Calculation of DC Fault Current and Analysis of Influencing Factors in MMC-based Medium Voltage DC Systems

The use of modular multilevel converters (MMCs) is an emerging trend in medium-voltage direct current (MVDC)-based transmission and distribution networks. In order to address possible dc faults in MVDC system, it is necessary to conduct fault analysis with numerical calculations. Since MVDC systems have more complicated topology structures and flexible source/load deployment, the conventional fault analysis method applied to the HVDC systems cannot be directly extended to MVDC networks. In this paper, an expanded RLC equivalent model is derived for state-space based analysis of fault current calculations. Results of MMC and branch currents in different cases reveal the unique characteristics in the MVDC system. The system fault level is evaluated and compared with respect to different influencing factors, i.e. fault locations and initial operating scenarios.