To ensure (n-1) reliability at distribution substations (DS) at 11 kV level, spare transformers are required at each DS. With no spare transformers at a DS, a failure of a transformer may result in loss of load connected to it. Spare transformer at each DS is very expensive. In a distribution system, where load transfer to the neighbouring units is not possible, the average time for unserved load could be 2-3 days. This loss of load would not be preferred by utility consumers. Also, this degrades the reliability indices of the utility. The paper proposes two methods to find out optimal location of spare transformers to provide maximum reliability. Optimal location for spare transformers has been found out via an Integer Linear Programming (ILP) problem with the objective of 1) minimization of the Value of Lost Load (VOLL), and 2) minimization of transformer failure probability. The proposed approaches are illustrated with a sample distribution company.
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