Transformer Problems

Status
Not open for further replies.
M

mgervais

Member
Hello,

I was hoping to find some help troubleshooting a transformer problem...

We have a typical 600V-120/208V delta-wye transformer that is providing some problems. The problem was first noticed when several flourescent lights went out due to low voltage.

I went out and did some measurements at the suppy transformer and found that the phase to neutral voltage was low, only on B phase (98V compared to 123V on A and C phase). There was no neutral to ground voltage found (showed 293mV). The neutral wire had a current of 55.3 A.
Using a little Trig I was able to find that the B phase voltage is out of phase by 137? with the A phase and 103? with C phase instead of the 120? that it should be.

If anyone has any idea how I could go about troubleshooting this problem please let me know.

The load side of the transformer is feeding several 3 phase panels that go on to feed a variety of equipment including small pumps, fans, lighting, and heating.
 
Re: Transformer Problems

Originally posted by jimwalker:
how about the amps on A,B,C ALONG WITH VOLTAGE.Any chance that B is over loaded ?
Click to expand...
The values that I measured were 189A on A phase, 121A on B phase, 186A on C phase.

The phase-phase voltages were 205V A-B, 214V A-C, and 173V B-C
 
Re: Transformer Problems

I had almost the same exact situation, but with a 2500 KVA substation transformer....also delta-wye....low voltage on a phase and low current on the same phase, when compared to the other two voltages/currents.

After some extensive trouble shooting (TTR's, micro-ohm meters, power factor, continuity checks on the taps, etc, we discovered that we had an open delta on the primary.

Suggestions/comments:
1. Over-loading should not be an issue because of the low voltage & currents.
2. If possible, perform a TTR test (turn ratio). The TTR might come out fine, but most likely it will not.
3. Meassure the resistance of the windings. Note if your meassurement of B phase is different than A&C. If it is different, note if the sum of results A + C = B. If that is right, then you have an open delta, and the resistance meassurement is reading across the remaining two legs of the delta A&C...which is the reason for the math.

This might sound complicated, but it's really not. If you have an open delta, you need to repair or get a new transformer.
 
Status
Not open for further replies.
Top