Transformer OC Test

Why OC test is conducted on low voltage side?

Questions by govardhanratna

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The test is to determine the shunt branch parameters of the equivalent ckt of the transformer. One of the winding is connected to the rated voltage and other winding is kept open circuited.

From the convenience point of view and availability of supply the test is carried out on LV side, while HV is kept open cktd. the no load current is so small( 2-6% of the rated value) and resistance and reactance of LV side are so small that the primary voltage V can be regarded as E by neglecting series impedances. so for all practical purposes the power input on no-load equals the core(iron) loss.

These are the values of the LV side, the parameters of the HV side can be calculated with the transformation ratio.  The test can be done from HV side too but the losses, the power consumption and the risks will be much higher!

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Sometimes the primary voltage required may be in kilo-Volts and it may not be feasible to apply nominal voltage to primary from the point of safety to personnel and equipment. If the secondary voltage is low, one can perform the test with LV side energized keeping the HV side open circuited. In this case the parameters that are obtained are in terms of LV . These have to be referred to HV side if we need the equivalent circuit referred to HV side.

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Before the transformer is put into service the oc test is conducted on lv side leaving the transformer hv side open,i.e the meters all are connected on lv side to determine the tranrformer no load current or magnetizing current which help in determining the core losses of transformer which does not change even in load conditions and magnetizing current helps to determine the parameters of the magnetizing branch which can be refered to primarey or secondarey side of the transformer.

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  • Jul 12th, 2010

Voltage injection in LV side is convenient, The cable attached to LV side also need insulation suitable to LV voltage rating. If LV ia in the range of 440V and HV is of the range of 11KV, we may inject voltage in steps also through variac of 440V , Which is not possible if inject voltage in HV side.

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  • Jan 26th, 2012

If want to conduct OC test on HV side we need more turns of auto-transformer, as an engineers we should have to decrease the cost required for the test, so normally OC test is conducted on LV side since we need less turns of auto-transformer

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