Use of Mobile Unit Transformers in High Voltage Load Stations

Electric utilities schedule power transformers in high voltage load stations (customer delivery or regional supply systems) for planned outages such as unit refurbishment, unit replacement, etc. that may last for a few weeks. During the planned outage time, companion units may fail causing a complete loss of supply to customers. Mobile unit transformers (MUT´s) could be used in these emergency situations to reduce the impact of the loss of the entire supply to customers. These MUT´s cost utilities money and utilities need to answer the question of whether the purchase of these units is justified or not. This paper describes a probabilistic method based on a Markov model for evaluating the benefit from the use of MUT´s in customer delivery systems. The benefit from an MUT is expressed in terms of annual reduction in total customer interruption costs. The estimated annual benefit can then be compared with the annual charge of carrying the MUT to see whether the use of MUT is justified or not. Sensitivity studies are carried out to evaluate the impacts of changes in some system parameters on the benefits from the MUT. A case study is presented to illustrate the proposed method and to compare the results with those obtained using a frequency and duration approach.