The Case Studies are numbered in reverse order, with the newer cases first. We would welcome any contributions, comments or suggestions.
Case 5. The Case of the Teleporting Squirrel.
Once upon a time, in a substation far away, two gentlemen of the engineering persuation were inspecting the installation of a Transgard Systems electric fence. By chance, they were standing inside the double enclosure (the substation perimeter fence and the electric fence) when they heard a frantic scratching. After a careful search of the area, they were forced to conclude that the sounds had to be coming from inside the tightly closed cabinet.
To their amazement, when the doors were opened, an hysterical squirrel flew out and raced around the area searching for an escape route. The engineers thoughtfully turned off the power and removed one of the panels of the electric fence. The squirrel found the opening and headed for the nearest trees at the speed of light. The bewildered engineers looked at each other with the same thought on their minds, "How in the world did the squirrel get inside the cabinet?"
Rising to the challenge, the engineers set out to discover the squirrel's point of entry. A closer examination of the cabinet revealed two interesting clues. First, bits of bone and fur indicated their hysterical friend was not the first squirrel to "teleport" inside the cabinet. Second, there appeared to be a rather large "hole" in the bottom of the cabinet. After a rather lengthy discussion, the engineers concluded that the "hole" was, technically, the squirrel's point of exit, not its point of entry. So, their mission was not yet completed.
They tracked the underground cable from the cabinet, through the substation yard, out to the street. There, they found a suspicious looking round object, often refered to as a "man hole". Deciding if squirrels could do it, engineers could do it too, they removed the man hole cover and climbed down inside.
The first things the intrepid engineers found were numerous bits of bones and fur. Many squirrels had passed this way and many squirrels had died here. Obviously, the engineers were on the right track. But, how did the squirrels get down there?
A quick look to the left...
A quick look to the right...
As intriuging as this discovery was, it still didn't explain where the squirrels actually gained entry.
Climbing out of the man hole, the engineers continued to track the underground cable until they ran into a...
As the engineers slowly gazed upward, their eyes lit upon the answer to the mystery of the "teleporting" squirrels.
Case Study 4. These are photos of flashed-over Kaddas perch-guard (18.5") that was installed between 2 phases on an 8' crossarm of a 34.5kV line. This flashover occurred during a light rain Sunday morning. It is suspected that the insulator skirts were contaminated with dust and became conductive with light wetting from a drizzle, which resulted in some surface tracking and corona discharges. There is only about a 1" air gap on both sides of the triangle from the insulator skirts. These perch-guards were also dirty and unlikely to be fully cleaned in any rain. The result was a phase-to-phase fault with an outage to a few thousand customers!
The utility feels they now need to have 9 inches of separation between the insulator side and the edge of the triangular guards. They need about a 16-18" air gap to achieve 35kV phase-to-phase voltage withstand for switching surges, and now considers the PVC material as conductive due to air contaminants such as dust (and about 7-12" air gap for 15kV). Unfortunately, this greatly reduces the effectiveness of the guards as birds will now be able to perch on the crossarms near the insulators. There is only 29 inches of separation between the wires.
From: Tim Chervick
Swift Creek Consulting
A smaller triangle guard(10-12") can be made for this situation which could be field tested for efficiency. The taller thinner triangle could be installed on the existing mounting bracket that is seen in the photo. Kaddas Inc. would have to change its molding measurements to accommadate a smaller guard but this could be done at the manufacturing plant. However, the clearances now required by the utility would prevent almost no guard installation at all for such a tight fit.(16"-18" air gap requirement). If the utility would relax the air gap new requirement, then a small triangle could be installed and tested. If not, then a snap-fit center pin insulator guard would have to be used as suggested by Rick Harness, EDM.
Since the retrofit products industry is still in its infancy, new product data will have to be collected when the products are installed and monitored under actual field conditions - dirt, moisture, and air-borne contamination. Regulatory requirements, however, require utilities to retrofit as soon as possible and not to wait 5 or 10 years for product testing to occur and so that is the dilemma we face at this time.
We will all have to be patient in this process, in order to reduce outages and electrocutions...
From: Donald Heald
Rural Utilities Service
The minimum clearance that the transmission design manual recommends between structure and phase is 3 inches (for insulators which swing and under extreme wind requirements). This value is based on a minimum dimension for flashover. Similarly for the arrangement with raptor guard, I would not think you need no more than three inches to prevent flashover. The NESC requires 12 inches for minimum clearance to structure. This is a clearance value associated with a 6 psf wind and suspension insulators. In substations for 34.5 kV, 12 inches are required for clearances from a guarded to phase situation. As a result, if they use 12 inches clearance, they will be conservative.
From: Ron Spencer
White River Electric Coopertaive and designer of the Raptor Guard Perching Deterrent.
Obviously the problem here is the air-gap clearance. Most any product, when contaminated with dust and rain, will flashover if there isn't a sufficient air-gap, as we have all seen on so many arrestors, bushings and insulators. The distance of the air-gap is one of the reasons that Raptor Guard Perching Deterrents were designed in the patented X shape. Because of the sharp angle at the bottom of the uprights, there is at least 8" of horizontal clearance between the insulator skirt and the closest point of the PVC uprights, and the Raptor Guard deterring birds from the entire crossarm. A perching deterrent should do just as the name suggests: Deter birds from perching on the crossarm. So in essence, a perching deterrent must fill the entire area between insulators, or as we have all seen, the raptors will perch even closer to the energized conductors. And what if the birds are wet and covered with dust....
From: Siegel, Steve
Although we never had a flashover, a PVC perch-guard was installed with less than 1 inch of clearance between( 2) 24.9 kV insulators. The pvc showed a spider web of burned track marks. We have a standard to supply at least 2-3 inches of air-gap to insulators. Using this spacing we have never had any problems. It is important to note that once the pvc or other perch guard shows any charring, we remove it. The charring is conductive and can provide a conductive path and result in an outage. A 7 inch or greater spacing will allow birds to perch and will not serve its function to prevent perching.
From: Chris van Rooyen
ESKOM, South Africa
So far we have not had any reported flashovers with our bird guards in South Africa . I am attaching one of the designs that we are using. Note that we have not used them on MV structures yet, only on 88kV to 400kV. I can provide the specs if you want it, we are not using PVC. Shout if you need more detail.
From: Dennis Rankin
Rural Utilities Service
Change out the pin insulators to narrower post type insulators. This will place the lower angle portion of the triangle near the bottom of the insulator and further away, since the post insulators require less space. RUS engineers are looking into the approporiate engineering spacing for a perch guard and will post the information later....
From: Bruce Hunt
Modify the perch guard to have a longer creep distance. Remanufacture the guard with polymer skirts.
Case Study 3. The following is a summary of a paper presented at the 1998 IEEE Rural Electric Power Conference (IEEE Catalog No. 98CH36188) by Richard E. Harness, M.S. of Engineering Data Management, Inc. Keywords: Steel poles, Distribution, raptors.
Summary - Steel Distribution Poles - Environmental Implications. Because of certain advantages over wood poles, steel poles are becoming increasingly popular in distribution line construction. However, construction in the traditional manner typically results in reduced phase to ground clearances. These reduced clearances are particularly lethal to raptors. Since raptors are protected by state and federal laws, the burden for preventing electrocutions is placed upon the utility.
Details - Steel poles have certain environmental advantages over traditional wood poles. These advantages include a high strength to weight ratio, resistance to insect, animal, and bird damage, and the fact that steel poles do not require treatment with chemical preservatives. Lastly, steel poles are recyclable. Despite these advantages, steel poles can be extremely lethal to raptors (Birds of prey).
Raptors are often at risk when perched on grounded pole tops, particularly under wet conditions. A golden eagle's tail can extend 10 inches below its perch and wet feathers will sustain an arc at 5 kV. A large raptor with wet feathers can be electrocuted while perching on a center phase pin if its tail feathers contact a steel pole.
Wood has been used as insulation between primary current and ground, with a wood impulse flashover value of approximately 80 kV per foot. The use of wood typically results in a rating in excess of 350 kV BIL between primary conductors mounted on wood pole tops and crossarms, excluding equipment. The Rural Utilities Service (RUS) advocates a minimum of 300 kV BIL on steel tangent poles and deadend steel structures to minimize flashover. To reach the required BIL, insulators must be added to deadends and pole top brackets must be installed to provide additional insulation and air gap. One solution currently employed to increase the center phase separation from the top of the steel pole is the use of a pole top pin mounted on a pultruded solid fiberglass rod. Although the increased distance eliminates the possibility of electrocutions to birds perching on the pin insulator, the modification makes it possible for a raptor to perch directly below the phase wire on the grounded pole top. This new condition can be lethal to birds sufficiently large to bridge the gap between the steel pole top and center phase wire. Therefore, steel structures using extended pole top pins need additional modification to keep large birds off the pole top or away from the center phase.
A typical RUS three-phase C1 pole constructed with 10-foot crossarms provides the 60 inches of clearance recommended by the Raptor Research Foundation. However, when a steel pole is substituted, the phase to ground separation is reduced to 53 inches or less. If all the conductors are supported on a single crossarm, the horizontal clearances can be further reduced to 15 inches, clearly placing small and large birds at risk. The reduced phase to ground clearances on steel poles can be mitigated by wrapping the pole with a band of 40-mil thermoplastic polymer membrane with a pressure sensitive adhesive back. The required clearances can also be obtained by snapping insulating Kaddas Bird Guards over problem phase wires and insulators. Perch guards can be mounted on nonconducting wood or fiberglass crossarms to keep birds away from the pole. Steel crossarms on steel poles should never be used in areas where birds of prey exist.
An alternative to constructing lines in a traditional manner is to frame them in a form that allows safe perching. This can be accomplished by suspending two of the energized conductors under the crossarm instead of supporting them on the crossarm. Suspending the conductors allows birds to perch safely on the crossarm although poletop caps must still be employed to discourage perching on the top of the pole.
Case Study 2. Fire ants have become a serious problem for some utilities in the South. The following is from an article that appeared in the Wall Street Journal on May 11, 1998. Keywords: Insects, fire ants, Distribution, Padmount.
Summary- Fire ants in Texas have invaded padmounted transformers, meters, relays, and control boxes, causing outages and placing maintenance crews at risk. One utility is spending $600,000 a year trying to contain the problem. The state of Texas estimates fire ants costs the state $300 million a year in medical expenses, equipment repair, crop and livestock losses, and pesticide costs.
Details - Houston Lighting & Power, plagued by fire ants for years, will soon begin testing a device developed by Texas Tech University. The device, hopefully, will prevent the ants from building mounds in utility equipment.
So far, commercial chemical solutions haven't worked, pesticides haven't worked, nothing seems to work. Some researchers are looking into other possibilities; hormone-growth regulators, introducing natural enemies, and a fatal fungus.
Last year, Texas lawmakers voted to spend $2.5 million annually for a state-wide ant management plan. In addition, utilities are planning their own efforts. Researchers have not as yet determined why fire ants are attracted to electrical devices, but there is no question they have been responsible for numerous outages and equipment damage. (We have some more information on fire ants in our "Search by Animal" section.)
The research resulting in the device being tested by Houston Lighting & Power is funded by Houston with both Texas Utilities and the Electric Power Research Institute (EPRI). The device consists of a long thin strip of metal with three tiny plates attached. Ants receive a fatal shock when they step on the plates. Once the ant has been shocked, it emits an alarm hormone that attracts more ants. More and more ants from the mound are attracted to the device instead of foraging for food. According to David Visconti, an engineering specialist for Houston Lighting & Power, while some ants are killed, the real benefit of the device is the disruption of the ant's normal foraging and building habits. And this, hopefully, will be enough to prevent mound building in utility equipment and to force the ants to abandon existing mounds.
In addition to being interesting and informative, this particular Case Study is a good example of how solving one problem can accidentally cause another one.
Case Study 1. Submitted by: John M. Bridges, Terrestrial Biologist, Western Area Power Administration, (303) 275-1712. Keywords: Raptor, Distribution, Poles.
Summary- A 12.5 kV distribution line, operated by the Western Area Power Administration, was built in 1984 in southwestern Colorado to provide power to irrigation pumping system associated with the Great Cut Dam. The line was designed to minimize raptor electrocutions. The line was later provided with anti-perching devices to prevent bird defecation from fouling the line insulators.
In the spring of 1985, Western's line crews, working in the area, noted and reported dead birds along the line. Cursory field examinations indicated the birds had died of electrocution. Western, in conjunction with the U.S. Forest Service and the Colorado Division of Wildlife, investigated the line to determine the cause of the electrocutions and develop corrective actions. Field Studies indicated that electrocutions were happening only at certain poles. The species involved included raven, red-tailed hawk, great-horned owl, and golden eagle.
After discovering the apparent cause (details below), the anti-perching devices were removed from approximately 75% of the line. Perches were provided on both structures where heavy use was apparent and to other poles considered to have a high potential for use. The perches were placed on the structures to minimize fouling of the insulators.
Details - The Great Cut-McPhee 12.5 kV distribution line in Montezuma County, Colorado, was constructed to provide power to irrigation pumps for agricultural activities in the Four Corners Region. The single pole distribution structure was designed to reduce the potential for electrocution by spacing the conductors four feet apart and placing them on opposite sides of the pole. To prevent birds from perching on the tops of the pole, defecating on and fouling the insulators, anti-perching devices were placed on the top of each pole.
In the spring of 1985, line crews found carcasses of various raptors during routine maintenance patrols. Based on the location of the carcasses in relation to the poles, electrocution was suspected. While waiting for the results of a necropsy, several more carcasses were found under the line. The carcasses were so decomposed that a cause of death could not be determined with certainty. However, because of the location, electrocution was again suspected. The Colorado Division of Wildlife and the U.S. Forest Service, who manages a portion of the land the line crosses, asked Western to minimize the electrocution potential of the structures.
Western conducted a preliminary survey of the distribution line in August, 1987. During the survey a raven carcass was found within three feet of the base of structure 2/10 (the tenth structure in the second mile of the line). The raven's feet were charred and clenched. Based on this finding, an intensive survey of the line was planned to determine which structures were being used heavily and therefore most likely causing the suspected electrocutions. The intensive survey took place by foot and horseback in October of 1987. Every structure in the first four miles of line was inspected for evidence of use. After structure 3/3 there was no indication that birds were using any structures. Based on the survey and other information, it appeared that the line was electrocuting approximately 12 birds per year.
Next, it was determined that the anti-perching devices were actually contributing to the electrocution potential rather than preventing it. Habitat conditions in the vicinity of the line allowed the 12.5 kV wood poles to be the tallest structures (approximately 30 feet above the ground) in the area. The poles were being used as hunting and feeding perches by raptors along half the length of the line. It appeared that the presence of anti-perching irons on the poles forced the birds down to the uppermost insulator, which sits approximately 24 feet above the ground. This is still approximately 10 feet higher than the surrounding vegetation. Once the bird perches on the insulator, a phase-to-ground electrocution becomes a relatively simple process.
To reduce the potential for this form of electrocution, the anti-perching irons were removed from all the structures up to and including structure 3/3. To reduce the number of flashovers from fouled insulators, perches were provided on those structures where raptor use had been documented or whose ecological or topographical location indicated they would provide good hunting, feeding, or roosting sites. All structures with documented electrocutions were provided with perches. The perches were constructed of treated wood two-by-fours and oriented on the poles to avoid conflicting with the overhead ground wire and other hardware. Surveys were conducted in following years to determine if electrocutions had continued along the line. These and on-going studies indicate that fewer than two birds per year are being electrocuted by the Great Cut-McPhee distribution line. Continuing surveys will determine if additional modifications will be necessary.