Automatic Rollover Between Two Different Voltages

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A customer is requesting for backup feed from another circuit to feed their facility in the event of a sustained outage. This entails installing an automatic open transition rollover switch tying two circuits together. If their primary service goes out, then the switch will automatically rollover the service to the second circuit.

The customer is fed from a 12 kV circuit and the closest adjacent circuit is 35 kV. To reduce cost in extending a 12 kV circuit for the second feed, it was suggested to provide rollover service between 12 and 35 kV. I heard this can cause circulating currents on the secondary side of the substation tranformers but not sure how. Or there any other reasons for concern with this proposal?

 

[desrod]

The problem will likely be with using the wrong voltage not circulating currents.
I suspect that you are missing a transformer somewhere (35/12 kV)
Circulating current might happen when you parallel 2 transformers. In your case it is a simple transfer switch and you want to disconnect from the faulty line before reconnecting to the other.

 

[davidbeach]

Connect a load expecting 12kV to a 35kV supply and things will get interesting. If truly an open transition switch there can be no circulation currents.