Analysis On Time-domain Algorithm Of Large Power Transformer Winding Under Very Fast Transient Over-voltage

In the power grid construction of the high voltage level,the gas insulated substation(GIS)has been widely used in the electric power system with the SF6 gas as the insulating medium.In the operation process of the isolation switch opening and closing,the very fast transient over-voltage with short wave head time and high oscillation frequency has been formed.If the overvoltage intrudes into the transformer and other power equipment,it is very likely to damage the transformer winding turn-insulating and the main insulation,resulting in a series of accidents caused by VFTO.At present,the VFTO test condition of large power transformer is not mature,and it can not get the over voltage distribution under the action of VFTO in the design stage.Therefore,it is necessary to study the voltage distribution of transformer windings under VFTO.On the basis of the previous research on transformer winding transient modeling,there kinds of time domain algorithms are proposed in this paper,and the comparison and analysis are carried out.Firstly,the transformer winding is modeled,and the calculation method of model parameters is presented.VFTO frequency up to several tens of MHz,due to the centralized parameter model has the upper limit frequency(usually within 1 MHz),does not meet the conditions of calculation,so the distribution parameter model is used to carry out winding modeling.Using the finite element software Ansoft,combined with the energy formula,the distribution parameters are analyzed and calculated.Secondly,Assuming the transmission line has no frequency dependent effect,each turn ciol of the transformer windings is treated as a unit to establish a multi conductor transmission line model,and the existing three kinds of time domain algorithm are improved,which is applied to the transient modeling of transmission line.The first method,based on finite difference time domain method,time and space discretization on the transmission line equation,using the two order central difference discrete method,push the transient voltage distribution calculation formula.The second method,the Lax-Wendroff difference method,using the hyperbolic characteristics of transmission line equations,based on Taylor series expansion and differential principle,deduces the calculation formula of over-voltage.The third method,high frequency modeling method based on time domain finite element method and weighted residual method,using Galerkin's method to carry out the weighted integral of telegraph equations,the time discretization is carried out by using the trapezoidal method for the weighted residuals,and finally the transient iterative formula of the voltage and current of the transmission line is obtained by combining the boundary conditions.With the increase of working frequency,the distribution parameters of the transmission line will change with the change of frequency,and the rational fitting of frequency parameters can be realized by introducing the vector matching method.Thirdly,the 500 kV transformer model provided by Baoding Tian-Wei group is used as the research object,using MATLAB to write the program to achieve the three calculation methods.The three methods are compared and analyzed through simulation results,then verify the correctness and feasibility of the method.Finally,based on the 252 kV simulation platform,the key factors affecting VFTO are analyzed,it is concluded that the effect of residual charge voltage and transformer coupling capacitance on VFTO is great.The analysis results can not only provide guidance to the design stage of the transformer insulation design,but also provide a reliable basis for the layout of the GIS substation,equipment selection and accident analysis.