Effects of Inner Loop and PLL on VSC Instability with Variable WTG Output Power

The instability of voltage source converters (VSCs) between wind farms and weak AC networks has recently attracted significant attention. This paper studies the relationship between variations in the inner loop of Voltage Source Converters (VSCs) and Phase-Locked Loops (PLLs) and their impact on system instability. The impedance analysis method is utilized to explore different oscillation modes influenced by the VSC inner loop and Phase-Locked Loop with a variable active power output of Wind Turbine Generators (WTGs). The stability analysis results reveal that the system stability trend exhibits various modes in different inner loop and PLL bandwidth frequency ranges. The theoretical analysis is validated by a time-domain simulation model of a wind farm using PSCAD/EMTDC.