The Contribution of Airborne Data Collection on Electric Utility Vegetation Management
Greg Ina Supervisor, GIS Technology, Davey Resource Group A Division of the Davey Tree Expert Co. 1500 Kent, Ohio, 44240, USA gregi@davey.com James Dow Chief Operations Officer, Airborne Remote Mapping A Subsidiary of American Indian Services, Inc. 560 Mitchell Field Rd, Bessemer, Alabama, 35023 westondow@aol.com Utility forestry is the management of vegetation that could affect the transmission and distribution of electricity. The objective of a utility forestry program is to apply scientific knowledge to maintain reliable electric service. This is achieved by protecting electrical conductors from tree interference. Another goal must be recognizing the environmental benefits of trees and the public’s awareness of the value of trees. A composite of information-based products and services to address strategic issues of the electric power transmission and distribution segment of the electric utility industry, as well as the associated support functions has been assembled by two industry vendors or our two companies. These entities, combined with other “team members”, together offer the electric utility industry a consistent, synergistic approach to gathering, analyzing, and reporting mission critical geographic information. An airborne survey crew photographs transmission line right-of-way using highresolution digital framing cameras. Individual video frames can be indexed to the geographical location (longitude and latitude) of any target within the right-of-way. The specific location of each facility is captured as the individual digital frames are post processed. Inventories of attributes are then collected using ground crews. Trained arborists inventory the vegetation and estimate the right-of-way maintenance workload affecting primary and secondary distribution lines and transmission lines in a project area. Data is collected using hand held (in some cases pen-based) computers. Once all the data is centralized, a vegetation management plan is developed. This plan establishes guidelines for prioritizing, routing, budgeting, contracting, scheduling, and updating of the initial inventory. Utility Forestry The infamous Idaho “brown out” in August, 1996 positioned utility forestry in the national spotlight. A stretch of transmission line outside of Boise Idaho, came into contact with a singe Western Cedar tree causing a thirteen-state disaster.(1) The tree shorted out the transmission circuit for periods ranging from several minutes to about nine hours which, in turn, shorted its connecting circuits until over 7.5 million customers were without electricity. This nearly happened again only twelve days later. when a circuit bordering Boise contacted a tree and shorted out. The second time, however, an alert foreman cut electricity to Boise, preserving the interrnountain transmission grid while “turning out the lights” only in Boise.(2) These incidents have prompted utility organizations to monitor their vegetation under and adjacent to the transmission lines in a much more proactive manner. Transmission right-of-ways are generally 300 – 400 foot wide corridors connecting power generation centers to electrical distribution sub-stations. The voltage of these lines range from 55 kilovolts to 500 kilovolts.(3) Due to the extreme electrical load these lines carry, clearance from the conductors are governed federally, at the state level, and by PUC (Public Utility Commission) organizations. Application of herbicides, mechanical mowing, and tree trimming maintain safe and legal clearance levels.(4) Aside from legal compliance, other motivators for utility operations to maintain transmission line clearance include un-interrupted service, safety, aesthetics, and the costs associated with energizing vegetation. Reliable service and safety leverage one another because the continuous flow of electricity presumes that the lines are safe for the public and those who manage them. Conversely, anytime that electricity is disrupted, it is reasonable to assume that an unsafe condition exists. Trees are often the cause of interrupted service and unsafe conditions because of the tendency for utility customers to expect the lines to be somewhat camouflaged by vegetation. When trees or areas of brush contact power lines, the utility company must absorb the cost of electricity lost through the line-tree-ground continuum.(5) In addition, fires started by tree-line contact can cost a utility millions of dollars each year, particularly in the Western United States where low rainfall during the summer months is common. Data Definition Once the decision is made to assess vegetation within a right-of-way, many techniques and technologies can be applied to gather raw data. In addition to capturing data, attribute data fields must be established to provide the most useful information to best manage the vegetation under and adjacent to the transmission network. Attribute selection varies from company to company and by regional restrictions. There are several attributes which most utilities inspect .(see table 1) After decisions have been made defining the focus within the transmission right-of-way, an appropriate technology is chosen to collect and store the database. Traditionally, all inspection work was conducted by surveyors on land. -However, the new technologies today include hand-held computers, pen-based computers, back-pack GPS units, digital cameras, and laser range finding devices. Table 1: Standard Transmission Inventory Data Fields and their Attribute Values (6)
Drawbacks of implementing ground units to collect transmission right-of-way data range from consistency issues to production monitoring. The most effective way to canvas the hundreds of miles of transmission line is to distribute line sections evenly among data collectors. When implementing global positioning systems technology, there is no variance in how trees and structures are located. With regards to attribute data, however, each person views and prescribes slightly different trim types and conditions of trees and brush. This lends itself to inconsistent data. Another key issue is travel and planning logistics. Many times these transmission easements lie on such rugged terrain that each collector must be equipped with four wheel drive vehicles and a means for safe communication. No amount of planning can take into consideration unpredictable dangers along the line such as poisonous snakes, mountain lions, and wild dogs. When inspecting areas with a reputation for wild animals or hazardous terrain, two collectors work together. At best, an inspector can patrol only seven to ten miles of line per day.(7) Costs associated with lodging and meals for long stays impact inventory projects significantly. Mission Planning The ideal mechanism for obtaining transmission line and vegetation data is the combination of airborne sensors technology with military precise ground control efforts. A myriad of factors impact flight mission planning. Weather forecasts are taken very seriously as they can make or break a flight mission. Airplane payload also plays a part in the planning process. Generally a representative from the utility company is required to observe each flight. This person’s weight can adversely affect the aircrafts ability to maintain steady altitudes. The number of collection days per job is decided during mission planning. In contrast to ground collection, an airborne unit can survey up to 900 line miles per day. Based upon the most current weather forecasts, the project manager will make a conservative estimate on the number of days an aircraft will need to be accessible to the contract. Mission planning software also assists each team before a mission. This defines all parameters for the survey mission, digitized flight lines, pilot guidance information, control commands, and settings for sensor systems. Each flight may be optimized by setting customized path width or flight height. In this way both the appropriate timing and the optimal shape and size of the survey area can be chosen. Airborne Sensor Systems The primary sensor on the helicopters is a scanned laser rangefinder that can rapidly survey topography with high levels of accuracy. Ideally suited to large scale mapping and engineering surveys, the system is especially suitable for surveying and surveillance of infrastructures such as transmission power lines. The laser rangefinder measures the distance between the helicopter and the ground at a rate of up to 7000 times per second.(9) Each individual laser sounding records up to five distances and is capable of detecting an object just 10cm in diameter, like a transmission line connector. The pulsed laser beam is scanned across the track of the helicopter and forward as a result of the advancing direction of the flightline. The resulting pattern of the scan across the ground is “Z” shaped. A video camera is mounted and aligned to the scanners field of view. The entire video-image is then recorded on tape. Each video frame is marked with a time and frame code to allow full synchronization with the measurements taken by the laser. It is also possible to mount a forward looking camera overhead to record overview images. To determine a highly accurate GPS position for the helicopter, it is necessary to align and process recorded GPS data with a Differential GPS (DGPS) position after the flights. The DGPS position is determined by an easily deployed ground reference station, also recording GPS information. To calculate the three coordinates, the various altitudes of the helicopter (roll and pitch) is measured by a high performance Internal Navigation System (INS). The INS information is used to stabilize the scanning systems. Table 2: Accuracy Comparison of Ground Survey vs. Airborne Survey ( 10)
Applications Taking into account the material characteristics of any specific conductor surveyed, TLSURVR (Transmission Line Survey and Rating) software can be used to compute the sag characteristics of each span at a user-specified conductor operating temperature. TLSURVR can also be used to determine the limiting sag/clearance characteristics of each transmission line span based on regional clearance requirements, resulting in a limiting conductor operating temperature complying to mandates. Subsequently, the TLSURVR user can specify any “weather” load case in terms of solar conditions (example: sun conditions on a winter/summer day, wind direction, wind speed, and a range of ambient temperature conditions), and TLSURVR will calculate the conductor’s limiting ampacity as a function of ambient temperature, at the limiting operating temperature. The conductor’s limiting operating ampacity as a function of ambient temperature and other weather conditions is graphed for each span of the transmission line surveyed to present an easy-to-read format for determining the transmission line’s “as built” limiting operating temperature, and hence the “thermal rating” of the transmission line segment. Since line sag can cause tree contact, even if significant clearance has been achieved, it is important to monitor re-growth and factors contributing to line sag. These factors include line diameter and age, structure distance, wind, temperature, and electrical load. In a deregulated market, electrical load can vary significantly as utilities are allowed to market their electricity across another utility company’s transmission lines, a term called “free wheel ing”( 11). With airborne data collection, the actual line sag can be calculated cost effectively which provides a tool to regulate electrical load and prevent outages. Vegetation Management Plans Data on trees and vegetation in right-of-way areas helps develop planned maintenance programs. Management reports detail an action plan for the utility forest maintenance managers based on the analysis of the inventory, and objectives determined within the rights-of-way. The management report includes an analysis of the current tree population, growing environment and maintenance needs, as well as long-range management recommendations. Management reports are customized for individual clients. Prepared by scientists, each plan includes the latest findings or trends within the green industry. Conclusions In summary, data collection along transmission rights of way has traditionally involved paper entries by ground crews, a laborious and relatively “low-tech” process. The introduction of new technologies such as hand held computers, GPS, Laser rangefinders, and digital cameras have improved productivity and reduced error rates. These technologies are extremely important in today’s de-regulated utility market where no one company controls the electrical load across a transmission line. References
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