A Novel Hybrid Modulation Strategy for MMC Energy Storage System

To balance the traditional nearest level modulation (NLM) lower switching times and the carrier phase-shift PWM modulation (CPS-PWM) excellent output harmonic characteristics, this paper proposes a novel hybrid PWM modulation strategy (NHPWM). NHPWM can arbitrarily configure the modulation mode of each sub-module, which provides a method for the system to coordinate device loss and output harmonic characteristics in real-time according to requirements. A rigorous mathematical solution method is proposed to accurately analyze the influence of NHPWM mode configuration on the MMC output voltage. Accurate analytical formula for the harmonic distribution of the converter output voltage under different modulation ratios and modulation mode distribution schemes is obtained, proving this kind of hybrid modulation strategy can suppress the low-order harmonics brought by NLM. In addition, the sub-module voltage equalization control method under NHPWM is given. A suitable voltage equalization control effect is obtained through reasonable sub-module working mode configuration combined with the common voltage equalization control method under CPS-PWM and NLM. Finally, the control strategy proposed in the theoretical analysis was verified through simulation and experimental verification.