Magnetic flux distributions in transformer core joints

This paper presents results of a rigorous analytical study performed in order to understand the role which joints play in determining the performance of cores of power and distribution transformers. The paper presents the details of the magnetic flux distribution and redistribution in the core joint region along the core stack height direction as a function of the operating flux density, core joint material, joint configuration, and joint design parameters. The results are presented for both conventional and step-lap joints. The results illustrate the benefits of the step-lap joint in reducing core losses, noise and excitation current. The results also explain the impact of overlap length, the number of laminations per step, and the contribution of the joint region to the whole core performance. The calculated flux density wave shapes in the joint region were found to be in good agreement with measured wave shapes. The results presented in this paper have been used in developing an accurate calculation of core losses for both power and distribution transformers. They have also been used to arrive at the optimum core joint configuration for a transformer core.