Interconnecting Very Weak AC Systems by Multiterminal VSC-HVDC Links With a Unified Virtual Synchronous Control

In this paper, a unified virtual synchronous control (UViSynC) is proposed for multiterminal voltage-source converter (VSC)-high-voltage dc (HVDC) (MTDC) systems, which can: 1) realize grid synchronization of the VSC stations by mimicking the rotor behavior of synchronous generators with the dynamics of dc grid and dc capacitor considered and 2) ensure the stable operation of the MTDC system that interconnects very weak ac grids. Unlike conventional master-slave control strategies that assign one master VSC to control the dc voltage, the UViSynC is capable of involving all VSC stations in dc voltage regulation. In this manner, each VSC station plays an equal role to control the dc voltage, which is a better way for dc voltage regulation compared with conventional master-slave control strategies. Grid synchronization mechanism for VSC station with UViSynC is particularly analyzed. Small-signal model of the MTDC system is developed to perform stability analysis and investigate the dynamic performance when interconnecting very weak ac systems. Simulation analyses based on MATLAB/Simulink and hardware-in-the-loop platform are conducted to verify the effectiveness of the UViSynC using a four-terminal VSC-HVDC test system.