A Closed-Loop Time-Domain Analysis Method for Modular Multilevel Converter

Time-domain steady-state analysis for modular multilevel converter (MMC) can effectively realize the performance evaluation, circuit parameters design, and semiconductor devices selection. However, the existing/conventional analysis methods could not always ensure the accuracy in the full operation range since they are mostly a kind of open-loop analysis method utilizing approximate switching function. This paper proposes a closed-loop analysis method for MMC utilizing the accurate switching function. This method first constitutes two closed analysis loops inside MMC based on the classical circular interaction. Along with them, two nonlinear key equations are established to solve the accurate switching function and circulating current, and finally to acquire all the other electrical quantities coupled by them. The proposed closed-loop analysis method presents improved analysis accuracy in the full operation range and the accuracy is irrelevant with the operation conditions. Moreover, this method reveals the explicit relationship between the switching function and operation points, and, consequently, can realize accurate ac-side voltage selection for MMC, which is very useful for the transformer design and transformer tap selection in practical applications. The effectiveness and accuracy of the proposed analysis method were verified by both simulation and experimental results.