Self-healing Characteristics of Metallized Film Capacitor under DC and AC Superimposed Voltage with Harmonics

Metallized film capacitors (MFCs) are reliable because of the self-healing feature and are widely used in the sub-module of the modular multilevel converter (MMC-SM). To reflect the practical working condition of MMC-SM, the self-healing characteristics of MFC in MMC-SM under DC and AC superimposed voltage with harmonics were studied in this paper. A film level experimental platform was built, in which the specimens were double-layer metallized film elements. A parallel superimposing circuit was adopted, where the DC voltage and the AC voltage containing harmonics were applied with independent sources. The self-healing breakdown voltages were recorded and analyzed by the Weibull distribution. It was found that the harmonic components have a significant effect on the self-healing characteristics of MFC. As the harmonic component of the applied voltage increases, the self-healing breakdown voltage decreases. Besides, the shapes of self-healing areas are distinct in different regions of double-layer films, which are highly related to the local film square resistances of the breakdown spots.