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 spare transformers shared by multiple substations can avoid considerable capital expenditure and still assure a sufficient reliability level. Using common spare transformers has been already a practice of some electric utilities in distribution substation transformer planning. This paper presents a probabilistic approach to determining the number and timing of spare transformers shared in a substation group. The proposed approach is based on the aging failure model of transformers, the overall reliability analysis and the probabilistic 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 reliability. A single transformer substation group in a nonurban region is given as an application example to illustrate the procedure of the method.