No one at Bonneville Power Administration (BPA) remembers a prior situation where so many employees were mobilized to inspect and document the condition of the entire 15,000 miles (24,000 km) of high-voltage transmission circuits. Previous vegetation-caused outages set the stage for this massive undertaking. During fiscal year 2008, thousands of man-hours and millions of dollars were spent on inspection, documentation and mitigation to ensure the transmission system was clear of subsequent potential vegetation-caused outages.

During this effort, five different inspection techniques were used to assess vegetative conditions within the right-of-way (ROW) corridors of BPA:

• Transmission line maintenance (TLM) working patrol field inspection

• Helicopter aerial survey with TLM observers

• BPA's Vegetation Clearance Categories

  Helicopter aerial survey with natural resource specialist (NRS) observers

• Contract field ground inspection conducted by a private firm

• Light detection and ranging (LiDAR) remote sensing.

Accuracy of Inspection Techniques

In assessing the efficacy of these inspection techniques, two questions were asked: Which vegetative sampling technique was most accurate in determining clearance violations? Which vegetative sampling technique was most cost effective in determining clearance issues?

The emphasis of the field and aerial inspections and the LiDAR remote sensing was to determine if any vegetation fell within three categories of vegetation-to-conductor clearance criteria that defined potential threats to circuits: danger brush, danger tree grow-in and high brush.

The term “brush” in BPA vernacular refers to any type of growth form: tree, shrub, side limbs and so forth. Danger brush is any vegetation located on the transmission line ROW extending into the minimum clearance distance at maximum sag from the conductor: less than 15 ft (4.6 m) for 287 kV to 500-kV lines and less than 10 ft (3 m) for 69-kV to 230-kV lines. A danger tree grow-in is a tree just off the ROW that may present a clearance hazard as its lateral branches or horizontal leaders elongate. High brush is located on the ROW extending into the minimum clearance distance at maximum sag from the conductor: 16 ft to 25 ft (4.9 m to 7.6 m) for 287 kV to 500 kV and 11 ft to 20 ft (3.4 m to 6.1 m) for 69 kV to 230 kV.

Danger brush is the most critical clearance distance because it has the highest potential for flashover. Clearance distance conditions in the high brush and danger tree grow-in categories do not pose an immediate threat to the transmission system. Vegetation at this distance may remain for another growing season followed by subsequent removal.

LiDAR outperformed all other field inspection techniques; it located the most clearance violations. This held true even when accounting for an approximate 12% rate of false positives found in verifying LiDAR reports in the field. The false positives were objects other than vegetation, typically transmission line hardware such as jumpers or other foreign objects such as light poles, abandoned wood poles or birds. What makes the LiDAR method stand out is the number of false negatives found in more subjective, or human-based, inspections. While the LiDAR approach locates false clearance issues, other methods miss real clearance issues.

In the danger brush comparison, the TLM working patrols found the least number of unique clearance issues (data points). The private firm found more unique data points than LiDAR in both the danger brush and high brush categories. BPA speculates the reason for this is because the private firm misclassified the specific data points. LiDAR would accurately report the data point in either category while human error, whether in measurements or data recording, would contribute to misclassifying these sites.

The same scenario is the reason why the TLM working patrol had more unique field data points than LiDAR in the high brush category. It is equally plausible that the TLM working patrols misclassified danger brush as high brush, therefore increasing the unique field data points for LiDAR in the danger brush category while increasing the unique field data points in the high brush category.

Another accuracy-related observation of the comparisons is that the private firm's ground inspection found almost twice as many field data points for danger tree grow-in as the next highest field survey with TLM working patrols. TLM ground patrols found the next highest number of field data points. Both helicopter survey techniques found the least number of observations.

However, an interesting error occurred when 2800 reports of vegetation were incorrectly identified as danger tree grow-in by TLM crews. Clearance distances were actually greater than the specification for this category. Though not a critical mistake, unnecessary time and resources were diverted from working on real critical problems in order to sort through data to differentiate the mistaken reports or, worse, to respond to them in the field. These diversions were costly, unnecessary and increase the risk of missing more critical work.

In the high brush category, LiDAR outperformed the other inspection techniques. The private firm's ground inspection and TLM working patrols found the next highest number of field data points. Helicopter surveys performed relatively poorly in identifying any high brush.

In terms of detecting vegetation-to-conductor clearance issues, there is only a slight gain in accuracy when using NRS versus TLM observers during helicopter aerial inspections. However, other program benefits are arguably important enough for BPA to consider instituting a vegetation-only helicopter aerial inspection with NRS observers as a new work practice into BPA's transmission vegetation management program. NRS observers gain a better understanding of the character of the ROW they manage by participating in an annual helicopter tour of their lines. The aerial perspective they gain imprints spatial relationships in their mind and enhances their memory of the corridors, ROW and circuits entrusted to them in ways that cannot be obtained from the ground-only perspective.

As slightly less than half of the NRS staffs are new hires — with less than one year of experience to become familiar with their districts — the annual aerial surveys can significantly accelerate their learning curve and skills. For this reason alone, it appears this work practice should be incorporated as a permanent part of BPA's transmission vegetation management program, until at least the new staff becomes more familiar with their districts.

Another benefit is that data gained through aerial surveys with NRS observers does not require subsequent ground verification by the NRS observers, thus eliminating one additional step in the process and saving on relative costs.