Third-Harmonic Voltage Compensation Control Strategy for Modular Multilevel Converter with Unbalanced Grid Voltage

Modular multilevel converter (MMC) owns high efficiency and its operation results in higher power quality. It has broad applications in the power system. However, unbalanced grid voltage leads to three-phase current imbalance and double frequency fluctuation power in MMC. In order to improve the performance of MMC under the unbalanced voltage conditions, this paper presents a control strategy for suppressing negative-sequence current based on the third harmonic voltage compensation method. The current output characteristics of MMC is analyzed under the situation of power grid voltage imbalance. For the purpose of maintaining the three-phase current balance, the mathematical model of the third harmonic voltage is established, which is used to Figure out the relationship between grid voltage unbalance degree and the third harmonic voltage compensation to keep the three phase current balance. The effectiveness and performance of the proposed control strategy are verified by the PSCAD/ EMTDC simulations.