A Hybrid Measurement Approach for Wideband Characterization and Modeling of Power Transformers

This paper describes a hybrid procedure for wideband characterization and modeling of power transformer behavior from frequency sweep measurements. The approach is based on measuring corresponding sets of voltage and current vectors that are associated with short circuit and open circuit conditions. These vectors are stacked side-by-side in two matrices which are used for computing the short circuit admittance matrix. This hybrid approach is shown to give more information about the transformer behavior than the traditional approach of measuring the admittance matrix directly under short-circuit conditions, which tends to corrupt the small eigenvalues associated with open-circuit conditions. From the obtained admittance matrix, a pole-residue model is extracted via vector fitting and passivity enforcement, thereby giving a model ready for EMTP simulation. The accuracy of the small eigenvalues is retained by using a modal formulation in the fitting and passivity enforcement step. Application to a distribution transformer shows that the hybrid approach can greatly improve the accuracy of time-domain simulations involving high-impedance terminations.