To mitigate subsynchronous control interaction (SSCI) in doubly fed induction generator (DFIG)-based wind farm, this paper proposes a robust controller for rotor-side converter (RSC) using fractional-order sliding mode controller (FOSMC). The proposed FOSMC can improve robustness and convergence properties of the controlled system, thus achieving SSCI damping under various operating conditions. Impedancebased analysis and time-domain simulation are performed to check the capability of the designed FOSMC as compared with conventional sliding mode control (SMC) and subsynchronous damping control (SSDC). Simulation results demonstrate that FOSMC can mitigate SSCI within shorter time and effectively reduce the fluctuation range of system transient responses under various operating conditions of wind speeds and compensation levels. Moreover, FOSMC also improves system robustness against parameter uncertainties and external disturbances, which is important for safe operation of realistic wind farms. K E Y W O R D S finite-time control, fractional-order sliding mode controller, subsynchronous interaction, wind turbine 1 | INTRODUCTION Electric power system connected to series-compensated transmissions is vulnerable to subsynchronous oscillation. 1-3 Traditionally, this oscillation results from interaction between synchronous generator and series-compensated lines, which is known as subsynchronous resonance (SSR). 4Conventional SSR includes three types: torsional interaction (TI), torque amplification (TA), and induction generator effect (IGE). 5 TI or TA occurs when the mechanical part of the turbine generator is engaged, whereas IGE occurs with the involvement of the electrical part of the machine. In recent years, the phenomenon known as subsynchronous control interaction (SSCI) emerged with greater penetration of doubly fed induction generator (DFIG)-based wind farm. 6 SSCI incidents detected in wind farms in China and United States has triggered the attention of engineers and researchers. The SSCI is characterized by the emergence of poorly damped resonance, with a varying resonant frequency lower than the synchronous frequency. 7 It is necessary to develop effective subsynchronous damping controllers.The band-pass filters-based controllers have been employed for SSCI mitigation. A supplementary damping control, consisting of gain, filters, and phase compensator, is designed for STATCOM to mitigate oscillations due to SSCI in wind power plant interfaced with series compensation. 8With the proposed controller, STATCOM is able to provide effective damping under different operating conditions. Based on genetic algorithm and simulated annealing, a band-pass filters-based damping control strategy is designed to depress SSCI in wind power system interfaced with series capacitor, and the optimized damping controller is robustified to mitigate multimodal SSCI under various resonant frequency, asymmetric fault, and symmetric fault. 9 A band-pass filters-based subsynchronous damping control (SSDC) is devised for rotor-side co...