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Delta/Star transformer for 480v to 4160v

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ricno

Electrical
Jun 13, 2008
1
I work on a small island and our power is currently generated by (800KW)4160 volt 3 phase generators that feed an island wide 4 wire distribution system (underground, 3 seperate phase conductors and a bare neutral/ground conductor that ties directly to the generator neutral connection. For the most part, our field transformers are 3 phase Delta/Star 4160/208v or 4160/480v with about a dozen single phase transformers tapped into the distribution system throughout the island. Now they have decided to replace the old 4160v generators with new 480v 3phase generators and step back up to 4160v at the power plant with transformers. No one has figured out why they went to 480v generators. But my concern is that the new transformers that will be used to step the voltage back up to 4160 are configured as Delta/Star which are the same as most of our other field transformers, but they plan to back feed these transformers to get back to 4160v. With our distribution system connected on the Delta side will this not have dire consequences for our single phase transformers since we would then basically have a 3 wire ungrounded system? If we utilized Star/Star transformers instead of Delta/Star transformers to step the voltage from 480 back to 4160 this should alleviate our problem but is their an easier fix or am I making a mountain out of a mole hill?
 
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Should be delta on the generator side and wye on the 4160 side. 800kW is really small for a 4160V machine, much more cost effective at 480V.
 
I agree with David on this. I have experience with a system of similar size to yours. (480:13200) Feeding a star/delta transformer bank involves several issues. Switching surges and over voltages if the wye neutral is floated and circulating currents and back feeds if the wye neutral is connected to the system neutral.
The delta/wye (feed the delta from the generators) spreads uneven currents across all three gen-set windings.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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