Applying the PD F-factor to verify the integrity of large motor stator insulation design

For large high-voltage motors it has been shown that there is a consistent relationship between motor Partial Discharge (PD) measurements recorded in Volts to PD measurements recorded in Coulombs, both measured quickly in succession [6,7,10]. This relationship, the ratio of PD measured in volts to PD measured in Coulombs, is expressed in a transformation factor called the F-factor. It appears that the F factor can be predicted based on core dimensions and insulation thickness and this paper examines if the F-factor can be directly related to the surge impedance of the winding. This paper also compares the F-factor to insulation design electric stress and a suggested rational is proposed for being able to predict the F factor based on stator physical data. In our experience an excessive insulation design electric stress degrades the integrity of a machine.