New Equivalent Circuit of Transformer Winding for the Calculation of Resonance Transients Considering Frequency-Dependent Losses

In this paper, a new equivalent circuit for the transformer winding is presented by which the eddy current losses in the transformer can be represented accurately at high frequencies. The new model enables representing the frequency-dependent impedances of a coil system using the idea of mutually coupled series Foster circuits. An effective method for the determination of model parameters is also provided which involves using the vector-fitting (VF) method for the rational approximation of the system impedance matrix using a certain number of common stable poles. A set of nonlinear equations is then established using the VF results that can be solved with the method of fixed-point iteration to obtain the parameters of the equivalent circuit in a fast and accurate way. The new model can be used for the calculation of resonance transients within the transformer, the situation wherein the maximal value of overvoltages is highly dependent to the damping effects or the loss phenomena in the transformer. Since the model only contains constant lumped parameters, it can be used for transient calculations in the time domain directly using the electromagnetic transients programs, such as the Alternative Transients Program or Electromagnetic Transients Program.