Electronic Relay Protection

[Power2all]

I have MV breakers in series: Main breaker to a Feeder breaker – Feeds another gear with a main and feeder breaker. I am wondering the minimum time that should be allowed for coordination. I think IEEE recommends 4ms but I think that was based on the old electromechanical relays, is 1ms sufficient? I have heard but not seen white papers recommending 2ms.
Thank you.

 

[dpc]

Old rule of thumb with electro-mechanical relays was 0.3 to 0.4 SECONDS minimum between relay curves. You have to allow for breaker opening time, CT error, relay error, overtravel, etc.

With digital relay coordinating with digital relay, I'm very comfortable with 0.25 seconds, and have been known to go down to 0.2 seconds without losing much sleep. It will take a digital relay much longer than 1 ms to even figure out that a fault has occurred.

 

[stevenal]

The down-loadable Alstom NPAG has a pretty good discussion on this. Their conclusion is .3 s for digital/numerical. After a misccoordination between a vacuum feede recloser and vacuum main breaker, both equipped with digital relays, at .2; I've been using .25s without incident.