A novel supplementary frequency-based dual damping control for VSC-HVDC system under weak AC grid

The Voltage-Source-Converter based High Voltage Direct Current (VSC-HVDC) system which is connected to weak AC grid, under the widely used Vector Current Control (VCC), may experience oscillations and even dynamic instability due to the Phase-Locked-Loop (PLL) dynamics. This paper proposes a novel supplementary control, i.e., supplementary frequency-based dual damping control (SFDDC), to enhance the stability margin for VSC-HVDC system under weak AC grid conditions. Based on the tracked frequency from PLL, the SFDDC approach introduces the power-based and AC voltage-based damping components to mitigate the instability induced by the larger PLL gains. A small-signal model of VSC-HVDC system is developed, and validated by the detailed electromagnetic transient (EMT) simulation. Then, the SFDDC dynamics are evaluated and the feasible region of the introduced damping coefficients are calculated. Finally, this paper investigates the maximum available power (MAP) and evaluates the critical short-circuit ratio (CSCR) for VSC-HVDC station, under the proposed control approach. The results show that, by proper selection of the dual damping coefficients within recommended range, the proposed SFDDC approach can effectively mitigate the instability issue induced by PLL, improve the maximum power transfer capability and enable the VSC-HVDC station operate in extremely weak AC grid conditions, even at SCR = 1.0. In addition, without control strategy switching, the over-current can be limited to an acceptable value by the proposed controller during the solid three-phase to ground fault scenario.