Break before make for EDG incomer

[bee79]

Dear All,

We have a 400V Switchgear fed from two 33/0.4kV transformers each on bus A and bus B. Then there is emergency incomer on bus E which is fed from 11kV emergency switchgear station via a 11/0.4kV transformer. Under normal operation emergency incomer breaker is open and bus tie BE is closed. Please refer to attached diagram for clear picture. When the supply from two normal transformer not available, bus tie BE will be opened and emergency incomer breaker will be closed(both are interlock to each other). The 400V switchgear now being supplied via 11/0.4kV transformer from 11kV emergency switchgear. At 11kV emergency switchgear there are two incomers as well. Normal supply is fed from 33/11kV transformer. When this supply is loss, EDG will start and supply the 11kV switchgear.

If the 400V switchgear being supplied by EDG only, during power restoration we can only do break before make i.e break 11/0.4kV incomer breaker and close bus tie BE? I was told by my senior this is due to the normal bus and emergency bus will not in synch since the downstream utility voltage has gone through completely different transformers. Please share if you have experienced a system configuration like this.

Appreciate your answers.

 

[davidbeach]

If your transformers are all delta-wye you have a 30 degree phase shift across your NO breaker on bus E.

Even if that isn't a problem, you'd still need permission from the power company to parallel your generation with their system.

 

[stevenal]

Yes I've experienced a system such as that. Replacing a 13.8/12.47 delta wye with a regulator locked off at the right tap fixed it at one location. Other locations require drop and pickup for certain transfers.

 

[crshears]

Even if the two sources are in phase electrically, utilities such as mine generally have a very jaundiced view indeed when it comes to supply sources being looped through customer stations; depending on impedances, very large circulating currents may occur immediately parallel is made, in some cases exceeding the breaking capability of the device it is intended to use to open the loop.

You really, really don't want to go there without doing your homework first and having all your ducks in a row as far as having the system studies performed [meaning load flow and short circuit analyses and fault current capability infeed sessments], having the utility sign off on your switching plan, and so on and so forth.

It may be helpful to know that at our own transformer stations we virtually without exception perform all station service and similar type switching using the break before make technique, with UPS's being provided to carry any critical loads through the transfer.



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

If your power is being supplied by the generators then there is no phase reference to the outside world. In principle the generator island could sync to either the 33kV system or the 400V system if appropriate instrument transformers and selectors are provided for the synchroniser.

If the 33kV infeeds are all from the same bus then there's no way to parallel the two substations when both are fed from the utility.

As David has already noted, most utilities will place specific requirements upon any generator which can parallel to the system and may simply refuse to allow it.

 

[bee79]

Dear All,

Thanks for your answers. The 2 normal supply transformers 33/0.4kV and emergency supply transformer 11/0.4kV are Dyn11. They cannot be paralleled even when the secondary voltage same i.e 400V?

 

[ScottyUK]

davidbeach noted this in the first reply to the thread. You have one 30° phase shift in the 33/0.4kV transformers connected Bus A and Bus B, but two such phase shifts in the 33/11kV and 11/0.4kV transformers feeding Bus E. This results in a phase displacement of 30° across Bus B and Bus E. You can not close this breaker with both sides energised.

 

[bee79]

Oh yes!. Now I understood. Thanks guys.