## 1 Introduction
In addition, both the control algorithms are more complicated than that of the proportional feedback. As another approach, a full-state feedback scheme is presented in [15; 16; 17]. In these methods, the feedback control of the capacitor voltage and inverter-side current is used to actively damp the resonance phenomenon. The advantages of these control methods are that the design procedure is standard and straightforward, thereby getting rid of any trial and error. However, there are still some drawbacks such as the computational burden and limited harmonic compensation [18]. The multiloop control and full-state feedback control structures are known to have large flexibility and easiness in implementation [18; 19]. The local nonlinear loads, AC machine drive, and saturated transformer often generate harmonic currents. The harmonic currents flow through the line impedance, causing distortion of the grid voltages at the PCC [20]. To ensure high-quality grid-injected currents even under distorted grid, several harmonic attenuation methods have been also presented with the above active damping methods, since the conventional proportional integral (PI) control method in the synchronous reference frame rotating with the fundamental grid frequency cannot effectively suppress the grid harmonic distortion. In order to eliminate the low-order disturbance caused by grid voltage harmonics, multiple harmonic compensation scheme is employed in [13; 14]. Even though this scheme works well in the stiff grid, there exists a smaller control margin when the grid impedance increases. To achieve the attenuation of current harmonics as well as the robustness against the grid impedance uncertainty, an active damping method by using the sliding mode control for the reduced model of a grid-connected inverter is presented [21]. Another approach uses a model predictive control [22], the plug-in combination of the PR control and the repetitive control (RC) [23], or multiresonant control [24] to mitigate the harmonic currents caused by grid voltage distortion. A full-state feedback controller which augments the resonant harmonic controllers of the grid-side currents is also proposed in [15; 16; 17]. The dynamic interaction of the inverter system with the grid impedance may degrade the power quality. Cable overload, saturation, long radial distribution feeders, and temperature effects are all reasons for possible variation in the interfacing impedance seen by the inverter [9; 20]. Therefore, the grid impedance should be considered in designing the controller parameters especially when the inverter system operates under the weak grid condition. Under weak grid condition, in which the grid impedance varies significantly, the performance of active damping as well as harmonic compensation is deteriorated severely, causing serious grid current harmonics or even system instability [5; 25]. The analysis on the system stability is given for the grid-connected inverter connected to weak grid in [26]. The research on the instability problem of multi-inverter system operation in [27] demonstrates that the weak grid condition yields output current resonance. Other research work in [28] concentrates to the effect of weak grid in the voltage feedforward control scheme. Based on the impedance analysis method, the instability mechanism and unstable area caused by voltage feedforward control are clearly derived. The authors in [29] present a different approach to implement the resonant compensators by means of the disturbance rejection concept. A comprehensive assessment of the controller under weak grid is also presented; however, the presented scheme is experimentally evaluated only with the grid impedance of two times grid-side inductor. This paper presents an assessment of the current controller stability and performance with harmonic compensators for an LCL-filtered inverter connected to distorted weak grid under the grid impedance variation.