In the offshore wind power transmission system via VSC-HVDC

the sending end converter (SEC) of VSC-HVDC generates AC voltage based on V/F control

and grid side converter (GSC) of direct-drive wind power locks phase based on AC voltage. The eigenvalue analysis shows that there exists a pair of low-frequency oscillation modes in the offshore station in which V/F control of VSC-HVDC and phase-locked loop (PLL) of wind power are jointly involved

which is prone to instability when offshore wind power output is high. In this paper

based on the correlation between this low-frequency mode and wind power PLL control

a second-order reduced-dimensional characteristic equation retaining the PLL equation was established based on the state space model of the system

and the low-frequency mode damping computation method based on the second-order characteristic equation was deduced. The damping was decomposed into the intrinsic damping of the PLL control and the additional damping generated by the external control

such as the SEC V/F. The method was applied to study the influences of PLL and V/F control parameters on the damping characteristics

and the time-domain simulation verifies the effectiveness and accuracy of the proposed method.