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.
Your breaker seems way too small.
50A @ 480 for a 37.5 kVA delta to wye dry type transformer would be a common primary CB feeding a 125A Main secondary breaker at 208Y120.
You now are reversing this....
Just loading up the coil...which is going to look like a dead short in the first cylce... will take a 50A into trip territory.
Up size your breaker...Your primary now ought to be the big one: 125A. Your Secondary needs to be OCPD at 50A.
Also check for any possible damage to the secondary (480) wiring load side of the transformer.
I've seen problems like Tesla describes.
The inrush current of the transformer can trip a C/B if it's not large enough.
Our problem was with a 500KVA xfrmr temporarily backfed by a 600A breaker. We remedied it by using a disconnect with dual-element fuses, which held through the inrush period.
just my two cents! I would go with tesla and electure on this one because it worked till the power outage its possible the original breaker had a problem and was letting it start up.
Greg,
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
@ Don:
In Europe an ungrounded system like this would mandatorily need a permanent isolation control, alalrming at the first fault and switching off at the second? (similar even for 24 VDC btw in Germany)
Same for you?
Wolfgang
Correct and correct. A bad case of headarectumitis here with no excuses. I hope deverson solved his problem.
I think my first option is to try a 100 amp CB. I looked into a 125 but was told Cutler Hammer didn't offer one, plus the 100 amp was available next day. Both of the transformers megged fine and approx the same. I'll keep updates current. No pun intended!!!
Dave
Wolfgang raises an important issue. The secondary of this transformer is a separately derived systems, and must _either_ be grounded or meet the requirements for ungrounded systems.
But gaaaaaa. I can't find the code reference that requires ground fault detection on the ungrounded secondary. Am I imagining that ground fault detection is required? The closest that I can find is 250.21(3), which applies to control circuits only. (2002 code)
An an additional issue, the 480V secondary on this system is a pure 480V, not 480Y/277. You must use 480V breakers, not 480Y/277 breakers.
-Jon
The 480 volt secondary will only feed a 5 ton Liebert air conditioning unit. I have the secondary fused at 20 amps, well under what the transformer is capable of handling. I didn't included much info as it wasn't an issue as I couldn't get the transformer working let alone the Liebert.
Thanks
Dave Everson
Installed a 3 pole 100 amp breaker today and everything worked like a champ. I did notice a slight dimming of the lights when it fired up. It was fine after a second and everybody was happy.
Thanks for the help
Dave
Telsa's right
37.5 x 1000 = 37500 divided by 359.84 (1.73 x 208)= 104.21 amps primary.
37.5 x 1000 = 37500 divided by 830.4 (1.73 x 480) = 45.18 amps secondary.
Yoop:
Why did you revive this thread? Did I miss something?
No problem with it, just wondering. I see names that have not been around in a while!