Closed Transition with a spot newtork

[living2learn]

I have a situation with (4) 2000A, 480V closed transition switches with (2) ASCO and (2) Russelectrics. They are fed from a 6MW, 480V grounded wye generator plant. The utility has (4) 1500kVA delta grounded wye connected in a spot network.
The situation arose when the secondary spot network tripped on instantaneous current which is set at %50.

Thoughts. Generators typically have a little higher voltage and operate at a higher frequency to prevent reverse powering. So the ATS switches made a decision to perform a closed transition and there was a great enough difference that allowed the generators to provide power to the utility thus tripping the network protectors.

Solutions:
Ensure enough time delays are established between the transfer switches (even though it would be hard for them to close all at once - supposedly two closed at exactly the same time)

Add active sync controls that will coordinate the utilities voltage, frequency, phase angle and have a much tighter transition.

Add a sync check relay to each ATS that routes through the ATS controller which provides a permissive signal when these ATS's can perform the closed transition.

Tighten up the closed transition settings on the ATS

Obviously change all ATS's to an open transition

 

[davidbeach]

Obviously change all ATS's to an open transition

You got it. If you have all or essentially all of the building load on generator and you want to get it back onto a spot network you have two choices: open transition or a very complex control system that does away with the transfer switches and uses breakers.

Assuming all the network protectors stay closed while on the generators, when you want to get off the generator and back onto the utility source you have to come in low and slow. The generators must be at a lower frequency, and a lower voltage than the utility source, and you have to close with the generators slightly lagging the utility source. It is essential that the generators are instantaneously offloaded by the oncoming utility source. If the generators are faster, leading, or higher voltage you can get an export transient; and that's all it takes.

If the unloaded network protector do what they usually do, you probably won't have all four closed at the time you begin to transition back. In that case you are even more susceptible to export transients and if you return your load too fast you may overload the transformer(s) available until the network protector catch up and close.