Partial Discharge Testing for Low Voltage Switchgear at High Temperatures

Low voltage switch-and control gear product standards of IEC, UL and CSA usually do not consider partial discharge testing. The creepage and clearance dimensions, which are material categories, mainly described through CTI and RTI values, are the tools subjected to insulation coordination. However, in principle, PD measurements are required for rated AC-voltages exceeding 500 V. IEC 60664-2 delivers rules for consideration and electrical dimensioning of solid insulation. Operation conditions changed due to two trends: miniaturization and increased application of power electronics. The first trend leads to increasing power densities as well as higher operational temperatures. Furthermore, lower dimensions of molded cases reduce the clearances in conjunction to solid insulation. The second trend addresses the use of pulse width modulation (PWM) converters for variable frequency drives (VFD) connected by long cables or lines. The dielectric stress and behavior of solid insulations changes when the time characteristic of the applied voltage becomes non-sinusoidal. For instance, one additional stress appears through reflected voltage waves. As result, these changing operation conditions enable partial discharges (PD) processes and insulation damages. Therefore, reconsideration of PD testing of low voltage switchgear is an essential item. PD processes are well known and the transfer to low voltage switchgear design is the prerequisite to evaluate the changed operation conditions. The paper presents two basic arrangements, which reflect typical design cases of insulating structures with solid insulation. The use of these test cases enables the calculation of PD inception conditions at higher ambient temperatures and the comparison with PD measurement results. AC voltage sources supply sinusoidal test voltages for the PD measurements. The temperature dependent PD characteristic delivers an additional parameter for comparison. Room temperature and elevated temperature up to the maximal operational temperature are specified as test conditions for the PD measurements. The interpretation of the PD measurement results allows proposing test procedures and test parameters for low voltage switchgear. PD measurement and simulation results of the test cases provide a first assumption for limiting values of the PD characteristics. Therefore, a PD test procedure is now available for future PD test considerations of low voltage switchgear. Actual failure mechanisms and consequences of PD induced failures are not investigated or discussed here.