Excessive Heat Primary Delt-Wye, WHY? 4

[helpdex]

We have burned up two 330kva Isolation Delt-Wye 480v transformers. Both transformers had the same primary winding burn up and all other windings were fine. We notice that the xformers would get really hot and then at time run normal temps, the heat was only on the phase a transformer and only on the primary side. The transform only supplies one DC drive and one 250HP motor. We see no problems with voltages and amps. we megged out primary and secondary sides. we did find one phase of line side wiring to be 47mohms compared to the other phases being 2000plus mohms. we have corrected this but the heat issue is still there. what could cause such a problem?

 

[Zogzog]

Winding resistance?
TTR?

 

[helpdex]

Megged out winding on both pri and sec both were perfect. Pri and sec were open and 0 to ground all legs.

motor windings megged out the same as should be around 40mohms across the armatures to ground.

We believe possible fault maybe comming from the Drive, we have an older relience MAXPac DCVS. seems that all Triac are firing, could loosing a triac or or a phase cause the overheating on the primary side of the Delta WYE transformer?

 

[Kiribanda]

helpdex,

It seems that there is some kind of drive (eg. VFD) between the transformer and the motors.

The only way I can think is that the DELTA winding may get heated up due to third harmonic current circulation inside the DELTA winding which may be injected from the secondary side.

1)Could you please post a copy of the SLD depicting the power distribution?
2)Is the WYE side secondary is grounded?
3)Have you ever conducted a harmonic measurement on the secondary side?

Thanks

 

[helpdex]

Yes there is a DC drive 480vac 3 ph to 0-500DC output going to 250HP DC motor.

1) I will try to figure out how. Delta to Wye no ground.
2) The wye side secondary is NOT grounded
3) we have looked at the output of the Drive and the wave does not seem to have harmonics. As a matter of fact what we did see however was that the DC ouput was not as ripple as we would have thought it should have been.

Further investigation is leading us toward two of the six triac's are not switching on and off. they seem to be stuck on and passing the whole wave. one is bad on phase A and the other on Phase B.

 

[itsmoked]

Sounds like circulating currents in the transformer due to phase imbalance. Could you have an open rectifier in the drive or some other single phase load happening.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KingRic]

Suggest you start monitoring current waveform. This may be possible with a clip-on ammeter and oscilloscope. Be careful that insulation of ammeter jaws is good for the working voltage. Current must go somewhere: from the primary it is either used to magnetise the core or it goes through to the secondary. Magnetising current can sometimes be excessive if transformer is incorrectly made (eg clamping bolts through laminations not properly insulated, which creates a shorted turn). If waveforms are well balanced and not too peaky, look to the SCR's or whatever. Look particularly for short-duration very high current spikes - these can be a sign of one device slow to turn off, with its opposite number turning on causing shoot through. Above all believe what the current probe tells you!! I am in UK and suitable probes from HEME / LEM. Not cheap but burning out a third 330kVA transformer will be much more costly. Good luck. Rich.

 

[Zogzog]

Winding resistance and megger test are not the same, winding resistance is the resistance of each winding and will tell you if you have anything that could cause heating, as could a TTR test (Which you nbever mentioned you performed either) that could indicate problems with you tap changer that could also be causing thr problems you are explaining.

Or it sould be a PQ problem form the VFD.