Closed Transition ATS's

[Nmattson]

Say you have 2 ATS’s that are closed transition and the utility is a separate substation feed for each ATS but the emergency power is common, will this create a problem if they CTTS transfer at the same time and are now tying the two utilities together (60 milliseconds or so) through the shared emergency circuit. Ideally this should not as they are both in phase but real world experience trumps expectations.

 

[ron]

The ATS will only do closed transition when the generator and utility are synchronized.

When going from utility to gen or visa versa, the gen is the common reference so there shouldn't be an issue.

Be sure to consider the contribution of all three sources for your short circuit calculations.

 

[GoldDigger]

Because of the sync requirement with the generator as a common reference I would not expect the two to close (transfer) at the same time. As long as the generator is at a slightly different base frequency from the utility power each ATS should transfer reasonably soon.
But if both ATSs drive the same emergency bus you will still have a problem until the second sync.
How is the emergency bus driven when both ATS are in the normal position????
Or are you calling the generator output rather than the emergency loads the emergency bus?

You definitely do not want to couple two stiff sources together even momentarily.
If the switching really is as you seem to describe it might be necessary to have one ATS open and *then* wait before closing??

mobile

 

[Nmattson]

Thanks for the replies.
The ATS's are connected to a common emergency generator bus.

 

[ghostbuster]

We just investigated a huge failure involving a "make before break" ATS system. The electronics for this switch got "confused" and paralleled 2 MW. of geny power out of phase with the utility.The main switchboard and all these electrical systems flashed over.The client was without main power and switchgear for sometime.Be careful.

 

[GoldDigger]

Thanks for the replies.
The ATS's are connected to a common emergency generator bus.

I would then expect that the only time both ATSs will transfer at the same moment is when both utility feeds are momentarily in sync too.

mobile

 

[Ingenieur]

Do both util sources feed a ommon bus in normal operation?
or is one a back-up and only closed on when the other fails?
if both fail then the gen picks up?
what is the sequence?
the chances of the 2 util being in phase is random
same utility?
same primary feeding each xfmr?