Dry Transformer Overheating

[farmaped]

At what level does the winding temp. of a dry transformer start to become a concern? For instance, there is a 1MVA xfmr 150 deg rise with a center winding temperature of 160 - 170 deg C. This seems excessive. Experience has shown similiar operating conditions typically in the 60 - 100 deg range.
I understand the xfmr can run at 150 deg C rise over 40 deg C ambient = 190 deg C.
That being said, from what I have seen this is a bit high and I was wondering what your experience has shown. If it is a concern, any recomendations for cooling the unit?
FYI this particular unit cont. load is 500-600KVA. There is a fan package and they are working. Ambient Temp approx. 25 deg C.
Thank you.

 

[whillis]

That seems pretty hot given the load and ambient temperature. My question is why the transformer is heating excessively. Did you look for harmonics on both the primary and secondary?

 

[Besoeker]

Some information on insulation classes here:
http://tristate.apogee.net/mnd/mfnrins.asp

 

[farmaped]

Thanks for the feedback. It seems high to me too. A monitor was connected on the secondary for 14 days. VTHD avg = 2.4%. ITHD avg = 15.3%. Not the best, but not the worst.
Transformer was recently cleaned.
Does a dry type transformer, under the same operating conditions, tend to escalate its operating temperature over time?

 

[steve66]

Thanks for the feedback. It seems high to me too. A monitor was connected on the secondary for 14 days. VTHD avg = 2.4%. ITHD avg = 15.3%. Not the best, but not the worst.
Transformer was recently cleaned.
Does a dry type transformer, under the same operating conditions, tend to escalate its operating temperature over time?

As far as I know, there isn't any reason for the temp. to escalate.

Maybe there is a bad connection somewhere. A thermal photo might be a good idea.

Also, maybe measuring the "KVA in" vs. "KVA out" and seeing if the losses are within specs. Or would you have to measure real power in and out to get any meaningful data?

Steve

 

[jim dungar]

I understand the xfmr can run at 150 deg C rise over 40 deg C ambient = 190 deg C.

Actually, most 150?C insulation is designed for a 40?C ambient + 30?C internal hot spot spot allowance for an overall temperature of 220?. This 'hot spot' allowance is intended to address the problem you describe; the differential between the coil surface temperature and that of the interior of windings of the center coil. While your transformer is running hotter than other units, it appears to be in the operating range of its insulation.

I believe over time the cooling ducts, in the transformer windings become clogged causing decreased heat transfer.

 

[templdl]

When a transformer is FA the contoller is set to turn the fans on a 190deg C and off at 190.
Alarm cantact closure is is at 200 and the shut down contact at 210.

 

[farmaped]

Indeed, typically the manufacturer would set the Fans ON at 190 deg C, this particular unit is set to come on at 150 deg C. Sounds like it is operating within the designed parameters but should be watched a bit more closely. Thank you all.

 

[templdl]

Oops,
On at 180 and off at 190.

 

[zog]

At what level does the winding temp. of a dry transformer start to become a concern? For instance, there is a 1MVA xfmr 150 deg rise with a center winding temperature of 160 - 170 deg C. This seems excessive. Experience has shown similiar operating conditions typically in the 60 - 100 deg range.
I understand the xfmr can run at 150 deg C rise over 40 deg C ambient = 190 deg C.
That being said, from what I have seen this is a bit high and I was wondering what your experience has shown. If it is a concern, any recomendations for cooling the unit?
FYI this particular unit cont. load is 500-600KVA. There is a fan package and they are working. Ambient Temp approx. 25 deg C.
Thank you.

Sounds like a high resistance connection, most likely in the tap changer from what you have described. When is the last time this unit was tested?