Hanging above the steep, angular surface of the mountainous terrain was a 450-meter-long section of four power lines that their customer, BKK Enotek, had just installed. After installing the cables, BKK had tried to determine and set the appropriate sag levels using a traditional sag scope. But the precipitous landscape and the span between the two towers made it too difficult to accurately calibrate the lines. So, the Geomatikk Survey crew took on the task of measuring the sag and assisting BKK in correcting the sag elevation to meet the height and tension tolerances. To spatially ground the project data, Sellevoll and Vestbø used their Trimble R10 GNSS receiver and SX10 total station technology to establish control. Standing beneath the utility tower at one end of the line, they aimed the SX10 directly overhead and collected a point where the cable connects to the tower, a process they repeated for each power line. They then set up underneath the other tower and acquired four more connection points. With the instrument’s controller, they determined a chord (straight line) between each power line’s end points and established a vertical offset from the chord down to the existing power line. Given the length of the cables, their above-ground height of 25 meters, and the current temperature of about 17°C, BKK specified that each cable’s sag needed to be 6.26 meters below the chord at the lowest point. “The ability to see and compare the chord line with the existing powerline on the controller’s big screen enabled us to see in real time how much each line was outside of tolerance,” Sellevoll said. “So when they adjusted and lifted the cable, we could monitor it on the controller and see that it needed 50 cm more or 10 cm more until it was in the correct position.” Measuring to an accuracy of plus/minus 3 cm, the Geomatikk Survey team determined that each of the four power lines was hanging too low — between 1.5 meters and 2 meters below the acceptable sag limit. With a BKK crew positioned at the transformer station down in the valley, Sellevoll and his colleague instructed the tensioning team via telephone. Starting with line one, he aimed the SX10 and once it had locked onto the cable, he told the crew how much adapting it needed. Then he followed the cable movements in real time, guiding the BKK crew until the line’s sag was in the correct position. “I had never worked like this before, so it was quite cool to watch the lines being adjusted right before my eyes,” Sellevoll said. “And because the SX10 locks onto the line, we only had to aim it once and follow it on the controller. With a traditional total station, we would have had to manually aim the instrument and measure the cable’s position each time it moved. If it had been windy, it would have been
When the Trimble SX10 scanning total station was pointed at the power line, the sensor automatically locked onto the line itself, even though there was no prism.
much more difficult to do.” A view to new business
Although BKK did not require a scan of the power lines, Sellevoll and Vestbø used the opportunity in the field both to test the scanning capabilities of the SX10 and to provide detailed 3D-derived documentation to their client.
A steep slope and the 450-foot span between two towers made it difficult to accurately calibrate the sag on four powerlines using a traditional sag scope.
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august 2018
csengineermag.com
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