Re: Connecting delta xfrmr to wye system
Sorry, the previous post was sent incommplete of some odd reason.
To continue:Wye-wye transformers require a 5 legged core and are $$$$$$$$$$$$$$$.
Step-up transformers are available as 208v delta-480y/277 secondaries.
This is how I understand it:
A 75kav transformer with a 208v pri is rated 208FLA. The allowable OCPD is 208 x 125%= 260a which is not standard where you are allowed to go to the next standard rating or 300a. Since the information that I have been provided by a former transformer design engineer tells me that the inrush for a 150degC 75kva transformer is about 10x the FLA, then the inrush would be in the are of 2080a which is well below the 3000a magnetic trip (300 x 10) of a 300a breaker. Therefore nuisance trips should not be a problem if the ori breaker is not sized to small. You could expect some problems with a 200a breaker and even with a 225a breaker. Please not that transformers with lower temperature rises 115 and 80degC) and K-factors can have much higher inrushes, increasing inrushes as much as 60% greater than a 150degC transformer.
Remember that any think that is done to a transformer to mack it more efficient and run cooler will increase the inrush.