Assessment of spare transformer requirements for high voltage load stations

This paper describes a probabilistic method based on a Markov model for assessing the number of spare transformers required for a group of similar transformers used in high voltage load stations. The proposed method accounts for a number of factors that affect the number of spare units. The factors include the group size, unit failure rate, unit repair time, installation time of a spare unit and the failure criterion defined for each load station. The method uses two criteria in determining the required number of spare units. The 1^st criterion assumes that a predetermined level of the group availability is given and the number of spare units is determined when the calculated group availability exceeds the pre-determined level of the group availability. The 2^nd criterion uses a cost/benefit analysis method in calculating the number of the spare units. In the 2^nd criterion, the number of spare units (optimal number) is determined when the total cost (spare unit capital cost and unit outage cost) is minimum. A sample of Hydro One power transformers is used to illustrate the proposed method of assessment.