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An On-line Capacitance and ESR Monitoring Method of Submodule Capacitors for Nearest Level Modulation Based Modular Multilevel Converters

The modular multilevel converter (MMC) has been rapidly developed and widely applied since it was proposed in 2001, especial in high voltage direct current transmission (HVDC), the MMC has been the primary choice. The reliability of submodule capacitors is a major concern since many practitioners consider it the most vulnerable component in power electronic devices. Therefore, it’s meaningful to put forward an on-line capacitor condition monitoring method to keep its safety operation. This paper presents an on-line monitoring method for capacitance and ESR values of submodule capacitors. In this method, the voltage and current equations are constructed by using MMCs operation characteristics and capacitor equivalent model to obtain the state parameters. Among them, the Cotes rule is used to simplify the calculation and Kalman filter is used to reduce the error. Simulation results verify the feasibility of this method. It shows that the capacitance calculation error is less than 1%, and the ESR calculation error is less than 4.2%.