Influence of High Frequency Voltage with Harmonic Contents and Temperature on Breakdown of Epoxy Resin Used in Power Electronic Transformer

Insulation in medium-frequency transformers of power electronic equipment usually suffers from the high-frequency voltage stresses, sharp-pulsed voltages and high temperature, leading to the dielectric breakdown failure. Besides, the harmonic contents are included in the high-frequency voltage waveform. It plays an important role in the insulation stress and dielectric loss under high-frequency fields. This paper studies the dielectric breakdown characteristics of the epoxy resin used in medium-frequency transformers under different frequencies voltages and temperatures. Particularly, the effects of harmonic voltages on breakdown strength are considered and discussed. The high-frequency breakdown test platform is used to measure breakdown voltage, including adjustable frequency sinusoidal voltage source (up to 20 kV) and oil bath heating system (up to 200 degrees C). The frequency of the applied voltage can be reached to 10 kHz. Weibull distribution analysis was employed to discuss the influence of frequency and temperature on dielectric breakdown characteristics of epoxy resin. The experimental results indicated that the breakdown strength of the epoxy resin decreases significantly with the increase of the frequency. Meanwhile, it decreases with an increase in temperature. The breakdown strength under 8 kHz is about 30% lower than 500 Hz at 120 degrees C. While it (120 degrees C) is about 15% lower compared to that of 30 degrees C at 8 kHz. It can be concluded that frequency has a greater effect on breakdown strength compared to temperature. In addition, the electric stresses of the contained harmonic voltages are enhanced under high voltage. Furthermore, the dielectric loss caused by the harmonic voltages presents a large proportion of the total dielectric loss. Hence, the breakdown analysis pointed out that the dielectric heating together with the high electric stresses contributes to the rapid breakdown of the epoxy resin.