This paper proposes a fractional order sliding mode control (FOSMC) strategy for grid-connected doubly fed induction generator (DFIG). The novel approach aims to directly control the active/reactive power output of a DFIG-based wind energy conversion system with FOSMC, which has never been utilized in the control of DFIG. The fractional order sliding mode control with disturbance is introduced and the Caputo derivative is employed in constructing the sliding surface. Meanwhile, an enhanced exponential reaching law is utilized in order to mitigate the chattering phenomenon in the sliding stage and accelerate the reaching progress in the meantime. A comparative study is conducted subsequently to verify the superior performance of the proposed FOSMC control scheme over that of the conventional sliding mode control and the vector control approach. A steady state simulation is first conducted to tune the parameters of the FOSMC. A variable wind condition is subsequently simulated to compare the performance of those methods. After that, simulations with background harmonics of the grid voltage and parameter perturbation are considered. Ultimately, an experimental test is conducted to further validate the proposed method. The superior performance and the effectiveness of the FOSMC are fully validated with the above four comparative tests.

This paper proposes a distributed fractional-order predefined-time sliding mode controller (DFOPTSMC) to adjust the external energy storage to improve the transient stability of the power system with time delay and input saturation. The time delay is able to worsen the performance of the power system, and even bring about instability. Based on the Artstein transform theorem, the model of the power system with communication delay is transformed into a model without delay. Meanwhile, the output of the external energy storage should be limited in practical power systems. The saturation compensation system is proposed to remedy the influence of input saturation of the external energy storage. Therefore, a DFOPTSMC considering communication delay and input saturation is proposed to adjust the energy storage device, which has fast response ability and strong regulation capability, to increase transient stability of the power system. The predefined-time stability of the power system under the DFOPTSMC is analyzed through the Lyapunov function. Finally, simulations are taken to verify that the proposed DFOPTSMC is able to raise the transient stability of the power system as well as ensure that the power system returns to stability region under a large delay margin.

Frequency and voltage are the most significant indicators in AC microgrids (MGs). To maintain the stability of grid frequency and bus voltage, this paper proposes a distributed secondary control scheme based on bi-limit homogeneity for frequency restoration, average voltage recovery, active and reactive power sharing among DGs in AC MGs subjected to non-uniform communication delays and actuator saturations. First, a state-transformation-based strategy by virtue of the past information of the control input is designed to deal with non-uniform delays, which can fully alleviate the adverse effects of communication delays. Additionally, the hyperbolic tangent function is introduced to approximate the non-smooth saturation function and to design anti-saturation fixed-time controller, which can mitigate the impact caused by actuator saturations. Then, by constructing an auxiliary bi-limit homogeneous system, the fixed-time stability of the close-loop control system with the constraint of actuator saturation can be demonstrated. Finally, several simulation cases are conducted to evaluate the performance of the proposed strategy on a test MG system, and the results are compared with other existing methods. Simulation and comparison results reveal the proposed method's validity, robustness and flexibility in restoring the MG's frequency/average bus voltage and achieving accurate active/reactive power sharing.