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Transmission crews complete repairs near Bowdle

Final repairs have been completed on two transmission towers damaged by a tornado last year.

Basin Electric Power Cooperative - October 28, 2011


	TSM crews erect a new transposition structure north of Bowdle, SD.

Final repairs have been made to a transmission line north of Bowdle, SD, nearly a year and a half after an EF4 tornado destroyed 11 of Basin Electric’s steel transmission structures. Two temporary transposition structures have been replaced with permanent ones on the Antelope Valley Station-to-Broadland, SD, 345-kilovolt line. The project was a joint effort between the Transmission System Maintenance (TSM) division and Basin Electric’s civil engineering group.

Bryan Keller, manager of TSM, said transposition structures are placed every third of the line length along a three-phase, high-voltage transmission line to change the positions of the conductors. “This balances the impedance of the line resulting in equal current flow and voltage drop in each phase,” Keller said. “The 300-mile long Antelope Valley-to-Broadland line has 1,200 structures, but only four of them are transposition towers. The tornado took out two of them.”

While Basin Electric had enough steel on hand to reconstruct the nine standard structures within a month of the tornado, materials for the transposition towers are not carried in inventory because of their specialized nature. Temporary emergency restoration structures were installed to replace the transposition structures until permanent structures could be fabricated.

New structures were fabricated in the following months, but TSM had to wait for a unit outage at Antelope Valley so the line could be removed from service so they could be installed. Antelope Valley Unit 1 is down for a scheduled maintenance outage through the end of October.

Duey Marthaller, manager of civil engineering, said erecting the towers poses special challenges. “The phase change of the bundled conductor results in a slight angle, so not only is there the vertical force but also a horizontal force posed on a crane that’s extended more than 100 feet in the air,” he said. “This kind of lift requires an experienced, specially trained crane operator and a very knowledgeable crew.”

Typically storm repairs are done with the conductor secured out of the way. “But since the other towers are intact, we’re building the new structures around the conductor that’s still hanging in place,” Keller said, adding another element of challenge to the project.

Keller said crews have been working safely and have completed erection of the towers. The line was returned to service on Oct. 21.

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