Improved practical method for low-inertia VSC-HVDC stability analysis in weak system.

The physical mechanism of the instability of the low-inertia voltage source converter (VSC) caused by the phase-locked loop (PLL) in a weak system is clearly revealed in a simplified way. Based on the complex torque method, the joint influence of the PLL and the system strength on the VSC is studied through the defined key stable component. To keep the VSC stable, the value of the key stable component must be positive. The component is decided by AC voltage, short-circuit-ratio (SCR), reactive power, and PLL parameters. Improper PLL parameters and weak AC system could make the key stable component negative. To keep the VSC stable in a weak system, the PLL parameters can be adjusted according to the curve of the key stable component, which means suitably decreasing kp and increasing ki. Finally, simulations are implemented to validate the theory, which matches well with the findings. [ABSTRACT FROM AUTHOR]