Experiences with parallel EHV phase shifting transformers

Phase shifting transformers (PSTs) are used to control unscheduled power flow in interconnected power systems. The authors discuss the application, protection, commissioning, and initial operating history of two sets of parallel, 300 MVA, 345 kV PSTs. The four Western Area Power Administration (WAPA) PSTs were energized in 1989 and are located in northwestern New Mexico in Shiprock and Waterflow substations. The installations are unique because two PSTs, using a series no-load and load tap changer (LTC) design, are operated in parallel at each location. A fault in the main LTC compartment later removed one unit from service, but the installation of parallel units at each location allowed for continued operation and control of the power flow on the two lines