Electric Power Systems Research 142 (2017) 36–46 Contents lists available at ScienceDirect Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr Substations SF6 circuit breakers: Reliability evaluation based on equipment condition Elaine A.L. Vianna a,∗ , Alzenira R. Abaide a , Luciane N. Canha a , Vladimiro Miranda b a Federal University of Santa Maria, Department of Eletromechanical and Power Systems, Campus Camobi, 97105-900 Santa Maria, RS, Brazil b INESC TEC and University of Porto, Porto, Portugal a r t i c l e i n f o Article history: Received 5 May 2016 Received in revised form 24 July 2016 Accepted 24 August 2016 Keywords: Substation Circuit breaker Maintenance Fuzzy Logic Reliability a b s t r a c t This paper presents a new methodology to define a priority scale for maintenance actions in substations, based on the development of a Composite Risk Index (CRI) associated with each device. Two auxiliary indices are built: Basic Condition (BC) and Operating Condition (OC), representing the physical and func- tional characteristics of the equipment that can compromise their performance and contribute to the occurrence of failures. Their evaluation is helped by a Technical Capacity Index (TCI), which evaluates how much the equipment has been affected by wear and tear, in the assessment of the Basic Condition, and the classification of the equipment defects by degrees of severity, in the assessment of the Operating Condition. Two cascading Fuzzy Inference Systems of the Mandani type are used, the first in defining the BC, and the second to obtain the equipment CRI denoting maintenance priority, which may then be used in planning maintenance actions. The methodology is verified through an SF6 circuit breaker CRI assessment, and its priority scale for maintenance planning. The method for evaluating the SF6 circuit breakers reliability is validated through a comparison with a statistical approach, using real data collected from equipment installed in Eletrobras Eletronorte Transmission System, in Rondônia, Amazon region of Brazil. © 2016 Published by Elsevier B.V. 1. Introduction Transmission substations are composed of a wide range of equipment and systems, subject to the occurrence of failures com- promising the performance of the electrical power system, their high availability results from a combination of operating with reli- able equipment and adequate maintenance procedure. In Brazil, as in other countries, the transmission substation operators are subject to penalties directly linked to the partial or global unavailability of the equipment, either from failures or from maintenance actions, depending on the degree and duration of equipment and substation unavailability. Maintenance policies adopting programmed interventions, equal for all devices of the same type regardless of their history and condition, are clearly sub-optimal. Nowadays, the system reliability evaluation has moved beyond programmed maintenance, to focus on condition based maintenance, for the reduction of maintenance costs and failure occurrence costs. ∗ Corresponding author. E-mail addresses: elainelimavianna@yahoo.com.br, elaine.vianna@eletronorte.gov.br (E.A.L. Vianna). Reviewing the literature related to reliability studies in power systems; paper [1] identifies the combination of maintenance tasks that achieve the best reliability within a limited financial budget; Ref. [2] focuses on the preventative maintenance on components with a high potential for improvement in system reliability; Ref. [3] proposes a monitoring system of the substation equipment, which allows migration from the practice of intensive corrective main- tenance for predictive maintenance, obtaining improvements in the equipment performance; and, Ref. [4] explains that the guaran- tee of non-occurrence of unscheduled shutdowns depends on the monitoring of the algorithms used in the data analysis and decision making. This paper focuses on establishing a maintenance priority table, at transmission substation level, for all pieces of equipment. Instead of following a fixed maintenance schedule equal for all devices of the same type, a decision to advance or postpone maintenance is made based on the actual condition of the device, its age and its history of past incidents and interventions. A set of cascading Man- dani Fuzzy Inference Systems (FIS) are adopted as building blocks for generating a final Composite Risk Index that will help in the decision making process by suggesting priorities for maintenance actions. The distinguishing feature of this paper, with respect to pre- vious literature, is to define the robustness and reliability, or a http://dx.doi.org/10.1016/j.epsr.2016.08.018 0378-7796/© 2016 Published by Elsevier B.V.