Research on Non-blocking Fault Ride Through Control Strategy for the All-DC Wind Power Transmission System Based on MMC-DCT

The modular multilevel converter-based DC transformer (MMC-DCT) is the critical component in DC power systems, yet research on its non-blocking fault ride through (FRT) control remains limited. A new All-DC wind power transmission system based on MMC-DCT is proposed in the paper, which can be further promoted and applied to large-scale long-distance renewable energy transmission scenarios. Firstly, a system-level steady-state control strategy is developed, and the energy balancing mechanism of the hybrid sub-modules (SMs) of full-bridge and half-bridge is investigated. Then the fault characteristics and distinctions of the temporary faults between low-voltage (LV) and high-voltage (HV) port are analyzed. For the fault on the LV side converter, a non-blocking FRT strategy based on active current control with energy balancing is developed, besides, the impact of reduced AC voltage on the DC FRT has been analyzed. For fault on the HV side converter, a non-blocking FRT strategy based on adjustable AC voltage under energy balancing is proposed, which can effectively reduce the proportion of full bridge sub-module (FBSM) of DCT. Both strategies are able to achieve rapid fault current clearance and restore normal system operation within 300 ms. The simulation results in PSCAD/EMTDC verify the effectiveness of the proposed control strategies.