RESEARCH ARTICLE A new approach of core structure model for 3‐phase distribution transformer Omar Sh. Alyozbaky 1,2 | Mohd Zainal Abidin Ab‐Kadir 1 | Mahdi Izadi 1,3 | Chandima Gomes 1 | Norhafiz Azis 1 | Maryam Isa 1 1 Centre for Electromagnetic and Lightning Protection Research (CELP), Faculty of Engineering, Universiti Putra Malaysia, Malaysia 2 Electrical Department, College of Engineering, University of Mosul, Iraq 3 Department of Electrical Engineering, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran Correspondence Omar Sh. Alyozbaky, Electrical Department, College of Engineering, University of Mosul, Iraq. Email: o.sh.alyozbaky@gmail.com Summary Reducing the losses of 3‐phase transformers is still considered a significant target by many transformer designers and manufacturers. Core joint design is one of the factors related to the losses. In previous studies, core losses and flux density distribution motivated researchers to design appropriate T‐joints. However, the variation of load and thermal profiles are the most important factors related to the losses in transformers. This study proposed a new T‐joint configuration for the core of the 3‐phase transformer. The proposed model was built after consider- ing the variation of load and thermal profiles. Results were compared with those of conventional T‐joint designs, such as butt‐lap and 45° mitered designs, that are cur- rently used. The no‐load losses of the proposed model were reduced by more than 6.8% and 10% compared with those of the butt‐lap and mitered T‐joint designs, respectively. Furthermore, the total losses, the hot spot temperature of the core, and the oil temperature decreased significantly. The proposed model contributes to the literature on new T‐joint designs for improving transformer performance. KEYWORDS core loss, finite element, flux density, joint design, power transformer 1 | INTRODUCTION The 3‐phase distribution transformer is a significant component of an electrical distribution grid. The losses of transformers consist of no‐load and load losses. Losses incessantly lead to loss of energy in transformers, which are con- nected to the network. Thus, more energy is lost and undesirable factors appear, which affect transformer performance. The power transformer is an important component of a power system, and its efficiency reaches 98%. 1 Core loss accounts for approximately 70% of the total transformer losses, whereas the operating (or energy) efficiency is 93.38%. Thus, a worldwide concern on core losses exists and should be reduced. 2 In electrical transformers, iron losses are important for thermal designs and electromagnetic analyses. 3 In the past few contracts, we have witnessed rapid changes in the field of transformer design and manufacturing. 4-6 As a general rule, increasing flux density by 1% increases losses by approximately 2%. 7 The efficiency of a transformer core is largely dependent on the design of the joints, such as corner and T‐joints. At the T‐joint area of the core of a 3‐phase transformer, the direction of magnetization in laminations may change during the magnetizing cycle, resulting in the increase in rotational flux. Therefore, designing the optimum core List of symbols and abbreviations: CRGO, Cold‐rolled grain‐oriented steel; PLC, Programmable logic controller; E, Electric field intensity; V, Voltage; B, Flux density; N, Number of turns; A, Core cross‐sectional area; f, Frequency; B m , Maximum flux density; D, Diameter of core transformer; MFD, Magnetic flux density; W, Watt; °C, Temperature Received: 10 March 2017 Revised: 3 November 2017 Accepted: 15 February 2018 DOI: 10.1002/etep.2572 Int Trans Electr Energ Syst. 2018;e2572. https://doi.org/10.1002/etep.2572 Copyright © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/etep 1 of 18