Investigation of Factors Influencing the End-of-Line Partial Discharge Testing of Electrical Machines

Recurrent partial discharges (PD) in the insulation system of electrical machines result in accelerated insulation degeneration and a premature breakdown. Determining the repetitive PD inception voltage (RPDIV) of stators for automotive traction machines at the end of the production line following IEC/TS 61934 is mandatory to ensure a PD-free operation over the service life. The RPDIV depends on various influencing factors, which impedes the comparability of RPDIV results from different test systems. This paper introduces a PD detection platform to systematically investigate the RPDIV differences between two exemplary commercial PD testers. The proposed fusion between the PD signals received by a Horn antenna and a high-frequency current transformer is a suitable reference with a higher PD detection capability than the state-of-the-art systems. Three main influencing factors are identified to explain the RPDIV differences of up to 15 % between the commercial PD testers. The first one is the sensitivity of the antennas for PD detection, which is affected by the antenna design, the relative position to the stator, and the frequency band utilized in the signal processing. Second, differences in the rise time of the high-voltage surge pulses between 220 and 670 ns do not result in a consistent overall RPDIV trend. However, the RPDIV tends to increase with a longer rise time for some stators. Finally, measuring the test voltage directly at the stator terminals or estimating the actual voltage by correction factors is essential to avoid RPDIV differences caused by an inaccurate voltage measurement.