Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes Operational vulnerability indicator for prioritization and replacement of power transformers in substation William I. Schmitz ⁎ , Dion L. P. Feil, Luciane N. Canha, Alzenira R. Abaide, Tiago B. Marchesan, Rodinei Carraro 1 Federal University of Santa Maria – UFSM, Av. Roraima, Santa Maria, RS, Brazil ARTICLE INFO Keywords: Substations Power transformers substitution Parallelism index Health index PROMETHEE ABSTRACT This work presents a new methodology for the classification and substitution of power transformers in substa- tions. The paper proposes the use of the operational vulnerability indicator, which consists of two groups of criteria, technical condition and operational condition. This indicator allows identifying which power trans- formers from a set of substations have priority for substitution, considering technical-operational information available in the substations. The technical condition is composed by the parallelism index, reserve transformer availability, nominal power and transformation ratio. Through criteria such as the parallelism index developed for this work, it is possible to identify the percentage of the load that is assured by a transformer in parallel at the moment of a contingency. The operational condition representing the physical situation of the power trans- former, typically found in the literature as health index. It assesses factors such as deterioration of the cellulose paper that makes up the insulation and oil, age of the equipment and loading factor, resulting in a life expectancy for the power transformer. The operational vulnerability indicator can be used as an important guide for the strategic planning of the resource application of electric power companies. The methodology is verified through a case study involving real data from 7 substations and 39 power transformers, belonging to a Brazilian elec- tricity company. In the validation of the method, the effectiveness of the proposed method is compared with the ranking obtained through the traditional health index. 1. Introduction The electricity distribution companies have a common character- istic of being constituted by several substations, which in turn have numerous power transformers in different states of conservation, ages, configurations and levels of supervision. This diversity makes the de- cision process of choosing the power transformer to receive main- tenance or be replaced complex and subject to many variables. Another point that reinforces this uncertainty at the time of the choice is the high values involved, representing a significant part of the capital of the substation, together with the disturbance caused to the reliability of the electrical system in case an equipment is out of operation, since they are equipment with long terms of manufacturing and delivery. The Health Index is an example of methodology adopted during the process of choosing the power transformer to be replaced, approaching factors related to the current condition of the equipment, it assists as a decision parameter indicating the most vulnerable equipment [1–5]. However, the strategic aspect of the substation in which the power transformers are and whether they operate in parallel or have reserves is hardly considered. Factors such as these can delay or indicate the need to advance the replacement of the equipment, contributing to an increase in the safety of the electrical system. Considering the factors presented, this paper proposes a metho- dology that presents a priority ranking for maintenance and replace- ment of power transformers in substations. This ranking indicator in- volves technical and operational aspects of power transformers to assist the decision maker in this crucial moment. Among the criteria that compose the proposed indicator, we highlight the parallelism index, an innovative calculation method that allows identifying the portion of the transformer load that can be reallocated to the transformer in parallel at times of contingency. The reserve transformer availability is another analyzed criteria. The methodology is verified through a case study involving real data from 7 substations and 39 power transformers, be- longing to a Brazilian electricity company named CEEE-Generation and Transmission. https://doi.org/10.1016/j.ijepes.2018.04.012 Received 11 January 2018; Received in revised form 1 March 2018; Accepted 17 April 2018 ⁎ Corresponding author. 1 Generation and Transmission Electricity Energy Company – CEEE-GT. E-mail address: ismaelschmitz@gmail.com (W.I. Schmitz). Electrical Power and Energy Systems 102 (2018) 60–70 0142-0615/ © 2018 Elsevier Ltd. All rights reserved. T