Impacts of Self-healing Properties on the Thermal Model of Metalized Film Capacitors Under AC Harmonics

The temperature rise of metalized film capacitors (MFCs) is commonly originated from the dielectric loss and conductivity of the dielectric film and metal parts. However, in practical applications, self-healing (SH) point is another inevitable heat source. In this article, the properties of SH points were firstly investigated. The results revealed that the carbon layers, generated during SH process, would increase the equivalent conductivity of SH point up to 10⁻⁶ S/m. Accordingly, a thermal model of MFC was established, including a primary part originated from the loss of dielectric and conductivity, and a dynamic part related to the SH points leakage currents. The rising of frequency enhanced both the primary and dynamic part, while the elongation of the ageing time further raised dynamic part. Furthermore, ageing tests were conducted under the dc voltage superimposed with ac harmonics from 0 to 250 Hz. Results showed that the MFC aged solely under dc voltage presented little temperature rise, while the temperature rise under 250 Hz harmonic was 6.6 °C, almost 8 times higher than that under 50 Hz. The study manifested the impacts of SH point to the capacitor thermal behavior, which could contribute to the design and the operation of MFC.