| When the transformer is subjected to lightning shock,operating overvoltage and other excitation effects,the coil voltage distribution will show a great gradient and accompanied by oscillation.In addition,when the transformer winding is deformed by a short-circuit shock,the frequency response curve of the winding will also change.All the above phenomena are an embodiment of transformer broadband characteristic behaviors.The span of these characteristics reflected in the frequency domain may be tens of hertz to tens of megahertz.Therefore,the research on basic problems such as transformer broadband domain characteristic modeling is the basis for in-depth analysis of the above characteristic laws.At the same time,it is of great engineering significance to improve the insulation design level of high-voltage transformer products,improve the accuracy of short-circuit fault diagnosis of large-capacity transformer and guarantee the operation reliability of high-frequency electrical system.This paper aims to establish an accurate and effective broadband characteristic model of the power transformer.Based on the nonlinear eddy current field model of silicon steel sheet,the curve of core equivalent permeability with frequency is calculated.The influence of magnetic permeability on the magnetic field is small when the non-measured coil is short-circuited.In this paper,the electromagnetic field finite element model is coupled with the broadband equivalent circuit to replace the nonlinear resistance parameter,and a field-circuit coupling model for efficiently calculating the broadband characteristics of the power transformer is obtained.This model uses the direct coupling between the circuit and magnetic field to replace the nonlinear inductance parameters in the circuit,so that the wideband characteristic model of transformer can be linearized within a certain analysis frequency range.Compared with the traditional equivalent circuit model,the calculation and extraction of complex element inductance and mutual inductance parameters are avoided.In order to verify the correctness of the electromagnetic coupling relationship among the coil units in the proposed equivalent model,an electromagnetic parameter test model is designed to study the influence of the frequency-varying effect of the core on the synthetic inductance parameters under different working conditions.The calculation method of capacitance parameters in the broadband equivalent circuit model is studied.The electromagnetic coupling coefficient and capacitance parameters are measured and analyzed by specific cases,and the validity of the model isverified.Finally,the transformer test model is designed.By constructing the transformer broadband characteristic model experimental test platform,the broadband response curves of different magnetic permeability of the transformer core are analyzed and tested.The results show that the linearization of the transformer field-circuit coupled broadband characteristic model is feasible when the non-measured coils under the condition of short-circuited and the effectiveness of the model for the analysis of broadband problems.The transformer broadband equivalent circuit under the condition that the non-measured coil is short-circuited can also be used for the analysis of the lightning lightning shock wave process and the impedance frequency response curve analysis. |
