Two damaged 230-kV transmission lines owned by the Turlock (Calif.) Irrigation District (TID) and the Modesto Irrigation District (MID) have been repaired by TID and returned to full service on May 23 after being derated due to conductor damage caused by vibration resulting from subconductor oscillation.
The 230-kV lines are the main feeds from California’s 500-kV transmission backbone to the service territories of TID and MID, according to Ed Jeffers, engineer with TID.
“When there’s an outage on the 500-kV system, the 230-kV system underlying it really loads up, so that’s why getting this fixed was important,” Jeffers told TransmissionHub June 6.
Damage to the Westley to Walnut and Westley to Parker transmission lines that feed the cities of Westley, Turlock, and Modesto, Calif., was discovered during regular ground patrols by utility crews. Vibration from wind and other sources had caused so-called hairpin spacers to damage the conductors, cutting nearly halfway through them in some places.
“The lines were originally built in the mid-70s so the knowledge about subconductor oscillation and the damage that can result has changed quite a bit since then,” Jeffers said. “We used new products, installed more of them, and used the science available today,” including employing asymmetrical spacing of the dog bone-style spacers, which use urethane bushings to cushion the conductor cable.
The Westley to Walnut line is owned by TID, while the Westley to Parker line is owned by MID, according to the California ISO. Due to the damage, the transmission lines were derated from 490 MW to 370 MW on April 30 and taken out of service for less than a day as line crews performed the necessary repairs.
To expedite the repairs and ensure the lines would be at full capacity for the coming summer peak demand, the utility opted for an approach different from its usual method. Instead of relying solely on ground-based crews in bucket trucks, the utility contracted with American Site Builders of Amarillo, Texas, to repair the conductors from a helicopter.
“The helicopter is more expensive per hour, but it takes [fewer] hours to do it,” Jeffers said, noting that that the bulk of the work was done from the chopper. “Time is important when you’re taking these transmission lines out of service.”
A ground crew followed up and repaired a further small stretch of conductor using compression splices and preformed armor rod-type repair splices. Using ground-based crews exclusively would have taken three crews and meant the lines would have been out of service for a week, at a cost of approximately $50,000 per day for each day the lines were down.