Design and analysation of DC voltage synchronisation control for a VSC-MTDC based on virtual synchronous generator

Voltage source converter (VSC)-based high-voltage dc (HVDC) transmission is widely utilised nowadays. However, a VSC-HVDC connected to a weak ac system still faces the problem of phase-locked loop (PLL) synchronisation. To tackle this problem, this study proposes a dc voltage synchronisation control (DCSC) based on the power synchronisation control and the power balance equation of the virtual synchronous generator. To analyse the dynamic stability of DCSC, voltage sensitivity indicator (VSI) is used to reflect the voltage fluctuations. Considering the deviations of dc voltage and converter losses, a closed-loop transfer function of dc voltage is designed. Three important parameters including VSI, virtual damping coefficient and inertia time constant are investigated. The dynamic stability analysis shows DCSC has strong controllability. Then, a three terminals VSC-MTDC model is simulated in PSCAD/EMTDC. Herein, DCSC is used in one inverter and PLL is used in another inverter. The waveforms obtained by using DCSC are compared with that by using PLL. Simulation results clearly depict that the inverter with DCSC has a normal steady state and a better transient response than using PLL when connected to a strong ac system, also provides better power transfer capability when connected to a weak ac system.