A study of the transient synchronization stability of PLL-GFC-based MMC-HVDC systems

Abstract

Phase-locked loop-based grid-forming converters （PLL-GFCs） can regulate frequency and voltage. However， the transient synchronization stability after their connection to the high-voltage AC power grid is unclear. Therefore， the transient synchronization stability mechanism of the PLL-GFC-based MMC-HVDC （modular multilevel converter-based HVDC） systems is studied. Firstly， the control strategy of the PLL-GFC-based MMC-HVDC systems is introduced， and the power response characteristics are analyzed. Secondly， the grid-connected MMC-HVDC systems' transient synchronization stability model is built. The instability mechanism of the converters after grid faults and the influence of the dynamic changes of the grid current on the transient synchronization stability are studied based on phrase diagram curves. It is concluded that the equilibrium point is crucial to the transient stability after the fault occurs on the inverter-connected power grid. Therefore， an enhancement measure to improve the transient synchronization stability by dynamically adjusting active current command values is proposed. Finally， an electromagnetic transient simulation model is built by PSCAD/EMTDC to verify the correctness of the instability mechanism analysis and the effectiveness of the enhancement measure.

Keywords

• transient synchronization stability
• phase-locked loop
• grid-forming converter
• mmc-hvdc
• phase diagram curve