Norton Equivalent Modeling of Current Source MMC and Its Use for Dynamic Studies of Back-to-Back Converter System

The current-source modular multilevel converter (CSMMC) is a good alternative for high-power applications with medium-voltage and high-current requirements. In such applications, to obtain the multilevel current output and for current scaling, a large number of submodules are required in CSMMC. The detailed representation of such systems in electromagnetic transient-type simulation programs brings computational complexity with a large computing burden. This paper presents a computationally efficient equivalent model of CSMMC based on the Norton equivalents. Each arm of the converter is represented by a two-node Norton equivalent in this model, which eliminates the internal intermediate nodes and, hence, significantly reduces computing time. The proposed model is implemented in PSCAD/EMTDC by using the module component coded in FORTRAN and results are validated with the full-size traditional model in PSCAD/EMTDC. Moreover, hybrid models are presented and validated for the submodule level studies such as fail-safe functionality with redundant submodules. This paper also focuses on the control method and dynamic studies of the CSMMC-based back-to-back system. The results of these studies are compared with the proposed equivalent model, and accurate matching of the results confirms the usefulness of the proposed model in such studies.