Equivalent Permeability of Step-Lap Joints of Transformer Cores: Computational and Experimental Considerations

Abstract

The paper develops an efficient computational method for establishing equivalent characteristics of magnetic joints of transformer cores, with special emphasis on step-lap design. By introducing an equivalent material, the method allows the real three-dimensional structure of the laminated thin sheets to be treated computationally as a two-dimensional problem and enables comparative analysis of designs. The characteristics of the equivalent material are established by minimizing the magnetic energy of the system. To verify the proposed approach, a series of experiments have been conducted. First, the anisotropic characteristics of the step-lap were established, and then space components of the flux density at specified positions measured. This enabled detailed analysis of the flux distribution in the step-lap region, in particular the way in which the flux travels between the laminations close to the air-gap steps. Encouraging correlation between the homogenized 2-D model and experiment has been observed.

Effects of magnetostriction and magnetic reluctances on magnetic properties of distribution transformers

“…An efficient computational method was described for establishing the equivalent characteristics of the magnetic joints of transformer cores. 6 The experimental results revealed a useful, practical method of predicting field distributions in various types of overlapping core joints.…”

Magnetic flux distribution in power transformer core with mitred joints

“…However, few 3D finite element simulations considered both the air gaps and the anisotropic magnetic properties of laminations. 5,6 Ref. 5 studied the simple butt and lap square core in 3D considering air gaps.…”

“…Moreover, the majority of simulations with finite element methods are focusing on the behaviour at normal operating flux densities. [2][3][4][5][6] Recent concerns on overflux challenges brought by geomagnetically induced currents or quadrature booster operations are now encouraging more flux distribution analysis at high flux densities. 7,8 This paper considers a realistic scenario of magnetic flux transfer, combining the effects of stacking air gaps, joint air gaps, and the anisotropic characteristics of laminations.…”

Modeling Overlapping Laminations in Magnetic Core Materials Using 2-D Finite-Element Analysis

“…Modeling and predicting the additional magnetizing current in transformers has been a research topic for decades [1]- [5]. As transformer cores only contain air gaps, or rather clearances, where the yokes and limbs are joined, whereas electrical machines have a relatively large air gap between the stator and rotor, most of the research has focused on modeling joints in transformers rather than in electrical machines.…”

“…A two-dimensional finiteelement (2-D FE) modeling method that accounted for the anisotropy of grain-oriented laminations employed in power transformers is devised in [3]. In [4], a Gaussian model is used to determine the magnetic flux density, permeability, and reluctance in the joining region, whilst accounting for saturation and anisotropy, and equivalent materials to represent the overlapping region have also been presented that employ an equivalent permeability in [5]. An efficient scheme for solving nonlinear magnetostatic problems is developed in [7] and is used to solve flux distributions in step-lap joints of transformer cores.…”