ORIGINAL ARTICLE Reliability assessment and improvement of digital protective relays Abdelkader Abdelmoumene • Hamid Bentarzi • Mahfoud Chafai • Abderrahmane Ouadi Received: 17 June 2014 / Revised: 20 September 2014 Ó The Society for Reliability Engineering, Quality and Operations Management (SREQOM), India and The Division of Operation and Maintenance, Lulea University of Technology, Sweden 2014 Abstract The development of digital protective relays is considered as a real revolution in the field of power system protection. This is due to their wonderful features not available with older relay generations such as: multifunc- tion, compactness, communication…etc. However, many experts have expressed their anxieties about the reliability of this kind of relays. In this paper, an analytical approach allowing the reliability assessment of digital protective relay has been presented. First, the different failure modes (common cause failures) have been discussed. Then, reli- ability modeling and evaluation has been developed in order to identify the points of weakness in digital relays. Subsequently, the measures that may be taken to prevent CCFs and enhance the reliability has been proposed and analyzed. Keywords Digital  Reliability  Protective relay  Power system 1 Introduction Ever since the apparition of the digital relay, the use of it growths rapidly in power system because of the advan- tages offered by the technology such as economical, high flexibility, multifunction, event archiving, communication, self-diagnostic and many other amazing features…things which make it a magic solution and the most preferable type for engineers and specialists in the field of protective relays. However, the low reliability level of this relay is, increasingly, noticed (Heising et al. 2000; Gurevich 2010). Reference (Stokoe and Gray 2001) noted that the integration of several functionalities in a single device may affect the performance and the reliability of the protective relay. (Polimac and Rahim 2007) indicated that typical service life of old electromechanical relays was 40 years, whereas the lifetime (LT) of digital relay is between 15 and 20 years. (Bickel 2009) declared that operating experience in nuclear power plants indicates digital systems (including digital protective relays) have the potential to exhibit unpredictable failure modes not found in the older types. Therefore, digital relay reliability assessment is of pri- mordial importance, and both manufacturers and users should pay special attention to how to improve the reli- ability of these relays generation. In this work, an analytical methodology for reliability assessment and enhancement of digital relay has been developed. The method involves not only reliability and LT estimation but also the identification of the points of weakness in digital relays and the measures that may be taken to reduce the occurrence probability of the dominant scenarios leading to the failure of digital relay. The first part of the paper is devoted to the presentation of the common cause failures. Then, fault tree model of the relay has been developed and analyzed qualitatively as well as quantitatively. Besides, reliability improvement using the redundancy and the barrier techniques has been applied and analyzed. A. Abdelmoumene (&)  H. Bentarzi  M. Chafai  A. Ouadi Signals and Systems Laboratory, IGEE, UMBB University, Boumerdes, Algeria e-mail: a_abdelmoumene@yahoo.fr 123 Int J Syst Assur Eng Manag DOI 10.1007/s13198-014-0300-z