Salt River Project, based in Phoenix, Arizona, was among the world’s first utilities to employ composite insulators along the entire route of a major transmission line. Insulation on this line – the 500 kV Mead-Phoenix Project, commissioned in the early 1990s – was designed so that, if necessary, it could one day be converted easily to HVDC for greater power transfer (although this has not yet happened). Silicone composite type insulators were selected at the time since they offered the most economical acquisition cost in relation to equivalent DC versions of glass or porcelain strings. Notwithstanding this long experience with composite insulators, in 2013 SRP decided in favor of using only glass and porcelain insulators for the 500 kV Palo Verde-Browning Line
With expanding need for electricity in several counties surrounding the city of Phoenix, Salt River Project (SRP) has strengthened its network by adding an important new 500/230 kV transmission line serving its key Browning Substation.
Planned to be energized in May, the final section of this mainly double circuit line will allow power to be transmitted here from the Palo Verde Hub – a wholesale electricity market serving Arizona and southern California – and will finally complete a loop that has involved sequential stages and nearly a decade of public hearings and construction. For example, the eastern portion of this line, running between Browning and Dinosaur Substations, was completed two years ago.
Although the new line bears striking similarities to other 500/230 kV lines in the region surrounding Phoenix, there are some important differences. One of these lies in the type of insulators selected. Unlike many other local lines that employ composite insulators, the new line is being equipped with only glass and porcelain strings.
According to Michael Voda, Project Engineer for the Palo Verde-Browning Line, the single major factor influencing selection of insulators for the new line was comparative ease when it comes to live-line working. “We do live line maintenance on most of our 500 kV lines,” says Voda, “so we decided to limit ourselves in this case to glass and porcelain. We consider the two basically equivalent but are not yet comfortable with polymeric insulators for this requirement.”
To illustrate, Voda points to the 500 kV Palo Verde-Rudd Line, which was built about 10 years ago by Arizona Public Service but is now operated by SRP. This line is equipped with polymeric insulators and, while he says that there have not yet been any reported problems, current work practices require that any change-out of an insulator on this line must wait for a scheduled outage.
Another difference of the new project lies in the line hardware selected For example, 50 miles (80 km) of the line route will see use of rods and clamps while there will be 100 miles (160 km) using armor grip suspension assemblies.
Voda and Sr. Project Manager for Transmission, Dan Hawkins, explain that the new line was seen as an opportunity to compare the long-term behavior of equivalent insulators and other line accessories. Says Voda, “we purposely mixed up the insulators on this new line and also the supports and dampers so that we could start to accumulate comparative experience with them under the same operating conditions.”
One of the biggest challenges for SRP they say relates to replacing components on old lines that have been in operation for at least 30 years. The new line will therefore allow them to evaluate different materials within the scope of the same project but without taking on any new risks or overspending on construction and testing.