An Asymmetrical Power Module Design for Modular Multilevel Converter With Unidirectional Power Flow

Abstract

Modular multilevel converters (MMCs) are widely used in grid scale power electronics. Due to the presence of a dc component in the arm current, the MMC submodules (SMs) show an unbalanced current loading profile. This may lead to some semiconductor chips being more stressed than others, significantly affecting the overall reliability. To level the junction temperatures and improve the SM reliability, this article proposes an asymmetrical power module design in line with the loading profile, allocating more chips to higher current parts. This is shown to be realistic if the power flow is unidirectional. The concept is investigated using simulation backed by experiment. It is shown that the designed power module can significantly reduce the maximum temperature and the total power loss can be reduced by 5%–8%. The chip utilization rate is also improved. The proposed module is applicable in a certain range of power factor and modulation index adjustment. This article provides a guidance for optimum design of the power module for the SMs in an MMC.