Energy-Based Virtual Damping Control of FB-MMCs for HVDC Grid

Full-bridge submodule-based modular multilevel converters (FB-MMCs) have attracted wide attention due to the dc fault blocking capability. However, the blocking of the FB-MMC can only suppress its dc terminal current, while the fault currents may still circulate along with the meshed dc network. To address this issue, energy-based virtual damping control is proposed, where the dc terminal current of the FB-MMC is regulated to follow the dc voltage in the event of a dc fault. The FB-MMC is, thus, controlled as a virtual damping resistor to actively absorb the residual energy in the dc network and quickly suppress the potential circulating dc fault currents. This enables fast fault isolation using dc switches and, thereby, fast fault recovery after fault isolation. The fault isolation time is significantly reduced from around 120 ms with the FB-MMC simply being blocked and 50 ms with the conventional FB-MMC fault control method to around 15 ms. The validity of the proposed control is verified in a three-terminal meshed dc network.