Feedforward Scheme Of Grid Voltage For LCL-Type Grid-Connected Inverters Under Weak Grid Conditions

Renewable energy based distributed power generation system is a good means to solve energy crisis and environmental issues.As the interface between distributed power generation units and power grids,the grid-connected inverter plays a significant role in converting DC power into high-quality AC power and feeding it into the grid reliably.When the grid-connected inverter is connected to a weak grid,since the grid impedance of the weak grid cannot be ignored and may vary in a wide range,the coupling between it and the inverter output impedance may cause system oscillation,or even instability.Besides,the noticeable background harmonics in the grid voltage will affect the power quality of the grid current.This thesis studies the control scheme applicable to the grid-connected inverter under weak grid conditions,guaranteeing the system stability and attenuating the influence of the grid voltage background harmonics on the grid current.This thesis firstly establishes the mathematical model of the grid-connected inverter.Then the feedforward scheme of grid voltage is deduced,which is essentially equivalent to increase the modulus of the inverter output impedance,so as to eliminate the adverse effect of the background harmonics in grid voltage on the grid current.Then,the effect of control delay on the feedforward scheme is analyzed,which causes the inverter output impedance can hardly be infinite in the entire frequency range.Besides,it may enhance the coupling between the grid impedance and the inverter output impedance,which is a serious threat to the system stability.With the help of the impedance-based stability analysis method,this thesis analyzes the stability of the LCL-type grid-connected inverter with feedforward scheme of grid voltage under weak grid conditions.It is pointed out that two basic conditions are required for system stability.One is that the inverter itself should be stable when connected to a stiff grid.The other is that the ratio of the grid impedance to the inverter output impedance must satisfy the Nyquist stability criterion.Furthermore,the two ways for solving the stability problem are presented: to adjust the grid impedance at the point of common coupling or the inverter output impedance.Aiming at the system stability problem of the grid-connected inverter under weak grid conditions,this thesis proposes a weighted feedforward scheme of grid voltage to adjust the frequency characteristic of the inverter output impedance,ensuring it match the grid impedance,so that the system stability conditions can be satisfied.In addition,the ability to attenuate the grid voltage harmonics of this scheme is evaluated,which indicates that the influence of the grid voltage harmonics under 33 th order can be effectively suppressed.Experimental results on a 6 kW LCL-type grid-connected inverter prototype built in the laboratory verify the correction of the theoretical analysis and the validity of the proposed weighted feedforward scheme of grid voltage.The result is also applicable for the three-phase grid-connected inverter system.