Partial discharge characteristics of slot liners for low voltage motors under high-frequency voltage stress

Recently there has been a strong interest in the influence of high frequency stress on insulation systems. This is caused by the increasing use of high frequency switches e.g. in frequency converters. The experience with these systems has shown that the periodic overvoltages caused by high frequency switching have a great influence on the lifetime of the insulation. The partial discharge (PD) characteristics of magnet wires for adjustable speed drives have been examined and discussed in a number of publications. These results are interesting for the winding insulation, but for the insulation system of the motor, the coil-stator insulation has to be regarded too. Here different insulation materials are used i.e. as slot liners, which protect the coil from mechanical but also from electrical stress. These materials are also stressed by the peak overvoltages, therefore there is a need for investigations regarding their resistance against high-frequency stress. In this paper we compare and discuss the PD characteristics (e.g. PD inception and extinction voltage, lifetime) of different materials used for slot lining at 100 kHz and 1 MHz