Impedance Heating of Dry Transformer

[7carisfast]

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.

 

[RRaghunath]

5.5% of 13.8kV, i.e. ¬700V gives you rated current flow of ¬105A on the primary side with secondary shorted. If it is 120V, 3phase applied across primary terminals the current should be around ¬20A - proportionate 105*(120/700).

Trust the above helps.

 

[7carisfast]

Thanks RR..
Can you tell me how you got to ~105A on the primary @ 700V?

 

[Skogsgurra]

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!

 

[ScottyUK]

2500kVA @ 13.8kV requires a current of 105A per phase on a three-phase system. Thus, 105A is the rated current of the winding.

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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If we learn from our mistakes,
I'm getting a great education!

 

[edison123]

For your trafo, 480 V, 3 phase supplied to HV with LV shorted should heat the trafo to about 70 to 80 deg C which is good enough for a dryout. Follow scotty's advice on size of cabling on HV and shorts on LV for the ampacity. It is advisable not to exceed oil temp of 90 deg C.

 

[busbar]

Is it possible to estimate the drying-circuit power factor for, say, proposed {3ø} 120-208-480V excitation?

 

[RRaghunath]

May be close to zero, say 0.2 or less, the transformer windings being predominantly reactive.

 

[busbar]

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

 

[RRaghunath]

Even for secondary-shorted transformer, the resistance to reactance ratio would be some thing like 1:10 atleast (for large transformers, may be 1:20). Hence, the power factor and the watts consumed will be low.

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.