Vegetation management for overhead lines predates the transmission of electric power by something like 30 years. Surprising? Well, the telegraph line was operational in the early 1840s, and the first electric transmission line was constructed in 1882. Whether it is a telegraph line or a power line, electricity does not like trees or brush.

If vegetation came in contact with a telegraph line, the line shorted out, and someone had to do a line patrol to fix the problem. Therefore, it made sense to keep those lines clear.

With the transmission of electric power, the significance of clearance is more dramatic. Trees contacting power lines often result in electrical shock hazards, fires and outages, hence the term “hazardous vegetation.” When trees and power lines come together, it is not good for either one of them. They are a dangerous duo.

Vegetation Management vs. Budget

Interestingly, vegetation management is an issue customers care about and is one of a utility's largest operational expenses, but it has only been getting real national attention in the past few years. Until recently, the only vegetation management regulations and standards utilities had to follow were voluntary for the most part, with no enforcement or penalties for noncompliance. As a result, vegetation management programs have always been an convenient place for budget cutting, but times are changing.

Presently, the electric utility industry is spending an estimated $2 billion to $3 billion each year to keep lines and vegetation apart. Recent events have shown the industry it is important to prevent vegetation-related outages and to be able to predict how the vegetation and power lines will interact. Hazardous vegetation must be removed, but very few understand the degree of sophistication required to be a utility arborist in this complicated digital world.

How often has this question been asked: If it is that important, why not make the right-of-way wider and cut everything?

This is a complex subject ranging from environmental issues to safety concerns to customer contact to expense.

Everything is intertwined, but the technology of the intelligent grid's light detection and ranging (LiDAR) systems offers new applications for asset and risk management when dealing with power line maintenance and national regulations.

The Department of Energy's Energy Information Administration estimates there were 163,480 miles of transmission line above 230 kV in the United States in 2007, the latest figures available. It projects there will be approximately 174,436 miles (above 230 kV) by 2012.

Utilities are challenged by this huge mileage of transmission lines. LiDAR-based technology offers a more-standardized approach to manage vegetation in the utility's rights-of-way and improve the maintenance cycle to identify hazardous vegetation before it causes problems for the grid.

Pesky Trees

What makes vegetation hazardous? According to industry definitions, there are two types of hazards. One is when a tree has grown in close proximity to a transmission line and may cause a flashover. The second is when a tree can fall into a line and cause a flashover or pull down a conductor. It sounds simple to go out and identify all the trees and bushes that fit these criteria and prune them back to safe dimensions; unfortunately, it is not that straightforward.

The transmission line's conductor moves and the hazardous vegetation has to be identified under all conditions. Conductor moves up and down (sags) and side to side (blowout caused by wind). In its simplest form, the movement is dependent on the wind and ambient temperature combined with the electrical load flowing through the conductor. As the load increases on the transmission line, the temperature of the conductor rises and the conductor expands, which is called sag.

In the summer, the ambient temperatures are high and so are the loads, which is the wrong combination. Add wind and the conductor swings in the direction of the wind and at least one phase gets closer to a tree somewhere along the right-of-way. Greater sag and higher wind velocity mean more movement and more vegetation coming into critical zones. When all of these factors hit in the correct proportions, there is the possibility of systemwide blackouts.

The blackout of Aug. 14, 2003, left more than 50 million people in the dark. It included eight northern states and most of Ontario, Canada, and lasted up to a week in some sections. The affected load totaled 61,800 MW. The outage-related financial losses were estimated at approximately $6 billion. The Federal Energy Regulatory Commission (FERC) investigation attributed the outage to trees contacting overhead lines.

Prior to the outage, customer demand had been very high and there were loading issues related to the transmission lines. As the lines loaded, their conductors sagged and eventually came in contact with trees. The report stated that three of the four lines contacting the trees had been inspected a couple of months before the outage and deemed to be within safe limits.

NERC Incentives

The blackout got everyone's attention. After the outage, FERC approved 96 new reliability standards that Joseph McClelland, then director of FERC's Office of Electric Reliability, summed up as the three T's: trees, training and tools.

The blackout led FERC's North American Electric Reliability Council (NERC) to develop Standard FAC-003 Transmission Vegetation Management Program, which was adopted in 2005. The program is designed to improve the reliability of the electric transmission system, which requires utilities to validate their vegetation management practices. The federal mandate is serious: Utilities are expected to eliminate all power outages caused by vegetation. Failure to meet the mandate can result in penalties from NERC for noncompliance ranging from $1000 to $1 million per day.

The NERC program has four goals: preventing outages from vegetation located on the right-of-way, minimizing outages from vegetation adjacent to the right-of-way, maintaining clearances between the transmission lines and vegetation on and along the right-of-way, and reporting all vegetation-related outages to NERC and the regional reliability organizations (RRO).

The RROs have three categories for outage reporting. Category 1 is outages due to grow-ins. Category 2 is outages from fall-ins from inside the right-of-way. Category 3 is outages from fall-ins from outside the right-of-way.

Modeling in 3-D

With all this attention on vegetation management, interest in using LiDAR systems for this exploded. The fact LiDAR has been a design tool for years in the engineering department was not lost on the arborists. LiDAR data is collected quickly over remote locations and in a very cost-effective manner. The technology has been used to record a transmission line's geometry and its placement in the right-of-way.

“The utility really needs a digital model of the entire transmission line right-of-way in order to fully understand conductor interactions with all vegetation under all loadings and weather conditions. Such a comprehensive understanding of these possible interactions defines NERC compliance,” said Brian Cormican, vice president of sales and marketing, Utility Risk Management Corp. “This data, when delivered to the field via intuitive tools and applications, enables the utility to demonstrate compliance by getting tree crews to the right trees at the right time and providing a historical record of the entire process.”

The 3-D models show trees and brush with vegetation encroachments such as blow-in or grow-in violations. Randy Miller, director of vegetation management for Pacifi-Corp, remembers when transmission engineers at his company told him about LiDAR.

“They had all this data showing the line, its sag and its sway in relation to the trees and brush,” said Miller. “They asked if it would be helpful to the vegetation management program.”

With approximately 15,000 miles of transmission lines and roughly 180 tree crews to supervise, Miller immediately saw it would be useful and used.

Vegetation Encroachment

One of the biggest challenges is identifying where vegetation will encroach and what are safe operating clearances to the conductor. So often it has been an educated guess based on years of experience, but today's standards of availability and reliability demand a more-rigorous approach.

Mike Neal, Arizona Public Service's manager of forestry and special programs, is hoping to save money with LiDAR surveys from GeoDigital International.

“Once the data is analyzed, contractors will be given bid packages with LiDAR data and models, which gives them an accurate understanding of what they are bidding on,” said Neal. “The contractors know what the terrain is like, where the access is and exactly what work needs to be done. They can see the trees needing to be removed, trees trimmed and brush cleared or treated. Their bids should be better, because it takes the guess work and unknowns out of the equation.”

LiDAR surveying and imagining offers a dynamic representation of the line and its surrounding vegetation. The visual information is so vivid that it is obvious to everyone what vegetation is encroaching and what is not.

Otto Lynch, vice president of Power Line Systems, said, “With all the utility departments finding uses for LiDAR data and models, it is more important than ever that all the stakeholders talk to each other before starting a project. It is more cost effective to specify the highest degree of data accuracy and fly the terrain once rather than go back a second time for better data required by someone left out of the loop.”

With the correct type of data-density selection and PLS-CADD models, it is possible to simulate the conductor and the vegetation's interaction. This allows the transmission engineer and arborist to work together and develop both vertical and horizontal clearance margins to prevent transmission lines from flashovers caused by vegetation encroachment.

These margins are identified as the minimum clearance distances to vegetation under normal sag conditions and high-temperature sag conditions. They also include wind-induced blowout clearance conditions. These models flag trees that could be missed by traditional rights-of-way surveys.

By combining LiDAR-based visual representations and geo-referenced digital cameras, and merging their data, an even wider range of right-of-way and transmission line information is available. As these models get better, dependability improves and utilities are more willing to rely on the results. Vegetation managers get quick reports that list each vegetation clearance violation found by the LiDAR survey, but this does not replace the need for field verification.

The LiDAR data is only as good as the technicians interpreting it. The representations they produce are extremely accurate, but there are always questions that can be answered only by a person doing field work.

Paul Adkins, marketing communications coordinator for Laser Technology Inc., points out how important it is to verify aerial LiDAR surveys.

“The aerial surveys cover a great deal of territory, but ground patrols are still necessary,” said Adkins. “Handheld range devices like the TruPulse 360 supplement LiDAR surveys by verifying questionable areas quickly. Suspect branches are identified, measured and recorded without large crews or heavy equipment, minimizing environmental impacts.”

Verifying LiDAR Data

Verification doesn't imply aerial surveys are faulty; only a small percentage of the data needs to be checked.

“PacifiCorp uses LiDAR models to find vegetation violations for various combinations of conductor temperature, wind and clearance requirements,” said Miller, adding that LiDAR flags trees that would never be seen otherwise. It also increases the efficiencies of his tree crews by dispatching them directly to problem areas.

“PacifiCorp verifies its ground data,” continued Miller. “When questionable results are found, a technician is dispatched. If there is a problem, a tree crew is sent out to correct it. This is a more cost-effective method to use your valuable resources.”

Hazardous vegetation poses a continuing threat to the reliability of transmission lines. Tree-related outages have a huge economic impact on our customers and can result in heavy fines imposed on the utility. The NERC vegetation management standards are forcing practically every utility in North America to increase expenditures on monitoring, measuring and managing vegetation encroachments.

Progressive utilities have determined that aerial LiDAR surveying is a priceless tool for vegetation management. Arborists can use it to identify problem areas. It also can be used to predict growth factors, helping utilities to set maintenance cycles. A tree's health can be determined from the laser scan (the amount of chlorophyll in a tree's leaves shows up on a reflected image). Dying and dead trees are more susceptible to fires, which are hazardous to transmission lines. There is literally no limit to the many ways utilities can use this technology, except if limiting the imagination.

Companies mentioned in this article:

ArborMetrics Solutions, www.arbormetricssolutions.com

Arizona Public Service, www.aps.com

GeoDigital International, www.geodigital.net

Laser Technology Inc., www.lasertech.com

PacifiCorp, www.pacificorp.com

Power Line Systems, www.powline.com

URMC, www.utilityrisk.com

W.I.R.E. Services, www.wireservices.ca