Impedance Heating of Dry Transformer
Impedance Heating of Dry Transformer
(OP)
We de-energized an outdoors dry transformer in the south on a very humid day. From the transformer cooling, we aborbed a lot of moisture that when we meggered the transformer's secondary, we measured 0.020MOhms. The transformer is a 2500KVA 13.8KV-480/277V.
One method is to short the secondary windings and to apply 120v to the primary. Assuming the transformer has an impedence of 5.5%, can so one walk me thru how to determine the secondary current thru the shorted windings? This would in the end help me determine how much current at 120V I would require. TIA.
One method is to short the secondary windings and to apply 120v to the primary. Assuming the transformer has an impedence of 5.5%, can so one walk me thru how to determine the secondary current thru the shorted windings? This would in the end help me determine how much current at 120V I would require. TIA.






RE: Impedance Heating of Dry Transformer
Trust the above helps.
RE: Impedance Heating of Dry Transformer
Can you tell me how you got to ~105A on the primary @ 700V?
RE: Impedance Heating of Dry Transformer
A proven method is to apply DC. Then you do not need to think about the inductivity and you can very often have the desired current with quite low voltages. It is even possible to series connect the windings since there will not be any mutual coupling between them, and their resistance is low.
You will need a DC source with the right current capacity and a voltage that is sufficient to overcome the R of the winding(s). All math needed is Ohm's law. Simple!
RE: Impedance Heating of Dry Transformer
The 5.5% impedance of the transformer means that with the secondary winding short-circuited, rated current (105A) will circulate in the winding with 5.5% of the rated primary voltage applied (700V).
Bear in mind that you will need shorting links capable of carrying over 500A on the LV side if you energise the HV side with 120V, or over 900A if you use 208V 3-phase. Be careful!
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RE: Impedance Heating of Dry Transformer
RE: Impedance Heating of Dry Transformer
Is it possible to estimate the drying-circuit power factor for, say, proposed {3ø} 120-208-480V excitation?
RE: Impedance Heating of Dry Transformer
RE: Impedance Heating of Dry Transformer
Very low PF is understandable for a no-load condition of the transformer. But, what if the secondary terminals are jumpered for drying? This concern is based on the likelihood of using a trailer-mounted generator for temporary power
RE: Impedance Heating of Dry Transformer
Catering the requirement from portable generator should not pose a problem, I think. It feeds kW equivalent of load losses in to the transformer and high current.
Generators can accept low power factor when the kW output is well below its rating, without any difficulty.
Hope the above is helpful.