Research On Equivalence And Characteristic Of District Power Grid

With the increasing demand of electricity,the development of the district power grid has drawn a lot of attention.To make a research on the influence of the fault occurs in district power grid,fault setting will make unnecessary loss to the grid.Therefore,it is necessary to build the model of the district power grid in simulation platform to simulate the dynamic characteristics of the grid.With the full implementation of ultrahigh-voltage grid and global energy interconnection,several large-scale power grids have been formed in China at present.It has formed the tendency of large-scale power grids interaction and it has performed well in power dispatching.However,when we focus on a district power grid,it is unnecessary and even impossible to establish the simulation model of the whole grid.Furthermore,considering the simulation efficiency,the scale of the system should not be too large.In conclusion,it is essential to make dynamic equivalence when we make aresearch on a district power grid in a large-scale power grid.Generally,only the internal system should be remained,while the external system which has small influence on it should be reduced.The reduced grid helps to improve the simulation efficiency without deteriorating the simulation accuracy.Recently,distributed generations,such as wind electricity and photovoltaic power generation,which possess low pollution,high reliability,high rate of energy utilization and flexible installation have made wide application in power grid.In consideration of the expanding scale of distributed generations and the elevated voltage level accessed to power grid,the equivalence and modeling of distributed generations becomes the fundamental research.In addition,distributed generations integration will certainly make an influence on the power gird.Therefore,the integration planning and operation strategy of distributed generations should attach great importance.Based on the above introduction,the equivalence and modeling of district power grid and the integration of distributed generations will be discussed in this paper.The following work is carried on:(1)The feature of common dynamic equivalent method is discussed.Considering of simulation efficiency and software limitation,the advantages and disadvantages of the existing dynamic equivalent methods for large-scale power grid are described.Then,the dynamic equivalent methods for wind farm and its integration are discussed.(2)Combined with the advantages and disadvantages of the existing methods,a multistep dynamic equivalent method based on district dividing is proposed when a district power grid in a large scale power grid is concerned.Based on the electrical distance,voltage level,and geographic location,the external system is divided into outer layer,inner layer,and buffer subsystem.Each district is separately reduced by different methods.Firstly,coherency-based equivalent method is used to reduce the outer layer to a large extent.Secondly,the backbone network of inner layer is reserved while the low voltage level grid of inner layer is reduced by physical-equivalence based method.Finally,the buffer subsystem is remained while the rest backbone network of external system is reduced.(3)The proposed method is illustrated by a real large-scale power grid in the southeast of China to attain the equivalent model of the district power grid.Compared with traditional single-step equivalent method,the verification of steady state characteristics and transient characteristics show that the main characteristics of the district power gird addressed by the proposed method could be reserved with higher equivalence accuracy.(4)Taking the application of distributed generation,wind power is chosen to be the typical form.A dynamic equivalent method for wind power based on clustering algorithm is proposed.What's more,how the wind speed fluctuation makes an influence on wind power integration to grid is discussed.