A Study of Common Mode Voltage Generation According to Modulation Methods and Reduction Strategies on MMC System

This paper proposes a method for analyzing common mode voltage(CMV) and three algorithms for reducing it in a low level MMC (Modular Multilevel Converter) system. An arm voltage, a pole voltage and a common mode voltage are defined according to the switching state of each submodule. In order to analyze characteristics of the MMC system, an independent space vector is used to express the upper and lower reference. This paper analyzed the principle of the modulation process of MMC system and proposed three common mode voltage reduction techniques based on NLC+PWM modulation method. DCRM (Discontinuous PWM CMV Reduction Method) to reduce the number of common mode voltage generation, PCRM (Partial CMV Reduction Method) to reduce the maximum value of common mode voltage, and CCRM (Complete CMV Reduction Method) method to completely reduce the common mode voltage to 0. The validity of the proposed common mode voltage generation analysis and reduction methods are verified by MATLAB simulation and experiments using a 5-level MMC set.