By Joe C. Pohlman
Changes in demands, and globally to way business is being done, mean the construction of new overhead lines will become a rarity. In order to ensure the effective delivery of power continues, a proactive approach will be needed for line maintenance.
In the time period between the early 1950s and the late 1970s, demand for electricity in the United States doubled every ten years. Utility decision making was in the proactive mode. System studies, made routinely, would define potential facility needs to maintain a proper balance between growth and stability. These needs would be investigated by research and development, engineering, operations, real estate and customer service departments to select the best combination of voltage level, capacity, configuration and location in terms of expected installed costs. Top management would authorize projects on the basis of their cost effectiveness and available funding. Simply stated, the major problem for utilities during this period was being able to build lines fast enough.
During the late 1970s, the general public started developing considerable concern about the proliferation of transmission lines where they lived, worked and travelled.
Today, demand for electricity is still growing, albeit at a much lower rate (2 to 3 per cent annually). It is not clear at this point in time what requirements might be to handle the wheeling loads expected to result from the recent deregulation of the (United States') power delivery market. One thing is sure: a significant portion of existing lines are old and deteriorated and that deterioration will continue. It is obvious the challenges of the short-term future can only be met if utilities implement effective management of their existing overhead lines in order to get the most out of them. To make sound decisions on what can be done, it is first necessary to know the present capability of each line. This is the essential starting point.
For each existing overhead line, there should be three pertinent data banks of information: physical description of the line configuration within its right-of-way conditions, information on the service experience of the line since its original energization and the results of all inspections made on the line. This data would be properly applied within an assessment discipline that would define the present capability of each line where capability is the product of capacity times availability. Once the present capability of that line is known, this information would be fed into the management decision making process. That's where the effectiveness of existing initiatives would be considered while trying to balance the quality of service against the costs of safety, public concern, regulatory and environmental requirements. This effort will produce a list of viable initiatives in descending order of cost effectiveness. These initiatives would aim to maintain capability (preventive maintenance), restore capability (refurbishment) and increase capability (increase capacity and availability).
Utilities which have attempted to assess the capability of overhead lines as a prelude to undertaking an asset management plan have discovered the difficulty of implementation is greatly influenced by the quality and quantity of historical information they have in their files.
Assessment Procedures
There are four things that need to be accomplished in order to assess the present capability of an
existing line.
Determine the capability of the line as it was built on its right-of-way (r-o-w) when the components were new.
Make a detailed inspection and assessment of the current condition of the line components.
Determine the present capability of the line reflecting the current condition of all line components. It will be expressed in terms of the capacity of the line at its weakest link along with an indication of the line's probability of survival over a specified period of time of interest. This period of time can be the desired remaining life of the line, the time until the next inspection, maintenance or any other length that is considered significant by management. As such, it becomes a factor in the decision making matrix for prioritizing lines for inspection and maintenance.
Determine the magnitude of risk associated with progressive line damage after the initial failure of a component. The risk after the initial failure of a component is line specific and the same for all components. For instance, if the probability of a mechanical separation of a porcelain insulator disc is determined for a particular line, it makes a difference whether the disc is in a suspension string or a dead-end string. In a suspension string, the event could lead to a dropped phase which may not result in progressive line damage. On the other hand, a failed disc in a dead-end string would also drop the phase, but it would also release full line tension which could be sufficient to set a line cascade in motion. Risk is equated to the cost of restoration and the lost revenues during the restoration. As such, it becomes a useful factor in the decision making process for prioritizing line upgrading.
Capability When New
Air gap clearances are specified in industry and/or company standards. Grounding assemblies are
made up of wires, rods and connectors which are manufactured to published specifications by the
suppliers. It is straightforward to select proper parts and arrange them into a predictable safety
subsystem.
Likewise, insulators, conductors, hardware and OHGWs (making up the electrical subsystem including the safety subsystem) are produced to manufacturing standards in families of designs with published electrical and mechanical properties. Again, it is straightforward to select the properly rated parts and assemble them into a predictable electrical subsystem.
The mechanical subsystem, however, is a different story. Conventional design practice has followed the sequence of :
The line configuration would then be adjusted to fit the topographical characteristics of the r-o-w.
Conventional deterministic design has many shortcomings, inasmuch as it does not provide a means to deal with the uncertainties of loading conditions, such as loads applied at angles instead of V/T/L and more extreme loads in micro environmental locations. It doesn't provide a means for predicting the performance of the line or a way to detect weak links in the line system. Nor does it provide a framework for decision making.
The rapid advances in the capabilities of computers permitted the introduction of three dimensional analytical design programs (see figure 1). Such programs overcome the shortcomings of past practice. Data on design criteria, installation loads, strength of components, clearances and corridor terrain are all fed into a computer to create a 3D model of the proposed line installed on its intended r-o-w. Once the line model is complete, questions can be posed to determine the impact of changes in the configuration of the line to meet local conditions. As well, the impact of those changes on the total installed cost of the line can be gauged.
Typical questions concern the impact of raising or lowering line height to maintain phase-to-ground safety clearances and adding angles in the line to avoid ground obstructions. Other questions cover adjusting the span in search of the optimum and increasing design loads to increase reliability levels.
The computer responds to each request with a revised line design. Such models were first used as a means to optimize the design of a final line configuration within its corridor. By preselecting the component desired to the weak link, and adjusting the strength level of all other components in relationship to that weak link, the line can be designed to meet a target reliability level.
It was soon discovered the process could be reversed just as effectively. The as-built line and terrain details are entered into the computer to create an accurate model of the line and corridor reflecting the properties of the components when new. After that loading conditions, such as different magnitudes of wind velocity at different angular directions, accreted ice or wind and ice, are applied to the model in increasing increments until the first component is loaded up to its capacity. That is the critical load for that component.
The probability of not experiencing that critical load at that location is equal to the probability of survival for that component.
The probability of its survival is an indication of the probability of survival of the total system under that critical loading event.
The process is repeated until all loading events of interest have been tried and the weakest link identified.
Probability of system survival is equal to the least probability of survival of all components or, in other words, probability of survival of the weakest link.
The quality of information available for assessing the capability of a line, based solely on deterministic design as opposed to 3D analytical analysis can be shown in a brief example.
The loading agenda (see figure 2) used in deterministic design of the tangent and light angle structure for 20-year-old double circuit 345 kv line are shown.
The design of different members (see figure 3) within the structure were under the control of different loading cases. There is no indication about the relative capability of the different members or what their weak link is.
The results of a 3D analysis of the structure (see figure 4) show the legs and tension cord of the conductor arm are under the control of 115 and 110 year return period loading conditions, the tension chord of the OHGW arm is limited to critical load conditions that have a 35 year return period. This is much shorter than the "50 year storm" conditions the original designer thought he was using. In addition, it was discovered the foundation capability was under the control of a quartering wind condition with a return period of only 25 years in that location. This made the foundation the weakest link in the line system. This is a condition which most utilities would consider undesirable.
Impact of Micro Environmental Conditions
It is recognized that micro environmental conditions can intensify the magnitudes of wind, ice and
wind loads over what would be expected based on global meteorological information for the
general area.
As an example, consider a transmission loop consisting of a double circuit line on steel poles lying east of the Rocky Mountains between Fort. Collins, Colorado and the Wyoming border. The loop is approximately 45 miles across. A person standing in the middle of the loop and looking in all directions would see little significant difference in the topographical features. The land is gently undulating, high altitude ranch country. Nevertheless, the line had a history of broken Stockbridge dampers, frequent unexplained trip outs, and in March of 1990, it experienced three broken cross arms from, apparently, extreme vertical load. The line dividing heavy and medium loading zones in the NESC passes north to south through the loop. The line had been designed for one inch of radial glaze ice, which was expected to be very conservative.
A comprehensive investigation was launched to identify micro environmental loading events.
In one zone, there was a highly unique situation where moisture from the Gulf of Mexico could come in contact with cold drafts down from the north following the front of the Rocky Mountains causing heavy, sticky wet snows. Accumulation of these wet snows on the conductors exceeded the vertical loads from one-inch radial ice. The area of the wet snow occurrences was too small to involve weather stations at local airports or military installations in the area, so they were not reported. Such wet snow events have a probability of occurrence of about 5 to 6 years at that location.
Another zone was found to be visited with hoar frost a few times every season. Hoar frost increases the diameter and weight of a conductor. Wind energy input into the conductor increases linearly with diameter. The weight of the hoar frost slows down the mode of vibration of the conductor. These effects combined to drive the Stockbridge dampers beyond the range of expected operating conditions, resulting in their failure.
Findings in yet another zone showed moisture bearing winds from the South had to pass up a very gentle slope. This caused rime ice on the windward side of the conductors. The horizontal pennant of ice effected the drag of a conductor setting it into horizontal excursions. These excursions were sufficient to cause conductors to move laterally, slapping together and causing trip outs. The event was predicted to happen several times each season.
There are skilled consultants available who are able to make such micro meteorological studies. There are also university and scientific groups working in the subject who have published interesting findings from studies they have made.
Once the critical loading event is identified and its probability of occurrence determined in terms of return period, this combination can be used to establish a rationale for decision making.
When only two possibilities exist, such as failure versus survival, the probability of occurrence for one is just equal to one minus the probability of the other.
The annual probability for the failure of a component is equal to one divided by the likelihood of occurrence of the critical loading event in terms of its return period. One minus this probability of occurrence is equal to the probability of that load not occurring at that point each year. Like a chain, the probability of survival of the line can be no greater than the probability of survival of the weakest link in the system. The probability of survival of the weakest link, therefore, is a convenient way to compare the capabilities of existing lines.
Thus: Pf = 1/Pocc
Ps = 1 - Pf
Ps = 1 - 1/Pocc
In the new condition, it is found that the weak link has a limit load with Pocc of one in 50 years:
Ps = 1 - 1/50 = 1 - .02 = 98% percent per annum
After aging, if the least return period limit load is once in 25 years:
Ps = 1 - .04 = 96 percent per annum
The probability of survival of a line over a period of n years is:
Ps(over period of n years) = Ps n(annl)
| Continuous Years | Ps for Pocc = 1/50 | Ps for Pocc = 1/25 |
|---|---|---|
| 1 | 98% | 96% |
| 10 | 82% | 66% |
| 25 | 60% | 36% |
| 40 | 45% | 20% | 50 | 36% | 13% |
This comparison shows that if the weak link in an as-built line is controlled by a critical load that occurs once in 50 years, the weak link and, hence the line, has a 36 per cent chance of surviving 50 years and a 60 per cent chance of surviving 25 years. The chances for survival with a critical load that occurs once in 25 years are 36 per cent and 13 per cent respectively.
There are several available 3D analytical programs for determining the limit load capabilities of components as described above. One of the most popular is PLS/CADD, a product of a cooperative effort between Electricite de France and Dr. Alain Peyrot of the University of Wisconsin.
Present Condition of Line Components
In order to assess the present condition of a line, it is necessary to know the present condition of
the individual components.
Many utilities find the amount of information in their service experience file is too sparse and indefinite to be much help in this assessment. Instead, they depend on inspections to detect the symptoms of deterioration of the various components and use on-the-spot judgement to make an appraisal of a present condition. If the condition appears serious, selective samples may be removed and lab tests performed.
An inspection is a single snapshot in time. Component assessments based on the results of a single inspection can be used to make decisions for corrective or preventive maintenance; but, information on the trend of deterioration of components is the most useful upon which to base predictive maintenance. Trend information can be developed in two ways. One is to repeat the inspection process and observe and record the relative deterioration over a period of time. Care must be exercised to make each inspection a literal repeat of previous inspections. The other way is to use the most effective inspection techniques available and compare the resulting assessments of present capabilities to new component capabilities.
Inspection
The complexity of an overhead transmission line creates a challenge for selecting and
implementing effective inspection procedures. The chosen procedures should detect and appraise
the symptoms of deterioration in order to assess the present condition of line components for
decision making.
The components of a transmission line are composed of different materials with different characteristics. These components are subjected to a variety of site conditions and system conditions, as well as a variety of extraordinary contingencies. The combinations of site system and contingency loads set in motion different mechanisms of deterioration depending on the response characteristics of the component. As deterioration occurs, it creates distinctive evidence of symptoms of potential problems. The purpose of inspection is to detect those symptoms and interpret their impact on the condition of the component so that an assessment can be made of any reduction in safety, electrical or mechanical reliability of the line.
As an example, if a porcelain insulator disc is subjected to contamination, it loses its leakage resistance resulting in the lowering of the electrical performance level of the line. If its ball and socket hardware parts are seriously corroded or worn down as a result of conductor motion, its mechanical strength is reduced lowering the mechanical performance level of the line. If the porcelain is punctured as a result of impact loading from dropped ice or galloping conductors, the safety level (for workers) is lowered.
In March of 1992, a workshop on assessment and inspection methods was held in Minneapolis, Minnesota with a number of objectives: Review what problems are experienced with transmission lines. Identify what symptoms these problems display before they actually occur. Identify diagnostic and sensing technologies currently available to detect those symptoms.
The utility representatives in attendance developed a list of problems in descending order of seriousness. Structure and foundation problems, including strength, remaining life of wood poles, cross arms and braces, were mentioned. As well, guy and anchors, overall structural integrity, corrosion and concrete spalling were included.
Several r-o-w problems were also identified. They include growth, encroachment, fault location, ground resistance, core corrosion, strand breakage, splices, hot splices, sag/clearance and tension.
Problems with insulators were also listed. They include electrical strength (puncture), contamination, mechanical strength and hardware.
While this is an interesting synthesis of the industry's prioritization of problems, it should be kept in mind that the significance of problems is line and system specific.
Inspection Methods In Current Use
There are many different inspection procedures available for use by utilities at the present time.
The most commonly used method is a trained lineman or inspector. The inspector relies on his senses of sight, feel and hearing. He will look at, and wherever possible, feel the component. It is obvious the quantity and quality of information gathered increases with his proximity to the component and the length of time he observes the symptom increase. He may also strike the component and listen to the sound it gives off.
Methods differ in how the inspector is transported to the component. There are several basic variations which include air patrol, air comprehensive, air hands-on, ground patrol, ground comprehensive and climbing.
An air patrol consists of a slow (60 to 100 knots) fly-by of the line in a winged aircraft or a helicopter. The inspector observes the line as it passes by and makes notes or red lines profile drawings. At that distance and within the short observing time, he can detect r-o-w encroachments, reduced air gap clearances and obvious broken components such as insulators, conductors, structural members and hardware. If something catches his eye, and he is in a helicopter, he can request the pilot to turn around and make another pass, go to the other side or hover until he finishes his observation. Most utilities make some use of slow fly-bys.
In a comprehensive air inspection, on the other hand, the inspector is usually located on a platform mounted on the side of the helicopter. He wears a hot line suit. The helicopter flies from structure to structure. At each location, the helicopter places the inspector as close as possible to the line. Using gyroscopically controlled binoculars, the inspector follows a discipline of looking individually at each different element located at that structure. If he detects an interesting symptom, he will ask the pilot to move him one way or the other so he can observe the symptom from two or more vantage points. The inspector gives a running dialogue of what he sees. Another crew member sits inside the cab and records the inspectors observations. If the symptom is particularly significant, the inspector will take a colored photograph of it and the location will be recorded. A photograph is particularly helpful since several people can look at it and form their own subjective opinions. The line is inspected at each structure location, on both sides and above. A shortcoming of the comprehensive air inspection is it doesn't provide an opportunity to inspect the parts of the grounding system close to the earth or the foundations. And, there are line locations where the use of a helicopter would be prohibited.
Air comprehensive inspections can be accomplished using a bucket truck instead of a helicopter, but a bucket truck is considerably less mobile than a helicopter. It's also much slower and more expensive. As well, there are some lines that cannot be reached by truck.
Air comprehensive inspection can be advanced to a hands-on inspection merely by bonding the helicopter to the energized line. There are many problems, like broken strands under a clamp, spacer or damper, that can't be detected visually. Once the bonding is accomplished, the inspector can inspect the conductor with ease.
Ground patrols are similar in purpose to air fly-bys. The difference is the inspector is on the ground and can inspect the r-o-w, the grounding assembly and the structure from the ground up, usually with stabilized binoculars. Unfortunately, he is unable to see the top of structures, the conductor or OHGW assemblies. He is further away from many critical components. The process is slow but access is usually not a problem.
Comprehensive ground inspections are frequently used for the assessment of a wood pole line.
With this type of inspection, the wood pole is inspected visually from the ground up, and it is sounded by hammer blows which are interpreted by a skilled technician. Borings are taken to check for suspected decay pockets, and the butt will be excavated at the ground line. Any rotted wood is stripped off and remaining good wood will be sized and appraised.
A big advantage of this type of inspection is that an on-the-spot decision can be made on preventive maintenance initiatives. A comprehensive report is generated for future reference.
Comprehensive ground line inspections have the same limitations, so far as inspecting details at the top of the structure and line, as ground line patrols. Guidelines for making such inspections can be found in IEEE Guidelines P 1218. They are most often implemented by contractors who are specialists in wood.
Climbing is something of a compromise of all of the above methods. Climbing places the inspector close to the symptoms to be detected; but once aloft, the inspector does not have the freedom of easy movement to view interesting evidence from more than one vantage point. He can inspect line details only on the structure side. He cannot view any evidence on the outside of the line components. Climbing is one of the most dangerous activities a lineman does. His attention is split between maintaining his own safety and making the desired observations. He is encumbered making it difficult to record his observations in any way except a hands-free two-way radio loop to the ground. It is a labor intensive, slow process. Safety can often be improved and the process speeded up by hedgehopping the inspector from structure to structure using an insulated sling from a helicopter.
Appraisal of the Effectiveness of Present Methods
All of the above methods rely on the personal observation by an inspector. It is generally believed
that all visual techniques have many limitations. Personal observations are subjective and
influenced by the proximity of the inspector and the number of different points from which he can
view the same symptom. The inspector may not be able to feel or sound the part so he must
depend solely on sight. Periodic inspections are generally not made on the same parts or
components, and are often made by different inspectors.
There are many single purpose sensor technologies available to supplement visual inspection. Some of the more promising of these are infrared, corona, electromagnetic, sonics and ultrasonics, x-ray, rf and tvi and field strength.
There has been a limited amount of experience with each of these.
Special Purpose Methods
There has been use of statistical sampling and destructive testing approaches to determine the
present condition of an aged line. These techniques have been used primarily on lines that can be
taken out of service for some length of time and the purpose of the sampling and testing has been
to check planned approaches to extend life or refurbish the line.
There have been many recent announcements about the marriage of space age technologies such satellite mapping and surveillance; laser distance detection, computer management of information system, pilotless helicopters and drones to accomplish an impressive array of highly specialized activities. Whereas it appears that such combinations of technologies can, in fact, accomplish some of the claimed purposes, it remains a moot point until proof-of-principle demonstrations of such technologies can be staged. Proponents of such technologies, under the present tight restraint of development monies, are seeking partners who have the practical problems that the technology can solve as well as the real world information and experience to establish performance criteria for the finished package of capabilities.
While some of these approaches are interesting and promising, there are no off-the-shelf advanced technologies ready for immediate application. Likewise, unless very large sums of money can be found by the proponents of these systems to stage controlled demonstrations in the field, it is not likely that such advanced systems will be available soon.
Present Capability
Regardless of how the assessment is made, the present capabilities of the components are
substituted for the original conditions in the 3D line model and the process, described in detail
above, is repeated. Since it is to be expected that deterioration of different components occurs at
different rates, it is not unusual for the weak link to change from one component to another within
the line section under study.
Extent of Line Damage After Failure of Component
Analytical technologies are also available for determining the extent of progressive mechanical
damage to the line system after the initial failure of a component.
In one case, the factor were a broken top conductor, a broken shield wire and a broken subcontractor. Under a non symmetrical discontinuity, the line system will distort in such a way as to redistribute the unbalanced forces depending on the response characteristics of the structure/foundation element. Routines are available to carry these unbalanced transient forces down the line system and predict whether the downstream elements have the capability to withstand them or not. In this way it can be determined how much progressive damage the line will actually experience. Loss of restraint can set off extensive and costly cascades.
Electrically, other components, such as the OHGW system that can fall into conductors and create line-to-ground faults or loose conductors or broken grounds that can set up dangerous electrified conditions, might be the contingency of interest.
The total risk associated with a progressive failure is equal to the lost revenues from the outage plus the costs of restoration. The probability of incurring the risk, therefore, can be equated to the probability of occurrence of the contingency loading event.
Proper determination of the risk expected after initial component failure is line specific.
The process of obtaining a permit to build new lines will become even more difficult in the future. Utilities will no longer have the alternative to build a new line to address a pending problem of quality of service or system stability.
The only practical solution to this problem is to implement a comprehensive program of managing the overhead lines to maximize their capability. The program should be system specific but is a monumental undertaking for a single company at this point of time.
Analytical tools are readily available to determine the present level of capability of a line based on the present condition of its components, but most utilities do not have the required input data on the physical details of the facilities, the service experience history or the results from periodic inspections.
Most utilities have substantial visual inspection programs but the value of the information effective techniques of use have not been clearly defined.
Joe Pohlman is a consulting engineer.