Improved carrier phase shift modulation and voltage equalization control strategy in modular multilevel converter

In order to solve the problem of traditional carrier phase-shift modulation with multiple ratios or PI controllers and cumbersome tuning parameters, this paper uses improved carrier phase-shift modulation. The total turn-on number of sub-modules each bridge arm is determined by comparing the sinusoidal modulated wave with the triangular carrier, and then the control signal is generated according to the capacitance voltage sorting result and the bridge arm current polarity. However, this modulation method uses a sorting method that causes the insulated gate bipolar transistor (IGBT) have an excessively high switching frequency. Therefore, a sorting trigger condition that can effectively reduce the switching frequency is used. The method determines whether to reorder based on the error between the voltage average and the actual value. For the circulation problem, the double-frequency negative sequence component is extracted by rotating coordinate transformation, and it is suppressed by PI control. A 21-level MMC model was built in MATLAB/simulink to analyze the sub-module capacitor voltage fluctuation, output current, voltage distortion rate and bridge arm circulation. It is verified that the modulation method can combine the sorting algorithm and circulation suppression method at the same time, and has better voltage equalization and circulation suppression effects.