Many of new electrical engineers may thought that the open circuit test to determine the no load loss of a power transformer is always applicable to any kind of transformer. In the case of the high voltage application transformers, this is false. In high voltage application transformers, the number of turns of the high voltage winding is very high such that its capacitance (between windings and the ground) is not negligible. In fact, its value may reach 20 or 25 percent of the nominal load of the transformer.
Transformer model considering the effect of winding capacitance
According to G. Oliver, et al, Frequency Response of HV Test Transformers and the Associated Measurement Problems, IEEE transactions on Power Apparatus and Systems, Vol. PAS-99, No. 1 Jan./Feb. 1980;
Conventional open circuit tests are based on the assumption that in the no-load condition, the current flowing in the primary of the transformer is equal to the current in the shunt branch. In the case of HV transformers, this is false. Indeed, a large part of the current flows in the equivalent capacitance of the winding, making the no-load test meaningless. In fact, this current exceeds the magnetizing current and causes the transformer to present a capacitive power factor at no load.