Serving the function of electrical isolation, power transmission, and scaling up/down the voltage level, medium frequency transformers are widely used in modern power electronic systems. The working voltage waveform of these transformers brought from semiconductor switches is usually PWM voltage that has the characteristics of medium frequency (much higher than 50-60 Hz, usually in the range from hundreds Hz to several kHz) and very fast voltage front comparing with the AC sinusoidal voltage used by traditional power transformers.

Because of this, these transformers may suffer from unusual electrical stress. Inter-turn and inter-layer overvoltage in the winding would be triggered due to the fast voltage rising front. In addition, since the frequency of the PWM voltage is much higher than that of power frequency, over temperature would be induced by increased core and wire losses. Both these factors can increase the risk of the partial discharge generation in the winding, especially in the parts that suffer from high voltage drops such as the region between two adjacent winding layers. Partial discharge may accelerate the insulation aging speed and lead to premature failure. Since the medium frequency transformers are developing to be smaller in size, which means shorter insulation distance along with poor heat dissipation, this potential problem should not be ignored.

Therefore, it is necessary to evaluate the insulation capability of medium frequency transformer with respect to partial discharge and deterioration under voltage pulses similar to the PWM voltage. This paper introduces a test system based on power electronic modules that can generate repetitive pulse voltage combined with a partial discharge detection apparatus. Then, by using this system, partial discharge inception voltage test under different temperatures and endurance lifetime test on Nomex paper, which is an insulation material usually used in the medium frequency transformer, are conducted. The results show the change trending of the partial discharge inception of Nomex paper with rising temperature and the fast aging speed of this material with the presence of continuous discharge. Based on these results, the threat from the PWM voltage to the insulation of medium frequency transformer has been confirmed and some suggestions can be given to cope with this threat.