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This question is in regard to switchyard/electrical distribution operations in a utility power plant.

During a startup, the electric distribution system is lined up so that the startup transformers will power the various 4.16kV buses. The startup transformers are 230kV/4.16kV.

When one of the main generators at the station synchronizes, it can provide station power to the 4.16kV buses via the auxiliary/station transformer (24kV/4.16kV).

For most of the 4.16kV buses, there are breakers for the startup supply, aux supply, and sometimes a diesel generator.

For one of these buses, is it expected that equal current will come from the startup and aux/station transformers when both are paralleled to the bus?

Usually the transfer for startup to aux/station power does not take long, but as I understand there is still time when the two sources are powering the distribution system in parallel.

Is there a good design/operation reference for explaining this?

I have been getting explanations involving circulating current with parallel transformers, but I believe the case I am looking at is different. There are two transformers from different supplies, and they are feeding a bus in parallel.

Also, there are synch switches and fast bus transfer on these feeders but there are interlock switches to override that logic and keep both supply breakers closed in the parallel arrangement. This override allows for testing of the supplies from the startup transformers.

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