Sub-Synchronous Oscillation Phenomenon Analysis of Grid-Connected Direct Drive-Doubly Fed Hybrid Wind Farms via VSC-HVdc System

When integrating offshore wind farms, comprising either direct-drive or doubly fed induction generators, into onshore power grids via voltage source converter-based high-voltage direct current (VSC-HVDC) transmission, subsynchronous oscillation (SSO) may occur, compromising system stability. Existing research predominantly focuses on single-type wind turbine systems, with limited attention paid to hybrid systems composed of both turbine types. To address this gap, this paper first develops three mathematical models corresponding to direct-drive, doubly fed, and hybrid wind farms connected through VSC-HVDC. A method for establishing the equivalent small-signal model of a hybrid system, comprising multiple turbines of both types, is then proposed. Using modal analysis, small-signal models for the three systems are derived, and various oscillatory modes are identified via eigenvalue analysis. The results show that systems containing doubly fed turbines exhibit SSO, whereas systems with only direct-drive turbines experience oscillations in different frequency ranges. Participation factor analysis further reveals that the doubly fed turbine is the primary contributor to SSO in the hybrid wind farm. Subsequently, the impacts of key parameters, such as the wind turbine’s DC-link capacitance, AC-side reactance, the VSC-HVDC AC-side reactor, and the PI controller settings of the doubly fed turbine’s rotor-side converter, on SSO frequency and damping ratio are examined. These findings are expected to provide theoretical support for the development of hybrid direct-drive and doubly fed wind farms.