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Applying Probabilistic Method in Determining the Number of Spare Transformers and Their Timing Requirements

This paper presents a probabilistic approach to determining the number of spare transformers for a group of transformers and the timing requirement for each spare transformer to meet the specific reliability criterion. The historical reliability performance designated as system average interruption duration index (SAIDI) is used to establish the specified reliability criterion. The proposed method considers both repairable and aging failures of transformers. The aging failure model of transformers is incorporated in the analysis in order to practically account for the fact that the electric power system in North America is aging, and a considerable number of equipment are approaching their end of life. The 138/25 kV 25 MVA transformer group in the BCTC system, consisting of both fixed turn ratio and on-load tap changing transformers, is used for an illustration in the paper. The detailed analysis in determining the number of spare transformers and their timing requirements during a 10 year planning period is presented.