Circulating Current Suppression Control of Modular Multilevel Converters Under Optimized Phase Disposition (PD) Modulation

This paper proposes an active low-order circulating current suppression control (CCSC) strategy for modular multilevel converters (MMCs) controlled by optimized phase disposition modulation (PD-PWM). Since optimized PD-PWM uses the same modulation reference for both MMC arms, the CCSC strategy is embedded into the pulse allocation stage, where it reallocates the submodule switching pulses to minimize low-order circulating current harmonics. While achieving a similar performance as a band change correction strategy to overcome the intrinsic limitation of PD-PWM patterns, the new approach can also suppress low-order circulating current components caused by small module capacitors, at the expense of slightly increased submodule switching frequencies. Simulation and experimental results using a five-level 2N+1 type MMC validate the effectiveness of the proposed CCSC technique.