A Collaborative Injection Strategy for Capacitor Voltage Fluctuation Reduction of MMC

The modular multilevel converter (MMC) has many advantages like excellent harmonic performance, low switching loss and so on. As a result, it’s widely applied in the field of large-scale renewable energy centralized power generation and high voltage flexible DC transmission. However, a large number of submodule capacitors in MMC result in its large size and high cost, which should hinder the large-scale promotion of flexible DC transmission. By optimizing the control strategy of MMC, the capacitor voltage fluctuation MMC’s submodules can be reduced, which is conducive to reducing the requirements on the capacitance value, volume and cost of the submodules. In this paper, the capacitor voltage fluctuation principle of the MMC submodule is analyzed. Then the collaborative injection strategy is proposed to reduce the capacitor voltage fluctuation effectively, it’s a combination of second harmonic current injection and the third harmonic voltage injection, and in collaborative injection strategy the second harmonic voltage controller is designed to control the second harmonic component of capacitor voltage. Finally, the effectiveness of the collaborative injection strategy is verified by simulation.