## 4 Performance Assessment under Distorted Weak Grid Condition
### Experimental Results  
In this section, the experimental results are presented to evaluate the current control performance of the integral-resonant state feedback controller under the grid impedance change by using the experimental system in Figures 15 and 16. A programmable AC source is used to implement distorted grid voltages. Figure 28: Simulation result of grid-side three-phase currents for integral-resonant state feedback controller with \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 10 uF under distorted grid voltages and \(L_{g}\) = 14 mH in Case 2. Figure 30: Simulation result for integral-resonant state feedback controller with \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 30 uF under distorted grid voltages when \(L_{g}\) is changed from 3 mH to 7 mH in Case 3. Figure 29: Simulation result for integral-resonant state feedback controller with \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 30 uF under distorted grid voltages without grid impedance in Case 3. Figure 31 shows the experimental results of the integral-resonant state feedback controller for Case 1 under distorted grid voltages in Figure 31a. Similar in the simulation, three-phase distorted grid voltages include the 5 th, 7 th, 11 th, and 13 th harmonics with the magnitude of 5% of the fundamental component. Figure 31b shows three-phase grid current waveforms and FFT result for \(a\)-phase grid-side current with the current reference of 4 A without the grid impedance. As expected, the disturbance caused by distorted grid is well suppressed, providing high-quality grid currents. Figures 32 and 33 show the experimental results of the integral-resonant state feedback controller when \(L_{g}\) = 7 mH and \(L_{g}\) = 14 mH are applied with the same conditions of Figure 31a. An external inductor is used to emulate the grid impedance as in Figure 15. Similar to the simulation in Figure 23, it is observed that the measured grid voltages are altered due to the inductive voltage drop by the grid current and \(L_{g}\). In addition, the harmonic rejection of the inverter is greatly enhanced with additional grid impedance. However, the injected grid currents oscillate seriously when \(L_{g}\) reaches 21 mH as shown in Figure 34. This also accords closely with the stability analysis in Figure 9 and the simulation in Figure 25. Figures 35-37 represent the experimental results of grid-side three-phase currents by the integral-resonant state feedback controller under the weak grid condition (\(L_{g}\) = 0 to 14 mH) with \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 10 uF as the LCL parameters. The current controller is also designed by using the given LCL filter set at stiff grid condition with distorted grid voltage of the 5 th, 7 th, 11 th, and 13 th harmonics. Until \(L_{g}\) increases to 14 mH, the inverter system maintains the stability, giving desirable grid currents. However, as \(L_{g}\) becomes larger than 14 mH, the grid currents become unstable, which is well matched to the simulation results. Figures 38 and 39 represents the experimental results for Case 3 using \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 30 uF as the LCL parameters. This LCL filter has the resonance frequency smaller than the critical frequency. It is shown that only 7 mH of \(L_{g}\) makes the inverter system unstable. To evaluate the injected current quality by two control schemes in the simulation and experimental results, Table 3 represents the THD values in the simulation results and the current harmonic magnitudes in the experimental results for three cases of the LCL filter parameters with \(L_{g}\). Figure 31: Experimental results for integral-resonant state feedback controller with \(L_{1}\) = 1.7 mH, \(L_{2}\) = 1.0 mH, and \(C_{f}\) = 4.5 μF under distorted grid voltages without grid impedance in Case 1: (**a**) Distorted grid voltages; (**b**) Grid-side three-phase currents.