequency. The relay contact can be used for load shedding of non-critical loads or for total disruption of the supply.
We use Under frequency relays to shed a certain portion of load automatically when the system frequently drops below the lower set frequency.
ANSI Code 81R – Rate of Change of Frequency( ROCOF Relay)
The relay monitors the rate of change of frequency. The relay outputs a tripping contact when the rate of change of frequency is not within the specified range.
The frequency and load have a close relationship. The frequency decrease when the load increase and, the frequency increase when the load decreases.
The generator frequency increase or decrease according to the loading on the generator. The turbine governor controls the steam to correct the frequency.
When load increase on the grid
The sequence of operation is ;
- Frequency decrease
- One generator trips
- More generator connected to bus trips
- Total blackouts
The solution to the above problem is;
- Frequency decrease
- df/dt relay senses the rate of change of frequency
- Partially load shedding