When high density and large capacity wind power is transmitted by HVAC/HVDC hybrid power grid, the interaction between them becomes more complicated. Large scale wind powers connecting to grid centrally haven’t be considered so far in the mechanism of AC tie-line power oscillation caused by commutation failure. Firstly accordingly the relationship between equivalent inertia of system and damping is researched, and the influence on damping made by large scale wind power connecting to grid centrally is quantified. On the basis, mechanism of AC tie-line power oscillation caused by commutation failure under the circumstance of large scale wind power connecting to grid centrally is first proposed. Fast algorithms for peak value and peak time calculation of power oscillation on AC tie-line after commutation failure are proposed. The mechanism and algorithms are verified by simulation analysis of a Chinese regional grid. It provides theory and technical basis for arranging operating schemes and control measures of interconnected power systems.