The commutation capacitor in LC oscillating branch of the mechanical HVDC circuit breaker plays an important role during the breaking process

and the withstood high-frequency voltage（HFV） of several thousand Hz may damage the polypropylene（PP） insulation in commutation capacitor. In order to improve the HFV breakdown strength

PP/boron-nitride composites doped with 1%

2% and 6% were prepared to conduct the experiments of polarization

thermal stimulation depolarization current

surface potential decay

and HFV（500

2 000 Hz） breakdown. The experimental results show that the HFV breakdown strength of the composites is increased compared with that of pure PP

and reaches the maximum at 1% content

which corresponds to the deepest trap energy. Based on bipolar charge transport and the space charge modulated breakdown model

the dynamic evolution of space charge and electric field is numerically simulated

including the dynamic variations of space charge distribution and electric field distortion rate with voltage frequency

trap energy

and boost-voltage time. The simulated results show that the charge injection under HFV voltage concentrates on the electrode-sample vicinity at a distance of several micrometers from the sample surface. The charge density and electric field distortion in the vicinity are more obvious under a higher frequency. The composite with deeper traps can improve the electric field distortion. The net accumulated charge amount under AC electric field is the most at 90°and 270°

and the electric field distortion is the largest at this time.