Determining number and timing of substation spare transformers using a probabilistic cost analysis approach

Compared to the N-1 security design principle in each substation, common sp are transformers shared by multiple substations can avoid considerable capi tal expenditure and still assure a sufficient reliability level. Using comm on spare transformers has been already a practice of some utilities in dist ribution substation transformer planning. This paper presents a probabilist ic approach to determining the number and timing of spare transformers shar ed in a substation group. The proposed approach is based on the aging failu re model of transformers, the overall reliability analysis and the probabil istic damage cost model for a substation group, the capital cost model for spares and the present value method. The spare transformer scheme obtained using the presented approach provides both cost efficiency and sufficient r eliability. A single transformer substation group in a non-urban region is given as an application example to illustrate the procedure of the method.