Control Strategy of Modular Multilevel Matrix Converter for Low Frequency Transmission System under Unbalanced Grid Conditions

With the purpose of reducing the transmission loss of offshore wind energy, the LFTS came into being under the background of offshore wind power generation. However, When a fault occurs on the grid side, the capacitor voltage balance of M
³
C is destroyed, which seriously affects the safe operation of the LFTS. This paper mainly studies the control strategy of LFTS based on M
³
C in the event of asymmetric fault. This method can not only maintain the symmetry of the power frequency side current, but also maintain the balance of the sub-module capacitor voltage, which ensures the normal operation of the system. Firstly, the operation characteristics of M
³
C bridge arm under unbalanced conditions are analyzed. Then, the positive and negative sequence components on the grid side were separated using the T/4 delay algorithm, and the negative sequence currents were suppressed by the dual sequence control strategy. And the balance of capacitor voltage is achieved by controlling the injected loop current using the PR controller. Finally, the effectiveness of the proposed control strategy is verified by the M
³
C simulation results under grid fault.