About the influence of the frequency on the partial discharge characteristics of enamelled wires

Due to the increasing use of high frequency switches e.g. in frequency converters there has been an increasing interest in the influence of high frequency voltages on insulation materials. For quality tests and evaluation of electrical insulation systems, the measurement of partial discharges (PD) is widely used. As an indicator for the quality of the insulation the PD inception and extinction voltages and the apparent charge at a certain voltage are common magnitudes. It is widely accepted that a 50 Hz PD test can not simply be compared with a high frequency PD test. For these reasons there is a need for measurements at high frequency voltages. For these measurements a powerful high frequency high voltage source has been developed. In this paper we show results of PD measurements at different frequencies between 100 kHz and 3 MHz. We discuss the different values of PD inception voltage (PDIV) and lifetime of twisted enamelled wires. Those are compared with data for 50 Hz. Our measurements show that the PDIV is greatly influenced by the characteristics of the test voltage source. If this aspect is taken into account, the PDIV is nearly constant in a frequency range of 200 kHz to 3 MHz. But regarding the lifetime during partial discharge activity there is an approximately double exponential decrease with increasing frequency.