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:

1. 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.

2. 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.

3. 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.