Abstract

The stability criterion of MMC (Multilevel Modular Converter) based on the sequence impedance model has been proved to be an effective method to analyze the practical resonance problems, such as the resonance problems of the interconnected system of wind farm, photovoltaic inverter and HVDC converter. In this paper, the topology and working principle of MMC are analyzed firstly. Then the impedance model of MMC ac port considering inner current loop and outer power loop is established. Based on the impedance model, the effect of circulating current suppression loop, power operation zone, time delay of control system and grid side voltage- feedforward on the stability of the ac side of MMC connected to the weak grid network is analyzed. Next, virtual damping additional strategy is proposed to enhance the stability of the system. Finally, a ±500kV 232 level MMC model is built in MATLAB/Simulink to verify the resonance mechanism and the effectiveness of the proposed measure.

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