Robust three degrees of freedom based on H∞ controller of voltage/current loops for DG unit in micro grids

Abstract

This study proposes a cascaded voltage–current robust control strategy for a distributed generator (DG) in microgrids. The main objective of the proposed controller is to improve the performance and power quality of a DG by injecting simultaneously a good sinusoidal voltage/current to the different loads connected to the DG. The proposed cascaded voltage/current controller consists of three degree-of-freedom controller, which is designed by using H∞ control theory based on mixed sensitivity specifications. The choices of appropriate weighting functions Wt, Wu, Wy that satisfy the preset goals to get a robust controller and a transform to a standard H∞ control design problem based on pre-compensator, feedforward and feedback connections are presented. The 3DOF-controller is based on MATLAB R/SimPowerSystems and using RT-EVENTS-toolbox. The controller is evaluated under different scenarios: transient responses with a resistive local load, and loads disturbances in steady-state responses with resistive, resistive inductive, resistive capacitive, and non-linear loads. Simulation and experimental results of resulting waveforms from a DG unit are presented, it confirms the effectiveness of the proposed method in effectively rejecting perturbation and robustness to loads disturbances; both good reference tracking and good transient response, and finally significantly lead to a very low total harmonic distortion.