Contribution Mechanism and Impact Analysis of AC System at the Diode Natural Commutation and Conduction Stage During Bipolar Short-Circuit Fault for Single-Terminal VSC-Based DC Distribution Networks

For single-terminal voltage source converter (VSC)-based DC distribution networks, the characteristics of bipolar short-circuit faults are a key factor for the design of protection schemes and the selection of power electronic devices. However, the contribution of AC systems is usually ignored at the diode natural commutation and conduction stage. There is no precise mathematical analysis model for this stage, and no influence mechanism of AC system has been demonstrated from theoretical perspective. This article presents a fault analysis model of the diode natural commutation and conduction stage, focusing on the transient feeding process of VSC converter during the bipolar short-circuit fault for single-terminal VSC-based DC distribution networks. According to equivalent circuits of different transient substages, accurate mathematical state equations are established. Expressions of each electrical fault variation during different substages are provided. A precise mathematical model is constructed, which considers the contribution mechanism of AC system at this stage. Then, based on this mathematical model, the influence of the AC system on electrical fault variations is analyzed from a theoretical perspective. The analysis results show that the influence is always small under different conditions and that ignoring the contribution of AC system can still meet actual engineering needs. A simulation model is constructed using MATLAB/Simulink software. The simulation results validate the effectiveness and superiority of the analysis model.