Abstract
The half-bridge-based modular multilevel converter (MMC) has emerged as the favored converter topology for voltage-source HVDC applications. The submodules within the converter can be constructed with either individual insulated-gate bipolar transistor (IGBT) modules or with series-connected IGBTs, which allows for 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 submodule voltage, are assessed in terms of overall converter reliability. Case studies were conducted to compare the reliability characteristics of converters constructed using the two submodule 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 submodules is greatly reduced.
Highlights
Multilevel Voltage Source Converters (VSCs) have been widely used in the High Voltage Direct Current (HVDC) applications
In an MMCSV, if redundant IGBT modules are included within a valve, each IGBT module is subjected to a lower voltage
If the failure rate of IGBT modules decreases by 30%, the mean time to failure (MTTF) of the MMCSV increases to approximately 3 years
Summary
Multilevel Voltage Source Converters (VSCs) have been widely used in the High Voltage Direct Current (HVDC) applications. Reliability models of MMCs are presented based upon the failure rate of IGBTs within individual sub-modules, considering the cases where each sub-module valve contains either individual devices, or series connected devices. Using this analytical model, the impact that the sub-module arrangement will have on the overall converter reliability is evaluated, and the relative merits of the two approaches are compared in terms of reliability and voltage capability. The influence of a reduced nominal voltage operating mode of the sub-module type containing series devices, on the converter reliability is assessed
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