| LCL-type grid-connected inverter is the bridge between new energy and power grid.Its control performance directly affects the grid-connected current quality of distributed generation system.Especially under the weak-grid environment,there will be coupling relationship between each inverter and between the inverter with the grid,which will aggravate the resonance problem of the inverter,reduce the quality of the grid-connected current,and seriously affect the stable operation of the distributed generation system.In this paper,the three-phase LCL-type grid-connected inverter is taken as the research object,the inherent resonance of the inverter,the resonance characteristics of single inverter and multi-parallel inverters system under weak-grid are studied respectively,and the control strategy of resonance suppression is proposed accordingly.Firstly,the mathematical models of three-phase LCL-type grid connected inverter in two-phase static coordinate system and two-phase rotating coordinate system are established respectively.After comparative analysis,it can be seen that the three-phase LCL grid connected inverter model in two-phase static coordinate system is easier to control,and there is no coupling component between the two coordinate axes.Based on the comprehensive consideration of inverter power loss,grid-connected current standard,design cost,resonance characteristics and other conditions,the parameter design method of LCL filter is given,and the relationship between LCL parameters and resonance characteristics is analyzed.Secondly,the inherent resonance characteristics of LCL grid connected inverter is analyzed,and the working characteristics of several passive damping methods is analyzed.After these analysises,it is found that the passive damping method has the problem of power loss.On this basis,the equivalent virtual impedance based on the active damping method of capacitance current feedback considering the digital control delay is deduced,and the equivalent virtual impedance is found that upper limit of its positive real part interval isf_s/6.In order to improve the upper limit frequency,an improved active damping method based on capacitor current feedback based on first-order leading link is proposed,which can effectively improve the upper limit frequency of the positive real part of the equivalent virtual impedance and reduce the possibility of the system becoming a non-minimum phase system.The influence of the first-order leading link on the stability of the active damping inner loop is analyzed by using the root locus method in the discrete domain.In addition,the phase margin of the output impedance of the inverter is very low when the traditional grid voltage feed-forward control is used under the weak-grid environment,and it can not suppress the harmonic coming into the control system well.The closed-loop output impedance model of LCL inverter using traditional grid voltage feedforward is established.An improved grid voltage feedforward strategy using band-pass filter is proposed to improve the robustness of inverter to grid impedance.At the same time,multiple harmonic compensators are paralleled on PR controller,which can not only suppress resonance caused by weak current grid,but also can suppress the distortion of grid-connected current caused by grid voltage harmonic.Finally,in view of the resonance problem of multi machine parallel system of inverter,the open-loop Davinan and closed-loop Norton models of multi machine parallel system of inverter are established and analyze its resonance characteristics.Based on the closed-loop Norton model of multi machine parallel system,the stability criterion based on Nyquist impedance ratio is used to analyze the stability of multi machine parallel system.According to the closed-loop Norton model of the multi machine parallel system,a global resonance suppression method based on the introduction of virtual admittance into the PCC point by the fundamental notch filter is proposed.Then the stability criterion of Nyquist impedance ratio is used to analyze the stability of the multi machine parallel system at this time.The working characteristics of the improved voltage feedforward strategy and the added virtual admittance strategy in suppressing the resonance of the multi-parallel inverters system are compared and analyzed,and the applicable occasions of the two control strategies are given. |
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