Online Capacitance Estimation of Submodule Capacitors for Modular Multilevel Converter With Nearest Level Modulation

This letter proposes an online capacitance estimation method for the modular multilevel converter. In the prevalent methods, the inherent second-order circulating current or the injected low-frequency current/voltage component is utilized to calculate the submodule (SM) capacitance. In this letter, the relationship between the voltage variation of an SM capacitor in one fundamental period and the driving signals switching angle under the nearest level modulation scheme is revealed. The voltage variation of each SM capacitor is a direct component and has no relationship with the second-order harmonic current and voltage ripples. Accordingly, an online capacitance estimation method for HVdc applications is proposed by simply monitoring the SM capacitor voltage variations at fundamental frequency. In the proposed scheme, neither the current/voltage injection nor the inherent circulating current is required, which reduces the estimation complexity. The recursive least squares algorithm is also adopted to guarantee the accuracy of the estimation method. Effectiveness of the proposed online capacitor estimation method is verified by the experimental results.