Capacitance Monitoring Method for Metallized Polypropylene Film Capacitor in MMC

Modular multilevel converter (MMC) is a promising converter topology for grid applications which requires high reliability. An MMC submodule (SM) contains large capacitors to stabilize the DC link voltage, and the capacitor reliability is an important issue in long term operation. While capacitor degradation is always associated with a reduction of capacitance value, detection this change on-line depends on the charging and discharging dynamics. A method of on-line monitoring the capacitance of metallized polypropylene film (MPPF) capacitor in each submodule is proposed in this paper based on model reference adaptive control (MRAC). It uses the available bridge arm current, the submodule capacitor voltage and the synchronous switching signal to obtain the capacitor voltage and current. Actually capacitor measurements are used as the reference model, and a mathematical model that tries to follow the reference as the adjustable model. The two models accept the same capacitor voltage as input and are expected to output the same current. The error between the output signals is used as feedback to modify the parameters of the adjustable model until it is zero. The capacitance value is then used to signify the capacitor condition. Both simulation and experiment verify the effectiveness and accuracy of the proposed method.