One of Avista’s substations was experiencing significant load growth which led to the need for two larger transformers and another two feeders to be installed to support this increased load. After evaluating many options, Avista settled on a complete rebuild so that transmission line sectionalizing and protection could be added to improve reliability. Commonwealth provided engineering design services for the rebuild of one of Avista’s substations. This design included the civil/structural, physical, protection and integration design, as well as, material procurement, transmittal of final design, construction support, and record drawings. The final substation configuration included two transmission line breakers, two power transformers with circuit breakers, and three distribution feeders each. A new concrete masonry unit (CMU) block panel house was also designed to accommodate the new protection schemes.
The final design configuration consisted of the 115 kV transmission line looped-through the substation. Each line utilized a 115 kV gas circuit breaker with disconnect switches on each side, tied together by a 115 kV main bus with a sectionalizing switch. Transformers (30 MVA each) with four-bay low voltage distribution structures (LVDSs) with auxiliary busses were designed. Each LVDS is complete with three feeders (plus auxiliary breaker position) using Square D Type FVR feeder breakers and 438A single phase regulators. The existing feeder exits were retained in their overhead configuration, with phasing changed to match substation standards. The new feeder exits were designed to be underground following the latest standards. The two new feeders were made to cross outside the yard to mitigate thermal rating issues so that the eastern feeder is off the western transformer, and vice versa. This substation is scheduled for rebuild during the Summer/Fall of 2019.