Design And Implementation Of Grid-Connected Inverter With LCL-Filter Based On H_? Robust Control

Grid-connected inverters(GCIs)are important interface devices in distributed power generation system(DPGS).The effect on stability of GCI caused by grid impedance perturbation should not be neglected.The perturbation will cause the fluctuation of LCL resonant frequency in large scale,affect the quality of the currents injected into the grid,and even make the GCI resonant.Furthermore,it will affact the point of common coupling(PCC)voltages,make the output of PLL instable,and reduce the quality of grid currents.In this paper,by constructing the small signal model of GCI,the range of grid impedance perturbation effect on the stability of GCI is discussed under proportionalintegral(PI)control strategy.The control structures of GCI are single current loop and current loop with capacitor current feedback.The allowance of grid impedance perturbation is small with single current loop control structure.For current loop with capacitor current feedback,an inherent instability frequency range is existed.PI control cannot guarantee the stability of system when resonant frequency is in this range.Furthermore,the perturbation effect on the output of phase-locked-loop(PLL)is analyzed.Design and implementation of H_? robust controller for GCI under grid impedance perturbation is presented.By constructing the mathematical model with parameters uncertainty,properly selecting weighting functions,a seventh-order controller is synthesized.Because the seventh-order controller needs more calculated amounts and data registers,a third-order controller is reduced which can keep the performance and robustness of the high order controller.In order to stabilize the output of PLL,the multiple complex-coefficient filters(MCCF)is added.In this paper,LCL-filtered GCI with PI and H_? robust controller is simulated and experimented.The comparison of simulation and experiment results between this two control strategies manifest the superiority and feasibility of the proposed H_? robust controller.