International Journal of Power Electronics and Drive Systems (IJPEDS) Vol. 13, No. 1, March 2022, pp. 223~236 ISSN: 2088-8694, DOI: 10.11591/ijpeds.v13.i1.pp223-236  223 Journal homepage: http://ijpeds.iaescore.com Resonant and a new disturbance-observer combined control for off-grid voltage source inverter Marco di Benedetto 1 , Mi Tang 2 , Alessandro Lidozzi 1 , Luca Solero 1 , Andrea Formentini 3 , Pericle Zannchetta 2,4 1 Department of Engineering, Center for Power Electronics and Drives (C-PED), Roma Tre University, Roma, Italy 2 Department of Electrical and Electronics Engineering, University of Nottingham, Nottingham, United Kingdom 3 Electrical, Electronics and Telecommunication Engineering and Naval Architecture Department (DITEN), University of Genova, Genova, Italy 4 Department of Engineering, University of Pavia, Pavia, Italy Article Info ABSTRACT Article history: Received Jan 17, 2021 Revised Jan 31, 2022 Accepted Feb 10, 2022 A new control strategy that combines the resonant controller and the repetitive-controller based on the disturbance-observer (RSCDO) for off- grid four-leg voltage source inverter (VSI) has been presented in this paper. The aim of the new controller is to regulate the inverter output voltage, reducing as much as possible the voltage total harmonic distortion (THDv) in every load condition. The resonant controller allows to adjust the voltage at the fundamental harmonic, while the repetitive observer is able to compensate the voltage distortions due to inverter nonlinearity. The proposed strategy has been at first validated by means of simulation results performed in MATLAB/Simulink environment. Then, the new control strategy has been implemented in field-programmable gate array (FPGA) using LabView environmental and experimental results are performed on the 40 kVA three-phase 4-leg VSI prototype. Keywords: Repetitive observer Resonant controllers Three-phase four-leg inverter non-linear loads Total harmonic distortion This is an open access article under the CC BY-SA license. Corresponding Author: Marco di Benedetto Department of Engineering, Center for Power Electronics and Drives (C-PED), Roma Tre University Via della Vasca Navale 79, 00146 Roma, Italy Email: marco.dibenedetto@uniroma3.it 1. INTRODUCTION The increasing penetration of renewable energy (especially photovoltaic, wind, and micro-turbine), and the growing need for reliability and power quality in the distributed generation (DG) units has, in the last few years, drawn more attention to direct current (DC) microgrids [1]–[3]. Compared to the alternating current (AC) distribution grids, the DC microgrids have several advantages [4]. In fact, thanks to the use of more efficient power electronic converter to connect the common DC-bus and DC sources, such as photovoltaic (PV), energy storage (ES), wind system, makes the distribution systems more efficient, and economical. A typical stand-alone DC microgrid is illustrated in Figure 1. In DC microgrids, the DC/DC converters can play an important role in the control of the DC-bus voltage and in maintaining the system power balance [5]. AC/DC converters, instead, are mainly used to manage the flow of power generated by micro-turbines and wind turbines. Another very important role is played by the DC/AC converter which is responsible for supplying the load starting from the DC-bus [6]–[9]. DC/AC conversion systems require high performance to ensure highly efficient utilization of DG units [10], [11]. Additionally, the quality of the voltage and current provided by the DG units must satisfy several international requirements, like EN-50160 and electromagnetic compatibility (EMC) EN-61000 [12]. In order to fulfill the mentioned standards, attention is paid to both the converter and the output filter employed in the DG units, as well as to the control