Modular Multilevel Converter Control Strategy Under Unbalanced Grid Condition

Upon the power analysis of modular multilevel converter (MMC), this paper proposed a comprehensive control strategy for the DC-link voltage, the capacitor voltages and the AC-side currents of MMC under unbalanced grid condition. Multi-hierarchy method was adopted to control the capacitor voltage. Under unbalanced grid condition, the AC-side currents were controlled to be symmetrical by adjusting active power distribution among three legs of MMC. Inner current loop employed arm current direct control, which can simultaneously control AC-side currents, DC bus current and circulating currents, removing the need for the three-sequence AC-side current controllers and the three-sequence circulating current suppressing controllers under unbalanced grid condition. A zero-sequence current canceller was proposed to add in arm current reference, and this could eliminate the zero-sequence fundamental-frequency current which is caused by the asymmetrical arm power losses and will flow to the DC-link. A 10kVA experimental prototype of the three-phase MMC was developed. The experiment results verify the feasibility and effectiveness of proposed strategy. © 2018 Chin. Soc. for Elec. Eng.