Lifetime Analysis of Metallized Polypropylene Capacitors in Modular Multilevel Converter Based on Finite Element Method

Metallized polypropylene capacitors (MPPCs) are widely used in the modular multilevel converter (MMC) for high-voltage direct-current transmission systems because of their lower power losses and self-healing capability. The performance of MPPCs in MMC deteriorates with time due to the increase in the equivalent series resistance and a decrease in the capacitance. Therefore, the reliability analysis of MPPC is critical. This article proposes a finite element method (FEM) to analyze the reliability of MPPC by considering corrosion failure. First, the equivalent electric model and actual thermal model of the MPPC are established to calculate power losses and temperature distribution of the MPPC. Second, the corrosion failure of the MPPC is analyzed and simulated by the FEM model, and the lifetime model of MPPC is established by the aging model of polypropylene film and verified by the traditional lifetime model of corrosion failure and the floating aging tests. Finally, the voltage of each submodule (SM) is extracted in the MMC model, and the lifetime of MPPCs in each SM is analyzed by combining the FEM model and the lifetime model. The results show that in the MMC, the SMs in each arm near the dc line or the middle part have a lower lifetime of MPPCs.