As service environments become cleaner, more wildlife and in particular birds return and play a proportionately greater role in what can adversely affect overhead networks. In particular, flashover outages attributed to birds, often included under the category of ‘unexplained outages’, have been a growing problem.
This edited past contribution to INMR by Igor Gutman (Sweden), Evgeny Solomonik (Russia) and Wallace Vosloo (South Africa) examined how birds and power lines co-exist in what is a close but mutually threatening relationship.
Past service experience from countries in Europe has suggested that up to 70-80% of outages on overhead lines can be directly attributed to birds. Although these types of outages are almost always characterized by successful automatic re-closure (i.e. from 94 to 97% of the time), there is still concern. In the past, this was due mainly to old type oil circuit breakers that required maintenance after each cycle of about 15 operations. More recently, the concern is due more to requirements for power quality and availability since many utilities set their own internal rules on average outages permitted each year per 100 km of line.
It is important in any discussion on problems of birds and power lines to distinguish the situation for distribution voltages from that in the case of transmission. The two are entirely different problems and therefore require different solutions. In the case of distribution systems, the central problem is electrocution of birds. Estimates of the numbers of birds killed each year are shocking. For example, one source claimed that 7 million birds die annually only in the European part of Russia. Another thorough investigation found a density of about 15 electrocuted birds for every 10 km of line. Especially dangerous in this regard are lines with vertically installed pin-type insulators since the conductor sits on the insulator, not below as in a suspension string.
What makes this situation the more tragic is that the threat posed to birds by distribution lines has been documented for many years. Furthermore, there is adequate knowledge on how to solve or least largely reduce the problem by insulating conductors close to the tower using protective devices (e.g. insulating covers) of different shape and design.
The principal requirement for such devices, usually made of polymeric materials, is that they have a service life similar to that of the components they are intended to protect, e.g. conductors, insulators, arresters, etc. In this regard, they must be resistant to deterioration from weathering and UV. Experience with such devices has been positive and the only issue is the cost necessary to provide sufficient protection in areas known to have high concentrations of birds.
Unlike the widespread carnage affecting birds along distribution lines, transmission networks face a challenge of an entirely different sort. Here, flashovers of suspension strings by streamer-type excretions are relatively common and can affect lines of up to 500 kV. In these cases, the birds fly away basically unharmed (although no doubt stressed by the effects of the spectacular flashover) while network equipment has to be relied on to quickly restore the line.
The problem of outages on overhead transmission lines due to streamer-type flashovers is caused by typical bird behavior after perching on a tower cross-arm. Before taking-off, they usually empty their bowels and release up to 60 cm3 of excrement mixed with conductive urine at one time and coming out at quite a pressure. This creates a conductive path that bridges the air gap between tower structure and conductors resulting in a flashover either in parallel to or a little apart from the insulator string.
Detailed investigation of streamers using high-speed cameras have been made in HV laboratories as well as in zoos. This research revealed that the continuous part of the streamer can attain lengths up to 2m and move at speeds of 2 to 5 m/s. The angle of the streamer to the cross-arm can even be 60-70° yet still lead to flashover. Resistivity of the dried excrement has been found to vary from 800-2000 Ω∙cm. In its natural state, this figure will vary slightly by species, e.g. storks (220-610 Ω∙cm); eagles (190-770 Ω∙cm); and the chicken which serves as a reference point (260-850 Ω∙cm). It should be taken into account, of course, that the average body temperature of a bird is 41.5°C and that actual resistivity will be less at lower ambient temperatures.
Experiments confirm that, given the above parameters, it is possible to flashover insulator strings up to 500 kV AC and ± 400 kV DC. In Russia, for example,research using excrement collected at the zoo showed that flashover occurs at a resistivity of about 600 Ω∙cm and a dielectric strength of 70 kV/m, i.e. less than the maximum operating voltage for the 110 kV insulators tested. Similarly, it was found in the U.S. that, at a resistivity of about 120 Ω∙cm (using samples of excrement mixed with raw eggs and salt),a 500 kV suspension cap & pin string flashed over at 320 kV, i.e. about 1.1 times nominal operating voltage. It is worth noting that during these experiments the streamer could be as far as 70 cm from the string yet still affect it. At Stellenbosch University in South Africa,a special bird excretion flashover simulator was developed and experiments performed at a resistivity of about 70 Ω∙cm. This figure was selected after analysis of samples from two species of eagle and the mixture consisted of cellulose, salt and tap water. Results were then used to investigate the physics behind bird streamer flashover.
Based on this data, it certainly seems clear that, from an engineering point of view, the problem of bird streamer flashover on transmission lines has been clarified and confirmed. The issue then becomes how best to protect a line from birds perching in the vicinity of insulator strings and periodically flashing them over. This requires asking, why are the birds there at all?
Streamer Flashover: Bird Perspective
There is an obvious reason why transmission towers attract large raptor species such as owls, vultures and eagles, i.e. the usual culprits behind streamer type outages. These structures provide ideal platforms for hunting, roosting and nesting. Indeed, the cross-arms of a typical 110 kV tower are located at the optimal height for buzzards to hunt prey along the right-of-way.
Moreover, once birds establish a roosting site, they continue to use it year after year. In Finland, for example, two lines of identical design and sharing the same corridor but constructed at different times had different operating histories. The older line suffered from unknown outages while the newer one did not. The reason was that the first line was already occupied by most of the large local birds that never moved to the second line. A similar example from South Africa showed that only half a line suffered from unknown type outages while the other half did not. In this case, the reason was that the problematic half was under control of a pair of eagles that settled near its end, while no other raptors watched over the other half.
This demonstrates that it can prove especially worthwhile to study the behavior of birds occupying any portion of a transmission line. In most cases, birds come to rest and sleep towards dusk and take their place on the cross-arm or in the nest. Sometimes, they empty their bowels for the first time right after landing – thereby providing the first peak of outages around 10 to 11 PM. The birds then empty their bowels a second time in the early morning (4 to 6 AM), most probably to reduce body weight before take-off. They fly away leaving behind yet another potential outage to be dealt with.
In summary, transmission towers offer a natural place for birds to rest and hunt. The only issues left to resolve are:
How to be certain that the flashovers are indeed bird-induced and, if so, what to do about it?
Clues to Look For
As discussed, the peak number of unexplained outages (probably from bird-induced flashovers) take place in the early morning. This normally occurs in clean or slightly polluted areas during conditions of high humidity and when insulators are wetted by morning dew. It is therefore not easy to pinpoint the exact cause of the flashover. For example, it could well be a classical pollution flashover or one related to a problem with E-field.
Recommendations to ascertain whether or not birds are involved in the outage include:
1. Inspect the problematic line by helicopter, checking for traces of bird contamination on the surfaces of insulators or on the cross-arms. Nests can also be observed on particular towers;
2, Contact local ornithologists for any maps that show density of different species in the area. Local biology students can be of great help in the field to observe bird activity near any affected line;
3. Establish presence of any nearby agricultural land, fish farms or sugarcane fields;
4. Check seasonal pattern of outages and relate this with natural departure/arrival of migratory birds and time of birth of their juvenile In the European part of Russia, for example, there is typically a sharp increase in outages during August, related mostly to juveniles being taught how to fly and feed;
5. Check daily pattern of outages and relate this with typical morning peaks;
6. Verify percentage of successful auto re-closures (typically around 95% if birds are involved);
7. Monitor the area below the line for dead birds or burnt feathers;
8. Check the appearance of flashed insulator strings. In the case of bird-induced outages, there are usually traces of the arc root on the conductor (at 1 to 1.5 m from the insulator), on the cross-arm above the insulator or at the top one or two cap & pin insulators.
Solving Problem in ’Bird Friendly’ Ways
A number of guidelines/standards exist with recommendations in these situations, including the IEEE Guide for Reducing Bird-Related Outages, Eskom Transmission Bird Perch Guidelines, Russian Regulations on Design and Reconstruction of Electrical Installations (for lines and substations). Still, engineering bodies continue to come under pressure. For example, the IEEE Guide noted “Disturbed by the continuing large numbers of birds electrocuted and colliding with power lines, the U.S. Fish and Wildlife Service has begun to step up enforcement of the Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act. All migratory birds are protected under the Migratory Bird Treaty Act. In a landmark case, a utility was sentenced to three years probation for electrocuting 17 eagles and hawks near Rangely, Colorado. The utility ultimately pleaded guilty to six violations of the Migratory Bird Treaty Act and the Eagle Protection Act. Under the settlement it agreed to pay $100,000 in fines and restitution to the U.S. Fish and Wildlife Service.” In Russia, a scientific workshop “Problems of bird electrocution and safety on overhead power lines of distribution voltage: modern scientific and practice experience” brought together ornithologists and engineers as well as representatives of the country’s environmental prosecutor’s office.
Preventing Outages Due to Roosting
The main device to prevent birds from roosting on a tower cross-arm is known as a perch guard, also known as bird discourager. Such perch management has been successfully applied in the U.S., Russia, Sweden, China, Germany and other countries to prevent problems due to bird streamers. Experience on one line in South Africa, for example, demonstrated how effective these can be in eliminating bird induced flashovers.
Perch guards are available on the market but must be dimensioned properly for each species – a factor that is extremely important for birds with long legs. To reduce cost and maximize effectiveness of this countermeasure, it is best to ensure collaboration between engineers and ornithologists to define the most suitable locations for these on each type of tower. Care should also be taken to cover the entire area potentially occupied by birds since they will quickly take over any open space. Many utilities still use mostly metallic guards made of wire or rods, which could harm birds while landing. This argues in favor of flexible guards made of plastic. Again, as for distribution lines, these must be weather and UV-resistant, with some periodic maintenance cycle to monitor for deterioration.
There have also been cases reported where contamination to insulators was due to excretions apart from streamers, although these are typically less a risk due to their lower conductivity. The solution in this case is to use a top insulator of larger diameter or install a protective plastic shield over it.
Preventing Outages Due to Nesting
Birds usually try to build their nests on the very same towers chosen the year before. For example, there are anecdotes from a utility that removed the nest of a white stork from the first tower entering a substation yet each time the stork returned to this same tower. After a ‘battle’ lasting 3 years, the substation was ‘bombarded’ by storks bringing branches and metal wire and the nest was finally left on the first tower.
Power supply utilities worldwide have reported on how many nests they have had to take down before the birds return. This is typically not an easy task since the nest of a stork can measure 2m in diameter and weight up to 50 kg. To remove such a construction might take a few hours. In fact, experience has demonstrated that, if anything, removing nests is not the ideal approach. This is because, during initial construction and subsequent re-construction, birds use branches and metallic wire, which can either fall causing an outage or which can expand downward from the nest, reducing the air gap and causing even more trouble. Therefore, if birds have already built their nest, it is better not to touch it before the birds vacate. Instead, the structure should, if anything, be protected by fence to keep out e.g. snakes, cats, which will be attracted to the nest.
Moreover, once the birds have raised their offspring and left the site, the nests should be carefully re-located. In the long run, this would be the most successful and ‘bird-friendly’ approach, namely providing the birds with a nearby nesting alternative – usually some kind of pole with an installed platform and located near the existing nest site. Such alternative platforms should be built alongside the line and should be somewhat higher than the original nests to make them an attractive alternative. Then, once the nest has been re-located, the original tower should be equipped with bird guards to discourage future re-nesting. Again, to establish the best construction of alternative platforms, there should be close co-operation between line engineers and ornithologists.
Power supply companies should do whatever possible to protect birds from overhead lines at distribution voltages and to protect overhead lines from birds at transmission voltages. Fortunately, there is a plenty of experience and proposals on which to base such actions. However, these are still not well structured by specific species, which in many cases may be a key issue in success or failure. To best solve the problem, there must be close collaboration between power engineers and ornithologists.