State-Space Based Analysis of Initial Fault Current Response in Bipolar MVDC Systems with MMCs

Medium Voltage Direct Current (MVDC) systems are increasingly used in modern power networks due to their high power density and transmission efficiency. However, the rapid rise in fault current within milliseconds presents significant challenges for protection systems. Conventional methods, such as switching-based simulations, offer high accuracy but are computationally demanding, while RLC equivalent models are faster but less precise during transient conditions. This study analyzes the initial fault current response in a bipolar MVDC system with full-bridge Modular Multilevel Converters (MMCs) and multi-ground neutral line. Two fault types are considered: Pole-to-Ground (PTG) and Pole-to-Pole (PTP). For each case, equivalent RLC parameters are derived, and two modeling approaches, an analytical RLC model and a State-Space Equation (SSE) model, are compared. Both models are validated against simulation data using cumulative Root Mean Square Error (RMSE) as the evaluation metric. Results show that the SSE model provides improved accuracy for PTG faults and comparable performance for PTP faults. These findings highlight the potential of the SSE approach as a fast and reliable tool for early fault current analysis and protection strategy development in MVDC systems.