Do what is right. Do what is safe. This core value has particular resonance with one of the riskier tasks that WAPA’s linemen undertake: live-line maintenance. Live-line maintenance refers to work on energized lines, which is required when a line is unable to be de-energized for upkeep or repair.
Due to the hazards associated with working on energized lines, live-line maintenance is performed in a highly controlled environment with multiple systems of protection, including strict work procedures; specialized tools, equipment and clothing; and engineered protective devices.
Recently, WAPA collaborated with the Electric Power Research Institute to develop a new protective device for live-line maintenance called a portable protective arrester. This device is expected to improve versatility and better transient overvoltage control.
Collaboration with EPRI began in early 2016 with the goal of pairing an off-the-shelf transmission line arrester with EPRI-developed sensor technology to create a device that meets technical requirements while being able to withstand the rigors of the worksite.
When the project reached the field testing stage in mid-2018, a team consisting of Electrical Engineer Jackie Brusoe, Foreman III Lineman Ed Hunt, Supervisory Electrical Engineer John Quintana and Electrical Engineer Gary Zevenbergen offered to incorporate the test into a planned maintenance activity on Rocky Mountain’s Ault-to-Craig 345-kilovolt line.
A major benefit of using a portable protective arrester is the reduction of minimum approach distance, or MAD, making live-line maintenance safer.
Energized conductors must be insulated from each other and from ground. For overhead transmission lines, air provides that insulation. Transmission towers are designed to support the conductors while maintaining the proper air gap around each conductor. This air gap must be large enough not only to withstand the operating voltage of the line and any voltage surges, but also any transient overvoltage.
A transient overvoltage is a voltage surge that travels along the transmission line at nearly the speed of light. It is created any time a circuit breaker connected to a transmission line opens or closes, such as opening to clear a fault on the line and then automatically closing to re-energize the line once the fault is cleared.
If a grounded object—such as a person in a bucket truck—breaches the air gap, the air insulation breaks down. A flashover occurs as the electricity creates a path to ground. To avoid flashovers while performing live-line work, linemen—using proper techniques, tools and personal protective equipment—must maintain a certain MAD from the energized conductor at all times.
Reducing MAD improves the working environment for the lineman. Working closer to energized conductors reduces their reaching distance with hot sticks and tools and reduces strain. Additionally, some transmission tower designs do not provide enough space to perform live-line maintenance while maintaining MAD.
Sierra Nevada’s Olinda-to-Tracy 525-kilovolt line is one example. With a hot-line order placed on the line, the MAD is 11 feet, 11 inches. Many of the lattice steel towers on this line were created by converting existing 230-kV double circuit towers to 525-kV single-circuit towers. This tower doesn’t provide enough space to perform live-line maintenance and maintain MAD.
MAD can be reduced if the transient overvoltage level for the line can be reduced. One way to do this is with a portable protective air gap, or PPAG, device.
The PPAG was developed in the 1960s as a means of controlling transient overvoltage and reducing MAD. It consists of a fiberglass hot stick which is mounted between the conductor and the tower. Two metal rods are mounted on the hot stick to form an air gap. One of the rods is installed at the end of the hot stick that is clamped to the conductor and the other is connected to a metal collar located on the shaft. The metal collar is grounded to the structure with a personal protective ground.
The PPAG controls the transient overvoltage by establishing a specific air-gap distance between the metal rods corresponding to the desired transient overvoltage level.
Each PPAG configuration is custom designed for a specific tower geometry and operating voltage. It cannot be used on other tower configurations. Using PPAGs on the Olinda-to-Tracy 525-kV line allowed the MAD to be reduced to 7 feet, 6 inches. With a portable protective arrester, though, the MAD could be reduced even further.
Getting closer with Portable Protective Arresters
PPAGs have been used successfully for many years to reduce MAD and control transient overvoltages during live-line maintenance. The portable protective arrester accomplishes the same tasks with two distinct advantages: it is more versatile than the PPAG and it provides a more consistent control of transient overvoltages.
PPAGs are custom designed for a specific tower configuration and must be installed on the towers adjacent to the worksite. In contrast, the portable protective arrester is not limited to a specific structure geometry—one type of arrester can be used on a variety of towers. Additionally, installing arresters at a single tower can provide transient overvoltage control for other towers up to five miles away.
In terms of transient overvoltage control, the portable protective arrester has a significant advantage over PPAGs because it is able to control the transient overvoltage without causing a fault to the line.
Installing a portable protective arrester on the 345-kV line mentioned above resulted in the MAD being reduced to 6 feet, 1 inch.
The results of this field test provided necessary validation of laboratory tests as well as information about connections, cables and tools that only a real-life jobsite environment can provide.
WAPA’s willingness to expend resources on this important effort will result in a significant advancement in worksite safety for linemen and system reliability for operators across the utility industry.