Research On The Instability Of Grid Connected Inverter With Point Of Common Coupling Voltage Feedforward Scheme

The renewable energy based distributed power generation system(RE-DPGS)is a doubleedged sword.On the one hand,it is one of the most important way to deal with energy crisis and environmental pollution.On the other hand,due to the increased penetration of the REDPGS,the power grid has gradually changed from a traditional strong grid to a weak grid.The grid-connected inverter as the interface between the RE-DPGS and power grid,its stability and output current quality will be challenged.Therefore,this dissertation devotes to the stability analysis and control strategies of the grid-connected inverter under weak grid,and the main work is as follow:First of all,the nonlinearity of the grid-connected inverter caused by the saturator is studied in this dissertation.The description function(DF)to analyze nonlinear problems and its application conditions are introduced.Moreover,the linear modeling method for single-phase grid-connected inverter is presented,when two sinusoidal components are assumed at the nonlinearity input.The mathematical model of saturator for single-phase grid-connected inverter is established.Based on this model,the system stability assessment method is presented according to the Nyquist stability criterion,which lays the foundation for the rest of the dissertation.Secondly,the mathematical model of the digitally controlled L-type grid-connected inverter with point of common coupling(PCC)voltage feedforward scheme is derived,and its stability under weak grid condition is assessed with the impedance-based stability analysis method.It is revealed that the PCC voltage feedforward scheme shifts the phase of inverter output impedance below-90° in the low frequency band,leading to the risk of instability in the weak grid.With DF,the instability of the L-type grid-connected inverter considering saturator under weak grid is analyzed,and the mechanism of limit cycle is revealed.Besides,according to the relationship between the PCC voltage and real grid voltage,and the sampling law of digital control,a real grid voltage extraction method using dual sampling mode is proposed.Using the grid voltage instead of the PCC voltage as feedforward not only ensures the system’s ability to suppress grid background harmonics,but also improve the stability of the gridconnected inverter under weak grid conditions.Simulations and experiments are performed on a 6-k W single-phase grid-connected inverter,which confirm the correctness of the theory and the effectiveness of the scheme.Finally,the above research thought is applied to the LCL-type grid connected inverter with PCC voltage full feedforward scheme.The impact of the grid impedance and the saturator of the grid connected inverter on the stability and the instability oscillation of the single-phase grid-connected inverter are analyzed respectively.It is revealed that the PCC voltage feedforward scheme shifts the phase of inverter output impedance below-90° not only in the low frequency band,but also in the mid frequency band.Based on the DF,the linearized model of the grid-connected inverter saturator is conducted,and the oscillation characteristics in different unstable area are analyzed according to the Nyquist stability criterion.Simulations and experiments are performed on a 6-k W single-phase grid-connected inverter,confirming the correctness of the theory and the effectiveness of the scheme.