IMC based circulating current controller for MMC-HVDC

The working performance of modular multilevel converters (MMC) is directly affected by the circulating current. This paper presents a circulating current suppressing technique based on internal model control (IMC) applied in conventional NLM modulation scheme and double loop upper control hierarchy. According to the instantaneous energy balance between AC and DC sides, the mathematical model of circulating current is analyzed firstly, which reveals its double-frequency and negative sequence features. Then, an IMC is developed based on double frequency rotational frame, and details of decoupling control block are given. A detailed Matlab Simulink model of MMC was built and corresponding simulation results are presented. Simulation studies with, without as well as PID based CCC are carried out, the effectiveness of the proposed control strategy is verified.