Experimental Investigation on Breakdown Characteristics of Metallized Film Capacitors Under AC and DC Superimposed Voltage

As energy storage elements in modular multilevel converter (MMC) sub-module of a unified power flow controller (UPFC), metallized film capacitors endure ac and dc superimposed voltages, which brings new challenges to insulation performance of the dielectric film. In this article, an experimental setup was created to investigate the breakdown characteristics of the dielectric film for sub-module capacitors. Factors which may affect breakdown strength under ac and dc superimposed voltages were studied experimentally. The results reveal that, as the dc voltage component increases in the range of 0–3200 V, the breakdown strength gradually increases by 16.36%; as the ac voltage component increases in the range of 0–1800 V, the breakdown strength slightly decreases by 3.52%; the breakdown strength is more affected by frequency as the ac voltage component increases and decreases with increasing frequency; as temperature increases from 20 °C to 95 °C, the breakdown strength substantially decreases by 31.12%; as the thickness of the film increases from 7 to
$10~\mu \text{m}$
, the breakdown strength slightly increases by 6.32%. For capacitors in 500-kV UPFC, the dc voltage component is approximately 1600 V; the ac voltage component, whose peak-to-peak value is within 800 V, mainly contains power frequency and the second harmonic components. According to the experimental results, since the ac voltage component is small and the order of harmonic is low, the breakdown strength of capacitors in UPFC has little difference compared with that of the dc capacitors. This article can provide a reference for insulation design optimization of capacitors in UPFC.