energies Article Power Quality Improvement through a UPQC and a Resonant Observer-Based MIMO Control Strategy Holman Bueno-Contreras 1, * , Germán Andrés Ramos 1 and Ramon Costa-Castelló 2   Citation: Bueno-Contreras, H.; Ramos, G.A.; Costa-Castelló, R. Power Quality Improvement through a UPQC and a Resonant Observer-Based MIMO Control Strategy . Energies 2021, 14, 6938. https://doi.org/10.3390/en14216938 Academic Editors: Andrea Bonfiglio and Andrea Mazza Received: 28 July 2021 Accepted: 15 October 2021 Published: 21 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Electrical and Electronic Engineering, Universidad Nacional de Colombia, Bogotá 111321, Colombia; garamosf@unal.edu.co 2 Departament d’Enginyeria de Sistemes, Automàtica i Informàtica Industrial, Universitat Politècnica de Catalunya (UPC), 08028 Barcelona, Spain; ramon.costa@upc.edu * Correspondence: hbuenoc@unal.edu.co Abstract: Performance degradation is, in general, regarded as a power quality problem. One solution to recover grid performance is through the application of a unified power quality conditioner (UPQC). Although these devices are multi-input/multi-output (MIMO) systems, the most common control strategies consist of two decoupled controllers, which neglect the coupling effects and add uncertainty to the system. For this reason, this paper proposes a multivariable resonant observer-based control strategy of a UPQC system. This method includes all significant coupling effects between this system and the grid. This strategy results in a stability-based compensator, which differs from recently proposed strategies that are based on signal calculation and cannot assure closed-loop stability. In addition, this paper introduces a simplified controller tuning strategy based on optimal conventional methods without losing closed-loop performance. It implies that the controller can be easily tuned, despite the complexity of the MIMO dynamic model. The UPQC with the resonant observer is verified on an experimental setup for a single-phase system, obtaining three relevant results for power quality improvement: (1) harmonics compensation tested with a total harmonic distortion limit of 5%; (2) sags and swells mitigation; and (3) power factor correction, achieving a unitary value on the grid side. Keywords: resonant extended state observer; power quality; resonant control; power factor correc- tion; UPQC 1. Introduction Power quality (PQ), as defined in the recommended practice IEEE-1159-2019 [1], establishes some characteristics for both the voltage and current signals to achieve good performance for a grid and its connected load. This concept provides some limits for electromagnetic phenomena based on the Electromagnetic Compatibility Standards defined by the International Electrotechnical Commission (IEC). Some of these phenomena cause performance problems for both grids and the connected loads. Voltage disturbances such as sags swells and harmonics cause issues that could lead to damage or malfunctions of loads. Furthermore, if a nonlinear load is connected, current harmonics cause a loss of grid performance. With non-pure resistive loads (whose power factor is different from one), the grid provides reactive power, which increases the drawn current, the losses, the maintenance, and the costs of the power system. The power electronics field has provided several solutions for PQ compensation to improve grid performance and robustness. Some solutions such as the dynamic voltage restorer (DVR) for handling voltage disturbances include [2,3], which provide voltage har- monics and amplitude variations compensation. Another solution is the active filter [4,5], whose main objective is to reject current harmonics caused by nonlinear loads. Both solu- tions (DVR and shunt active filter) use DC/AC converters, with the main difference being the DVR is series-connected with the power grid, while the active filter is shunt-connected Energies 2021, 14, 6938. https://doi.org/10.3390/en14216938 https://www.mdpi.com/journal/energies