Capacitance Parameter Design Of Hexagonal Modular Multilevel Ac/Ac Converter

This paper mainly studies the reasonable design of capacitance parameter of the hexagonal modular multilevel AC/AC converter. Based on the brief analysis of working mechanism, the minimum required capacitance of the submodule is designed from the viewpoint of the power and energy pulsation of the converter's branch and the voltage ripple factor is also taken into consideration. Firstly, the system structure and operating principle of the hexagonal modular multilevel AC/AC converter including the branch voltage and the minimum number of submodules n required in the branch are analyzed and verified by simulation. Secondly, for two different working conditions of f(1)not equal f(2) with Q(g1)=Q(g2) and f(1)=f(2) with Q(g1)not equal Q(g2) where f(1)/f(2) and Q(g1)/Q(g2) denote the frequency and reactive power of both systems respectively, the branch voltages, currents and the expression of the voltage fluctuation of the capacitor are derived, then the branch instantaneous power is obtained and its containing frequency components are analyzed in detail. Furthermore, the energy pulsation of the branch and submodule are deduced. Based on the relationship between the capacitor's required energy storage and its rated voltage when taking into consideration the voltage ripple factor, the function expression of the capacitance parameter of the submodule is obtained.