Unknown system dynamics estimator based control for sub-synchronous oscillation of direct-drive wind farms

Aiming at the problem of sub-synchronous oscillations induced by direct-drive wind farms with series-compensated lines, this paper proposes an unknown system dynamics estimation-based PI controller to achieve sub-synchronous oscillation suppression. According to the mathematical model of the direct-drive wind farm grid-connected system, the relationship between the direct-drive wind turbine grid-side converter current inner-loop PI controller and the sub-synchronous components is first established. Secondly, the uncertain sub-synchronous current components and voltage components in the series-compensated lines of the direct-drive wind farm are taken as the total disturbance, and an unknown system dynamic estimator-based PI controller is designed by introducing the first-order low-pass filter operation. Then, the stability and convergence of the closed-loop system are proved by Lyapunov theory. Finally, the Prony method is used to analyse the current signal output by the direct-drive wind turbine, and the inherent characteristics of the negative damping of the SSO induced by the series-compensated line of the direct-driven wind farm are revealed. A comparative numerical simulation is carried out to demonstrate that the sub-synchronous oscillations of the direct-drive wind farm with series-compensated lines can be suppressed under different operating conditions.