Degradation Behavior and Mechanism of Metalized Film Capacitor Under Ultrahigh Field

Metalized film capacitor degradation under ultrahigh electric fields is crucial for the reliability of voltage source converter (VSC)-HVDC systems. In the present study, systematic investigations were performed that metalized film capacitors were aged under the dc electric field ranging from 300 to 400 kV/mm. Results showed that under moderate electric field, the capacitance reduction of sample capacitors presented two stages with different decay rates. With the increase of the electric field, the rapid degradation occurred immediately on the starting of the aging process, suggesting a voltage threshold determining the capacitors' stability. In addition, the degradation mechanisms of both metalized films and biaxial oriented polypropylene (BOPP) base films were investigated by statistical analysis and structural experiments. Below the voltage threshold, more shallow traps were generated and the activation energy of molecular motions was reduced due to PP molecules scissions, resulting in the decrease of breakdown strength (BDS) of BOPP films. While above the voltage threshold, the accelerated failure could be attributed to two main causes: the fast breakdown of originally formed weak points and the dramatically increased self-healing area. The study can contribute to the design and evaluation of dc-link capacitors both experimentally and theoretically.