Research On Dynamic Short Circuit Ratio Estimation Technology Of Grid-connected Inverter Based On Impedance Identification

New energy power generation and traditional thermal power generation have different impacts on the power grid.The high penetration of new energy sources makes the traditional power grid appear weak grid characteristics.At the same time,there are series compensation capacitor devices in the electrical line of the power grid,which can cause its impedance is capacitive in partial frequency bands.The complex impedance characteristics of the grid have an adverse effect on the stable operation of new energy grid-connected systems.It is a good way for grid-connected inverters to estimate the grid impedance characteristics online and use them for inverter adaptive control.At present,most of the researches on grid impedance identification in grid-connected inverters are based on the grid impedance model of resistance series inductance,which is not suitable for describing the grid impedance characteristics of series-compensated grids and multi-inverter grid-connected systems.In the research of active method to identify impedance,the injection method of adding disturbance amplitude signal to the reference of power control loop is often used,which leads to uncontrollable phase of disturbance signal and coupling between disturbance injection and power control.In view of the above problems,this paper improves the disturbance signal injection method and disturbance signal extraction method,and applies them to the research of the series compensation grid and the multi-inverter system grid impedance identification,and then estimates the dynamic short circuit ratio(DSCR).First,this paper introduces the background and significance of this research,and summarizes the advantages and disadvantages of impedance identification methods.The definition of short circuit ratio(SCR)for grid-connected systems is investigated,and the limitations of existing impedance identification methods and disturbance injection methods is analyzed.Based on this,the research on DSCR estimation of new energy gridconnected system based on impedance identification is proposed.Secondly,the working conditions of the new energy high penetration grid-connected system considered in this paper are described.Based on this,an improved method of disturbance extraction is proposed and compared with existing methods.Aiming at the shortcomings of superimposed disturbance injection methods in the power loop,a disturbance injection method based on amplitude closed-loop control is proposed,and the advantages and disadvantages of the two disturbance injection methods are compared.Thirdly,corresponding DSCR estimation methods are proposed for the two working conditions of series compensation grid and multi-inverter system.In the estimation of weak grid and series compensation grid,according to whether the grid-connected inverter system has harmonic resonance phenomenon,the disturbance injection method and the disturbance-free signal injection method are respectively proposed.Proposing an iterative solution method to estimate the series compensation capacitance value,and the convergence of the method is proved.So the DSCR estimation of weak power grid and series compensation power grid is realized.In the DSCR estimation of the multi-inverter system,the double disturbance injection method and the single disturbance full injection method are proposed from the perspective of the system and one of the inverters in the system.The theoretical deviation of the double disturbance injection method and the fundamental impedance theory expression of single disturbance full injection method are derived.And the influence degree of each factor on the estimation result of the single disturbance full injection method is analyzed.So the DSCR of the multi-inverter system is realized.Finally,a hardware-in-loop simulation platform and an experimental platform are built.The composition of the platform,the hardware part of the inverter control adapter board and the control program structure are introduced in detail.And the proposed disturbance signal injection,disturbance signal extraction and DSCR estimation methods are experimentally verified.The results obtained verify the effectiveness of the proposed method.