Power System Relay Co-ordination

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Substation relay coordination
Substation relay coordination

What is relay co-ordination:

The relay co-ordination is nothing but a tripping of protecting relay in a sequence or order in electrical power system. Relay coordination is very difficult task for relay engineers. Relay co-ordination is required to isolate the faulty part with minimized relay & circuit breaker operation.

Substation relay coordination
Substation relay coordination

Consider four number of substation, Substation A, Substation B, Substation C, Substation D. Here Substation A is generation station and B, C and D are distribution stations. In this, if the fault (short circuit or earth fault) occurs in Substation D means, the substation D relay has to operate, instead of that, the substation A relay operated means such system said to be poor relay coordinated power system. It causes the total power system shutdown or unnecessary zone trips. Because, there is no fault on substation A, B and C but the substation A operates unnecessarily. In order to avoid such relay operation, we have to set co-ordination between all 4 substations.

Relay co-ordination Procedure for Earth Fault:

  • All the substation relays current, voltage setting and time setting values will be noted and tabulated.
  • Check the current setting and operating time of the relay which is associated with the fault. i.e if you are coordinating for earth fault means, you should consider all the earth fault setting.
  • Adjust the setting i.e the substation D should have minimum operating time and current or voltage setting. Then only substation D operates. If the value is higher than the remaining substation, the substation D relay does not operate under fault condition.
  • The coordinated setting values would be substation A > Substation B > substation C > Substation D. Also the setting value of substation should not be exceeded its safe limit.

Key Points of relay co-ordination:

The coordinated power system operating time will be substation A > Substation B > substation C > Substation D.

Also see: Anti Pumping Relay 

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