Wide band modeling of large power transformer windings for very fast transient overvoltage (VFTO) analysis

There are some difficulties in using multi-transmission-line (MTL) model for wide band modeling of whole windings of the large power transformer. In this paper, the normalized MTL model is firstly derived, with which not only the difficulty of modeling windings with different turn-lengths using MTL can be solved, but also the model can be extended to the modeling of the multi-winding transformer. Secondly, both MTL model and the lumped circuit model on turn basis are mathematically compared in validation of the frequency range and it is pointed out that the lumped circuit model on turn basis is generally valid below 2.5 MHz for EHV and UHV power transformers. Finally, based on the MTL equations, a novel lumped circuit model is derived and it is shown that the valid frequency range of the new circuit is extended to about 4 MHz for modeling large EHV and UHV power transformer windings.