DC-Link Energy Control in Voltage Droop Strategy of Multi-Terminal DC System for AC Grid Frequency Support

This paper presents an inertia emulation control method with the voltage droop strategy for voltage-sourced converter (VSC) based multi-terminal DC (MTDC) system. In the voltage droop strategy that all VSCs of MTDC system participate in the DC voltage control, the electro-static energy in MTDC capacitors cannot be delivered to a specific terminal: This may become an issue when the MTDC interconnects asynchronous AC power systems and one of the AC systems needs an immediate support from the MTDC. The proposed method aims to deliver the DC-link energy to one of the AC systems. This methodology is performed by adjusting the DC voltage reference depending on the frequency of each connected AC system. Subsequently, it improves the frequency stability of AC grid by utilizing the DC-link energy in MTDC system. The efficacy of the proposed method is demonstrated through PSCAD/EMTDC simulation studies for three-terminal DC system of Jeju Island in South Korea.