Cost/Benefit Analysis for Use of Mobile Unit Substations in Customer Delivery Systems

Mobile unit substations (MUS´s) have been used in distribution systems for a long time while their use in customer delivery systems (regional supply systems) has been limited. This is mainly due to unit design limitations and due to the fact that most customer delivery systems are designed with redundancy. As the population of transformers in customer delivery systems grows older, electric utilities need to refurbish or to replace older units and the risk of losing the entire supply to customers due to the overlap of scheduled and forced outages of these transformers will be higher. Utilities, therefore, need to prepare some emergency plans in case of the entire loss of supply to customers. MUS´s can provide an alternative to reduce the risk of losing the entire supply to customers. This paper describes a frequency and duration (F&D) method for evaluating the benefit from the use of MUS´s in customer delivery systems. The benefit from MUS´s 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 MUS´s to see whether the use of MUS´s is justified or not. The various factors affecting the benefit from MUS´s will also be discussed. A case study using the Hydro One customer delivery system is presented to illustrate the proposed methodology.