Many power companies use a cause-and-effect approach to maintain their transmission and distribution systems. When a tree crashes down on a power line, they dispatch their vegetation management crews to conduct a closer inspection of the trees on that line. Then upon a closer look, they try to prevent a future outage and often will remove the trees to resolve the problem.

By directly addressing the trees, however, utilities might be managing the symptom rather than the underlying cause. At Seattle City Light (SCL), the utility is working with Arbor Intelligence LLC on proper system assessments that can help to minimize outages.

Taking a Closer Look

Vegetation management (VM) was once handled on a fire-drill or “emergency” routine basis at SCL. In the late 1990s, the utility moved its VM program to the back burner due to budget cuts and other priorities. Because the company wasn't taking a proactive approach to its VM program, SCL found it challenging to manage the tree-trimming needs on a limited budget. As a result, reliability suffered.

Three years ago, however, the company moved to a four-year trim cycle with the support of the executive staff, the mayor and the city council. By moving to a selective trim cycle, the company was able to get the program back on solid footing. The company is now in its last year of its first four-year cycle, and the benefits of the new approach have been better than expected.

Late last year, the company hired Arbor Intelligence to assess the system and review SCL's vegetation program on the distribution side. The company used its knowledge of density factors, probability and tree growth species to determine its target strategy for moving SCL into more of a selective trimming program.

According to Arbor Intelligence, an appropriate system assessment is more than an inventory. Instead, it is a diagnosis of the effect of the current management practices. Traditionally, VM crews take random samples of the species, location and trim cycles, but these often give very little information on root causes of outages and failure modes of species and alike.

A good system assessment is designed to gather very specific data to analyze, discern and interpret, using scientific methods, the causes of system outages, avoidable costs, the effect of the contracting strategy, quality issues, the cost of difficult landowners, the risk of the current policies, management styles, cycle length and the composition of the workload.

Arbor Intelligence spent three months assessing SCL's VM program. In the second quarter of 2010, SCL presented the report to its management team, who then devised a plan for moving from a more reactive to proactive approach to VM.

Putting the Plan into Practice

Following the system assessment, SCL began prioritizing the areas that needed the most attention. Some parts of Seattle are more densely populated than others, and the Pacific Northwest and heavy vegetation often go hand in hand. Compounding the challenge, each area of Seattle has a different mix of tree species and types of vegetation.

The company first focused on those areas that had the most dense vegetation. In these regions, a significant amount of trees were causing day-to-day reliability issues with distribution feeders. During the first year of its four-year cycle, the company zoned in on these areas and hired tree-trimming crews from Asplundh Tree Expert Co., J.T.S. and Kemp West to get the vegetation under control.

After taking control of the areas with the lowest reliability and highest tree density, the company then moved on to regions with a lower density of trees and slower-growing vegetation.

Throughout its tree-trimming process, the utility was careful to strike a balance when it came to environmental sensitivity and the need for tree trimming, removal and plantings.

Over the last four years, the utility has managed about 2,470 miles of 26-kV distribution feeders and 624 miles of transmission right-of-way. The company plans to restart that program next year but place a more selective approach to get back on track. SCL is also working on continuing to better understand the species growing cycles and the trim needs of the trees in the Pacific Northwest.

On top of these concerns, the utility also had to find a way to comply with the right-of-way transmission standards. Being proactive and up to date with best management practices in VM is expected by the North American Electric Reliability Corporation, which requires companies to comply with the FAC-003 Transmission Vegetation Management Standards.

Effect on the Field Workforce

Now that SCL has moved into its selective trim cycle, the linemen have noticed a dramatic drop in tree-related outages. In the past, the lion's share of the outages and related fusing issues were a direct result of trees contacting lines. Of the 12,000 tree-related trouble calls to SCL before the start of the four-year cycle program, most of those calls were being taken care of one at a time or as an emergency trim.

By implementing the program, the number of tree-related outage calls has plunged to less than 1,000. This is good news for the line crews, who can respond more quickly to outage calls. Rather than spending their professional time clearing trees, linemen can stay focused on restoring power and establishing new services.

As a result, SCL has noticed a significant decrease in the amount of time spent by line crews to clear vegetation. Rather than serving as both tree trimmers and linemen, the field crews can focus on their duty to set poles, string new wire and restore power to SCL's customers.

Due to the increased focus on VM, SCL also has seen fewer field crews dispatched following a storm. From October 2009 to April 2010, the utility experienced a dramatic change in the number of outages caused by tree branches as a result of wind and heavy rain.

For example, during one particular storm, many of the SCL managers were surprised that the company only had to send one crew out that evening. The wind storm was significant, yet the damage was minimal because of the company's improved VM program. This was a good sign that the program was going as planned.

So far, SCL has seen significant gains by implementing the four-year cycle and winning the support of the local government. The company can now be more efficient by taking more of a planned approach to vegetation management.

Bernie Ziemianek (Bernie.Ziemianek@seattle.gov) is the director of energy delivery operations for Seattle City Light and has been with the company for five years and in the industry for 33 years.

Companies mentioned:

Arbor Intelligence LLC www.arborintel.com

Asplundh Tree Expert Co. www.asplundh.com

Kemp West www.kempwest.com

North American Electric Reliability Corporation www.nerc.com

Seattle City Light www.cityofseattle.net/light