Medium-frequency and sub-synchronous oscillation analysis of direct-drive wind farms connected to the parallel-compensated AC grid

Parallel compensation is widely used for reactive power compensations, which improves the voltage quality of power grids. However, the direct-drive wind farms connected to the parallel-compensated AC grid face the risk of oscillations. In light of this, the small-signal model of multiple direct-drive wind turbines connected to the parallel-compensated AC grid is firstly built. Then, the medium-frequency and sub-synchronous inside-wind-farm/wind-farm-grid oscillation characteristics are analyzed with the eigenvalue-based method. Finally, the oscillation interactions among multiple wind turbine generators in the homogeneous/heterogeneous wind farms are investigated. The analytical results show that the inside-wind-farm medium-frequency oscillation is mainly affected by the DC voltage outer loop control of wind farms, and the wind-farm-grid sub-synchronous oscillation is mainly affected by the phase-locked loop control of wind farms and the parallel-compensated AC grid. Meanwhile, the wind turbine generators in the homogeneous and heterogeneous wind farms present strong oscillation interactions and weak oscillation interactions, respectively. PSCAD/EMTDC simulations are performed to verify the theoretical analysis.