In Norway’s Sogn og Fjordane County, energy companies have typically used traditional survey methods to adjust the sag of newly constructed power lines. They may be changing, after a breakthrough project in 2017.
It was August, and a team from Geomatikk Survey was commissioned to measure and adjust the sag of a new, 450-meter-long section of four power lines recently installed by BKK Enotek. The lines were in the mountainchallenging terrains of Leikanger. The challenging terrain and the long span made traditional measurement virtually impossible.
The survey crew devised a simple plan: use a newly acquired Trimble SX10 scanning total station to collect the measurements, go to the office to calculate the adjustment, and then return to the field. But a funny thing happened. “When we pointed the SX10 at the first power line, the sensor automatically locked on to the line itself, without a prism,” said Even Sellevoll, a survey engineer with the company. “That meant we could not only easily and accurately survey the lines, we could adjust the sag in real time, right in the field.”
That changed everything. Sellevoll and coworker Svein Ivar Vestbø used their Trimble R10 GNSS receiver and SX10 total station technology to establish project control. Then, standing under one utility tower, they aimed the SX10 and collected a point where the cable connected to the tower. They repeated the process for each line. Then, after setting up underneath the other tower, they acquired four more connection points.
With the instrument’s controller, the team determined a chord between each power line’s end points and established a vertical offset from the chord to the existing power line. BKK calculated that each cable’s sag needed to be 6.26 meters below the chord at the lowest point. Measuring to an accuracy of 3 centimeters, the Geomatikk Survey team determined that each line was between 1.5 meters and 2 meters below the acceptable sag limit. Sellevoll then aimed the SX10 at a line. When it locked onto the cable, he phoned a BKK tensioning crew, which was standing by in the valley’s transformer station. He followed the cable’s movements in real- time, guiding the BKK team until the line was in the correct position, then repeated this process for the other three cables. Although a scan of the power lines wasn’t necessary, Sellevoll and Vestbø used the opportunity to test the scanning capabilities of the SX10 and provide detailed 3D-derived documentation.
Setting up near each tower, with the cables about 18 meters overhead, they scanned the entire span in two scans, one from the top of the slope and one from the bottom. Collecting 26,600 points per second, each scan took just 10 minutes to complete.
Back in the office, the technicians used Trimble Business Center software to integrate the point cloud and survey data. In just one day, they produced a vector-based report to verify that the power lines were adjusted to BKK specifications. Not only was the new technology a game changer in the field, it is proving to be a valuable business development tool. Following their success in Leikanger, BKK commissioned Geomatikk Survey to perform sag measurements on another section of the power line. “The ability to switch between total station surveying and scanning with one instrument allows us to be more efficient, both in time and costs. The SX10 is giving us the ability to take on projects that require real-time surveying and opening up a whole range of business development opportunities,” said Even Sellevoll, survey engineer for Geomatikk Survey.