Sequence Impedance Modeling And Interaction Stability Research Of Three-phase Grid-Connected Inverters Using DDSRF-PLL

As the energy conversion device between the distributed generation system and the grid,grid-connected inverter plays a very important role.It not only needs to convert the power generated by renewable energy into high-quality AC and feed it to the grid,but also undertakes the important mission of the safe and stable operation of the distributed generation system.With the increasing penetrations of the renewable energies these years,not only the grid characteristics become weak,grid impedance more obvious,but also the grid voltage may be unbalanced.DDSRF-PLL has the ability to eliminate the negative-sequence fundamental wave,but the current research mostly focuses on its dynamic and steady-state performance,and it is still blank in its frequency characteristics analysis.In impedance modeling,most papers regard the grid-connected inverter as a single-input single-output system,ignoring the influence of frequency coupling,so the stability analysis of the low frequency band of the inverter can be inaccurate.Therefore,the dissertation aims at the three-phase LCL-type grid-connected inverter based on DDSRF-PLL.Using impedance analysis method,the frequency characteristic model and output impedance model of DDSRF-PLL are established,and the grid connected interactive stability is studied.In this thesis,DDSRF-PLL is adopted to enhance the stability of the system under unbalanced power grid conditions.Considering the frequency coupling relationship of DDSRF-PLL,the multi-harmonic linearization method is used to model and analyze the frequency characteristics of the phase-locked loop,and the control parameters of PLL are designed.Secondly,the multi harmonic linearization method and symmetrical component method are used to derive the positive and negative sequence output impedance model of three-phase grid connected inverter,and the frequency coupling mechanism is analyzed.On this basis,the equivalent positive and negative sequence output impedance model of grid connected LCL inverter considering frequency coupling is derived.Then,based on the inverter impedance model and frequency characteristic model of DDSRF-PLL,the Nyquist stability criterion is used to study the interaction stability between the grid-connected inverter and the grid,and the influence of frequency coupling characteristics on the stability of grid connected inverter system is analyzed.For ensuring that the interconnected system has enough phase margins under weak grid conditions,the optimal design methods and impedance reshaping methods are proposed to improve the impedance characteristics of the inverter and enhance the interactive stability of the system.Finally,a 15 k W three-phase LCL-type grid-connected inverter platform is built in the laboratory to verify the impedance model and stability analysis of the inverter.The experimental results prove the correctness of the theoretical analysis.