A path analysis method to study the subsynchronous oscillation mechanism in direct-drive wind farm with VSC-HVDC system

Subsynchronous oscillation (SSO) exists in direct-drive wind farm with VSC-HVDC (DDWFV) system. In the process of mechanism analysis, the existing analysis methods are difficult to reveal the disturbance transfer process and the coupling relationship between direct-drive wind farm and VSC-HVDC. In this paper, a path analysis method is proposed to study the SSO mechanism, and the oscillation mode dominated by DC capacitor link of the direct-drive permanent magnet synchronous generator (D-PMSG) is investigated. Firstly, the linear-ized model of the DDWFV is established, and the transfer path of subsynchronous disturbances between direct-drive wind farm and VSC-HVDC is identified based on the transfer function block diagram. Then, the SSO damping characteristics in the DDWFV are studied, in which the damping of interaction can be obtained by the damping decomposition. Finally, the impact of the controller parameters on the damping characteristics is analyzed, and the effectiveness of theoretical analysis is verified by time-domain simulations. The results show that the disturbance transfer path between direct-drive wind farm and VSC-HVDC presents a four-loop coupling relationship, which will lead to the SSO. By optimizing the controller parameters, the SSO damping is increased, and thus improving the system stability.