A Research On Multiple Targets Control Strategy And Robustness For Input-Series-Output-Parallel Grid-Tied Inverter System

Input-series-output-parallel(ISOP)grid-tied inverter system is suitable for power conversion applications with high input voltage and high output current,such as the distributed power generation system(DPGS)based on wind and solar-power.The combined system is conducive to improve the reliability of grid-tied system.For the ISOP system,firstly select control variables need to meet its multiple control objectives,and secondly it is necessary to propose an effective redundancy operation scheme to improve the reliability of the grid-tied system.In addition,the long distance transmission lines in the DPGS can result in the variable grid impedance between the inverter and the grid,and this weak grid has more stringent requirements on grid-tied inverter control technologies.The first part of this paper aims at the ISOP grid-tied inverter system.For control targets of power balance,LCL resonance suppression and high grid-tied power factor,the select control variable can be inverter-side or grid-side inductor current.Analyze and compare system performance when control variable is inverter-side or grid-side inductor current.The analysis shows that,when the control variable is inverter-side inductor current,output current sharing can be realized directly,high grid-tied power factor can be achieved indirectly,and the necessary condition for LCL resonance suppression is f_r<f_s/6.When the control variable is grid-side inductor current,LCL filter parameters of each module should be matched to realize output current sharing indirectly,high grid-tied power factor can be achieved directly,and the necessary condition for LCL resonance suppression is f_r?f_s/6.The second part takes ISOP grid-tied inverter system whose control variable is inverter-side inductor current as an example,a redundancy operation scheme is proposed to improve the system reliability.In addition,the fault module drops out by means of bypass,and the spare module is put in by means of mode-switching.What's more,the specific hot-plug timing sequence is given for smooth switching.The proposed redundancy scheme is verified by a three-module ISOP grid-tied inverter system prototype.The third part aims at the condition that control variable is grid-side inductor current.Taking the single LCL grid-tied inverter as an example,when capacitor-current-feedback active-damping is adopted,analyze its effect on capacitor-current-feedback active-damping.And find that the digital-control-delay may reduce the system robustness against grid impedance,thus a phase-lead compensation method is proposed.Then,the system stability margins under different resonant frequencies are analyzed,and it is demonstrated that the system with phase-lead compensation has good robustness against the grid impedance.Next,the design of capacitor-current-feedback coefficient is developed,and the select coefficient is checked by the closed-loop pole maps with and without phase-lead compensation.Finally,the validity of the proposed theoretical analysis has been proved by testing a 1-kVA prototype.The fourth part applies the phase-lead compensation method above to the ISOP grid-tied inverter system,to enhance the system robustness against grid impedance.With each module equivalent to a Norton model,the system's equivalent model is obtained.According to the impedance-based stability criterion,the system robustness against grid impedance with and without phase-lead compensation is compared and analyzed,and the corresponding simulation verification is given.Finally,the proposed theoretical analysis are verified by a two-module ISOP grid-tied inverter system prototype.