An Extended Drought on the Island of Oahu in Hawaii resulted in slower vegetation growth. So, from the late 1990s through 2004, vegetation- or tree-caused outages were not a major factor for Hawaiian Electric Co. Inc. (Honolulu, Hawaii, U.S.). However, in 2005, the weather patterns affecting the island began to resume a more normal pattern for the area, resulting in increased rainfall throughout the island. As the rain fell, vegetation began to thrive. The accompanying increase of customer interruptions caused by trees and vegetation rose to No. 3 for the years 2005 to 2007, up from the previous ranking of No. 9 in 2003.
TREES AND OUTAGES
Historically, the top-five outage causes explain about 70% to 75% of the System Average Interruption Frequency Index (SAIFI) annual reliability results. Hawaiian Electric focuses on the top-five outage causes to prioritize its maintenance and capital programs. Equipment deterioration — primarily overhead poles and lines — and underground cables are perennially the most frequent causes of customer interruptions. Auto accidents and weather-related causes, such as high winds or lightning, are also major contributors. By 2007, vegetation- or tree-caused outages contributed about 16% to SAIFI.
Prior to 2004, the average annual rainfall measured at the Honolulu International Airport was about 18 inches (46 cm) per year. In 2005, twice the normal rainfall was measured. In 2006, rain fell in all areas of the island for an unprecedented 40 consecutive days. The result was an explosion of tree growth throughout the island after a period of prolonged dormancy. By 2007, the vegetation management expenses were almost double that of the 2005 expenditure. Expenses were conservatively projected to double again within a few years.
THE ASSET MANAGEMENT APPROACH
To address the increasing concerns over the number of outages due to vegetation- or tree-related causes and the rising expenses, Hawaiian Electric selected an interim vegetation management director, with project management and system operation experience, and a superintendent in the system operations department to apply an asset management approach to the situation. Vegetation management is staffed with arborists, and the utility hires private contractors to perform the physical tree-trimming work. The utility established the following goals for the new approach:
There will be no island-wide blackouts caused by vegetation (the utility is isolated with no interconnections to any other utility).
Vegetation- or tree-related outages will be below fifth in outage causes.
Optimize costs by balancing it against performance and risk.
The new group was able to categorize the work into three major areas:
Right-of-way work consists of six districts and 115 line segments covering about 370 miles (595 km) of overhead transmission and sub-transmission lines. The routine trim cycle for this work can vary from one to five years, depending on the vegetation under the line segment.
Roadside work consists of 18 districts with about 1200 miles (1931 km) of overhead sub-transmission and distribution lines. The routine trim cycle for these districts can vary from 12 to 15 months.
Customer requests, or hot spots, include trimming of vegetation, mainly near service drops to individual homes or small commercial businesses. Hot-spot work includes any emergency trimming work where the vegetation is reported to be encroaching on a line that is not part of the routine trim cycle.
The new vegetation management group reviewed the current tree-trimming work. This evaluation showed that customer requests and the hot-spot work were consuming increased resources compared to all other work. This, in turn, was generating an increased backlog of the unaccomplished routine trim-cycle work. There were also more outages affecting entire circuits on the distribution and sub-transmission systems. Additionally, there was a shortage of qualified labor to do the trimming work next to energized lines. Thus, additional crews were hired from the U.S. mainland and mobilized to the islands on a rotational basis to perform the trimming work.
NEW STRATEGIC PHILOSOPHIES
In order to achieve success, the vegetation management group made several conscious changes to its strategies. As a group, the members first asked: "What is our core business?" The group came to realize that Hawaiian Electric is not in the business of cutting trees. Rather, the group determined: "Hawaiian Electric is in the business to achieve zero failures caused by vegetation." This paradigm shift allowed the group to prioritize the work into strategic areas and develop the following strategic guidelines:
- "Focus on the critical few" versus the trivial many
- "Save the backbone" and then the laterals
- The rest can wait.
An asset management approach requires an inventory of critical assets (the "critical few"), a health assessment and then work prioritization based on available resources. Transmission is more critical to the system than sub-transmission or distribution. This is true for any electrical system, whether it is an isolated system such as Hawaiian Electric or an interconnected utility with ties to neighbors. Even within the family of transmission lines, some are more critical than others. A transmission line to a power station may be more critical to the system than a transmission line to a rural service area. This philosophy allows the utility to focus on the critical system components first and then prioritize addressing others.
The second major strategic approach is to "save the backbone" on a distribution circuit. Hawaiian Electric defines the backbone as the part of the distribution circuit that is protected by the substation breaker. Laterals on a distribution circuit are protected by fused cutouts. The fuse protection of laterals allows for the largest potential improvement to reliability results for distribution circuits, because it impacts a relatively small number of customers on the lateral, compared to the larger number of customers affected by a backbone interruption.
In consultation with some industry experts and from research in publications, Hawaiian Electric came to understand the phenomena that there is a higher probability of tree-branch contact causing an outage on a 12.47-kV three-phase distribution line with a phase-to-phase fault than compared to a tree-branch contact causing a high impedance line-to-ground fault on a 7.2-kV single-phase distribution line. The basic design of a distribution circuit, and the lower probability and consequence of a single-phase fused lateral outage, supports the circuit segmentation strategy of "save the backbone."
Requests for trimming service lines to single-family homes and customer hot-spot trimming had increased so significantly by 2007 that seven of the nine contractor crews were assigned to this type of work, while only two crews were doing routine trim-cycle maintenance. By using the asset management approach, Hawaiian Electric decided to reassign five of the crews from hot-spot work to routine trim-cycle work and focus on the roadside distribution backbone. This allowed Hawaiian Electric to focus on the backbone, which provided the best reliability results. However, the utility also had to effectively manage the increased backlog of customer requests, since most were due to vegetation physical encroachment near the service lines and not due to the reliability of their electrical service.
Hawaiian Electric used the Pareto principle (also known as the 80-20 rule and the law of the vital few), which states that 80% of the effects will come from 20% of the assets or system. Hawaiian Electric anticipates that this philosophy change will result in a reduction in the backlog of routine trim work for the distribution backbone within two years instead of the four years originally estimated. Setting the correct priorities also required teamwork and concurrence with other operating areas within the energy-delivery process area and with the community relations personnel, since more customer complaints are anticipated.
Hawaiian Electric also employed a concept called Quick Operations Intelligence, which uses existing teams to effectively share information. A cross-functional group called the system reliability team meets weekly to review the past outage causes, and a vegetation management team member is part of the reliability team. This provides for a much quicker response to systemic problems affecting outages.
Another cross-functional team meets weekly to review the upcoming (several weeks up to a month) scheduled maintenance work on the transmission, sub-transmission or distribution system. Routine trim-cycle work is coordinated with this work schedule so that inspections can be done on critical backup circuits before the scheduled pole and line work is done. The maintenance work is also coordinated so that tree trimming will not inadvertently drop a branch on an energized circuit.
Hawaiian Electric was able to increase its contractor crew productivity by working with its contractors to closely audit the crew output and performance. Hawaiian Electric compared its contractor crew productivity against an industry benchmark and found it to be about 60% of the industry average. The reassignment of crews from hot-spot work to routine trim-cycle work resulted in significant productivity gains. Hawaiian Electric was also able to reduce the "windshield" time for its contractor crews by using its facilities, such as power stations, to relocate the baseyard closer to the physical work locations.
Before 2008, Hawaiian Electric was using the services of only one tree-trimming contractor. The utility was able to increase the available labor resources and competition for trimming services by hiring three additional contractors, including a ground clearing specialist. Hawaiian Electric currently uses the services of Davey Tree Expert Co., Asplundh Tree Expert Co., Trees of Hawaii (a local contractor) and Vegetation Control Services (ground rights-of-way clearing).
Hawaiian Electric also employed several other changes to continue to improve the productivity of its contractor crews. The most productive crew size is a two-man crew. However, if one person in the crew was absent, the contractor would often reassign the remaining person to another two-man crew, which increased the crew cost by 50% but with no increase in productivity. Thus, Hawaiian Electric and its contractors instituted a two-man-crew-only rule, so if one person is absent in a crew, the second person is also released from duty for that day.
A customer request is normally evaluated by a Hawaiian Electric arborist to define the scope for the contractor crew. Many of these requests were for trimming work that was close to but not affecting the electrical service, or was only impacting communication lines. The arborists were provided with tree pruner tools, so that they could perform incidental trimming for customer requests rather than sending a two-man crew to the job site.
From 2003 to 2007, the number of customer interruptions due to tree-related causes increased by 400%, resulting in customer dissatisfaction. The budget for vegetation management was estimated to require a 300% increase to address the situation.
By using the principle of "focus on the critical few" and balancing costs against performance and risks, Hawaiian Electric was able to reduce its projected costs by 23% in 2008. The increased specialization of contractors, competition and productivity improvements resulted in a 100% increase in contractor crew productivity, as measured by man-hours per tree by the second quarter in 2008. There is still a significant variation in the productivity results depending on the density of the vegetation in a particular district, because the island has areas of dense vegetation but also has desert-like microclimates in other areas of the island. The comparison of the productivity benchmarks against past performance will continue in the future.
The reliability from tree-related-outage causes improved by 54%, compared to the similar period in 2007, and the routine trim-cycle backlog will continue to be significantly reduced. Hawaiian Electric plans to refine its vegetation management work by developing a portfolio of segmentation strategies to further balance the costs against the system performance and risks. Hawaiian Electric has selected a permanent vegetation management director to manage the program going forward.
|Trees and branches in the lines||8||9||6||3||3||3|
|Yearly outage ranking by cause.|
Dean Mizumura (firstname.lastname@example.org) has gained more than 30 years of experience working for Sargent and Lundy Engineers in Chicago, Citizens Utilities on Kauai and Hawaiian Electric Co. in Honolulu. His current responsibilities at Hawaiian Electric include asset management for the energy-delivery system, system reliability and emergency response. Mizumura holds a BSEE degree from Purdue University and a MBA from the University of Hawaii.