Improved Voltage Balancing Method Based on MMC Nearest Level Modulation

Nearest level modulation is one of the main modulation strategies in the modular multilevel converter (MMC), which has obvious advantage in the converter with a large number of voltage levels. To ensure the effect of voltage balancing, the capacitor voltages of submodules are usually required to be collected and sorted all the time, and a great deal of voltage acquisition modules are needed to achieve this goal in practical engineering. In view of the above, a replacement method is proposed, the integral amounts of bridge arm current to modules conduction time is used to sort , that indirectly realizes the effect of voltages sorting. and the method can effectively reduce the voltage sensors. Meanwhile, on the basis of the replacement method, this paper introduces the maximum voltage deviation among submodules, on the premise that the system output characteristics in allowable range, it can greatly reduce the frequency of IGBT switching caused by sorting. The model of MMC with 21 levels is realized by RT-LAB, the IGBT switching frequencies and submodule capacitance voltages under two voltage balancing methods are compared. the simulation results show that the proposed method is feasible and effective.