Study On Control Parameter Optimization And Improved Feedforward Strategy Of LCL Grid-connected Inverter

With the depletion of fossil energy and the increasing emphasis on environmental protection,the development of renewable and clean new energy is urgent.LCL type gridconnected inverter is the grid-connected interface of new energy power station,and its operation performance has a huge impact on power transmission.Due to low cost and powerful functions of digital chips,more and more converters use digital chips to achieve control functions,but the limited operating frequency brings digital control delay problems,which have a large negative impact on system performance.In addition,changes in LCL filter parameters and undesired grid voltages at PCC will affect the operating characteristics of grid-connected inverter.In this paper,LCL-type grid-connected inverter under digital control is taken as the research object,and damping characteristics of the controlled object,closed-loop resonant peak suppression,robustness to filter parameter changes and suppression of grid harmonics are studied.In this paper,the mathematical model of LCL grid-connected inverter with capacitive current feedback active damping under digital control is derived.On this basis,the stability problem of the controlled object is analyzed.It is pointed out that when the capacitive current feedback coefficient takes different positive values,the controlled object will be stable or unstable.Then the characteristics of controlled object damping ratio are detailed analyzed.It is concluded that digital control delay causes the controlled object damping ratio to exhibit a non-monotonic relationship with the capacitive current feedback coefficient.Controlled object damping ratio has a maximum value,and its value is much smaller than the value under analog control.For the whole LCL-type grid-connected inverter system,the stability constraints are analyzed from the perspective of nyquist stability criterion.The relationship between the closed-loop resonant peak amplitude and the capacitive current feedback coefficient is analyzed.The feedback coefficient optimization design method is given to minimize closed-loop resonance peak of the system and to improve the output waveform quality.Then the problem of system robustness when LCL filter parameters change is analyzed.It is found that when the control parameter takes value close to stable region of the controlled object,that is,the PI parameter and the capacitance current feedback coefficient take a small value,the system is more robust to changes of filter parameters.In the case of weak grid,LCL-type grid-connected inverters will be unstable when using conventional proportional feedforward.A conventional multi-resonant feedforward strategy has been proposed in the literature,which can greatly enhance the stability of the weak grid.But due to digital control,the delay will cause phase lag of the feedforward component,resulting in insufficient suppression to grid harmonics.In this regard,this paper proposes a compensation method of the phase lag based on changing the initial phase of ??/dq transformation of resonant feedforward components,so that the output impedance of system is greatly increased.In addition,for the variation law of the output impedance of system near resonant frequency,this paper optimizes the value of the resonant frequency by mathematical analysis,and further improves the output impedance of system.In order to prove the correctness of the theoretical analysis,this paper uses a threephase LCL grid-connected inverter as a platform.Experiment results verify the analysis correctness of the influence of capacitive current feedback coefficient on the closed-loop resonant peak and improved resonant feedforward improves the harmonic suppression capability.