I have an industrial client who we've designed two new unit substations for, with a Main-Tie-Main configuration on the 480V secondary. We originally had a traditional interlock arrangement on the M-T-M, where only two of the three breakers can be closed at a given time. The client wants the ability to close the tie before opening a main, so the downstream loads never see an interruption. I'd like to make sure we've thought through everything before proceeding with this.
- We've calculated the fault current assuming paralleled sources, representing worst case.
- The arc flash calc will look at all switching scenarios.
- The transformers in the unit substations have identical ratings (voltage, turns ratio, kVA, impedance, delta-wye, etc.).
- The 13.2kV primary consists of dual feeders (customer owned), but both are fed from the same utility source, so are always in sync.
- Because we're not doing a closed transition with a utility source, there are no power company requirements to comply with.
What else do I need to consider?