Wye/ Wye transformer/ distrobution bus problems

[mwk]

We have 6 oil filled ground mounted transformers at my facility. All are locked type, ground mounted transformers with primary and secondary feed below ground. Primary feed(power company side) 12470VAC, capacitor bank (PF correction network), then 12470 to 480 (in plant distrobution). Five of these units are delta-wye, one of them is wye-wye.

The wye-wye unit feeds a bus distribution system and we are experiencing failures at the bus, some of them catastrophic with the bus plug being physically being blown off the bus by the arc. It is only on the wye-wye rail. Any suggestions what could be causing these failures.

 

[alehman]

This is very strange. I can't think of any reason a wye-wye transformer would cause that. Wye-Wye connections are very common for this application.

Can you provide any more information on the bus plugs? Can you provide more detail on what happens when they 'blow off' of the busway? Is there anything going on in the plant at the time?

If the bus plugs are being blown off, there is either something wrong with the way they are installed or for some reason there is a short circuit flowing through the bus plug that exceeds it's short circuit withstand rating.

There will be a phase angle difference between they wye-wye transformer and the delta-wye. That shouldn't be a problem if there is no interconnection between their secondaries. Is your system somehow paralleling the output of the wye-wye transformer with one of the others (through a bus plug??).

 

[jghrist]

The 480 volt system on the secondary of the wye-wye transformer will be out of phase with the rest of the system. If you try to parallel the systems, the results will be catastrophic.

If you aren't trying to parallel the systems, I'd guess that the failures don't have anything to do with the fact that the transformer is wye-wye. I expect that faults on the bus are caused by insulation failure or something getting into the bus. If there is higher arc energy, it is because the protection is different, resulting in longer clearing times, or the transformer impedances are lower causing higher fault currents.

 

[mwk]

The bus plugs are standard ITE four conductor style, nothing special about them. They are being installed correctly, I put the last one on myself. Inspection of the failed plugs displays arcing on two phases, but never ground. The bus occasionally shows arc damage. I have used a FLUKE 1735 and performed analysis yielding <2%THD but I have never been able to capture any events. Two bus plugs failed last week and were clear by thermal scan in March during our annual survey.


None of the transformers secondaries are connected in parallel, that was the first thing that I thought but I can't find anything that shares a connection. A lot of our equipment has come from Canada and have 480 to 600 transformers at the machines and I am looking to see if somehow this voltage is making it back to the bus somehow but doesn't appear likely. Also I would expect this to occur at the same machine or machines if it was related to one of these step up transformers.

One of the delta transformers shares a primary feed with the wye. Not sure what this could mean if anything.

 

[alehman]

I wonder if you're getting some sort of high voltage transients that initiating an arc. Still the tap units should not blow off of the bus unless they are not installed correctly or the fault current well exceeds their rating. Are you sure all required clips, clamps, etc are being installed?

 

[mwk]

Why would the voltage transients only damage the bus on the rails fed by the wye-wy can? That's the dinger...

Pretty sure the boxes are connected well. While I can't speak for everyone, I know the quality of work that most of the techs perform.

 

[alehman]

Could the supposed transients be coming from within the plant, i.e. the process equipment?

Have you verified that the transformer neutral is properly grounded?

 

[edison123]

Could the neutral swinging in wye/wye trafos be the problem ? Just my $ 0.03.