Research On Resonance Suppression Method For Grid-connected Inverter Based On Adaptive Impedance Reshaping

The LCL-filter circuit of grid-connected inverter is a third-order system,which has serious resonance issues at the resonance frequency,and the grid-connected inverter stability will be destroyed.The problem seriously affects the grid current quality,which restricts the large-scale application of photovoltaic grid-connected power generation technology.In multi-parallel inverters system,the equivalent grid impedance of grid-connected inverters will affect the resonance frequency points,and reduce the control system stability.In order to mitigate the effect,this paper proposes a resonance suppression method for grid-connected inverter,which is based on adaptive impedance reshaping,and the method has the advantage of adjustable control parameters in a simple system.The control system stability is improved.First,based on the open-loop and closed-loop models of multi-parallel inverters impedance network,the resonance mechanism is analyzed,and the variation law of equivalent grid impedance is pointed out.The open-loop model is established by Norton equivalent circuit model.Considering the inverter numbers,grid load,grid impedance and other factors,the frequency-domain characteristics of impedance model are analyzed.It is concluded that the inverter numbers and grid impedance are not conducive to the system stability.The closed-loop model is established by Thevenin equivalent circuit,which is compared with the output current expression of a single inverter.It is concluded that the equivalent grid impedance is equal to the product of the inverter numbers and grid impedance.Next,resonance suppression methods for grid-connected inverters are summarized.passive damping and active damping methods are analyzed for resonance suppression.Resonance suppression methods for multi-parallel inverters system are summarized in two aspects:resonance suppression method at PCC,resonance suppression method of single inverter.Impedance reshaping control method based on“inverter-side current feedback+proportional feedback of capacitor current”is adopted to suppress resonance.The adverse effect of equivalent grid impedance on control system is pointed out.Then,an adaptive impedance reshaping control method is proposed to improve the adaptability of control system to the variation of equivalent grid impedance.According to the equivalent grid impedance value,impedance coefficient R_ir?proportionality coefficient k_p and resonance coefficient k_r of control system are changed in real time,which ensures the multi-parallel inverters system stability.The impedance reshaping unit is used to improve the output impedance characteristics of grid-connected inverters.The Quasi-Proportional-Resonant?QPR?controller unit is used to maintain the current loop stability.The design processes of system parameters are introduced in detail.Finally,the correctness of the proposed method for resonance suppression are verified by simulation and experimental results.The simulation model of LCL grid-connected inverters is built to verify the resonance suppression in MATLAB.A 2kW grid-connected inverter is designed.From the experiment results,it is proved that the proposed control method can effectively improve the adaptability of the control system to the change of equivalent grid impedance.