Ali Eslami - Zanjan EDCOZanjan- Iran
Adel Kazemi - Zanjan EDCOZanjan- Iran
Payam Javadi - Zanjan EDCOZanjan- Iran

Transformers are one of the most importantequipments in distribution networks. To minimize the effect ofcatastrophic failures of transformers in electric distribution system,due to the high expenses of the N-۱ security design in eachdistribution substation, some spare transformers can be used for agroup of distribution substations. Compared to the N-۱ securitydesign principle in each substation, common spare transformersshared by a group of substations can avoid considerable capitalexpenditure and still assure a sufficient reliability and availabilitylevel. This paper presents a probabilistic approach to determiningthe optimal number of spare transformers for a group ofdistribution substations based on representing the systemoperational states by a Markov model and cost-benefit analysis. Thespare transformer scheme obtained using the presented approachprovides both cost efficiency and sufficient reliability. ConsideringZanjan Electric Distribution Company data, the optimum numberof spare transformers for any distribution substation group isdetermined.

Distribution substation; cost-benefit; markovmodel; reliability; spare transformer; unavailibilty.