Stability Analysis Of Grid-connected Inverter With Background Harmonics And Resonance Suppression

With the increase of demand for electricity in production and life,the proportion of distributed power supply in power system also increases gradually.Because of the task of energy conversion between distributed generation system and power grid,grid-connected inverters are paid more and more attention.The actual operation of the grid voltage contains the background harmonic component,the existence of the background harmonic will lead to the grid current will also produce harmonics,affecting the safety,stability and high quality of the system operation;Under the condition of weak power grid,the grid side impedance cannot be ignored and varies in a large range.The resonance generated by the inverter will also affect the system stability.In this paper,for the grid-connected system of three-phase LCL inverter,the suppression method of the influence of background harmonic voltage on grid-connected current is studied.At the same time,considering the resonance problem between the larger internal impedance of the grid and the inverter,the suppression method of resonance is studied.The work content of this paper around harmonic voltage and resonance is as follows:Firstly,the topology structure and control mode of the three-phase LCL grid-connected inverter are introduced,and the mathematical model of the grid-connected system in the two phase static and rotating coordinate systems is established.After comparison,the control strategy analysis is carried out in the two phase static coordinate system in order to facilitate analysis and avoid the influence of coupling phase.The transfer function of LCL filter and current controller is analyzed.On this basis,the control block diagram of grid-connected system is given and the expression of grid-connected current is derived.It is pointed out that the influence of background harmonic voltage on grid-connected current is divided into two aspects: The indirect effect of the given value error and the direct effect on the grid-connected current also lay a foundation for the subsequent selection of the suppression of background harmonic voltage in the grid.Secondly,based on the influence mechanism of background harmonic voltage on grid-connected current,a fully feedforward control method of grid voltage is selected to suppress the influence of background harmonic voltage on grid-connected current.Combined with the influence mechanism of harmonic voltage on grid-connected current,the principle block diagram of the full feedforward strategy is given.The control block diagram is used to analyze the mechanism of suppressing the influence of harmonic voltage on the grid,and the feedforward function is derived.The admittance model of grid-connected inverter system is established,and the output admittance model of grid-connected inverter before and after full feedforward control strategy is compared.It is pointed out that adopting full feedforward control will change the output characteristics,and make the phase frequency characteristics of the Bode diagram of output admittance shift up as a whole,thus affecting the stability of the system.Based on admittance stability criterion,the resonant mechanism of grid-connected system is analyzed and verified by simulation when the grid voltage feedforward strategy is adopted under the condition that the grid impedance varies in a large range.Finally,aiming at the problem of system resonance caused by large grid impedance,a method of parallel active dampers at common junction points is proposed to suppress resonance.The structure control diagram of the active damper is given,and the mechanism of suppressing the system resonance is analyzed.It is pointed out that the active damper is equivalent to a pure resistor,and the phase frequency characteristic of the output admittance is closer to 0° after it is connected in parallel with the inverter.The output admittance model is established.Based on admittance stability criterion and Nyquist criterion,appropriate equivalent virtual resistance value is selected to control the phase Angle margin of the system within a reasonable range.The shunt active damper will lead to the increase of the amplitude-frequency characteristics of the output admittance of the inverter,and then lead to the decrease of the system cutoff frequency.When the cutoff frequency drops to a certain extent,the system will still produce resonance.In order to suppress the resonant problem caused by the decrease of cutoff frequency,a method of adjusting the control parameters of the inverter is proposed to keep the cutoff frequency unchanged before and after the shunt active damper.Finally,the time domain simulation model is used to verify the effectiveness of the proposed method.