Working under PWM voltage, medium frequency transformer would suffer from electrical stress that is quite different from that of conventional power frequency transformer. Previous study showed that, due to steep wave front of the PWM voltage, inter-turn and inter-layer overvoltage would appear in the winding. Moreover, since the frequency of PWM voltage is much higher than that of the conventional sinusoidal wave voltage (50-60 Hz), skin effect and proximity effect would be induced, leading to a higher temperature in the transformer. Due to these electrical and thermal stresses, partial discharge may be triggered with high probability within the winding, especially for some regions that suffer from high voltage drops such as the region between adjacent winding layers and between two windings.

Partial discharge is a well-known factor that can greatly threaten the insulation reliability. Continuous discharge can degrade the insulation quickly and lead to a premature breakdown. Therefore, it is necessary to improve the insulation capability of the medium frequency transformer. This task is rather important in the future since the medium frequency transformer is developing to be smaller in size, in order to reach higher power density and to have lower costs, which also means shorter insulation distance and poorer heat dissipation. To achieve better insulation capability for the transformer, selecting better insulation material is an important part. This paper compares three different materials including polyimide, polyester and Nomex paper, which are often used for the medium frequency transformer’s insulation. Based on the test system that can generate repetitive pulse voltage, comparative partial discharge inception voltage tests under different temperatures and endurance tests are conducted. The results show that obvious difference exists among these materials with respect to resist partial discharge in high temperature and continuous partial discharge deterioration under repetitive pulse voltage. With these results, guidelines about choosing appropriate materials can be given for the medium frequency transformer’s design, in order to improve its insulation reliability.