Partial discharge inception voltage characteristics at low frequencies: The role of electrostatic charges

Standards like the IEC 60502 and IEEE 400.2 support the possibility of utilizing Very Low Frequency supplies in testing of MV and HV cables insulation. Those systems have been employed for several years and have the advantage of requiring relatively small sized equipment. Partial discharge measurement is a common test done to highlight and identify different defects in the insulation system and supposedly obtaining a result reasonably close to the outcome of a test done in operating conditions. The aim of this paper is to investigate the discrepancies introduced by this method, from a theoretical and experimental point of view. In this paper, the case of an internal defect will be analyzed. A simple mathematical model for a real defective insulation, and experimental validation on XLPE and polypropylene, assess the importance on field distribution (hence Partial Discharge activity) of electrostatic buildup on internal surfaces. Depending on the properties of insulating materials, this has a deep effect on the inception voltage at low frequencies, increasing or decreasing PDIV in VLF with respect to the values found at industrial frequencies.