Improved Single-Loop SMC Strategy With Hybrid Sliding Surface and Torque Observer for Double-Controlled DFIG

Doubly fed induction generator (DFIG) system suffers from parameter variations, external disturbances, complex control structure, and slow dynamic response speed under traditional cascaded double-loop control strategy. The friction loss is always ignored in DFIG model, and a large switching coefficient is required to resist the influence of time-varying term of friction loss, which would cause large system chattering. In this article, a single-loop sliding-mode control (SMC) strategy with hybrid sliding surface and torque observer is proposed for double-controlled DFIG to improve transient and steady state performance. There are three improvements in this article. First, a damping torque observer is designed according to Lyapunov function to improve system model accuracy and lower switching coefficient of SMC. Second, a single-loop control structure without intermediate link is adopted to improve transient performance for reduced-order DFIG system. Third, a hybrid sliding surface which consists of control target and intermediate variable for SMC strategy is proposed to overcome the shortcoming of inner rotor active current loop absence, which improves system dynamic performance and reduces intermediate current chattering. Finally, through simulation and experiment, the feasibility and effectiveness of proposed strategy are verified.