Robust grid-forming and grid-following control of DFIG-based wind energy conversion systems using multi-objective optimized sliding mode control

Since renewable energy resources are penetrating modern power systems more than ever, the wind energy conversion system (WECS) requires fast dynamic response and robustness as well as reliable operation under strong-grid and weak-grid conditions. One example is the doubly fed induction generator (DFIG), which remains an attractive option within variable-speed wind generation technologies, as it has a low converter rating, high efficiency, and flexible power control capability. Yet, DFIG-based WECSs control methods are still typical in nature and their performance degrades with wind speed fluctuation, grid interference, model instability and parameter perturbation. In this paper, we proposed a robust control approach for DFIG-based WECSs that merges grid-forming and grid-following operation in the same multi-objective optimized sliding mode control (SMC) design framework.
To improve the active and reactive power control, rotor speed regulation, dc-link voltage balancing as well as fault ride-through capability with reduced chattering effect encountered in classical SMC, the proposed concept approach is devised. When working in grid-following mode, the controller accurately synchronizes and injects power into the utility grid. It then operates in grid-forming mode, providing voltage and frequency support, thus improving operation under weak-grid and islanded conditions. A multi-objective optimization procedure is applied to find the SMC parameters that minimize settling time, overshoot, steady-state tracking error (SSTE), and control effort simultaneously. Simulation results show that active power settling time is decreased from 0.42 s to 0.18 s and dc-link voltage overshoot is resided from 14.6% to 4.1% in comparison to a traditional PI-based controller using the proposed approach. Moreover, compared to the conventional observer, rotor speed tracking error is reduced by 31.8%, and stator current total harmonic distortion decreases from 4.9% to 2.1%. When subjected to a 30% sag in grid voltage, the proposed controller retains closed-loop stability and recovers nominal operating conditions within 0.12 s versus 0.31 s for the benchmark controller. In addition, these findings verify that the proposed MOO-SMC called upgrade substantially increases the dynamism performance, stabilizing and quality of power with respect to conventional SMC for various operating conditions applied to DFIG-based WECSs.