Transforming partial discharge data in volts and coulombs for six high-voltage motors

It has recently been demonstrated from a limited database of large motors that there is a consistent relationship between Partial Discharge (PD) measurements recorded in Volts to PD measurements recorded in Coulombs [6]. This relationship has been expressed in a transformation factor called the F-factor (the ratio of PD measured in volts to PD measured in Coulombs) which was characterized from PD data taken in the field under a variety of operating conditions. Because past effort was limited to two motor designs, this paper has extended the work to include the PD data from four additional designs. The motor database also includes machine physical data for six different winding designs ranging in size from 7,000 HP to 21,000 HP, induction and synchronous, with windings manufactured from 1980 to 2002. This paper looks at two ways to relate the F-factor to motor design. The first utilizes the machine physical parameters to calculate the surge impedance of each motor design, and the second explores a simpler method of predicting F-factor based on core and winding data. The calculated surge impedances could not be satisfactorily correlated to the empirically derived F-factors, but a simple and robust correlation derived from core and winding data is demonstrated. The F-factor can be used to determine if the insulation margin of the machine being tested is substantially different from modern practice by comparing it to simple core volume.