Analysis of Short-Circuit Electromagnetic Forces in Distribution Transformer Windings with Axial Asymmetry Using the Finite Element Method

Abstract

Short-circuit faults subject transformer windings to severe electromagnetic (Lorentz) forces that can cause radial buckling, axial collapse, and permanent deformation of the winding structure. Conventional design calculations often assume that the low-voltage (LV) and high-voltage (HV) windings are of identical effective height, i.e., are axially symmetric. In practice, however, the LV winding of a distribution transformer is commonly wound as a single-layer helical winding, which introduces an axial asymmetry with respect to the HV winding. This paper presents a two-dimensional finite element method (FEM) study, carried out in FEMM 4.2, of the short-circuit electromagnetic forces developed in the windings of a 630 kVA, 11000/433 V, three-phase, three-limb distribution transformer under (a) an idealized, axially symmetric winding arrangement and (b) the actual, axially asymmetric arrangement. Radial force, axial force, and axial compressive force are computed cell-by-cell for both windings under the worst-case three-phase symmetrical short-circuit condition. The results show that while the total radial force is only marginally affected by axial asymmetry, the axial force distribution changes fundamentally: it is identically zero for a symmetric winding pair but rises to 125.6 kN (LV) and 145.5 kN (HV) once asymmetry is introduced, with the two halves of the winding experiencing oppositely directed forces. The location of maximum axial compressive force also shifts from the winding centre (symmetric case) toward the winding ends (asymmetric case). These findings indicate that neglecting axial asymmetry in short-circuit strength calculations can significantly underestimate the mechanical duty imposed on distribution transformer windings.