Research On Control Method Of Modular Multilevel Converter

With the continuous progress of society, the requirement of flexible HVDC transmission is also increasing. The converter as the core component of the direct current transmission needs to improve the quality to meet the requirements of the transmission. In many converters, due to the high voltage level and low switching frequency, the modular multilevel converter(MMC) has received more and more attention. This paper introduces the related control method of modular multilevel converter. The main contents are as follows:Firstly, Based on the introduction of MMC topology, working principle,mathematical model and the selection of capacitor voltage value for sub module,this paper studies the block pre-charging control method of MMC. It mainly includes the analysis of block pre-charging process and equivalent circuit. On this basis, when capacitance voltage reference value of sub module doer not exceed the maximum value of block pre-charging, it use the partial pressure of series resistance, making capacitor voltage of all sub module in converter to achieve the desired value after a cycle.Secondly, this paper studies the carrier phase shifted pulse width modulation(CPS-PWM) method of MMC. It analyzes the principle of the single modulation wave and the double modulation wave CPS-PWM method. Besides,it carries out the simulation to verify this method. According to the conditions of MMC stable operating, it can known that double modulation wave CPS-PWM method is more suitable for MMC. Then, it analyzes the third-harmonic injection in double modulation wave CPS-PWM and carries out the simulates to verify this method. The simulation shows that this method can reduce the capacitance voltage fluctuations of sub module and the capacitance voltage harmonic content of sub module.Finally, this paper studies the voltage balancing of sub module method of MMC. In view of the capacitor voltage balancing control of sub module containing large amount of calculation, this paper improves the method. The improved method is described in detail. At the same time, it also introduces theprocess of the overall control of MMC. According to the diagram of MMC system overall control, the simulation module is constructed. The rationality of the voltage balancing control and MMC integrated control is verified by simulation.