Abstract With the attractive features, modular multilevel converters have been widely utilized and studied in large-capacitor power conversion field. This paper derived the expressions of capacitor voltage fluctuations from sub-module and the operating number of bridge arms. As for the fluctuation calculation of capacitor voltage as well as the complexity and diversity of performance analysis of MMC circulating current controllers, a composite circulating current control method was proposed. Herein, the feedforward control based on estimated voltage fluctuation, and the dual-closed-loop feedback control of voltage and current, were included. The proposed control method can effectively combine fast tracking performance of feedforward control with closed-loop tracking performance of feedback control. Accordingly, it can optimize the calculation. The method falls into four parts: estimated feedforward calculation of sub-module capacitor voltage; calculation of 2nd circulation voltage reference based small signal model; operating number calculation of sub-modules; sequence of sub-module capacitor voltage and operation of sub-modules. The 2nd circulation voltage reference was obtained through dual-closed-loop controller, while the dynamic performance of the dual-loop controller can be analyzed by equivalent closed-loop impedance. Then the appropriate control parameters can be selected with the best suppressing effect. Simulation and experimental results verify the validity of the proposed control method.
Yue Yufei,Xu Qianming,Ma Fujun等. Dual-Loop 2nd Circulating Current Strategy Based on Estimated Capacitor Voltage Fluctuation of Sub-Module for Modular Multilevel Converter[J]. Transactions of China Electrotechnical Society, 2017, 32(10): 20-32.
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