For several years, Eskom, KwaZulu-Natal, South Africa, successfully provided the lowest-cost electricity possible to further growth and prosperity. Quality of supply, however, posed a problem, because of natural phenomenons such as lightning, and veld and cane fires. As a result, line faults, particularly those of unknown cause, became the subject of Eskom's research. The utility studied several mechanisms responsible for these line faults, including light wetting, light pollution and streams of bird excreta known as “bird streamers.”
Although air-gap breakdown resulting from bird streamers was first suggested in 1923, the concept is still regarded with suspicion in certain areas. Experiments with simulated bird streamers had demonstrated they could cause flashover, but eyewitness accounts of the process had remained rare.
In 1995, however, Eskom fitted bird guards to two 275-kV lines in the KwaZulu-Natal area after a re-insulation (glass-to-silicon) exercise failed to reduce line faults. Positive results on these two lines prompted Eskom to extend the project to 18 more lines in the grid. The expanded project showed a strong correlation between the presence of the bird guards and a decrease in line faults from bird streamers. Therefore, the Line Technology Group of Transmission produced a report on the correlation, and the results were sufficient to continue fitting guards to the rest of its national transmission system.
Setting the Scene
To pursue its goal, Eskom formed a partnership in 1996 with the Endangered Wildlife Trust (EWT), which advises the utility on interactions between wildlife and the electrical infrastructure. In early 1999, Eskom appointed a multi-disciplinary team. In addition to the project manager and engineers, the team also included a statistician and an environmental consultant from the EWT whose contributions proved invaluable in the project assessment.
In Search of Proof
The “other” fault category comprises faults that could not be conclusively linked to pollution, lightning, hardware failure or cane fires. The bulk of this category was simply classified as unknown. Because bird streamers are relatively unknown as a fault mechanism, the team suspected that a significant number of unknown line faults could have resulted from streamer activity.
The team's first challenge was to find a method to determine which of the “other” line faults possibly could have been caused by bird streamers. The fault statistics for lines fitted with bird guards were subjected to a statistical test known as the decomposition method. This test showed a statistically significant decrease associated with the fitting of the bird guards with regard to “other” faults.
Figure 3 illustrates the impact the various interventions have had on the line-fault trend on one of the two 275-kV lines initially marked with welded-rod bird guards. The line faults (12-month moving average) are shown in green, with the line-fault trend calculated using the decomposition method in blue. The purple and yellow lines represent the re-insulation and cane and veld fire programs, respectively. (The cane and veld fire program clears excess vegetation and sugar cane to reduce the risk of fire beneath the line, which causes phase-to-earth faults). The black line represents the bird guard program.
Figure 4 shows the impact the various interventions have had on the line-fault trend on the 18 275-kV lines in KwaZulu-Natal, which subsequently were fitted with bird guards.
To determine which of the “unknown” faults bird streamers caused, Burnham's time-of-day approach was used. Burnham's work in the United States showed that line faults resulting from bird streamers have a bimodal temporal distribution.
The team found that 85% of the line faults presumably caused by birds on the original two 275-kV lines occurred on the center phase. Time-of-day and faulted phase formed the basis of the approach in the determination of the magnitude of the streamer problem and consequently led to the financial justification of the national bird guard project.
Figures 5 and 6 show the distribution of all line faults on the national 275-kV network before and after applying the above criteria.
Applying this analysis to the entire transmission grid, the team found that bird streamers caused about 600 (20%) of the approximately 3000 annual voltage dips on the transmission system. It also showed the bird guard project could conceivably eliminate some 428 (14.2%) of these.
The Eskom transmission group uses four main drivers for capital and maintenance expenditure:
Investments in infrastructure to ensure a significant increase in sales
Investments to reduce operating expenditure
Investments to improve system reliability (for example, quality-of-supply-related capital expenditure)
The first two categories of investments are evaluated using discounted cash flow techniques, and justification is recommended if the projects realize a positive net present value over the expected life of the proposed new assets.
The investments in the third category are evaluated using an economic cost-benefit analysis that takes into account the impact and cost of power supply interruptions to customers. The objective of this economic cost-benefit analysis is to maximize the net benefits (minimize the total costs) associated with the supply and consumption of electricity. A potential investment is justified and recommended if the marginal reduction in customer interruption costs exceeds the incremental costs to transmission to construct, commission and operate the new infrastructure.
The national bird guard project satisfied the third category. The required capital was motivated on the basis of expected improvements in the quality of supplied voltage to customers at the various supply points affected on the network. Hence, the project was approved on the basis of a positive effect on the “sustainability index” and on customer satisfaction, two aspects that underperformed during 1999.
Line Selection for Bird Guards
Eskom selected the lines to fit with bird guards by:
Estimating the percentage of the load sensitive to dips according to the type of industry connected to the network.
Obtaining the predicted number of outages attributable to bird streamers from fault statistics.
Calculating the cost of fitting bird guards from the actual number and type of towers for each line using the unit costs obtained from the KwaZulu-Natal project, which used commercial contractors.
Calculating the expected energy not supplied (for example, kilowatt hours were calculated for 1%, 3% and 10% load-loss scenarios and the break-even cost of energy not supplied) for these expected load losses.
Selecting lines for the marking or bird guarding exercise. The selected lines showed a value below R12 per kilowatt-hour (US$1.74 per kilowatt-hour) for the break-even cost of energy not supplied for the 3% load loss case. This value is higher than the R8 per kilowatt-hour (US$1.16 per kilowatt-hour) value normally used to justify projects such as substations. The R12 per kilowatt-hour (US$1.74 per kilowatt-hour) is used for this assessment because the total load is not considered — only the more expensive dip-sensitive portion of the load.
The voltage dip cost the national economy R60,000 (US$8700) per dip, a value determined in 1993 and subsequently confirmed by surveys with customers. Therefore, the 428 voltage dips that the project proposes to eliminate would save the South African economy R25 million (US$3.62 million) per annum.
Selecting the Right Bird Guard
Devices to stop the negative effects of bird streamers have been used with varying degrees of success since the problem was first recognized in the 1920s. To a large extent, the unsuccessful ones did not recognize the resulting air-gap breakdown as the cause of the fault, but instead were designed to prevent the insulator string from becoming polluted by excrement (Fig.1). The successful devices modified bird behavior by keeping them away from high-risk areas on the tower.
Recent experience favors a sturdy, visible device that creates a physical barrier to birds on critical zones on the tower and causes the least interference with maintenance. Universal application across all tower families and voltage ranges and ease of installation also are important factors. Many devices can successfully mitigate bird streamers, but standardization ensures low prices and also permits a comparison of effectiveness among different regions and tower types.
The welded-rod bird guard is the device that has been used most comprehensively (Fig. 2). Because it is the only device that had been widely used within Eskom, it was used by default as the preferred device for transmission after a superficial comparison with other devices.
Meanwhile, further development has started on improved designs and materials other than galvanized mild steel. So far, three companies have produced prototypes, and two other companies have produced concepts. Eskom has investigated methods to prevent interference with live-line work and to minimize the risk of injury to personnel and birds. The utility installed the devices on selected towers to assess universal application, ease of installation and effectiveness in field conditions. The need for standardization soon became apparent, and a multi-disciplinary team established the following criteria for testing and selection:
Cost effectiveness (supply delivery and installation, speed and ease of installation)
Impact on line maintenance
Examples of the tested devices are shown in Fig. 7.
Positioning Bird Guards
The project team was faced with the choice of performing detailed studies for each line. Because of time constraints, however, some general guidelines were developed and later modified in view of the experience gained in KwaZulu-Natal and other regions:
Faulting phase. The KwaZulu-Natal experience showed that the center phase on the 275-kV network faulted the most as a result of streamer activity, and marking the center phases in that province proved effective. However, this was not so on the Warmbad-Witkop 275-kV line in the northern province. Also, it became evident that even marginal areas, such as the landing plates or earth peaks, are exploited by certain species and require bird guards to eliminate faults.
Time-of-day analysis. Although Burham's work provides the best way to identify bird-induced faults, the bird population in Africa is unique in terms of species, numbers and behavior. Hence, fault patterns caused by streamers in Africa differ from those in North America.
Impact on quality of supply. Initially, Eskom planned to fit bird guards on 70 towers of the 400-kV lines on either side of a substation, as this was considered financially justifiable. However, prior to fitting the bird guards, the utility noted that almost all the faults occurred outside this zone. Hence, the original decision had to be revised to accommodate local conditions. The following factors are now used to decide where bird guards should be fitted:
Topographical features. Vultures, in particular, favor high spots for roosting and perching, and transmission towers present a comfortable perch that is higher and safer than most trees. The higher the perch, the farther the bird can glide before expending energy for flight.
Availability of food. This is perhaps the single most important factor. Vultures, for example, often establish semipermanent roosts on power lines close to feeding areas that are hundreds of kilometers away from breeding areas. Other species are influenced by the rainfall patterns and the resulting increase in food.
Breeding sites. With species such as the Cape Griffon (Fig. 8), breeding colonies are recorded and relatively well known. Foraging ranges of adult birds have been estimated and can influence the decision to place bird guards. The recovery of ringed birds also indicates the distance these birds travel. African Whitebacked Vultures breed in loose colonies in trees and on towers. The presence of these nests influences the placement of bird guards.
Line-fault history. The use of historic line-fault data plotted on a geographic information system (GIS) in conjunction with data sets, such as breeding and feeding sites, forms the core information for determining the best areas to fit guards.
Local knowledge. The ultimate success of the marking strategy is influenced by the local knowledge of bird behavior and presence. Crucial to this process is the information provided by local line personnel and landowners.
Design specification. The electrical clearance between steelwork and phase also has a major influence on whether a line will fault as a result of streamers. The two Matimba-Spitskop 400-kV lines illustrate this point. The clearance on line No. 1 is 3.2 m (10.5 ft), while the clearance on line No. 2 is 4.2 m (13.8 ft). All of the bird-induced faults occurred on line No. 1, with no recorded faults on the adjacent No. 2 line.
The only way to ensure the elimination of all bird streamer faults on the transmission system is to fit bird guards to every tower — a practice that, for financial reasons, is not possible.
At the start of the KwaZulu-Natal project, Eskom maintenance personnel fitted the bird guards. Today, private contractors perform this job and face the problem of gaining access to the line. In some cases, they have to take great detours between two consecutive towers. Because circuit outages have to be scheduled to fit the bird guards, contractors are pressured to complete the job in the shortest possible period. Some contractors are able to install 3000 bird guards during a 12-hr shift. At this rate, a typical 400-kV line, such as the Matimba-Witkop, can be completed in three working days. This line required 7800 bird guards to be fitted to 360 of the 549 towers of the line, using 22 teams and 140 people.
Eskom has identified bird streamers as a prominent cause of line faults and has developed solutions to overcome the problem. Financial constraints and variety in the behavior of birds prevent these solutions from working 100% of the time, but system operational experience shows correctly positioned bird guards are up to 80% effective. By identifying and preventing these line faults, Eskom has effectively improved the quality of supply and significantly reduced the number of unknown overhead line faults.
Hein Frederich Vosloo graduated from the University of Pretoria in 1974. He is a member of the Institute of Land Surveyors; is registered with Plato, the professional body for the survey profession; and serves on the executive board of the South African Rights-of-Way Association. He joined Eskom in 1993 as a land survey manager for the transmission group, during which time he was involved in the development of an “invisible tower” as well as a project to piggyback a 22-kV line on a 400-kV line to aid negotiations with landowners. He formed the National Bird Guard group in 1999, for which he coordinates a multi-disciplinary team.
Chris Van Rooyen is project manager for the Endangered Wildlife Trust (EWT). He is the coordinator of the Eskom/EWT strategic partnership, which addresses negative interactions between wildlife and electricity structures countrywide. The efforts of this initiative were recognized in the United States in 1997, when the partnership won the Edison Electrical Institute's Common Goals Award for outstanding electric-utility customer and community relations programs. In December 1999, EPRI invited Van Rooyen to present a paper on his work in the United States. He also co-authored a chapter in the book Birds and Powerlines, sponsored by the Spanish utility RED Electrica.
The Sustainability Index monitors and reports the sustainability status of the organization. It combines 24 weighted indicators into a composite index. These measures include key environmental indicators in addition to specific performance and safety measures. The purpose of the measure is to reflect the overall technical performance of the organization. It balances low-cost production against long-term reliability. The Sustainability Index, through this monitoring and alarm system, ensures the long-term technical operation of Eskom in a sustainable manner and also provides an index to evaluate senior management performance.