Optimal Robust Control of LCL-type Grid-Connected Voltage Source Inverters against Grid Impedance Fluctuations

LCL filters are used in grid-connected voltage source inverters (GC-VSIs) due to their lower cost and higher efficiency. These filters attenuate high order harmonics effectively; however, the grid impedance fluctuations (GIFs) may cause resonance and system instability. The GIF can reduce the resonance frequency to lower than a permissible threshold, then, the resonance may happen and prevent some nonlinear loads from being fed. On the other hand, very high resonance frequencies cause capacitor of the filter being actually short circuited. Consequently, the nature of the filter changes and reduces the power quality of the network. In this paper, considering the possible range for GIFs and using Kharitonov control theorem (KCT), the coefficients of the PI controller and the current feedback of the filter capacitor are obtained simultaneously to ensure safe and desired operation of the GC-VSI against the GIFs. As a result, the proposed closed loop system shows optimal response to variation of the reference current. Genetic algorithm (GA) is used to determine optimal values of the coefficients and the gain. Simulation results carried out in MATLAB/SIMULINK show accuracy and effectiveness of the proposed method.