Research On Modeling And Simulation Of Grid-connected Converter For Active Distribution Network

In recent years,against the overall background of energy upgrading and transition,the electric system faces two technological challenges,that is,renewable energy of high permeability and a large proportion of power electronic equipment connected to the power grid,which bring a series of new technological demands.In the new power generation system,grid-connected converter as an important power electronic converter is the necessary interface to connect the distributed generation and grid and the core device of the gird-connected system.Therefore,the accurate model of converter is the basis and premise for analyzing and researching on a series of key problems about distributed generation.Due to the lack of products designing information caused by various kinds,control strategy and specific parameters of distributed generation and the confidential protection of commercial converter’s technology,most traditional modeling approaches is difficult to meet the needs of the active distribution network simulation research.Even if all the data can be obtained,it is not suitable for the simulation of large scale active distribution network,because of the complexity of model which is established in detail.The experimental modeling of converter has gradually become a new research hotspots and potential directions,due to its less modeling information,high accuracy and less resource consumption,etc.In this thesis a black-box modeling method of three phase grid-connected converters is proposed on the basis of experimental modeling.The identification of the model is based on the analysis of the converter transient response.The following study has been conducted in this dissertation:(1)The basic contents about ideas,principle and method of black-box modeling are introduced in detail.Firstly,the topological construction and input/output of the black-box model are derived in detail according to the equivalent circuit of two-port network.Secondly,analyze the basic principles of experimental design for identification and basic steps of black-box modeling.Finally,to solve the problem between parameters coupled caused by cross-coupling effects,an effective method is proposed to eliminate the parameters coupled due to using the decoupling matrix.(2)A real time simulation platform based on dSPACE and Typhoon is built.Then,build a grid-connected converter adopts the doubled closed-loop control strategy which contains inner current loop and outer voltage loop based on the dSPACE-Typhoon real-time simulation platform,as a real grid-connected converter,modeling its black-box as the proposed method above.And the experimental results show that the black-box with high accuracy and high fitting.By the building of the real-time simulation platform,verify the correctness of the modeling method.(3)The technology of virtual synchronous generator is the necessary ability of fhe power electronic equipments of the future active distribution network,so the penetration of the power electronic equipments with virtual synchronous generator technology will increase rapidly in the distribution network.However,it is almost impossible to model the power electronic device in detail in the analysis of active distribution network,the black-box modeling is a good implementation method under this condition.Firstly,the basic idea,the control target and the control strategy of the virtual synchronous generator is analyzed in detail.Secondly,the model of grid connected converter with VSG is built based on the Simulink,and the correctness of the control algorithm of the virtual synchronous generator is verified in two modes which are VSG connected to the grid and VSG parallel with the traditional synchronous generator.And compared with the common converter,the simulation result proves that the control algorithm of VSG can work in frequency regulation of system when the system frequency has unusual changes,to reduce the frequency fluctuation.Finally,the black-box model of grid-connected converter which based on the control strategy of the virtual synchronous generator is built.In order to make the black-box model can instead of the detailed model of grid-connected converter,the current of DC side of the black box model is modified using the function ralation between frequency and the current of DC.The simulation result shows that the final model can accurately fit the active power-frequency and reactive power-voltage static characteristic of VSG control algorithm.At the same time,it also proves that the black-box model has correctability and extendibility,confirms the application value of the black-box model.