A hybrid modulation strategy suitable for MMC with small quantities of sub-modules

Abstract

As the voltage level is relatively low under medium and low voltage distribution network,the number of modular multilevel converter (MMC) sub-modules is also small. Therefore,the modulation mode of MMC with small quantities of sub-modules has an important influence on the performance of the MMC system. In order to improve the output quality and output capability of the MMC with small quantities of sub-modules,a hybrid modulation strategy suitable for MMC with small quantities of sub-modules based on level step switching at a high modulation degree is proposed. On the basis of the nearest level modulation (NLM),the MMC is controlled to switch between NLM and the carrier phase shift pulse width modulation (CPS-PWM) at the output level step point in real time. At the same time,the circulation control and the sub-module voltage equalization control are combined to further ensure the normal operation of MMC with small quantities of sub-modules under the hybrid modulation. Then,the simulation model of MMC hybrid modulation with four sub-modules is established,and a simulation study is carried out on the MMC hybrid modulation strategy of the MMC with small quantities of sub-modules. The results show that the proposed hybrid modulation strategy not only takes into account the characteristics of low output harmonics and low switching loss,but also improves its output quality. And its operation in high-profile system increases the DC voltage utilization rate and improves the output capacity of the MMC with small quantities of sub-modules.

Keywords

• medium and low voltage distribution network
• modular multilevel converter (mmc)
• nearest level modulation (nlm)
• carrier phase shifted pulse width modulation (cps-pwm)
• harmonic analysis
• voltage equalization control