Primary and Backup Relay Timing

[lyla1711]

I've been asked to create settings for an MV switchyard with primary and backup relays. So regular MV feeder breakers will have two relays connected to them. The trip outputs will be in parallel in the control circuit (on a single trip coil). Should I have a time delay between them, or should they fire simultaneously? We'll have a breaker fail scheme with a 15 cycle delay.

 

[davidbeach]

It depends on what you mean by primary and backup. If you're doing two of the same things, more Main A and Main B, then set them the same. But if you're meaning what we used to do, one good relay and one so-so relay then you may want to consider a different approach. What we do in that case is let the good (relative, there's still a few straggling DPUs in that role) relay do the work and set the so-so relay 10 cycles or so slower. The good relay will do the reclosing, it will do any instantaneous tripping that might be called for, and if it's working correctly the other one does nothing. But if the good relay fails the so-so relay will trip one and done.

If you're going to have both relays doing reclosing they need to incorporate exactly the same reclosing logic. One of the good/so-so combos we went through was the SEL-351/SEL-551 combination. To begin with they were set with the same tripping and reclosing settings. There were enough subtle differences between the two relays that we got some interesting results. Don't remember the details, it's been a decade or more since, but I think one of those combos produced 5 trips to lockout rather than intended 3 trips.

But you also have to watch our for two of the same, right at the margins. Where we have a non-delayed instantaneous on the first trip we've had a few cases where we just barely get one relay to trip (sometimes it's be just a quarter cycle above pickup) and it trips and recloses. But the other relay, seeing the breaker open without recloses initiate, goes to lock out. Then when the breaker recloses the inrush gets a much more definite instantaneous trip, but no reclosing because it's already at lockout. The other relay, now with instantaneous blocked, doesn't trip and goes to the lockout state. Two instantaneous trips, one on inrush, and it's all over. Delayed instantaneous or just time overcurrent solves that.

 

[lyla1711]

In this case, we will have two SEL-351s, and we don't have to deal with reclosing, so that will make it pretty easy.

I've come across similar arrangements in existing subs, but I did not fully understand why they were set that way - thanks for the detailed reply.

 

[RRaghunath]

In this case, we will have two SEL-351s, and we don't have to deal with reclosing, so that will make it pretty easy.

I've come across similar arrangements in existing subs, but I did not fully understand why they were set that way - thanks for the detailed reply.

It seems the two relays are for redundancy in protection and not exactly Main & Backup. Hence, the practice is to set the relays identically.