Bath redesigned the protective relay schemes and control circuits at the Citgo West 138/12.5kV substation to change operation from a normally open 12.5kV bus tie arrangement to a normally closed 12.5kV bus tie arrangement. The Citgo West substation contains two 24/32/40MVA, 138/12.5kV step-down transformers with a 12.5 kV main, bus-tie, main bus arrangement. Bath designed the closing circuits of the two 12.5kV main breakers and the bus-tie breakers to be supervised by a programmable logic controller.
Bath performed short circuit, load flow, and relay coordination studies to analyze system operation under the new operating philosophy. As a result of the studies, Bath found that four 15kV switchgear line-ups needed to be upgraded from 500MVA to 750MVA. Bath performed the design engineering for the four 15kV switchgear upgrades and retrofits. Bath also supplied relay setting for the overcurrent and voltage relays at the main 138kV substation as well as the four down stream 12.5/4.16kV and 12.5/.480kV distribution substations. Bath provided new, revised, and demolition drawings for all five substations. The short circuit and load flow models contained 50 buses, 200 components, and 75 branches. All short-circuit, load flow, and relay coordination studies were performed with the aid of SKM PTW-32 power systems analysis software.