The effect of distribution of a primary winding on the short-circuit forces of a current transformer

Abstract

An accurate estimation of the mechanical forces on the primary conductors of a current transformer (CT) is a complex task, since it essentially is a function of leakage flux between the primary and the secondary windings. The leakage flux, in turn, depends strongly on the geometry of the CT. Due to variations in the geometry of the CTs, a simple analytical or empirical expression for force calculation in all types of CTs cannot be derived. In this work, three cases of possible primary winding geometry were considered: i) concentrated, ii) semi-distributed, and iii) uniformly distributed over the core. A 69 kV, 200/5A current transformer was modeled using field-circuit coupled finite element method (FEM) to compute the axial and radial forces on the primary conductors under a short-circuit condition. It was observed that the uniformly distributed winding conductors experienced much less axial and radial forces than the other two cases. It was shown that the nature of ‘physical distribution’ of the turns of the primary winding on the core of a CT can have a significant impact on the short circuit forces, although the number of conductors (and ampere-turns) of the primary remain unchanged. Furthermore, the effect of physical distribution found to be more pronounced on the end conductors when the number of turns on the primary is smaller.