Metallized Polypropylene Film Capacitor Condition Monitoring in MMC Based on Impedance Measurement

The healthy condition of the capacitor in each submodule (SM) is of great importance for the safety and stability operation of modular multilevel converter (MMC). An MMC structure often contains hundreds of SMs. Although the metallized polypropylene film (MPPF) capacitors with superior performance are widely used in current MMC projects, the aging and failure of those capacitors are still inevitable. Considering the most common cause of failure (self-healing and end-spray detachment), equivalent series capacitance (ESC) and equivalent series resistance (ESR) are employed to account for condition parameters of the MPPF capacitor. A new SM impedance measurement technique is proposed to monitor the condition of MPPF capacitors in MMC. Measurement technique extracts arm current, switching sequence and voltage signal of SMs which are already existed in control system, SMs capacitor voltage and current is then calculated by above signals. After processing its various frequency components by fast Fourier transform (FFT) algorithm, ESC and ESR of MPPF capacitor can be figured out with desired frequency. A 2.1kVA MMC system is established in MATLAB/Simulink. Simulation results verified the effectiveness and accuracy of the monitoring technique.