Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes An improved method for harmonic mitigation and stability improvement of the grid-connected inverters under local load variation and weak grid condition Elham Samavati, Hamid Reza Mohammadi ⁎ Faculty of Electrical and Computer Engineering, University of Kashan, Kashan, Iran ARTICLE INFO Keywords: Variable virtual admittance (VVA) Grid-connected inverter Variable grid impedance Impedance-based stability criterion Variable local load ABSTRACT Variable voltage and current harmonics appear to be critical challenges for grid-connected inverters at the point of common coupling (PCC). The nonlinear local load and grid impedance variations contribute a lot to the problem. In this paper, a novel control strategy is proposed using the variable virtual admittance (VVA) and series active filter concepts capable of improving harmonic mitigation and stability in grid-connected inverters. The magnitude and phase angle of the VVA are adaptively determined according to the local load variation. Moreover, the series active filter injects a series voltage to reduce the grid impedance effect. The proposed control method can be simply implemented, where sinusoidal grid-injected current and sinusoidal local load voltage can also be achieved under local load variation and weak grid condition. The theoretical approach is verified using several simulation case studies in Matlab/Simulink software. 1. Introduction The increasing application of renewable energy sources in power system has imposed extensive changes in transmission and distribution levels. Recent widespread application of nonlinear local loads and the grid voltage harmonics have led to increased grid-injected current harmonics in the Distributed Generation (DG) systems. The harmonic pollution however, is deteriorated in cases of nonlinear local load and grid impedance variations. This problem can be solved by designing appropriate power quality compensators [1]. The grid-connected in- verters should be controlled in such a way that the grid-injected current has the minimum harmonic content [2,3]. The IEEE 1547 standard determined the allowable limits for both voltage and grid-injected current harmonics at the point of common coupling (PCC). If the Total Harmonic Distortion (THD) of the PCC voltage exceeds its limit, then the THD of the grid-injected current is increased excessively. Conse- quently, the protective system operates and disconnects the DG unit from the grid [4,5]. The major challenge is hence related to the effect of the grid impedance and nonlinear local load variations, where har- monic instability of the DG unit occurs due to the deteriorated har- monic pollution. On the other hand, it is worth noting that resonant frequency variation is caused by the grid impedance variation. In such conditions, the grid-connected inverter remains stable if the ratio of the equivalent grid impedance to the inverter output impedance satisfies the Nyquist stability criterion [6]. In recent years, several control methods have been introduced aiming to suppress harmonic dis- turbances at the PCC using: multi-loop Proportional Resonant (PR) controllers [7], grid voltage feedforward control [8,9], ∞ H repetitive controller [10,11], employing an auxiliary inverter [12,13], control techniques based on virtual impedance [14–25], and inverter output impedance shaping methods [26–29]. Moreover, to address the re- sonant problem and achieve better harmonic rejection, the active damping control loop using capacitor current feedback was employed, leading to improved stability of the control system [30,31]. In a study [12], an auxiliary inverter containing a series LC filter was used, where by adjusting the parameters of the series LC filter, the main inverter output impedance was increased leading to harmonic suppression at the PCC. In another study [13], two parallel inverters were employed for simultaneous harmonic compensation of the PCC voltage and grid-injected current. The first inverter reduced the har- monics of the local load voltage, while the second one reduced the current harmonics produced by the nonlinear local load. Additionally, in a number of studies [14–18], virtual impedance was applied in series with the grid to reduce the voltage harmonics at the PCC. Harmonic voltage references were also generated in other works [19–21] based on harmonic virtual resistance and inductance in positive and negative sequences. Load voltage harmonics were suppressed by adaptive virtual impedance elsewhere [22], where the magnitude of virtual impedance https://doi.org/10.1016/j.ijepes.2020.106310 Received 9 December 2019; Received in revised form 12 June 2020; Accepted 16 June 2020 ⁎ Corresponding author at: Faculty of Electrical and Computer Engineering, University of Kashan, Ravand Blvd., P.O. Box 8731753153, Kashan, Iran. E-mail address: mohammadi@kashanu.ac.ir (H.R. Mohammadi). Electrical Power and Energy Systems 123 (2020) 106310 0142-0615/ © 2020 Elsevier Ltd. All rights reserved. T