High frequency circuit model of transformer windings based on frequency responses for VFTO studies

In order to study the effects of VFTO to transformers, the high frequency model for transformer windings must be set up. In this paper, a systematic methodology is presented for establishing the high frequency two-port circuit model of transformer windings under VFTO based on its frequency responses. The pi-equivalent circuit is constructed based on the Y-parameters transformed from the measuring S- parameters. A correction to the rational function of Y approximated by vector fitting (VF) is calculated, which enforces the real part of Y is positive definite. This enforcement can assure the passivity of the whole model, which could bring a stable simulation. The values of the synthesized RLCG elements can be easily calculated form the rational functions. This kind of pi-equivalent circuit model could do stable and exact simulation compared with the experiment results, which shows the validity of the method.