General Pulse Distribution Method of MMC under Phase-leg Based PD-PWM Modulation

Under the phase-leg based PD-PWM modulation, the upper and lower arms of the modular multilevel converter (MMC) are treated as a whole and coupled with each other. To reasonably distribute pulses in this scenario, this paper proposes a general pulse distribution method on the basis of finite state machines. This method does not need to know the whole state machine, but starts from the local state. Upon multilevel variations, the next state is selected through self-defined rules, which ensures active control of circulating current, minimization of total switching commutations, and even distribution of switching among submodules. This method has the following four features: 1) The circulating current is only dc and does not contain low-order harmonics; 2) There is only one submodule in response to each multilevel change and no redundant switching action exists; 3) A pseudo-random process is used to select the next-state, ensuring self-balancing of the submodule capacitor voltage over a large time scale. No active voltage control is required and the hardware cost is reduced; 4) It is suitable for cases where the number of sub-modules is arbitrary. Results from simulation and experiment validate the correctness and effectiveness of the proposed method.