Insulation Power Factor Test. Insulation power factor test is the ratio of the power dissipated in watts to the apparent power (volt-amperes) under a sinusoidal voltage. The applied 60 Hz voltage of this test is generally lower than the operating voltage on the transformer. The Doble Test Set is designed specially to carry out this test. Portable versions are used to measure the insulation power factor of transformers in the field. This test usually must be done by a trained technician. The test results are temperature corrected to a reference temperature of 20 degree C.
Ref:Power Transformers: Principles and Applications
By John J. Winders