Distributed Secondary Control Based on Bi-Limit Homogeneity for AC Microgrids Subjected to Non-Uniform Delays and Actuator Saturations

Abstract

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.