Detailed High Frequency Models of Various Winding Types in Power Transformers

-In this paper, techniques are described which demonstrate how a highly detailed internal transformer model can be obtained systematically with Matlab and how it can be prepared for subsequent transient analysis. The input of such a model will mainly be the description of the cross section of a single transformer leg, including all turns and windings. From the input data a three phase equivalent circuit is calculated and corresponding nodal impedance matrices are found for appropriate frequencies. Vector fitting is then applied which produces an excellent rational function approximation to the nodal impedance matrix in the valid frequency range. From the approximation an equivalent circuit can be extracted and implemented in circuit simulation software. A small but novel step makes it possible to compute not only the voltage distribution but also the internal currents from the extracted equivalent circuit. Finally a new circuit extraction technique is proposed for vector fitted impedance matrices for more efficient computation.