Grid side reactive power support strategy for MMC-HVDC connected to the wind farms based on unloading resistor.

• A mode switch control and reactive power support strategy are proposed. • Voltage sags are mitigated or eliminated with proposed control. • The real-time local control enables inverters to respond to voltage drops swiftly. • The probability of AC voltage sag impact on windfarm is reduced with proposed control. In recent years, VSC-HVDC has been widely used in windfarm integration owing to its ability of connecting the weak AC system. And the hybrid modular multilevel converter based HVDC(MMC-HVDC) has attracted more attention because of its DC fault blocking capability. However, most of the researches about MMC-HVDC connected to windfarm are focused on frequency response and wind farm fault ride through, without assisting AC system to maintain the voltage via the converter's reactive power control capacity. Based on this, this paper proposes a mode switch control and reactive power support control scheme for the grid connected inverter connected to windfarms. The switch mode control scheme provides three operation modes for MMC to adapt to different operation situations. The reactive power support coordination control scheme is an advanced control scheme for cooperating converter, unloading resistor and wind farm. With the cooperating control, the PCC voltage sag due to the fault is alleviated while reducing the impact of grid failures on the wind farm. Finally, the presented control strategy is simulated in PSCAD/ EMTDC, which verifies the effectiveness of the scheme proposed. [ABSTRACT FROM AUTHOR]