Determination of Optimal Number of Spare Transformer Using Probabilistic Method: A Study Case of Pematangsiantar Distribution Unit

Transformer failure is an event that is difficult to avoid, especially in distribution networks where thousands of distribution transformers must be maintained for reliability. A Transformer is an important piece of equipment in the distribution of electricity to customers where one distribution substation has only one transformer. The case study of the Pematangsiantar Distribution Unit has more than five thousand transformers (20kV/400V) to serve low voltage customers, namely 0.45 kVA - 197 kVA power. The transformer capacity which is the object of research is 25 kVA-250 kVA. Determination of the optimal number of spare transformers is carried out using the Poisson Distribution probability method by comparing the desired level of reliability, the Mean Time Between failures (MTBF), and Statistical Economics. The result of the case is the most number of spare transformers is MTBF, Reliability, and Statistical Economic criterion respectively.