We have a problem with a 3 phase transformer at the university where I work and we need advice. Here's the setup A 37.5kva 3 phase transformer wired backwards with the 208 Y side being fed with a 50 amp 3 pole breaker. There is no connection to the XO 3 phase 208 volt INPUT. The "secondary" side is a 480 volt delta hookup feeding a disconnect that supplys a AC unit that was surplus. after the original installation the setup worked. After a power outage the 50 amp main breaker tripped instantly. We megged the transformer and there were no grounds. We swapped out the breaker with a used one and had the same results. I changed out the transformer with another one and we still have the same problem. There is no load on the secondary as we fed a disconnect and it's still open, not an issue. What is the best place to look for a problem? I pulled the conductors out of the primary and secondary and there was no sign of failure. any suggestions would be greatly appreciated Thanks Dave
Since the secondary side of a transformer is a new service there must be a grounded conductor. Since there is no XO from the primary the secondary must be corner grounded. In other words one of the secondary phases (it doesn't matter which one but H3 is cutomary) must be bonded to the EGC. This phase will then read 0 volts to ground while the others will read 277 volts and voltage phase to phase will be 480. When this is not done properly the problems you have described will frquently occur. Usually the whole thing will fail to function at all, but I have seen what you are describing happen, in exactly that way. If you have never done this, it seems counter intuitive and it makes you cringe when you turn it on.( I know I did the first time) Also the grounded phase must be color coded gray or white.
It sounds top me like there is an internal short in the transformer. I'd start with a visual check, to make sure there are no blatant line-to-ground faults, and then us an ohmmeter to check each winding for infinity to ground, and for any measurable differences between any one winding compared to the other two.
Thanks for the response. I will give that a try Monday. From the first time I worked on this I had a bad feeling about the "XO" and no "grounded" secondary. I realize this isn't a customary hookup and I've been hesitant about going back to it. It isn't too often anymore that I get stumped but this one got me. Thanks
We megged the transformer and it megged good. I even swapped out the entire transformer with a differant one. The transformers have been use to feed old freq drives that were no longer in service. Hopefully I can get it running monday, computer rooms like it cold.
I didn't mean to make it as though my suggestion was the only possibility. Larry's suggestions should be done first, but your post sounded as if you had already done these things. If not, check the transformer first, thoroughly.
Since the secondary side of a transformer is a new service there must be a grounded conductor. Since there is no XO from the primary the secondary must be corner grounded. In other words one of the secondary phases
Not true...there is no requirement for a 480 volt delta system to be grounded.
In other words one of the secondary phases (it doesn't matter which one but H3 is cutomary) must be bonded to the EGC. This phase will then read 0 volts to ground while the others will read 277 volts and voltage phase to phase will be 480.
Also not true. The grounded phase will read "0" volts to ground, the other two phases will be 480 volts to ground, and the phase to phase voltage will be 480 for all combinations. Don