Relay for high impedance grounded/ungrounded multiple machines 1

[jhonsom]

1. Which Relay is the best for detecting fault in high impedance grounded/ungrounded multiple synchronous generators in parallel?
What I think is voltage based ground fault detection is not very sensitive/selective for this type of ground. 59G, 64G (VN, VN3) are all good as backup protection only.
67G uses I0 and 3V0 where I0 is math calulated from 3 phase CT and 3V0 math calulated from 3 phase PT (broken delta not used).
67N uses IN and 3V0 . I think 67N should be the primary protection.
we currently have only 1A,5A nominal CT's, thinking of including 50mA,0.2A CT's to be more sensitive for this type of grounding.

2. Also is it required to add 67N funtion for this type of grounding in distribution feeder relay?

Please share your experience/opinion on this.

 

[dpc]

Traditionally, a 59 voltage relay, with harmonic filtering, has been used for ground fault detection in high resistance grounded system. In a properly designed system, the ground fault current is very low, less than 5 amps. It is difficult to selectively coordinate fault currents of this magnitude using standard overcurrent relaying.

It is true that the voltage relaying will not be selective. That is why high-res grounding is typically used for unit-transformer installation where each generator has a dedicated transformer. For multiple generators connected to a common bus, high-resistance grounding has some problems with selectivity.

Some attempts have been made to use current relaying, using sensitive digital relays. I have doubts as to the reliability of this approach.

There was an IEEE Working Group that developed a hybrid system of generator grounding that involved switching between high and low resistance grounding via a contactor. You might want to see if you can Google that up.

David Castor

http://www.cvoes.com