Impedance Analysis and Stability Optimization based on Phase-Optimization PLL of VSC-HVDC Connected to Weak AC Grids

For voltage-source converter (VSC) based high-voltage direct-current (HVDC) connected to weak AC grids, the grid current control interacts with phase-locked loop (PLL) under weak grid, via point of common coupling (PCC) voltage. However, the dynamic of PLL might result in system instability under weak ac grid. To mitigate this problem, a phase-optimized PLL (PO-PLL) technique is proposed for VSC-HVDC connected to weak ac grids. Then, the small-signal output impedance models of VSC-HVDC connected to weak ac grids under based-PLL synchronous and based on PO-PLL synchronous mode are derived in the synchronous reference (d-q) frame. The impedance and stability under PLL-based synchronous and based on PO-PLL synchronous mode are analyzed. Finally, numeric simulations on MATLAB/Simulink are conducted to validate the correctness of the proposed PO-PLL synchronous mode.