I. Introduction
Capacitors play an important role in harmonic filtering and reactive compensation in HVAC system, and also in voltage support in HVDC system. In addition, capacitors as energy storage devices are also indispensable in many technical fields. Polypropylene (PP) dielectric films with uniform structures or properties are easy to be produced by biaxial stretching technology. Such as biaxially oriented PP (BOPP) films are considered to be the most advanced (state-of-the art) capacitor dielectric film. However, the low dielectric constant of PP limits its energy storage density to 3-5 J/cm3, which is difficult to meet the increasing requirement for high energy storage density or miniaturization of capacitors. In addition, the working field strength of power capacitor dielectrics is usually about 1/3 of the breakdown field strength, and under such high field strength, insulation damage accidents of PP capacitors often occur.