Reliability analysis of MMCs considering sub-module designs with individual or series operated IGBTs

摘要

The half-bridge based modular multilevel converter (MMC) has emerged as the favored converter topology for Voltage-Source HVDC applications. The sub-modules within the converter can be constructed with either individual IGBT modules or with series connected IGBTs, which allows different redundancy strategies to be employed. The main contribution of this paper is that an analytical method was proposed to analyze the reliability of MMCs with the consideration of submodule arrangements and redundancy strategies. Based on the analytical method, the relative merits of two approaches to adding redundancy, and variants created by varying the sub-module voltage, are assessed in terms of overall converter reliability. Case studies were conducted to compare the reliability characteristics of converters constructed using the two sub-module topologies. It is found that reliability of the MMC with series connected IGBTs is higher for the first few years but then decreases rapidly. By assigning a reduced nominal voltage to the series valve submodule upon IGBT module failure, the need to install redundant sub-modules is greatly reduced.