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This paper describes a probabilistic method for determining the number of spare transformers (regular spare transformers and mobile unit substations) required for a group of distribution transformers in order to meet a predetermined level of the group availability. The method is based on a Markov model and accounts for a number of factors that affect the number of spare units. The factors include the use of both regular spare transformers (RSTs) and mobile unit substations (MUSs), type of transformer failure and the various parameters of regular transformers, and spare transformers. Sensitivity analysis is performed to determine the factors that affect most the number of spare units. The proposed method is illustrated using a sample system of the Hydro One's distribution stations.
Date of Publication: Feb. 2011