Comprehensive Study on the Grid Fault Behavior of Grid-Forming Control for Modular Multilevel Converters

This comparative study evaluates the performance of various Grid-Forming techniques, namely, Virtual Synchronous Machine (VSM), Droop Control, and Proportional-Integral (PI) control. Additionally, it explores the structures used for fault current limitation, including the Voltage-Current cascade structure, the Synchronous Power Controller (SPC), and the Virtual Impedance. The study focuses on evaluating the performance of these Grid-Forming topologies and structures in terms of fault current limitation under various operating conditions and fault scenarios. Simulation-based investigations are conducted using a Modular Multilevel Converter (MMC) and a representative power system. A simple and effective control strategy based on a virtual impedance is also proposed to limit the fault current.