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__2 Transmission Line Siting
__3 Sequence of Line Construction
__4 Conductor Pulling Plan
__5 Conductor Stringing Methods: Slack or Layout Method • Tension Stringing
__6 Equipment Setup
__8 Overhead Transmission Line Maintenance: Introduction • Overhead Transmission Line Inspections • Transmission Line Inspection Software • Transmission Line Fault Investigations and Corrective Action(s)
__9 Transmission Line Work: Live Line Work • Worksite Grounding • Vegetation Management
__10 Data/Information Management and Analysis
__11 Emergency Restoration of Transmission Structures
Electric transmission lines are constructed to provide a path for electricity to flow from a generation source to a specific service area or to intertie with the transmission grid. Transmission of electricity is categorized by voltage level and can range from 69 through 765 kV and higher. The majority of new transmission line construction is being done at higher voltages commonly referred to as extra high voltage (EHV) or voltages greater than 230 kV. The higher voltages reduce power losses inherent in long-distance transmission of electricity, thus allowing the system to operate more efficiently. EHV line construction requires more specific construction procedures and constructor expertise than construction of lower voltage lines.
Transmission lines are engineered and designed to meet National Electric Safety Code Standards and the criteria of regulating authorities such as North American Electric Reliability Council (NERC) overseen by Federal Energy Regulatory Commission (FERC). Design and construction must also com ply with regulations, rules, and laws set forth by federal, state, and local authorities. The engineering and design must also consider constructability of the line design and future maintenance requirements.
Additional information regarding transmission line construction can be found in IEEE Standards 524-2003 and 1441-2004.
__2 Transmission Line Siting
Historically, transmission line easements or rights-of-way (ROW) followed as direct a route as possible from the generation source to the service territory or grid interconnection. Today, consideration also has to be given to the proximity of populated areas, archeological, geological, and environmental concerns. EHV lines ROW require wider easements not only to facilitate construction and future maintenance but to insure that NESC rules are not violated. These requirements must be met or mitigated before a permit or license to construct and operate will generally be issued by the regulatory authorities.
Above: Fgr.1 Stub angle braced and set in foundation pier.
Above: Fgr.2 Tower assembled from layback to bridge on ground and lifted onto the tower body and legs for final assembly.
__3 Sequence of Line Construction
Prior to construction, decisions as to structure types, foundation requirements, conductor size and type, insulation, and line hardware have all been determined. The ROW has been surveyed and the transmission centerline, structure locations, and edge of ROW have been marked. Next, the ROW is made ready for construction activities by constructing access roads, clearing obstacles, and removing vegetation that may hinder construction efforts. Once this is accomplished, line construction can begin.
A typical line construction sequence is as follows:
• Foundations installed as applicable
• Structures and hardware delivered to the designated construction sites
• Structure assembly and erection
• Hardware and insulators assembled and installed
• Conductor travelers installed
• Install pulling lines
• Pull conductor
• Sag and clip conductors
• Install vibration dampers or spacer dampers as applicable Factors to consider during the construction process include the following:
• Foundations are constructed according to engineered design but should be checked for correctness regarding location, type, orientation, and the bisector verified prior to structure erection.
FGR. 1 illustrates a stub angle braced and set in a foundation pier.
• Structure assembly should be as complete as possible prior to the structure erection, which reduces the amount of aerial work required prior to conductor stringing. This may include all hardware, insulator assemblies, and stringing blocks. Attention must be paid to total weight of the assembled structure so that the lifting capacity of the equipment used in the erection process is not exceeded. An example of this is illustrated in FGR. 2 where the tower from the layback to the bridge was assembled on the ground, lifted, and set to the tower body and legs.
• Consideration needs to be given to how damaged and incorrectly fabricated steel members and gussets are going to be addressed. Incorrect bolt hole placement or patterns can be corrected in the field using a "fill and drill" process, as long as the new hole spacing and edge distance con forms to the engineering design or applicable structural steel standard(s) for bolted connections.
Bent lattice needs to be assessed on an individual basis to determine if a field repair is possible.
FGR. 3 shows a gusset with incorrectly placed holes.
• Aluminum conductors with steel reinforcement (ACSR conductors) have been used extensively in the past, but conductors with higher operating temperatures and current carrying capabilities are becoming more prevalent with today's EHV transmission line construction. The design of these types of conductor employs aluminum stranding that is softer than conventional ACSR and requires considerably more care in handling during installation. Careful planning and a thorough understanding of stringing procedures are needed to prevent damage to the conductor during stringing operations.
• Conductor reels are supplied by the conductor manufacturer and can be either the nonreturnable (NR) wood or the returnable metal (RM, RMT) type. Reel size is dependent on type and size of conductor specified by the engineering design or conductor manufacturer. The most commonly used reels are the RMT 96.60 size often referred to as 8 ft reels. Wooden reels are not recommended for use in transmission stringing.
• A worksite grounding plan is necessary to provide a safe electrical environment for all on-site personnel. This is accomplished by evaluating all electrical hazards and soil resistivity within the worksites with respect to the placement of equipment and personnel. Typically, the greatest electrical hazard faced by transmission line construction workers is induced current when working in congested transmission line corridors or when working near energized distribution or transmission lines.
Above: Fgr.3 Gusset with incorrect hole placement.
__4 Conductor Pulling Plan
A conductor pulling plan must be developed for the entire line prior to the start of conductor stringing.
The conductor pulling plan is dependent on a variety of factors:
• Conductor size and lengths on reels
• Span lengths
• Structure heights
• Ground clearance during stringing operations
• Obstructions, such as railroad crossings, road crossings, other line crossings
• Line angles (individual and cumulative)
• Allowable stringing tensions
• Terrain and elevation changes
• Dead-end structure locations
• Equipment setup locations
• Midline splice location access
• Capacities of pulling and tensioning equipment
With all of these factors taken into consideration, it’s desirable to make the pull section as long as allowable but generally not more than the length of the wire on two conductor reels. Also a pulling plan should incorporate dead-end structures whenever possible as the termination of a pull section.
__5 Conductor Stringing Methods
__5.1 Slack or Layout Method
With this stringing method, the conductor is dragged along the ground by means of a pulling vehicle, or the reel is carried along the line on a vehicle from which the conductor is laid on the ground. Usually, a braking device is provided to control conductor payout. When the conductor is dragged past a supporting structure, pulling is stopped and the conductor is placed in travelers attached to the structure before proceeding to the next structure. This method is only applicable to the construction of new lines and for cases in which the conductor surface condition is not critical. This method is not usually economical in urban locations where hazards exist from traffic or where there is danger of contact with energized circuits.
__5.2 Tension Stringing
Tension stringing is preferred for all transmission conductor installations. Using this method, the conductor is kept under tension during the stringing process, which keeps the conductor off the ground, minimizing the possibility of conductor surface damage and facilitates overcoming obstacles such as road crossings and also for maintaining clearance from other overhead lines. In a typical tension stringing operation, a pilot line is pulled through travelers installed on the structures for the length of the conductor pull section. The pilot line is then used to pull in the heavier pulling line. The pulling line is then attached to the conductor with a swivel and a woven grip, commonly referred to as a pulling sock, and then pulled in from the wire setup to the puller.
Pulling speed is an important factor in achieving a smooth stringing operation. Speeds of 3-5 miles/h usually provide for smooth passage of the running board over the stringing travelers. Slower speeds tend to result in unnecessary traveler and insulator hardware swing as the pulling grip and running board pass over the traveler. Faster speeds reduce the time to react in the event of an equipment malfunction.
Stringing tensions should be kept low as possible during the stringing process to minimize conductor creep. Conductor creep is a function of time, temperature, and conductor tension which results in permanent elongation of the conductor. Conductors that have been subjected to excessively high stringing tensions or that have been allowed to remain in travelers for an extended period of time will experience an abnormal amount of creep. If this occurs, the sag tables should be corrected to compensate for the additional creep elongation. If not, the initial sag tensions will be higher than designed and could result in conductor damage due to vibration.
Major equipment and tools required for tension stringing include the following:
Reel stands: A device designed to support one or more conductor or ground wire reels and can be skid, trailer, or truck mounted. Reel stands can accommodate conductor reels of varying sizes and should be equipped with reel brakes to prevent the reels from turning when pulling is stopped. They are used for either slack or tension stringing.
Tensioner: A device designed to hold tension against a pulling line or conductor during stringing operations.
The tensioner consists of one or more pairs of urethane or neoprene-lined single or multiple groove bullwheels in which each pair is arranged in tandem. Tension is accomplished by friction generated against the conductor that is reeved around the grooves of a pair of the bullwheels. Some tensioners are equipped with their own engines, which retard the bullwheels mechanically, hydraulically, or through a combination of both.
Puller: A device designed to pull a conductor during stringing operations. The puller can be either the drum or bullwheel type. It can be truck or trailer mounted and is normally equipped with its own engine, which drives the drum mechanically, hydraulically, or through a combination of both. The pulling line can be either synthetic fiber or wire rope.
Pilot line winder: Pilot line winders usually have multiple drums to provide pilot lines for several phase or ground wire positions. They have operating characteristics similar to drum-type pullers. Pilot line winders are used to pull in the larger pulling lines that will in turn be connected to the conductors to complete the pulling sequence. They can also be used to pull in overhead ground wires if the capacity rating is sufficient.
Pulling vehicle: A suitably con figured vehicle used to install pilot lines or pulling lines in accessible ROW.
Helicopter: Helicopters are sometimes used to install pilot lines, especially in rough terrain or where vehicle traffic is restricted on the ROW. Helicopters have proven to be efficient and cost effective when compared with traditional methods of pilot line installation. When a helicopter is used to install pilot lines, it’s necessary to have the travelers equipped with outrigger arms that guide the pilot line into the throat area of the traveler. Spring-loaded gates keep the line from being pulled out of the traveler throat as the helicopter continues the installation to the adjacent travelers. Bundle conductor travelers may have additional guides to deposit the lines into the pulling line sheave of the traveler. Bundle travelers are directional as the guides or gates must open toward the puller and away from the wire setup location.
Helicopters are also used in tower erection to lift structural elements and assemblies as well as trans porting men and materials in difficult terrain.
Travelers or conductor blocks: Travelers must be sized correctly for the size and type of conductor being installed. It’s recommended that the sheave be at least 20 times the conductor diameter as measured from the bottom of the conductor groove. The radius of the conductor groove should be 1.10 times the radius of the conductor. The flare of the groove should be between 12° and 20° from the vertical to facilitate the passage of swivels, pulling grips and to contain the conductor within the groove, particularly at line angles. Travelers are available with single sheave or with multiple sheave combinations to accommodate different conductor bundle configurations. Sheaves should be lined with neoprene or urethane material to prevent damage to the conductor. Travelers must run freely or they will adversely affect stringing and sagging operations. Also traveler efficiency must be considered when determining pulling/sagging section lengths. Finally, always ensure that the manufacturer's safe working load for the traveler does not exceed the traveler stringing loads.
Grounded travelers: The same as a standard traveler but have additional unlined rollers that make electrical contact between the conductor and the assembly which is connected to ground. They are usually placed at several locations along the pull section with a set relatively close to each end of the pull and at both sides of any energized line crossings.
Running grounds: These consist of spring tensioned unlined rollers that ensure constant contact with the conductor. The unit is connected directly to a suitable ground. Running grounds should be placed between the reel stands and the Tensioner, between the Tensioner and the first structure out and also on the pulling cable/wire rope between the puller and the first structure out on the pull section. Running grounds are also known as rolling grounds.
Pulling lines: Pulling lines can be either wire or synthetic rope as long as the rope is of a sufficient rated strength with appropriate safety factor to withstand the applied stringing tensions. Pulling lines should also be non-rotating, i.e., the rope won’t imply twist or torque to the conductor.
Swivel: A device used to connect pulling lines to the conductor or from conductor to conductor. It’s constructed so that each end will spin or rotate independently, thus reducing the transfer of rotational torque from one line to the other.
Woven grip: Also referred to as Kellems grip or most commonly as a sock. It’s similar to a Chinese finger grip and constricts to grip the conductor when tension is applied. One end is open to allow the conductor to be inserted and the other end is fitted with an eye that facilitates attaching a swivel.
Running board: A device used for pulling multiple conductors with only one pulling line. A running board is also referred to as an alligator or gator.
Above: Fgr.4 Flying in pilot line, note distribution line crossing and crossing structure.
Above: Fgr.5 Five-drum pilot line puller (two statics and three conductors).
Above: Fgr.6 Drum-type line puller.
Pull line Running board Swivels Socks and conductors
Above: Fgr.8 Running board with three conductors.
Above: Fgr.9 Running board with conductors approaching, (a) passing, and (b) the traveler.
Above: Fgr.7 Tensioner and reel stands set up for conductor pull.
__6 Equipment Setup
Once pull sections are identified, then the pulling, tensioning, and reel stand placement can be done.
The physical location of each piece of equipment integral to the wire and puller setups is of the utmost importance. Distance from the first structure out to the puller or tensioner must take into consideration the loading capacities designed into the structure and traveler supports. The general rule is that the distance from the structure be a minimum of 3 ft horizontal for every 1 ft vertical to the traveler attachment point. This is also applicable to the conductor snubbing location which will be between the tensioner or puller and the structure. The snubbing locations are sites where the conductors are temporarily fixed/ anchored to allow for sagging, conductor splicing, and serve as the pull site for the next sag section.
The reel stands should be located a sufficient distance from the rear of the tensioner to allow for enough fleet angle of the conductor leaving the reel and entering the bullwheels of the tensioner so that no damage or scuffing of the conductor can occur. Also all equipment at each location must be grounded and bonded together to ensure that no difference in ground potential exists.
Examples of conductor stringing and stringing equipment are presented in Fgrs. 4 through 11.
Above: Fgr.10 Pressing outer sleeve of splice.
Above: Fgr.11 Snub location being prepared for wire letup.
Overhead conductors are flexible and uniform in weight and when suspended between two supports form the shape of a catenary. Sagging transmission conductors is the process of obtaining the proper catenary shape based upon the sag-tension design requirements. The two most common methods used to sag transmission conductor are the transit method and stop watch or time of return method. The transit method is typically preferred as it provides for greater accuracy and control of the sagging process.
The sagging process begins by identifying the sag spans where the measurements are made to obtain the desired catenary shape. Sag spans should be located near each end of the sag section with preference given to longer more level spans. Long sag sections will require additional sag spans in the middle of the sag section. Sagging conductor in hilly terrain presents the additional difficulty of balancing the horizontal conductor tensions. The imbalance of the horizontal tension is due to gravity pulling the conductor through the travelers to the downhill end of the sag section. This increases the sag and lowers the tensions in the downhill spans. To restore the horizontal tension or pull the conductor uphill, a clipping offset is used. The clipping offset is a calculated distance, measured along the conductor, from the plum mark to a point on the conductor at which the center of the suspension clamp is to be placed.
Conductor is usually progressively sagged from the tensioner end to the puller end of the sag section.
Prior to sagging, bundle travelers at structures with line angles of more than three degrees should be exchanged for single sheaves for each conductor and suspended as closely as possible to their respective clipped in position to allow for more accurate sagging. Also, when sagging bundled conductors, all subconductors in the bundle should be sagged at the same time in order to maintain their mechanical characteristics relative to each other. Conductor sagging should be initiated as soon as possible after all conductors are pulled in. Should the sagging operation be delayed, the conductor tension should be reduced as much as possible to avoid causing excessive conductor creep prior to sagging. Once sagging is complete, all conductors should be marked at the suspension point on each structure prior to any conductors being clipped in the section. Clipping-in is the process of installing the conductor in the permanent suspension clamps that replace the stringing travelers on the support structure. Finally, if dampers, spacers, or spacer dampers are required, they should be installed on the conductor immediately after clipping to prevent vibration damage due to the wind. Installation locations for this type of line hardware are important for optimal conductor damping and/or spacing and are usually provided by the manufacturer.
__8 Overhead Transmission Line Maintenance
Overhead transmission line maintenance is the management of all transmission line assets (structures, hardware, conductor, and insulators) with the goal to optimize the risk, reliability, and operation over their respective life cycles. The typical overhead transmission line maintenance program is comprised of
• Transmission line inspections
• Transmission line fault investigations and corrective action(s)
• Transmission line work
• Vegetation management
• Data/Information management and analysis
• Emergency restoration of transmission structures
The organization and performance of these tasks is driven by a utility's work practices, system age and build, voltage, environment, and regulatory requirements. Regardless of how a utility chooses to organize and perform these basic maintenance tasks, it’s imperative that they are all integrated so that information derived from or technological advancements to any individual task can drive process changes to the others. Not integrating these maintenance tasks, i.e., treating them as separate entities, will result in less than optimal maintenance practices.
Above: Fgr.12 Examples of basic types of visual inspections: (a) ground, (b) climb and shake, and (c) air.
__8.2 Overhead Transmission Line Inspections
One of the primary sources from which transmission line maintenance work is derived is by inspection of the transmission line assets. Inspection methods work since the majority of overhead transmission line failure modes are time or cyclically dependent and typically exhibit visible or measureable distress prior to failure. Examples of the type of failure modes encountered include; wear, fatigue, corrosion, loosing of mechanical fasteners, electrical breakdown/tracking, in addition to fungal and insect attack on wood assets. Random damage events, vandalism, for example, typically don’t result in immediate failures and are often identified within the inspection cycle. Severe structural loading events, such as extreme weather or a vehicle/airplane striking the transmission line or structure, often result in immediate failure and/or operational loss. These types of events cannot be prevented by transmission line inspections and are better addressed by an emergency restoration plan which is discussed later in this section.
Transmission line inspection plans are mainly developed from experience by compiling information such as past failures, material/environment degradation mechanisms, voltage, risk to transmission system and grid, public risk, and regulatory requirements. Once the inspection plan has been established, it’s important to follow the plan while continually assessing its effectiveness. Any changes to the inspection practices and plan should be well documented as to why the process is being changed. Failure to follow your established inspection plan and adequately document your findings could result in regulatory fines and increase the risk of litigation.
TABLE 1 Typical Transmission Line Findings from Visual Inspections
Below grade wood pole inspections typically expose 18-24 in. of the wood pole below grade and inspect for exterior decay and termites. Interior degradation is determined by drilling several holes and using a probe to search for decay or termite pockets or by "sounding," which is hitting the pole with a hammer and listening for a hollow sound upon impact.
Split wood pole tops and cross arms. Other above grade wood pole findings include, but not pictured; pole top degradation, wood pecker holes, shell decay or sloughing, large length-wise checks or splits, local buckling or compression failure, and mechanical damage due to vehicle strike.
Electrical tracking resulting from stray currents that destructively breakdown a dielectric material. Tracking occurring on wood poles and cross arms usually results from poor or inadequate grounding that in turn can result in pole fires. Tracking can also occur on polymer insulators where the dielectric material has degraded.
Degradation of insulated fiberglass guy rods. Loss of the outer coating on fiberglass line hardware results in exposure of the fiberglass, hence increasing the risk of mechanical and electrical (tracking) failure.
Corroded guy anchors are typically identified by slack guy line(s) and a leaning guyed pole. If anchor corrosion is problematic, specific inspection techniques are available to identify anchor corrosion before it results in a mechanical failure.
Corrosion of steel transmission structures. Over time the environment around steel structures can change resulting in earthen contact or standing water which will result in accelerated corrosion of the steel. If necessary, specific corrosion inspection techniques are available to more thoroughly evaluate steel corrosion.
Transmission line hardware failures are often the result of loose or missing bolts and cotter keys, corrosion, fatigue, and improper or poor installation.
Flashed insulators typically result from a lightning strike or foreign debris, such as a string of balloons, shorting the insulator to ground. Flashed insulators usually remain functional but should be replaced at the first available opportunity.
Broken porcelain insulators result from the propagation of microcracks, cement expansion, lightning puncture, excessive corona discharge around the insulator cap, and vandalism (gunshot). Industrial and coastal areas are also prone to cap and pin corrosion. Similar problems are also encountered for toughened glass insulators; however, the material failure mechanisms differ.
Insulator contamination can compromise the dry arc distance of an insulator or insulator string resulting in a flash over. Common contamination sources include birds, industrial sources, and salt fog in coastal environments. If natural processes cannot clean the insulator(s) sufficiently, protective screening, bird deterrents, or insulator cleaning could become necessary.
Mechanical failure of polymer insulators is often due to handling damage, binding hardware, vandalism, manufacturing issues, and exposure of the fiberglass rod to the environment as a result of excessive corona cutting of the seal or sheath, sheath damage due to lightning, and electrical tracking under the sheath.
Conductor damage can be the result of vandalism (gunshot), line hardware wearing against the conductor, incidental contact such as from a crane boom, or during transmission line construction.
Design issues such as conductor uplift, left photo, and missing or improperly installed hardware such as the absence of a standoff for the optical ground wire, right photo.
Foundation issues such as erosion, burying of foundation caps, or foundation cap cracking.
Encroachments due to construction, stock piling of materials, and vegetation within the transmission line ROW. Also blocking or limiting access to the ROW is considered encroachment and should be noted in the inspection report.
Bent tower lattice can be due to buckling from high wind loads, foundation settling/movement, impact from vehicles or earth, and linemen climbing/stepping on smaller members. Failed steel members are usually the result of fatigue at bolted connections, corrosion, and major loading events such as vehicle impact and extreme weather.
Visual inspections are the primary means of identifying overhead transmission line problems from which maintenance work is derived. They are normally conducted at predetermined time intervals and can be via ground, climb and shake, or air ( FGR. 12).
Visual inspections can be general, looking at all the transmission line assets, or they can be specific to a particular asset or component due to a previous identified failure mode. TABLE 1 lists the types of findings that usually result from visual inspections. It’s not uncommon to supplement or combine visual inspections with other inspection technologies such as radio frequency (RF), infrared and corona cameras, corrosion potential measurements, Light detection and ranging (LiDAR) mapping, and acoustic/ultrasonic methods for a more comprehensive and in-depth evaluation of the transmission line structures, conductor, insulators, and hardware. Examples of the uses for the aforementioned inspection technologies include the following:
• LiDAR mapping for line rating and vegetation management.
• Ultrasonic techniques and corrosion potential measurements for steel poles, tower stub angles, and anchor rods for corrosion management.
• Corona camera evaluation of polymer insulators to identify excessive corona discharge that can damage the seal at the end fittings and/or the silicon sheath resulting in exposure of the fiberglass rod. Exposing the fiberglass rod to the environment significantly increases the risk of a mechanical or electrical failure.
• Infrared temperature measurements to evaluate poor electrical connections in connectors, switches, and splices.
An excellent resource covering all aspects of transmission line inspection methodologies and technologies is EPRI's "Overhead Transmission Inspection and Assessment Guidelines" also known as the EPRI Yellow Book.
Above: Fgr.13 Example of inspection software entry screen(s). (a) The software is GPS enabled and highlights the closest structure, and (b) allowing for menu-driven inspection entries/findings.
__8.3 Transmission Line Inspection Software
Most of today's transmission line inspection software is built on a geographic information system (GIS) platform in addition to being global positioning system (GPS) enabled. The reporting format of transmission line problems or issues within the software is usually standardized or menu driven, FGR. 13. Reporting standardization gives consistency to the inspection data, thus making the data more searchable and easier to analyze. Comment sections, while valuable, should only be used to document problems not found within the standardized report or to enhance the report with specific information related to the problem or transmission asset.
__8.4 Transmission Line Fault Investigations and Corrective Action(s)
One of the primary objectives of any transmission line maintenance program is to prevent line faults.
Therefore, it’s important that when a fault does occur, it’s investigated to determine if it can be effectively addressed by preventative maintenance measures. One method for investigating transmission line faults and assessing the potential for preventing future faults is the root cause analysis (RCA) process.
RCA is usually systematic and standardized with regards to conducting and documenting the investigative process. Upon completion of the investigation, work is initiated on identifying the root cause(s) and potential corrective actions. Finally, the potential corrective actions are evaluated for their cost and potential to prevent future fault occurrences. FGR. 14 illustrates the RCA process for a lightning investigation.
Another method by which faults can be evaluated is by control charting. Control charting of trans mission line faults can be useful in assessing transmission line maintenance and inspection programs by determining the statistical characteristics of faulting associated with a particular transmission line, voltage, or fault type. An example of this is illustrated in FGR. 15 for a 115 kV transmission system located in the Southwestern United States in which all faults that occurred from 1997 to 2009 are charted.
By knowing the stable faulting characteristics, one is better able to assess the maintenance impact on the transmission system and thus distinguish between changes driven by special variation versus those that are inherent.
Above: Fgr.14 Example of the RCA process for a lightning investigation. (a) Lightning strikes a transmission structure resulting in a line fault, (b) fault location analysis estimates the probable location from relay data and this is compared to the lightning strike data from the National Lightning Detection Network (NLDN), (c) a field patrol verifies the fault location, assesses damage, and investigates the root cause, (d) the root cause report is created and stored for future trending purposes, and (e) as appropriate, line improvements are made, such as the installation of lightning arrestors.
Above: Fgr.15 "C"-type control chart for annual faults on 115 kV transmission system located in the Southwest United States.
__9 Transmission Line Work
Scheduling the necessary maintenance repairs starts by assigning all maintenance findings with a work priority based on a timeline for repairs to be completed. Most utilities have multiple work priority levels ranging from repair immediately to monitor. Work priority durations are developed based upon a utility's work practices to address the particular transmission line problem and the problem's impact to public safety, as well as the transmission system and grid. It’s important to be realistic in developing and assigning work priorities as a poorly designed and/or applied work priority methodology will result in work inefficiencies and could create unintended regulatory issues. TABLE 2 presents typical trans mission line maintenance repair work.
__9.1 Live Line Work
The term "live line work" refers to working with the conductors in the energized state by either the bare hand technique or insulated hot stick. This method of transmission line maintenance is preventative and preferred for transmission lines where de-energizing is cost prohibitive, adversely affects reliability, or results in extensive customer outages (radial feed). Live line maintenance is highly specialized work and utilities that perform live line work have the following:
• Clear and consistent guidelines for the performance of such work.
• Equipment and vehicles specifically designed for the energized environment.
• Regular live line training to maintain qualified/certified personnel.
• A transmission system that allows for the minimum approach distance (MAD) to be maintained while working under energized conditions. A simple definition of MAD is the distance upon which the air gap provides sufficient insulation from electrical sparkover to ground due to a potential overvoltage. FGR. 16 is a simple illustration of the work envelope and MAD for live line work. The distance, D, in FGR. 16 must be equal to or greater than the MAD which is a function of phase voltage, potential overvoltage, and altitude.
Additional information regarding live line working methods, tools and terminology can be found in IEEE Standards 516-2009 and 935-1989 as well as EPRI's Live Work Reference Guide also known as the EPRI Tan Book.
__9.2 Worksite Grounding
The purpose behind worksite grounding is to protect transmission line personnel while working on electrically isolated (de-energized) lines in the event the lines become accidently energized or due to induction from adjacent parallel lines. Two fundamental rules are universal to worksite grounding:
1. The installation of protective grounds is considered to be energized work. The circuit is always treated as energized until properly grounded.
2. The line worker should never place himself or herself in series with the grounding system.
Voltage differences commonly referred to as step, touch, and transfer-touch, occur when there is a difference in potential between two points as a result of accidental energization or induction, FGR. 17. This difference in voltage can result in potentially life threatening current flow for line personnel within the voltage gradient zone. Worksite grounding is designed to reduce the voltage difference within the work zone, thereby minimizing the risk to line personnel working on electrically isolated lines. Information on the temporary protective grounds used for grounding can be found in ASTM F855-2009.
In addition to grounding, two other approaches are available for protecting against voltage differences.
They are insulation and isolation. Insulation is achieved through the use of insulating (non-conductive)
mats, footwear, and gloves. Isolation involves limiting physical access to the work zone usually by barricading or fencing.
__9.3 Vegetation Management
Transmission line vegetation management is regulated by the North American Electric Reliability Corporation (NERC) via Standard FAC-003-Transmission Vegetation Management Program. This standard was developed in response to massive regional blackouts that were the direct result of vegetation related faults. NERC requires the transmission owner to prepare and keep current all objectives, practices, procedures, and work specifications associated with preventing outages from vegetation located on and adjacent to the transmission line ROW.
TABLE 2 Typical Transmission Line Repair Work
Steel pole repair due to impact. Repairs involve welding a similar thickness patch over the dented/damaged area, if damaged beyond repair the pole is replaced.
Replacement of wood poles, H-structures, and associated members such as cross arms and bracing are usually due to wood degradation or damage/failure as the result of vehicle impact or severe weather.
Wood pole ground line reinforcement usually involves driving a steel C-channel down the side of the pole and attaching it with steel bands. The preferred placement of the steel C-channel is with the open end of the "C" in line with the conductor.
Replacing broken or flashed porcelain or toughened glass insulators. The photos to the left are examples of live line or energized insulator replacement. Polymer insulators are also replaced for similar reasons, however, the appearance and failure mechanisms differ from those of porcelain and toughened glass insulators.
Conductors that have broken strands are typically repaired with patch rod. Patch rod consists of a set of helical wound rods that are formed around the damaged area to provide mechanical strength and electrical continuity.
Conductor splicing is required for damage that is beyond the mechanical capacity of patch rod or for situations where additional conductor needs to be added to existing conductor. The photos to the left show the circumcision of the aluminum to the steel and the completed compression splice.
Replacement of dampers, spacers, spacer dampers, and other line hardware. The photos to the left are examples of live line spacer-damper replacement from a conductor buggy and boom truck.
Insulator cleaning is sometimes necessary in areas of high contamination. The dry cleaning process uses compressed air to force suitable abrasive media such as corn cobs out of a nozzle, mechanically removing the contamination.
The dry process can be done while energized. The wet cleaning process uses pressurized water and must be done under de-energized conditions.
Foundation caps and aboveground retaining structures are used to protect the structure at the foundation-structure interface near and above the grade line. Foundation cap cracking and deterioration are normally the result of poor concrete, environmental degradation, or impact damage.
Above: Fgr.16 Illustration of the work envelope, which is the ergonomic work area for line personnel to operate and the distance, D, to grounded conductive elements. The distance, D, must be greater than or equal to the MAD as determined from the phase voltage, potential over voltage, and altitude.
Above: Fgr.17 Illustration of an electrical fault or induced voltage and the resulting voltage rise curve. Touch and step voltages occur when there is a voltage difference between two points. Transfer-touch is similar to touch voltage except the voltage is passed through a conductive element prior to the touch.
Above: Fgr.18 Spring type spacer-damper failures and analysis. (a) Failure is due to two body wear between the steel damping spring and the aluminum housing, (b) Weibull failure probabilities were determined from inspection and GIS information, and (c) used to develop a failure forecast.
__10 Data/Information Management and Analysis
Effective and efficient management of the transmission line assets can only occur by integrating the data and information from all aspects of transmission inspection and work management programs with that of the GIS or asset management database(s). In today's environment this means that all the software platforms must be capable of interfacing to the necessary databases and perhaps to each other. Software that is not well integrated often results in less than optimal work practices and missed data opportunities that make any follow up analysis difficult. However, data and information that is well integrated provides the opportunity for better understanding of the entire transmission system and analysis of this combined data often results in improved transmission line maintenance operations and asset life cycle performance. Guidance on the collection and management of transmission line inspection data can be found in IEEE Standard 1808-2011.
An example of analyzing data from several sources is presented for 500 kV transmission lines with failing tri-bundle spring-type spacer dampers. The failure mechanism for this type of spacer damper is two body mechanical wear between the steel damping spring(s) and the aluminum body which eventually results in separation of the spacer damper. Data from transmission line visual inspections and installation dates from the GIS database was used to construct the Weibull failure probabilities. Using the Weibull probabilities, a spacer-damper failure forecast was developed which resulted in implementation of a replacement plan suited to the exposure risk and failure characteristics of the spacer dampers. The failure mechanism and analysis process for the tri-bundle spring type spacer damper is given in FGR. 18.
Another example of analysis utilizing data and information from several different sources is presented for the asset management of 69 kV tangent wood poles. This analysis combines information from the GIS database such as pole material, treatment, dimensions, framing, the presence of under built distribution, span lengths, and age with that from the ground line inspection and local wind gust data to provide a structural risk assessment of all tangent wood poles in the circuit. This type of analytical approach to evaluating 69 kV wood poles is much more effective in regards to establishing replacement budgets and managing 69 kV system risk rather than solely basing decisions on the wood pole inspection data alone. FGR. 19 illustrates the analysis process.
Above: Fgr.19 The structural risk assessment process for individual 69 kV tangent wood poles is accomplished by combining and analyzing data from multiple sources. (a) Wood pole inspection data, (b) is combined with loading data, (c) and (d) analyzed to assess wood pole and circuit risk.
Above: Fgr.20 Collapsed 500 kV tower from a microburst and subsequent installation of emergency restoration towers.
__11 Emergency Restoration of Transmission Structures
Planning for high impact, low risk of occurrence events that result in loss of transmission structure(s) requires evaluating the economic and operational impact of the downed transmission line to the local transmission system and regional grid. If conditions are such that the line can remain out of service until construction of new structure(s) are complete, then the restoration plan needs to consider maintaining a reasonable number of structures in inventory necessary for line restoration. However, if the economic and operational impacts are such that a prolonged outage is intolerable, the restoration plan's main focus should be on the quick and efficient restoration of the line. Planning for the rapid restoration of transmission line structures at voltages 345 kV and higher usually requires the use of modular emergency restoration structures. These structures consist of column sections fabricated from light weight, high-strength aluminum alloy, which are easy to transport, and once on-site can be arranged to construct a variety of guyed structures. An example of this is presented in FGR. 20 for a 500 kV tower which was collapsed due to extremely high localized winds (microburst). This restoration took approximately 4 days from mobilization to line operation.