Evaluation of distribution transformer banks in electric power systems Juan Carlos Olivares-Galvan 1 * , Rafael Escarela-Perez 1 , Pavlos S. Georgilakis 2 and Issouf Fofana 3 1 Departamento de Energia, Universidad Autonoma Metropolitana de Azcapotzalco, Ciudad de Mexico, D.F., 02200, Mexico 2 School of Electrical and Computer Engineering, National Technical University of Athens, GR-15780, Athens, Greece 3 Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), Université du Québec à Chicoutimi, Québec, Canada SUMMARY This article compares the total mass and the total owning cost (TOC) of three-phase distribution transformer banks with standard three-phase distribution transformers. The comparison is based on the minimum TOC. This is achieved through a field-validated distribution transformer design program that automatically minimises the objective function (TOC). In particular, 12 oil-immersed distribution transformers are designed: 6 three-phase transformer banks and 6 three-phase transformers; these designs meet all the requirements of a given transformer standard. As a result, curves of minimum TOC versus transformer rating are obtained for three- phase transformer banks and three-phase transformers. Moreover, similar curves from seven transformer manufacturers are collected; the advantage of this collection is that these different manufacturers have different types of transformers: oil immersed or dry type, core or shell type, various voltage classes and power ratings, and so on, and consequently more general conclusions can be drawn regarding the comparison of three-phase transformer banks and three-phase transformers. From these investigations, it was found that from the view- point of minimum total mass and minimum TOC, three-phase transformer banks should be recommended in case of small-size transformers (rating lower than 45 kVA). This is an important finding that is not emphasised in recommended practices reported in transformer textbooks. Copyright © 2011 John Wiley & Sons, Ltd. key words: cost of transformer materials; total owning cost; transformer total mass; distribution transformers; single-phase distribution transformers; three-phase distribution transformers 1. INTRODUCTION Transformers are essential components in the electrical power system. A typical transformer consists of coils of copper or aluminium conductors (that may be insulated with paper insulation for large units), which are wound around a magnetic core. Transformers are filled with dielectric fluid, which has two important functions [1]: a) to strengthen the dielectric properties of solid insulation by impregnation and to electrically insulate active parts from grounded ones, and b) to remove heat generated by the windings during service. There are three main reasons why three phases are used in electrical power systems: a) a three-phase machine can generate up to 95.5% of an ideal machine with infinite number of phases [2], b) the use of three conductors in a three-phase system can provide 173% more power than two conduc- tors in a single-phase system [2], and *Correspondence to: Juan Carlos. Olivares-Galvan, Departamento de Energia, Universidad Autonoma Metropolitana de Azcapotzalco, Ciudad de Mexico, D.F., 02200, Mexico. E-mail: jolivare_1999@yahoo.com Copyright © 2011 John Wiley & Sons, Ltd. INTERNATIONAL TRANSACTIONS ON ELECTRICAL ENERGY SYSTEMS Int. Trans. Electr. Energ. Syst. 2013; 23:364–379 Published online 30 December 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etep.665