Robust Hierarchical Control of VSC-Based Off-Grid AC Microgrids to Enhancing Stability and FRT Capability Considering Time-Varying Delays

In this article, a new robust hierarchical control scheme is proposed for off-grid (autonomous) voltage-sourced converter (VSC)-based alternative current (ac) microgrids (MGs). The main focus of this article is devoted to the primary layer, including the cascade structure of inner (current), outer (voltage), the virtual impedance, and droop control loops. In the inner control loop, a robust controller is designed based on an adaptive backstepping integral nonsingular fast terminal sliding mode control (ABINFTSMC) strategy to regulate and track the reference of current signals in the presence of unknown bounded uncertainties and external disturbances. The outer loop is designed based on a mixed $H_{2}/H_\infty $ control strategy by utilizing the state feedback control law to generate the inner-loop reference signal and to achieve stability and robustness versus perturbations, and sufficient conditions are derived based on linear matrix inequalities (LMIs). The performance of the controllers is improved to consider time-varying delay (TVD). The droop control and virtual impedance loops are applied to enhance the power-sharing quality of the system. Also, a distributed consensus-based protocol is used in the secondary layer. Finally, to evaluate the performance of the control laws and to demonstrate the effectiveness of the proposed robust control scheme, offline digital time-domain simulation studies are carried out in MATLAB/Simulink software environment. The obtained simulation results and comparison with previous work prove that the proposed robust hierarchical control scheme can effectively, and robustly enhance the transient response and steady-state performance, and fault ride-through (FRT) capability of MG when faced with small- and large-signal disturbances, and show better robust performance over conventional proportional-integral (PI)-based nested-loop control strategies.

I. Introduction

Widespread contribution of distributed energy resources (DERs), in the form of either distributed generation (DG) or distributed energy storage (DES) systems, will make the whole system more reliable, but this will add another concern to previous ones. The basic and the most commonly used control structure for microgrids (MGs) is the hierarchical control, including primary control (droop control and primary stabilization), secondary control (restoration of voltage and frequency), and tertiary control (optimal operation). MG can be operated both in grid-connected or islanded mode. In islanded operations, the task of voltage and frequency regulation is the responsibility of the primary layer. As due to primary layer operation, the voltage and frequency have some deviation from their nominal values, and the secondary controller is responsible for compensation of these deviations [1]–[3].