Submodule Capacitance Monitoring Approach for the MMC With Asymptotically Converged Error

To reduce noise interferences and improve the steady-state estimation accuracy of submodule (SM) capacitors, a novel SM capacitance monitoring approach has been proposed for the modular multilevel converter in this article. First, the fundamental frequency (FF) component is analyzed to be dominant of the capacitor voltage squared. Then, based on the energy equation W = Cv ² /2, half of the FF capacitor voltage squared is selected as the input variable, and the FF capacitor energy calculated by power integral is selected as the feedback component. By proper design of the estimation law, closed-loop capacitance monitoring can be achieved with reduced noise interference. Furthermore, based on the Lyapunov stability criterion, the estimation parameter is designed so that the estimation error asymptotically converges to zero. Compared with the conventional methods, the proposed approach is easy to implement and requires no operation interruption of the monitored SM. Simulation and experimental results indicate that by the proposed approach, the estimation error of the SM capacitance can be limited to 1%.