标题：Modular Multilevel Converter Control Strategy Based on Arm Current Control Under Unbalanced Grid Condition
作者：Ou, Zhujian; Wang, Guangzhu; Zhang, Lanhua
作者机构：[Ou, Zhujian; Wang, Guangzhu] Shandong Univ, Minist Educ, Sch Elect Engn, Jinan 250061, Shandong, Peoples R China.; [Zhang, Lanhua] Texas Instrument 更多
来源：IEEE TRANSACTIONS ON POWER ELECTRONICS
关键词：Arm current control; circulating current; modular multilevel converter; (MMC); multi-hierarchy control; unbalanced grid condition
摘要：The existing control strategies of modular multilevel converter (MMC) balance the capacitor voltage on the premise that the active power of ac side is balanced with that of dc bus. Thus, the symmetrical ac-side current references and the unevenly distributed dc current references in three legs are obtained by coordinate transformation, precise calculation, and numerous filters under unbalanced grid condition. However, by controlling capacitor voltages, the active powers between ac side and dc bus can self-regulate to balance, and this could simplify the obtainment of ac-side current references and dc-bus current references. Based on this idea, this paper proposed a control strategy, which combines the multi-hierarchy control with the arm current control for MMC under unbalanced grid condition. Within the multi-hierarchy control, the symmetrical ac-side current references and the unevenly distributed dc current references in three legs could be obtained easily by three voltage controllers in the abc coordinate, avoiding coordinate transformation, precise calculation, and numerous filters in the existing methods. Besides, the employment of the arm current control removes the need of the three-sequence ac-side current controllers and the three-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is designed for arm current regulator, which can perfectly track its reference and is easy to design, avoiding the complicated design of proportional resonant (PR) controllers. Both system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.