Research On DQ Impedance Modelling Method Of Grid-Tied Inverters

With the increasing proportion of renewable energy and power-electronics in power system,the issues of broadband resonance and harmonic stability caused by the dynamic interaction between grid-tied inverters and the power grid becomes increasingly prominent.The dq impedance method based on the small-signal model of frequency domain in synchronous rotating coordinate is an effective tool to study the broadband harmonic resonance of three phase inverters-grid connected system.Aiming at the shortcomings of existing dq impedance modeling and harmonic stability research,includeing fuzzy physical interpretation and weak design-guidance,inverters-grid connected stability assessment,white-box inverter impedance modeling and oriented reshaping of inverter impedance basised on black-box impedance model are performed in this thesis.Firstly,small-signal models of pivotal physical links and the inverter are established,where the the inverter impedance is regarded as a black-box model.Moreover,the impedance characteristics of the grid-tied inverter under different control methods,control parameters and power points are compared and analyzed.Then the dq impedance measruement method based on disturbance injection in abc coordinate is discussed,results of Simulink simulation and RTDS hardware in-loop experiment verify the correctness of the modeling method and impedance model.Subsequently,dq impedance models of the inverter-side and the grid-side are combined in this thesis to establish the small-signal model and equivalent circuit of inverters-grid connected system,which provides the harmonic stability analysis with physical foundation.Furthermore,the connection between frequency domain characteristics and time domain characteristics are discussed,and a new harmonic resonance stability assessment method combining impedance ratio and damping characteristics are proposed in this thesis,which was applied to the stability analysis of inverters-grid connected system with different power factors and different grid impedances.The theoretical assessment of other stability analysis approaches,Simulink-based simulation results and RTDS platform-based experimental results are compared,which verifies the effectiveness of proposed stability analysis method.Afterwards,the definition of branch admittance based on the original inverter small-signal model are intruduced in this thesis.The active power admittance and reactive power admittance in branch admittance matrix take advantages to analyze the influence mechanism of power points on the inverter impedance and system stability.Furthermore,the reduced-order impedance modelling method are discussed with reduced matrix operation and simplified coupling terms.In the reduced-order inverter impedance model,impedance terms are derived with independent expressions,which reveals connections between the internal physical segments and external impedance characteristics.As a contribution,discussed modeling method proviedes the original black-box impedance model with the characteristics of white box model,which is helpful to guide the oriented reshaping of inverter impedance.Finally,based on the research conclusion of the small-signal model of the inverters-grid connected system,non-ideal regions of the inverter impedance on the bode diagram are defined,and it is pointed out that the goal of impedance directional reshaping is to reduce the intersection of inverter impedance curves and non-ideal regions.Furthermore,the physical connections revealed by the reduced-order impedance model are used to guide the control structure improvement and parameter optimization.The theoretical analysis in frequency domain and simulation results in time domain are consistent with each other.The research on impedance modeling,stability analysis and impedance reshaping effectively enhances the physical interpretation of dq impedance and the design-oriention capacity during the process impedance stability assessment.