Innovative New Approach Enables Helicopter Live Line Repair
In a first-of-its-kind project, EPRI recently conceived an innovative solution that allowed FirstEnergy Corporation to perform a helicopter live line repair of a 500-kV transmission line. The new method allowed the utility to avoid a prolonged line outage and saved the significant labor and equipment costs that would have been required by a ground crew—all while ensuring the safety of the aerial live line crew.
The challenge arose when a helicopter inspection of FirstEnergy's Conemaugh-Hunterstown 500-kV transmission line detected spacer-induced conductor damage. The spacers causing the damage needed replacing, but taking the line out of service for an extended maintenance interval was not a desirable option.
At the same time, keeping the line energized during the repair also did not seem feasible. Rugged, forested terrain made access to each span difficult, so using ground-based crews would be labor-, time- and equipment-intensive. Use of a helicopter was hindered by the small clearances between phases. Overvoltages from switching operations create the risk of phase-to-phase sparkover when the helicopter is operating between the middle and outside conductor phases.
In November 2005, Cal Hoppe, FirstEnergy's Director Transmission Maintenance, brought the issue to an EPRI task force meeting on live working and described it to George Gela, Senior Project Manager in EPRI's Overhead Transmission Program. Gela proposed an innovative solution involving the use of Protective Portable Air Gaps (PPAGs, or arc horns) installed between phases to control overvoltage at the work site.
PPAGs are live working tools consisting of two diagonal electrodes; the distance betweenthe electrodes is the air gap. Under most conditions, the PPAG does not operate (spark over), and the line remains energized. However, when an excessive overvoltage occurs, the PPAG sparks over and causes circuit breakers to open and clear the line. This action temporarily de-energizes the line, but ensures worker safety and is far preferable to de-energizing a line for the duration of a repair project.
PPAGs have been used successfully on 500-kV lines for more than 30 years to control phase-to-ground overvoltages in ground-based live work. However, PPAGs had not previously been used to control overvoltages between phases, or to protect helicopters engaged in aerial live work.
To support the project, EPRI provided FirstEnergy with expert consulting and theoretical background. Working closely with FirstEnergy transmission planning and protection personnel, the EPRI team performed overvoltage studies to identify additional worker-protection measures. Study results called for disabling automatic reclosing on all line terminals at Conemaugh and Hunterstown substations, switching capacitor banks out of service during the work interval, and installing PPAGs every four miles and within two miles of the line terminals.
The result? FirstEnergy successfully completed the spacer replacement project on schedule and under budget, without the undesirable long-duration outage. The use of PPAGs and other precautions enabled helicopters to operate safely between the middle and outside conductor phases. FirstEnergy crews replaced 6,208 spacers over 112 miles of line in 22 days.
According to Hoppe, "Performing the repair with the line energized was the key to the project's success. EPRI's consultation on the use of Protective Portable Air Gaps enabled FirstEnergy to use helicopters and crew to perform the live-line repair safely, efficiently, and economically. As a result, we avoided a prolonged line outage, saved on labor and equipment costs, and avoided millions of dollars of congestion charges."
The project was so successful that EPRI and FirstEnergy have subsequently embarked on a follow-on initiative to develop a next generation of PPAGs that can be used for phase-to-phase overvoltage control. The new PPAGs, which will limit the fault current when the PPAG operates, will be designed to be permanently installed and "plugged-in" as needed, and can be installed on wood poles where ground connections are more difficult to establish than on steel lattice structures.
3420 Hillview Avenue, Palo Alto, California 94304
© 2007 EPRI. All rights reserved