Variable DC Voltage Threshold-Based Control Considering Emergency Mode-Switching for MTDC System

The voltage source converter-based multi-terminal DC (MTDC) system is being developed as a gateway system for offshore wind farm integration and as an inter-regional transmission system. MTDC systems potentially offer greater flexibility and higher efficiency for power flow control; however, some technological issues must be overcome for practical realization. Two major control strategies for MTDC systems, the dc voltage droop control and the dc voltage margin control, were developed for dc power flow management and dc voltage regulation. However, these methods do not consider operation under transient conditions, e.g., when a dc fault occurs on a dc transmission line. Therefore, this paper proposes a new control strategy that maintains stable operation even when the MTDC system is partially isolated because of a dc fault occurrence. The proposed method's effectiveness is confirmed through simulation analysis of a six-terminal MTDC system.