A power plant tripped off-line seven years ago, leading to a series of events that caused the 2003 blackout. Transmission lines also were sagging into trees that were growing beneath the conductors. Due to the role of vegetation management (VM) in this blackout, the North American Transmission Forum started working on this practice immediately following the creation of the organization.
Today our Vegetation Management Practices Group comprises 155 foresters, VM staff and right-of-way directors who work for the Forum's 60 members. Over the past three years, the group has written VM practices that cover line patrols as well as procedures for notifying system operators when field personnel discover vegetation that's approaching too close to a transmission line. In addition, our members have described work-quality inspections and annual work plans. And the Forum also has a “model” transmission VM program that members can tailor to their own needs.
- Maintaining proper clearance
As part of the Forum's focus on VM, we also recently completed a practice on VM techniques based on ANSI standard A300, Part 7, “Integrated Vegetation Management Standards.”
One of these practices is maintaining a “wire zone” vegetation profile under the conductors. If the right-of-way is wide enough, utilities also can establish a “border zone” along the edges, as shown in the image above. The wire zone is typically a band of low-growing grasses and shrubs, while the border zone may have taller species that, at their mature height, would not grow too close to the conductors. We refer to these taller border-zone plants as “compatible species,” which means “incompatible species” would not be allowed. This wire zone?border zone concept resulted from research begun in 1953 by Purdue University researchers William Bramble and William Byrnes to measure the effect of VM on wildlife habitat.
- Managing risks
The North American Electric Reliability Corporation (NERC) implemented reliability standard FAC-003, “Transmission Vegetation Management Program,” in 2007. Utilities are mandated to comply with that standard. In fact, they face potential penalties of up to US$1 million for each day that they allow a single tree to encroach within a specified distance of a conductor. In other words, a utility can be fined if vegetation grows to within a specified distance of the conductor, even if no physical contact occurs.
The reliability standard does not specify what VM methods utilities must use. Practicing integrated VM as described in ANSI A300, however, can significantly reduce the risk of trees contacting the transmission lines. In turn, this practice helps maintain transmission system reliability and avoids potentially huge financial penalties for the utility. The Forum's VM practices are based on our members' experiences in implementing ANSI A300 standards as well as reliability standard FAC-003. We also provide additional recommendations that help in our pursuit of operations excellence and compliance.
Soon after NERC implemented FAC-003, many utilities began more aggressive transmission VM programs. These companies strived to bring their rights-of-way in line with ANSI A300 and achieve a true wire zone and, if room allows, a border zone as well. In many situations, this meant removing incompatible yard and other trees that had been allowed to remain in the past. The financial consequences from non-compliance penalties became too great to allow incompatible species in either zone.
- Providing new habitats
Removing trees where they don't belong doesn't mean the environment suffers. Indeed, not all plants and animals live in forests. As the research from Bramble and Byrnes shows, the grassy habitat in the wire zone and low-growing border zone provides a haven for small animals that would otherwise fall prey in a forest where they could be more easily spotted from the branches above. Many of those animals will also eat the seedlings of what could have been an incompatible tree in the right-of-way.
To help educate landowners, customers and communities about the importance of responsible VM, the Forum also shares educational materials developed by our members. By taking a proactive stance to VM, the Forum has helped our members as well as their local communities to understand the importance of quality VM practices in the reliability of our electric system.
Don Benjamin (firstname.lastname@example.org) is the executive director of the North American Transmission Forum. He previously served as vice president of operations at NERC.