Research On Establishing Method Of Broadband Nonlinear Model For Transformer Based On Duality

Transformer occupies an important position in the power system.The establishment of the transformer model is very important for the research and analysis of the characteristics and the influences of transformers in various transient and steady-state processes.However,due to its complex structure and operating conditions,it becomes a weaker part of the modern electroma gnetic transient simulation software.The general problem of the transient transformer models available for rapid simulation is that the applied range is limited and the role of the iron core is not fully taken into account.The establishment of an accurate broadband nonlinear model can provide the basis for transient analysis of power system,over-voltage protection and transformer design,which has theoretical and application value.Based on the principle of duality and the saturation nonlinearity of the iron core,a wideband circuit model of transformer is established in Matlab / Simulink which both considers the core nonlinearity and the frequency-dependent eddy current effects.The applicable frequency of the model is in the range of power frequency up to megahertz.Firstly,the overall structure of the model is obtained based on the duality principle.Secondly,a nonlinear frequency dependent eddy current model of the iron core is established.The improved vector fitting method is used to fit the frequency dependent eddy current admittance parameter of the core laminations.The Foster parameters of the core model are obtained through circuit synthesis theory.The no-load U-I curve of transformer is measured by using signal generator,power amplifier and power analyzer and is converted to flux-current characteristic curve of the core.The nonlinear flux-current characteristic is fitted by a modified polynomial,which is added into the Foster circuit as a nonlinear inductor.Then,the calculation of winding resistance,inductance and capacitance parameters is carried out.Resistances and inductances used to describe the winding eddy current and the leakage inductance are calculated by analytical solution.The scattering parameters are measured by the spectral impedance analyzer and transformed into transmission parameters in the range of 50Hz-1MHz.So the capacitance parameters can be calculated and optimized by taking the measured and calculated transmission parameters as the objective function.Finally,a broadband nonlinear model is established by taking the laboratory standard Epstein square as an example to simulate the time-domain response under different frequency.The simulation results and experimental results are compared to verify the effectiveness of the modeling method.