Research on Control Strategies of Modular Multilevel Converter Based on Three-Level Controllers

The topological structure and basic operation principle of the modular multilevel converter (MMC) are introduced. An implementation method of a new PWM modulation algorithm suitable for MMC is improved, and the equalization strategy of sub module capacitor voltages is analyzed. In the improved algorithm, only one modulation wave is required for the upper and lower arms of the same phase, and there is no need to calculate the number of sub - modules in the on and off states and the PWM duty - cycle information of sub - modules in the PWM state, which reduces the system complexity. Meanwhile, a control system architecture based on a three - level controller is proposed, and the design methods of controllers at all levels are analyzed. Finally, a five - level experimental prototype is built for experimental verification. The experimental results verify that the improved algorithm is correct and feasible, and the control system operates stably. (Abstract)