Effects of topology and feeder capacity on substation distribution transformer loading

As loads on substation distribution transformers (SDT's) increase, engineers must decide at what point in time a second transformer bay needs to be added to keep loss of load expectation (LOLE) values at acceptable levels. An alternative to adding an additional transformer bay is to add ties to physically-close SDT's to provide backup power on outage; adding ties improves the SDT LOLE for any given loading level. Presented in this paper is a method for assessing the effects of interconnection topology, SDT forced outage rate (FOR), feeder capacity limitation, and load duration data on customer reliability. Included in this paper are the theory and results associated with an application software program that is being used by utility engineers to calculate LOLE for an interconnected SDT system.