Three dimensional finite element modeling of a current transformer and analyzing its operation under normal condition

As the secondary signal of the Current transformers (CTs) is used in measuring and protection schemes, the accuracy of their operation is so important. In this paper, a real current transformer is modeled using three dimensional (3D) finite element method (FEM). Then its operation is analyzed under normal current of the primary conductor. First, the primary is positioned at the center of the current transformer. In the second stage, the operation of the CT is analyzed when the primary conductor is eccentric with respect to the CT core. At the third stage, the effect of the abrupt bends of the primary conductor on the CT magnetic field distribution is studied. Finally, two dimensional (2D) FEM is used for analyzing the effect of the stray flux on the CT magnetic field. This stray flux is caused by the adjacent bus bar which is under an over current condition. It is shown that the situations mentioned above can cause to an asymmetric magnetic field distribution in the CT core which in turn may lead to the error in the CT secondary current and so undesirable operation of the protection schemes fed by that CT.