XIE Rui1，HU Lie-xiang2，XV Jian-guo1，ZHOU Zhi-chao1，QIAN Feng1，GAO Zhi-ling1

（1. Zhejiang Electric Power Design Institute，Hangzhou 310012，China；

2. State Grid Zhejiang Electric Power Company，Hangzhou 310007，China）

Abstract： Aiming at comparing the different sub- modules of modular multilevel converter（MMC），the operation process and characteristics were analyzed. The equivalent circuits of output voltage and circulating current were obtained by the detailed mathematical expressions. Also the applications of these different sub-modules of MMC were summarized. In order to compare the different modulation strategies，the accurate expressions were calculated to analyze their linearity and harmonic characteristics. Then the performances of

different capacitor voltage balancing methods were concluded from the perspectives of system cost，controlling complexity and system loss distribution. The voltage level，system capacity and operation characteristics of four MMC based HVDC projects in China were compared with each other. The researching results indicate that nearest level modulation（NLM）method is easy to be realized and usually adopted for the large scale MMC with the capacitor voltage sorting algorithm. Carrier phase shift pulse width modulation（CPS-PWM）can obtain

the performances of high equivalent switching frequency and low distortion，and can be used with capacitor voltage sorting algorithm or closed-loop balancing method even when the number of sub-modules is small.

Key words：modular multilevel converter（MMC）；high voltage direct current transmission；stair-case modulation；carrier phase shift

pulse width modulation；capacitor voltage balancing method