Research On Control Strategy Of Interface Converters In PV-ES DC Micro-grid Based On Virtual Generator

PV-ES DC micro-grid,as one of the effective ways to access new energy,is conducive to speeding up the construction of a low-carbon and efficient modern energy system.However,at present,the interface converters in PV-ES DC micro-grid often use “rigid”control strategy,which aggravates the severity of the low inertia and weak damping of PV-ES DC micro-grid.Therefore,in order to solve the problem of the lack of inertia of PV-ES DC micro-grid and ensure the stable operation of PV-ES DC micro-grid under the disturbances of photovoltaic output fluctuation,load switching and grid frequency mutation,this thesis takes the PV-ES DC micro-grid as the research object and conducts studies on the maximum power tracking control of photovoltaic,and the control strategies of interface converter(DC/DC)on the DC side at energy storage as well as interface converter(DC/AC)on the AC side.Firstly,the overall structure of PV-ES DC micro-grid is introduced.Then the topological structures of interface converter(DC/DC)on the DC side at energy storage and interface converter(DC/AC)on the AC side are described,and the mathematical model is established according to the working principle.Secondly,in order to improve the convergence speed of photovoltaic maximum power tracking in the later stage under local shading,the maximum power tracking control strategy based on the combination of grey wolf algorithm and disturbance observation method is studied.The range of the maximum power point is determined by using the global search ability of the grey wolf algorithm,and then the perturbation observation method is utilized to carry out the local search,which reduces the convergence time while tracking the accurate maximum power point.Thirdly,in order to ensure the stability of bus voltage on the DC side of PV-ES DC micro-grid when the illumination under local shading and load power change abruptly,the control strategy based on the virtual DC generator of interface converter(DC/DC)on the DC side at energy storage is studied.The mathematical model of virtual DC generator control is established by analogy with the operation principle of DC generator,and the influence of parameter changes on system stability is analyzed according to the small-signal model.On this basis,an adaptive control strategy of virtual inertia and damping is designed to improve the dynamic stability of DC bus voltage.Finally,in order to realize the flexible connection between PV-ES DC micro-grid and AC power grid,the control strategy based on virtual synchronous generator of interface converter(DC/AC)on the AC side is studied.By establishing the mathematical model of the virtual synchronous generator control,the root locus analysis of the key parameters is carried out to clarify the relationship between the parameter changes and the system stability by utilizing the small-signal model.On this basis,a cooperative adaptive control strategy of virtual rotational inertia and damping is designed to improve the dynamic regulation performance of grid frequency on the AC side.The research results show that virtual DC generator control and virtual synchronous generator control are respectively applied to the interface converter(DC/DC)on the DC side and the interface converter(DC/AC)on the AC side of PV-ES DC micro-grid under partial shading environment,which can not only ensure the stability of bus voltage on the DC side and grid frequency on the AC side,but also improve the inertia and transient characteristics of PV-ES DC micro-grid,which has a stronger adaptability to the penetration of large-scale new energy.