A novel virtual synchronous control of VSC based HVDC transmission for inter-area connection

In recent years, VSC-based inverters have been widely utilized in power systems for its fast regulation speed and control flexibility. Traditional grid-connected converter usually relies on phase-locked loop (PLL) to obtain the accurate phase of the grid in the normal operation. However, this PLL-based grid following control might induce a negative damping and result in the instability of the overall meshed AC/DC system when the grid short circuit ratio (SCR) is low or the control bandwidth of PLL is high. To overcome these drawbacks of traditional PLL based control, this paper proposes a novel virtual synchronous control for VSC based HVDC system without the need of PLL for inter-area interconnection of the power grid. In the proposed control, the droop property between the DC-link voltage and the frequency of the receiving-end system is artificially coupled. As a result, the emulated inertia support can be provided from the DC-link capacitor for the system under the severe system frequency deviations. Moreover, the sending-end AC system can sense the frequency disturbance of the receiving-end system without the remote communication by utilizing the local DC-link voltage and current variations of the VSC based rectifier. Accordingly, the sending-end system can provide the primary frequency control for the other side disturbed system by fully emulating the primary frequency regulation of traditional synchronous generator (SG). Compared with the traditional PLL based control of VSC, the proposed control strategy stands out itself by fully utilizing the grid-forming and grid-support potential of VSC for the disturbed AC system without relying the PLL and remote communication, and the stability of the receiving-end system is largely improved. The simulation results of sudden load variations in PSCAD/EMTDC have validate the effectiveness of the proposed control.