Temperature Rise of Metallized Film Capacitors Under AC and DC Superimposed Voltage

The metallized film capacitors in modular multilevel converter (MMC) submodules of unified power flow controller (UPFC) endure ac and dc superimposed voltage, which raises a new problem to the research of capacitor temperature rise. This article presents an experimental setup to perform the capacitor temperature rise experiment under ac and dc superimposed voltage. The effects of ac voltage, dc voltage component, and frequency on the temperature rise of metallized film capacitor are studied experimentally. The experimental results show that dc voltage has no effect on the temperature rise of the capacitor, and the temperature rise can be calculated using the ac voltage component and equivalent series resistance (ESR). According to the equivalent circuit model of the capacitor and experimental results, it can be considered that the equivalent resistance of conductor loss and the dielectric loss factor of the polypropylene film remain unchanged in the frequency range of 0–250 Hz. With the increase in frequency, the proportion of conductor loss power in capacitor increases significantly, and it should be considered in capacitor design. The research results can guide the design and selection of metallized film capacitors in MMC submodules of UPFC.