The Research of Capacitor Voltage Balancing Strategy of MMC

Abstract

Compared with the two level voltage source, the modular multilevel converter (MMC) adopting the modular design dose not have the disadvantages such as the direct series connected Insulated gate bipolar (IGBT),high generation of harmonics, etc. MMC is currently studied and applied in high voltage and high power conversion applications. But the main problem in the new potential MMC topology is the voltage balance of the capacitors of their sub modules to protect the capacitors voltage fluctuation within the same voltage levels, so the voltage balance strategy plays an important role in improve it's performance. In order to achieve voltage balance, this paper ameliorates the traditional balance arithmetic through changing the charge and uncharged time; what is more, a capacitance voltage diversity index(CVDI) is defined firstly to adjust the sorting frequency, so the switching frequency of IGBT is reduced under satisfying the balance demand. At last, the simulation is carried out to verify these algorithms validity. Keywords-Modular multilevel Converter; voltage balance; sort sequence balance; capacitance voltage diversity index.