Improved voltage equalization control strategy in modular multilevel converter

Abstract

In order to solve the problem of traditional carrier phase-shift modulation with multiple ratios or PI controllers and cumbersome tuning parameters, this paper proposes an improved carrier phase-shift method. The control signal is no longer generated by a large number of proportional or PI controllers, but rather by the number of turn-on sub-modules and sorting principles. However, because the method uses a sorting method, the IGBT switching frequency is too high. Therefore, a sorting trigger condition that can effectively reduce the switching frequency is proposed, and the lower switching frequency is realized by reducing the times of sorting. At the same time, the method has better modulation wave following characteristics, and it is easy to achieve the suppression of the circulation. For the circulation problem, the second-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 sorting algorithm and circulation suppression method at the same time, and has better voltage equalization and circulation suppression effects.