Modular multilevel converters with integrated arm inductors for high quality current waveforms

Abstract

This paper deals with the system structure and operating principle of a modular multilevel converter (MMC) with integrated arm inductors for improved performance. The proposed integrated inductors provide inductances not only for circulating current suppression, but also for switching ripple mitigation. Compared with the conventional MMC structure implemented with two separate inductors connected in both upper and lower arms by two magnetic cores, only one core is required for the arm inductor of each phase. Hence, the overall size, weight, and cost of magnetic components will be much lower than discrete ones. In addition, the relationships between the number of voltage levels, the equivalent differential inductance of the integrated inductor, and the total harmonic distortion (THD) of the phase voltage is analyzed based on the designed integrated inductor. Without differential inductance, the number of voltage levels should be more than 12 with N+1 phase shift PWM (PSPWM) or 8 with 2N+1 PSPWM to bring the THD below 5 %, while this goal can be achieved by 4 sub-modules MMC with only 2 mH differential mode (DM) inductance if N+1 modulation is applied, or 0.5 mH DM inductance if 2N+1 modulation is adopted. Simulation results for a three-phase inverter system are provided to support the theoretical considerations.