Short time Parallel - Inter trip / Interlock

[NickParker]

Hi Everyone,

I have found the below statements in a SLD. I could not ascertain the reason for these statements.

1) For Downstream breakers - 2oo3 interlock with short time parallel for max 500ms. This shall be inhibited when Upstream Tie CB is in open position. Why cant the short time parallel happen when the upstream tie breaker is open?
2) If the MV transformer feeder circuit breaker opens or trips, the corresponding LV circuit breaker shall automatically open. the LV circuit breaker cannot be closed if the MV circuit breaker is open or racked out. All the feeders in the low voltage system are contactor controlled feeders, So why should it intertrip the upstream breaker.

Any inputs are appreciated!

 

[xnuke]

1) If the upstream tie is closed, the two sides of the MV bus are synchronized, so paralleling the LV sides with all three breakers shut for a short time period isn't likely to cause large currents to flow. I wouldn't allow paralleling at the LV buses across the LV tie unless I was pretty sure that both sides would be in sync. The upstream tie closed doesn't guarantee that, but if it is open then it is more likely that the two LV sides will be out of sync.

2) This is fairly normal operation for breakers around a transformer. If the primary breaker trips, open the secondary breaker. If the primary breaker isn't closed and racked in, don't allow closure of the secondary breaker. Energizing a transformer from its secondary can produce very large inrush currents - significantly higher than if energizing from the primary.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickParker]

1) If the upstream tie is closed, the two sides of the MV bus are synchronized, so paralleling the LV sides with all three breakers shut for a short time period isn't likely to cause large currents to flow. I wouldn't allow paralleling at the LV buses across the LV tie unless I was pretty sure that both sides would be in sync. The upstream tie closed doesn't guarantee that, but if it is open then it is more likely that the two LV sides will be out of sync.

The Low voltage switchgear has synch check relays at the Incomers and bus tie to sync the power sources. Even if the upstream tie breaker is open, these synch check relays could check and close. right?

 

[xnuke]

Sync check relays only determine if sources are synchronized, they don't make sources sync; synchronizers actually cause synchronization to occur by controlling prime movers. Do you only have the relays, or do you have a synchronizer?

If the MV tie is open, why would the MV buses be synchronized? It's likely they wouldn't. If the MV buses are out of sync based on phase angle because they aren't tied and both are controlled at a common frequency, they'll never synchronize, so the sync check relays would be useless. If one bus happens to be at a slightly different frequency than the other, the voltages are within tolerance, and the phase sequences match, eventually the phase angles will come into the tolerance band and the sync check relay will operate allowing breaker closure. The time it takes this to happen will be inversely related to the frequency differential, though, so it could be a while.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.