Paralleled Unit Substations

[Hv&Lv]

So did the OP get it tied?

 

[powerpete69]

You are talking of synchronization of both transformer checking voltage, phase rotation and frequency ?

I believe I'm talking about the phase to phase voltage at the line and load side of the tie breaker. If all is well, it should read zero volts between phase A and Phase A on line and load side for example I believe.

 

[Hv&Lv]

I believe I'm talking about the phase to phase voltage at the line and load side of the tie breaker. If all is well, it should read zero volts between phase A and Phase A on line and load side for example I believe.

You are correct, that’s the way we check them now. 480 can be done with a CAT3 VOM. The same as about every electrician carries, although CAT4 is becoming more popular. We went to CAT4 Only about 10 years ago.

as a side note, YEARS (30 or more) ago, we used to check 14.4kV regulator neutral position with a cat 3 meter. It wasn’t but 90 volts per step, so we were good to go up to about 6 steps. The clock dial was never off that bad, usually only one or two off...

 

[mjr5097]

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.

1. We've calculated the fault current assuming paralleled sources, representing worst case.
2. The arc flash calc will look at all switching scenarios.
3. The transformers in the unit substations have identical ratings (voltage, turns ratio, kVA, impedance, delta-wye, etc.).
4. The 13.2kV primary consists of dual feeders (customer owned), but both are fed from the same utility source, so are always in sync.
5. 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?

How are you/client handling the arc flash labeling? Are you labeling for the worst case, normal configuration scenario or multiple scenarios?