Research And Analysis On Grid-Connected Inverter Of Resonance And Control Strategy Based On LCL Filter

With the increasing capacity of renewable energy power generation systems,single-inductor L-filter has been unable to meet the design requirements of high-power grid-connected inverter.LCL-filter replace the L-filter gradually with its characteristics,small size,and has been widely used of the high-power grid-connected inverter.However,the parameters of LCL-filter designed complex,serious shortcomings of the resonance problem.In this thesis,the resonant problem of LCL-filter in three-phase grid-connected inverter system is studied,and a strong robust current feedback active damping resonance suppression method is proposed to cope with the severe change of grid impedance.The main works are as follows:In this thesis,the mathematical model of the three-phase grid-connected inverter based on L and LCL-filters is established respectively.The resonant characteristics of the two filters are compared and analyzed.The effects of the LCL-filter parameters on the resonant frequency and the ripple attenuation and the parameters of LCL filter are designed.In this thesis,the passive damping and active damping resonance suppression strategies are studied respectively,and the four passive damping methods are compared and analyzed.In this thesis,the active damping method based on single state variable feedback is systematically analyzed,and a unified analytical model of active damping based on feedback is constructed.By analyzing the root locus of the unified model,the filter capacitor voltage feedback,the filter capacitor current feedback,current damping effectiveness of three active damping methods.Since the actual system has a control delay,the grid-connected current feedback active damping method is equivalent to a virtual impedance with frequency change.This equivalent impedance will change the frequency of the system’s actual resonant peak in the case of a damped resonant spike and there are two right-half plane poles in the system loop gain when the equivalent impedance at that frequency contains a negative resistance component,the stability of the system,increasing the difficulty of closed-loop control parameter design.In this thesis,the method of improving the robustness of the system by improving the current feedback regulator is presented.The effectiveness of the improved active feedback damping method is verified by simulation and experiment.In the end,according to the Thevenin’s theorem,the parallel system of multi-inverters is simplified and the mathematical model of the multi-inverter parallel system is obtained.Based on the mathematical model,the resonant characteristics of the system.In order to suppress the resonance effectively,the active damping control is formed by grid-connected current feedback and the control strategy of the multi-inverter parallel system with active damping is established.Finally,the active damping method is validated by simulation.