## 5 Conclusions  
This paper has presented an assessment of the stability and performance of current controllers with harmonic compensators for an LCL-filtered grid-connected inverter under distorted weak grid conditions. Two typical current controllers with both the active damping and grid harmonic compensation are selected to investigate the closed-loop system stability and current control performance under uncertainty of the grid impedance in weak grid conditions. The system stability is analyzed by investigating the discrete-time closed-loop poles when the grid impedance varies. The assessment of the stability and current control performance is carried out for three sets of LCL filter parameters in view of current harmonic attenuation and resonance damping in the presence of uncertain grid impedance and distorted grid. It is clearly demonstrated that the LCL parameter selection mainly affects the control robustness. The lower the resonance frequency of the LCL filter is, the tighter the control bandwidth and stability margin yield with respect to the grid uncertainties. Furthermore, since the grid is commonly distorted and the controller is equipped with harmonic resonant controllers, the control stability investigation in this condition is very important for a stable inverter operation. In order to support the stability analysis of the current control schemes, the whole control algorithm is implemented on 32-bit DSP TMS320F28335 to control 2 kVA grid-connected inverter. Comprehensive simulation and experimental results are presented under distorted weak grid conditions to demonstrate theoretical analysis. Based on the presented results, it is confirmed that the state feedback controller is more flexible in the control of an LCL-filtered inverter in the environment of distorted grid and uncertain grid impedance. Furthermore, when the grid is subject to large grid impedance variation, the choice of high resonance frequency of the LCL filter is preferable since it extends much the stability margin of inverter system under unexpected weak grid condition. Conceptualization, S.-J.Y. and K.-H.K. ; methodology, S.-J.Y. and T.V.T. ; software, S.-J.Y. ; validation, S.-J.Y., T.V.T. and K.-H.K. ; formal analysis, S.-J.Y. and T.V.T. ; investigation, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{** LCL Filter**} & \multirow{3}{*}{**Grid Inductance**} & \multicolumn{4}{c}{**THD in Simulation**} & \multicolumn{4}{c}{**Harmonic Magnitude in Experiment**} \\ \cline{3-8}  & & **Integral-Resonant State** & **Direct Grid** & **2 h** & **5 h** & **7 h** & **_1I_ h** \\ \cline{3-8}  & & **Feedback Control** & **Current Control** & **2** & **5** & **5** & **5** \\ \hline \multirow{3}{*}{Case 1} & \(L_{g}\) = 0 mH & 3.96\% & 3.59\% & \textless{}0.8\% & \textless{}0.4\% & \textless{}0.2\% & \textless{}0.1\% \\  & \(L_{g}\) = 7 mH & 2.16\% & - & \textless{}1.5\% & \textless{}0.2\% & \textless{}0.1\% & \textless{}0.1\% \\  & \(L_{g}\) = 14 mH & 2.09\% & - & \textless{}0.8\% & \textless{}0.6\% & \textless{}0.9\% & \textless{}0.1\% \\  & \(L_{g}\) = 21 mH & - & - & - & - & - & - \\ \hline \multirow{3}{*}{Case 2} & \(L_{g}\) = 0 mH & 3.86\% & 2.54\% & \textless{}0.7\% & \textless{}0.1\% & \textless{}0.2\% & \textless{}0.1\% \\  & \(L_{g}\) = 7 mH & 1.12\% & - & \textless{}2.2\% & \textless{}0.1\% & \textless{}0.2\% & \textless{}0.1\% \\ \cline{1-1}  & \(L_{g}\) = 14 mH & - & - & & & & \\ \hline \multirow{3}{*}{Case 3} & \(L_{g}\) = 0 mH & 3.04\% & - & \textless{}0.7\% & \textless{}0.2\% & \textless{}0.2\% & \textless{}0.1\% \\ \cline{1-1}  & \(L_{g}\) = 7 mH & - & - & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Injected current quality in the simulation and experimental results. Figure 39: Experimental result of grid-side currents for integral-resonant state feedback controller with \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 30 μF under distorted grid voltages and \(L_{g}\) = 7 mH in Case 3. S.-J.Y. ; resources, S.-J.Y. ; data curation, S.-J.Y.