Research On Resonance Suppression Method Of LCL Grid-connected Inverter

As an important way to develop and utilize renewable clean energy,distributed power generation system has become an important driving force to promote green and low-carbon energy structure.The grid connected inverter is the energy conversion interface in distributed generation system,so its normal and stable operation is the key to improve the power supply reliability of the system.Although the LCL grid-connected inverter can reduce the volume of the filter while having good output performance,its inherent resonance characteristics will bring new challenges to the stability of the system.Therefore,based on the current research status,this article makes a detailed study of the resonance damping method and grid-connected current control strategy.In this article,the mathematical model of LCL grid-connected inverter under digital control is established according to the active damping method of capacitor current proportional feedback.Based on this model,the damping mechanism of capacitor current proportional feedback is analyzed and the research results show that the delay of digital control will make this damping method equivalent to the circuit form of capacitor in parallel with variable impedance.In addition,the characteristics of the equivalent resistance will affect the damping effect,and the equivalent reactance will shift the resonance frequency.When the actual resonance frequency is higher than 1/6 of the sampling frequency,the value of equivalent resistance will be negative,and at the same time,the open-loop transfer function of the system will contain a pair of unstable poles located in the right half plane.Therefore,according to the relationship between LCL resonant frequency and sampling frequency,the constraint conditions for system stability under different situations are analyzed.And the research results show that the unstable condition of f_r=f_s/6 will exist in LCL grid-connected inverter with proportional feedback of capacitance current,so this situation should be avoided when selecting filter parameters and sampling frequency.The active damping method of capacitive current proportional feedback has the advantages of simple implementation and good damping effect,but it requires an additional current sensor for the inverter to realize the closed-loop control of grid-connected current,which will increase the equipment cost.To solve this problem,a new inverter-side current control strategy is proposed.The phase compensation link introduced by this control method can effectively compensate for the phase lag in the system.Compared with the traditional inverter-side current feedback control,the stable region of the system after phase compensation will increase from the original[0,0.167f_s]to[0,0.22f_s],so this strategy can significantly expand the stable region of the system.Besides,the current outer loop design method based on system stability margin is also proposed,which can provide a reliable parameter selection range for system design.Even thought the inverter side current feedback with phase compensation can achieve good performance of current control,this method is essentially an indirect grid current control for the system and the characteristics of the power grid will have a noticeable impact on its control performance.Therefore,in order to achieve low-cost and robust grid current control,this article proposes an active damping method based on grid current delay feedback.By analyzing the system model in continuous domain,it can be seen that this damping method is equivalent to connecting the variable resistance and reactance in parallel with the inductance on the grid side.When the equivalent resistance is positive,the resonant effect of LCL can be effectively suppressed.Compared with the previous two damping methods,the proposed method has great flexibility in parameter selection.For different filtering parameters,it can construct corresponding effective damping regions.In addition,the introduction of grid current delay feedback will make the frequency domain characteristics of the damping inner loop have a wide range of selectable stability margin,so this method will make system have an excellent robustness against the change of power grid impedance.Finally,based on the performance evaluation index of grid-connected system in practical application,this article makes a systematic comparative analysis of above three control methods.The comparison results point out the advantages of phase compensation control of inverter-side current and delay feedback damping method of grid current.