O.C. Test of Transformers

I don't think it really matters: I suspect that it's normally easier to find a low voltage test source.

The "NO-LOAD LOSS AND EXCITING CURRENT TESTS" in this page says

"rated voltage is applied to either the high voltage or low voltage side while the other side remains open circuit."

This OC????

HV?

LV?

Good acronyms. What do they have to do with a generator?

What do you think would happen to the transformer if you short the HV side rather then leaving it open circuited, and then apply rated voltage to the LV? Maybe some smoke would come out of the transformer or the shorting cable. Or your LV source would experience an unhappy event...

OC? Over-Current Testing?

If Over-Current testing; Because it could be dangerous to personnel and harmful to the transformer as well as anything/everything attached to the high voltage terminals should there be a winding insulation failure.

If you were to short the HV terminals the xfmr windings might be damaged before the test set trips.

Applying rated voltage to the LV side is a true, accurate method of testing the windings turn-to-turn ratio and the insulation condition.

Is the transformer new, first-time-in-service or an in-service transformer?

Acceptance test? Or preventative-maintenance testing?

It is important that the transformer OEM is contacted and then follow their recommendations for all testing.

Good luck and stay safe!

I am from a company called A&D Power in South Africa.

Short circuiting the HV side of a TX while applying rated voltage on the LV side can result in the TX being damaged and workers getting hurt.

When we test a HV / LV TX we normally do a insulation resistance test with a megger on double the working voltage before we supply voltage to the TX. Then we follow with supplying the LV side with rated voltage and testing the voltage on the HV side to ensure all coils are in working order. If all that readings are within spec then we go ahead by doing a final test, we short circuit the LV side and then supply the HV side with rated LV voltage,then we tong test the tails on the LV side to ensure the same current is flowing. This is the only way that we have found are sufficient testing without having expensive testing equipment needed.

I hope this helps.

Very tidy explanation of your method.

You leave one side open and apply power to the other because your goal is to get the "No-load" losses. You would short circuit one side and apply a reduced voltage (equal to the voltage required to circulate rated current) on the other to get the rated full load losses and the impedance.

Magic smoke does not appear if this is done by qualified personnel, but it is useful for drying out a moisture laden transformer.

The reason for using the low voltage coil for the open circuit test is that is usually the coil closest to the core and the coupling is slightly better, especially in high voltage step down transformers. If the voltage is high enough you might have as much as 7 inches between windings on radial build which would mean more stray losses included in the open circuit test which would incorrectly add to the core loss measurement or open circuit test. If the LV winding is on the outside, and farther from the core, then the HV winding should be used. This is for core form transformers only, the analysis gets more complicated in shell form transformers.

OC is conducted to measure no load losses and magnetizing current. For this you need watt meters, ammeters, voltmeters. If you apply input voltage from low voltage side, you will need all instruments suitable for low voltage, arrangement or set up is also required suitable for low voltage.Only voltmeter for measuring secondary voltage will be suitable for higher voltage. All this is safer and cheaper as compared to the same for high voltage side application. Theoretically speaking, there should be no difference whichever side you apply the voltage.