A while ago I posted a transformer question concerning using an open delta trany to convert 480V to 575V. My question was, "would such a transformer work with either a 480V wye or delta".
Seems like the answer is *no*. I was getting weird ground current from the thing on a 480V wye supply. It produced the 575V fine, but the fault current was way to high. )
Took it to another building with 480V delta, and all those funny problems disappeared....
Count it as a lesson learned I guess.
uray
| A while ago I posted a transformer question concerning using an open delta | trany to convert 480V to 575V. My question was, "would such a transformer | work with either a 480V wye or delta". | | Seems like the answer is *no*. I was getting weird ground current from the | thing on a 480V wye supply. It produced the 575V fine, but the fault | current was way to high. ) | | Took it to another building with 480V delta, and all those funny problems | disappeared.... | | Count it as a lesson learned I guess.
A transformer with a delta primary should work fine hooked to a source with a wye secondary. If I recall a previous posting you made, what you had was probably not a "fault" current, per se, but a high circulating current. That, plus I don't thing the transformer was "correct" with it's existing 3rd winding not being used and possibly shorted to ground. The third winding would in effect be the closed end of the otherwise open delta just backfeeding between actual ground and the corner point of the open delta (circulating back through the source secondary which was serving as the current limiter). To have been correct, the transformer would have had to have two cores separate from each other, as in two single phase transformers. Then it should have worked.
If your source was wye, then buck/boost could have gotten the 277 volt phase to ground voltage up to the 331 to 346 volt level you needed. The kVA rating of the buck/boost transformers would only need to be at a level to gut enough supporting amps at the voltage step-up it is providing.
For example, consider taking three 480->120 volt single phase transformers, and connecting them to get 277 volts in instead of 480. Then it would give you 69 volts out. Boosting the 277 volts with 69 volts gives you 346 volts, which translates into 600 volts phase-to-phase. The rating needed would be the amperage you're using. For example, if you need 100 amps at each phase (346 kVA), the boost transformers would be 100 amps at the design of
120 volts, so 12 kVA each (but you be getting 69 volts at 100 amps).
Would you elaborate on how you determined there was a problem?
--s falke
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