Abstract—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. The high grid impedance can destabilize the grid current control loop and lead to sustained harmonic resonance or other instability problems. To mitigate this problem, a phase-optimized PLL (P-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 P-PLL synchronous mode are derived in the synchronous reference (d-q) frame. On the basis of the work above, the impedance characteristics under PLL-based synchronous and based on P-PLL synchronous mode are analyzed. Results show that: Compared with PLL-based synchronous mode, the output impedance of VSC-HVDC has negative resistance in a very small frequency range, being of importance for grid-tied system stability. Finally, numeric simulations on Matlab/Simulink are conducted to validate the correctness of the proposed P-PLL synchronous mode.

VSC-HVDC, PLL, P-PLL, impedance, Nyquist stability