A Discrete RMRAC-STSM Controller for Current Regulation of Three-Phase Grid-Tied Converters with LCL filter Guilherme V. Hollweg Federal University of Santa Maria email: guilhermehollweg@gmail.com Paulo J. D. de O. Evald Franciscan University email:paulo.evald@gmail.com E. Mattos, R. V. Tambara and H. A. Gründling Federal University of Santa Maria Abstract—This article presents a discrete robust adaptive control structure, gathering a Robust Model Reference Adaptive Controller (RMRAC) with an adaptive Super-Twisting Sliding Mode (STSM) controller. The resulting control structure is applied to current control of a voltage-fed three-phase inverter, connected to the grid by an LCL filter. The main contribution of this control proposal is its adaptability, maintaining the robustness characteristics of the controllers that compose it with good regulation performance. Moreover, as the adaptive Sliding Mode action is high-order (Super-Twisting), the chattering phe- nomenon is significantly mitigated. Thereby, its implementation is simplified, using a first order reference model. For this, the dynamics of the LCL filter capacitors are neglected during the modeling process, considering it as an additive unmodeled dynamics. To validate the viability of the proposed control structure, Hardware in the Loop (HIL) results are presented. Keywords – Adaptive Sliding Mode Controller, Robust Model Reference Adaptive Controller, Grid-tied Converter, LCL filter, Hardware in the Loop. I. I NTRODUCTION In recent years, world energy consumption is increasing, contributing to the exhaustion of natural energy reserves and consequently creating a run for new renewable energy sources. In this way, there is a popularization of micro and mini genera- tion [1]. To use energy from a renewable source, it is necessary to convert continuous to alternate energy. For this purpose, a DC-AC converter is normally used [1]. For attenuation of the harmonic distortion present at the Point of Common Coupling (PCC) between the inverter and the electrical grid, as well as for attenuation of the high frequency harmonic components generated by the full bridge key switching, output filters are fundamental parts of grid-tied structures. The attenuation of these harmonic components is necessary for the system to meet the IEEE 1547-2018 standard, which limits injected current THD (Total Harmonic Distortion). Therefore, for good power quality, L or LCL filters are commonly used to connect the inverter to the electrical grid [2]. An L filter satisfactorily fulfills the function of suppressing harmonics, as required by current standards. However, due to its size and high cost, its use in power systems greater than 1 kW is not a viable alternative [2], making the LCL filter a better option [2], [3]. In addition to choosing a suitable output filter, another important issue in the design of a grid-tied DC-AC converter is the robustness of the current controller, since the inductance of the electrical grid is uncertain [4], [5]. Note that its impedance varies depending on local energy system quality, and may have a high inductive content (weak grid). The inductive content of the electrical grid is a challenge for control system, since it drastically increases the required controller efforts to keep the system stable and with the currents properly regulated [4]. The current control of a DC-AC converter with LCL filter has already been implemented using several control techniques, such as: Proportional-Integral Controller [3], P+R (Proportional+Resonant) Controllers [6], Robust Quadratic Linear Regulator [7], Robust Model Reference Adaptive Con- troller (RMRAC) [4], modified RMRAC [8], among others. Also, recently [9] proposed a robust P+R with a first order Sliding Mode (SM) controller. However, control structures that use sliding mode terms usually presents chattering, generated due to their high frequency non-linear action, as shown in [10] and [11]. It is noteworthy that chattering phenomenon can cause instability in the control system and problems with electromagnetic interference [11]. An alternative to mitigate chattering is to use higher-order Sliding Mode controllers, such as Super-Twisting, proposed in [10], [12]-[13], which maintains a good regulation performance and provides a significant mitigation of these persistent oscillations. Thereby, this paper proposes the development and appli- cation of a discrete-time Robust Model Reference Adaptive Controller (RMRAC) with an adaptive Super-Twisting Sliding Mode (STSM) controller, called RMRAC-STSM. The con- troller is applied for current control of a three-phase grid-tied Voltage Source Inverter (VSI) connected to the electrical grid through an LCL filter. Also, the proposed controller uses a reduced-order reference model. For this purpose, a simplified model of the LCL filter is considered, where the dynamics of the LCL capacitor are neglected (equivalent to the pair of complex conjugated poles in the complete plant model), as presented in [14]. Then, the controller is robust to unmodelled dynamics. Besides, the digital implementation of the controller becomes simpler, once that a reduced order system require less parameters to adapted, impacting directly on the computational cost. https://doi.org/10.53316/sepoc2021.033