A Control Strategy for Grid-connected Inverters With LCL Filters

The LCL type filter becomes an attractive grid interface for grid-connected voltage source inverters(VSI).In contrast with L filters,the LCL filter can attenuate effectively high frequency current harmonics by using smaller inductance than L type filter,and specifically,the LCL filter has obvious advantage in high-power grid-connected inverter with lower switching frequency.However,the system of the LCL interfaced grid-connected inverter is instable when only the direct grid-current control is used.This paper proposed a dual-loop control strategy for regulating the grid current directly,the strategy integrated an outer loop grid current regulator with inner loop bridge current regulation.The out-loop of grid current feedback could render the grid current harmonics attenuation and control the power factor of the grid current directly.The inner-loop by converter bridge current regulation was adopted for enhancing active damping and stabilizing the system.The relationship among resistances,control parameters and stability margins was quantified precisely based on system modeling and stability analysis.Simulation and experimental results prove that the proposed control strategy is feasible for grid-connected inverters with a LCL filter,the resonance of grid currents is avoided,a high in-grid power factor is achieved,and the grid-connected inverter system has very good dynamic and static properties.