Stability Analysis Of LLCL Grid-connected Inverter And Research On Impedance Remodeling Method

With the increase of new energy power generation units,the use of PWM technology has become more and more widespread,but this has also introduced a large number of harmonics at the switching frequency to the grid-connected system,resulting in stringent challenges for grid-connected power quality.When the grid voltage has DC interference,the working performance of the traditional phase-locked loop will be seriously reduced.In addition,many new energy power generation units in my country are generally located in remote areas,and long transmission lines and a large number of voltage transformation links cause grid-connected inverters to operate in weak grid conditions.The grid-connected inverter is the energy conversion interface between the new energy power generation unit and the grid.It generates impedance interaction with the weak grid,which may cause oscillation problems in the gridconnected system and affect the safe operation of the grid-connected system.How to ensure the safe operation of new energy power generation units under weak grids is of great practical significance for improving the power quality of grid-connected currents.Based on the impedance analysis method,this paper has carried out research work on the stability of the LLCL inverter grid-connected system.The main work content is as follows:(1)The mechanism of LLCL filter suppressing the harmonics at the switching frequency and the influence of filter parameters on the ability of harmonic suppression are analyzed;In order to ensure the grid-connected operation of the grid-connected inverter under the condition of unit power factor,the grid-connected current is used as the controlled object for negative feedback control,and a mathematical model is established for the structure of the LLCL-type grid-connected inverter;Aiming at the problem of poor DC suppression in the traditional phaselocked loop structure,an improved phase-locked loop structure that can effectively suppress DC interference is proposed;It is verified by simulation that the LLCL grid-connected inverter has a strong suppression effect on the harmonics at the switching frequency and the improved phase-locked loop has a strong suppression effect on DC interference.(2)The impedance stability criterion of the LLCL inverter grid-connected system is analyzed.Under the condition that both the LLCL inverter and the grid can operate independently and stably,the stability of the system can be judged according to the ratio of the grid-side equivalent impedance to the inverter impedance;The impedance model of the LLCL inverter is derived.From the impedance expression,it can be seen that the parameters of the LLCL filter,the current controller parameters and the bypass current feedback coefficient will all affect the impedance model of the inverter;On this basis,the range of control parameters for the stable operation of the inverter itself is analyzed;Aiming at the stability problem caused by the interaction between the impedance of the LLCL grid-connected inverter and the gridside equivalent impedance,the influence of filter parameters,control parameters,and grid-side equivalent impedance on the stability of the grid-connected system is analyzed.It is verified by simulation that the increase of bypass capacitor,resonance gain coefficient,and grid-side equivalent impedance will reduce the stability margin of the grid-connected system.(3)The identification method of grid-side equivalent impedance is studied,and the influence of identification parameters on the quality of grid-connected current and identification accuracy is analyzed;The inverter impedance reshaping method based on phase compensation is analyzed.By adding a phase compensation link after the current controller,the stability of the grid-connected system can be enhanced;An adaptive impedance reshaping strategy including grid-side equivalent impedance identification and phase margin compensation is proposed.This strategy ensures that the inverter grid-connected system still has a high stability margin during the change of the grid-side equivalent impedance.And considering the identification error,the proposed adaptive impedance reshaping strategy can still ensure that the grid-connected system has good dynamic performance.(4)Based on the RT-LAB hardware-in-the-loop platform,an experiment model of the LLCL inverter grid-connected system is built.The experiment verify the following conclusions:1)The improved phase-locked loop structure proposed in this paper has a very good suppression effect on DC interference;2)The increase of bypass capacitor,resonance gain coefficient,and grid-side equivalent impedance will reduce the stability margin of the gridconnected system Degree;3)The adaptive impedance reshaping method proposed in this paper can ensure that the LLCL inverter grid-connected system still has a relatively high stability margin during the change of the grid-side equivalent impedance.