Stability and Adaptability Analysis for PLL-Synchronized VSC-HVDC with Frequency Regulation Scheme Under Islanded Grid

Voltage-source converter (VSC)high voltage direct current (HVDC)connected to islanded grid is becoming a normal system operation mode. However, there is little research on the stability analysis of VSC-HVDC connected to islanded grid. On the basis of previous work, this paper analyzes the stability and adaptability of PLL-synchronized VSC-HVDC adopted with the additional frequency regulation scheme (FRS)under islanded grid. By defining the instantaneous frequency and regarding it as a state variable, the small-signal model of PLL-synchronized VSC under both grid-islanded mode and grid-connected mode is synthetically established. On this basis, the impact of FRS on the stability of PLL-synchronized VSC under islanded grid is analyzed, and then the adaptability of FRS to different grid strengths is investigated via considering different short current ratios (SCRs), including islanded grid (SCR=0), weak grid (SCR<;2)and strong grid (SCR>5). The analysis results show that the FRS has important role on enhancing the small signal stability of the islanded grid, and also has excellent adaptability to the wide change of grid conditions, so as to further improve the possibility of the FRS applied in practical projects. These results finally are verified by time-domain simulations.