Fractional Model Identification And VFTO Distribution Calculation Method Of Oil-immersed Transformer Windings

The operation of disconnector or circuit breaker or short circuit fault in Gas insulated substation(GIS)may induce very fast transient overvoltage(VFTO),which has a rise time of several nanoseconds to tens of nanoseconds,and with a frequency range from hundreds of kHz to at least tens of MHz.When VFTO surge into a transformer,it will cause uneven voltage distribution and may excite local electromagnetic oscillations in the windings.Therefore it is very important to determine the high frequency circuit model of transformer windings.The frequency-dependent characteristics of the impedance parameter and the admittance parameter in the transmission line model of the transformer windings are mainly caused by the skin effect and the complex permittivity's variation with the frequency of oil paper,respectively.However,the frequency-dependent characteristics of the oil paper permittivity are often neglected in the conventional models or the frequency-dependent admittance parameters are determined on the basis of the parallel plate and the coaxial cylinder capacitance formula,which affects the accuracy in transient overvoltage calculation.In this paper,author do the research of fractional model identification and VFTO distribution calculation method of oil-immersed transformer windings.Firstly,the linear relationship of the complex capacitance and the complex permittivity is obtained based on the theory of dielectric polarization,different values of complex capacitance can be obtained by using the finite element software FEMM.Secondly,combining with the complex capacitance fitting model,the fractional admittance parameter of the transmission line model is derived,and considering the depth of skin effect in the different types of rectangular cross section conductor is different,then the parameters of the fractional transmission line model of the transformer windings are determined.Thirdly,based on the time domain finite element method for the multi conductor transmission line model,frequency-dependent part processing by fractional order differential term,and the complex vector matching process is avoided.Fourthly,as a practical application,through the simulation of the fractional transmission line model with new parameters of practical windings,compared the voltage waveform with experimental measurement and the other voltage waveforms,verify the accuracy of the parameters in the fractional model.Finally,the model identification and VFTO distribution calculation method of transformer windings are used for the voltage distributions calculation of large transformer windings.