Lightning Arresters Definition and How It Works

LIGHTNING ARRESTERS DEFINITION and HOW IT WORKS/ OPERATES TUTORIAL  A Tutorial About About Lightning Lightning Arrester 

There are two basic voltage surge that may cause damage to the system and its corresponding equipment and apparatus. These are: 1. Switching Surge 2. Lightning Surge Switching Surge - These are voltage abnormalities that are caused by changes in the operating state of the power system, that involves switching (literally) of breakers, disconnect switches, and other switch gears. It happens as trapped tr apped energy are released during the event. Lightning Surge - These are voltage abnormalities caused by the phenomenon of lightning. Damaged may be experienced through direct stroke or hit, or by induced voltages. Lightning are harmful, and runs in millions of voltages, which makes the equipment vulnerable without protection. The risk of having your equipment exposed to both of these abnormalities can be greatly reduced with the application of surge arresters. Not to be confused, surge arresters refers to devices which could protect from the aforementioned abnormalities. Lightning arresters on the other hand are specifically named protection device, designed more for lightning protection, but in itself is capable enough to protect it from switching surge. Below are selected article links to help you understand further what is a lightning arrester and how lightning arrester works:

LIGHTNING AND.... SURGE PROTECTION Opto Isolation, Transformer Isolation, Surge Protection Hi - here's some technical information for your perusal.

Tutorial by Kenneth Schneider PhD

Telebyte USA Lightning has long fascinated the technical community. Ben Franklin studied lightning's electrical nature over two centuries ago and Charles R Steinmetz generated artificial lightning in his General Electric laboratory in the 1920's. As someone concerned with premises data communications you need to worry about lightning. Here I will elaborate on why, where and when you should worry about lightning. I'll then discuss how to get protection from it.

6.1 WHY WORRY ABOUT LIGHTNING?  It is unfortunate, but a fact of life, that computers, computerrelated products and process control equipment found in premises data d ata communications environments can be damaged by high-voltage surges and spikes. Such power surges and spikes are most often caused by lightning strikes. However, there are occasions when the surges and spikes result from any one of a variety of other causes. These causes may include direct contact with power/lightning circuits, static buildup on cables and components, high energy transients coupled into equipment from cables in close proximity, potential differences between grounds to which different equipment’s are connected, miswired systems and even human equipment users who have accumulated large static electricity charge build-ups on their clothing. In fact, electrostatic discharges from a person can produce peak Voltages up to 15 kV with currents of tens of Amperes in less than 10 microseconds. A manufacturing environment is particularly susceptible to such surges because of the presence of motors and other high voltage equipment. The essential point to remember is, the effects of surges due to these other sources are no different than those due to lightning. Hence, protection from one will also protect from the other. When a lightning-induced power surge is coupled into your computer equipment any one of  a number of harmful events may occur. Semiconductors are prevalent in such equipment. A lightning induced surge will almost always surpass the voltage rating of these devices causing them to fail. Specifically, lightning induced surges usually alter the electrical characteristics characteristics of semiconductor devices so that they no longer function effectively. In a few cases, a surge may destroy the semiconductor device. These are called "hard failures." Computer equipment having a hard failure will no longer function at all. It must be repaired with the re sulting expense of "downtime" or the expense of a standby unit to take its place. In several instances, a lightning-derived surge may destroy the printed traces in the printed circuit boards of the computer equipment also resulting in hard failures. Along with the voltage source, lightning can cause a current surge and a resultant induced magnetic field. If the computer contains a magnetic disk then this interfering magnetic field might overwrite and destroy data stored in the disk. Furthermore, the aberrant magnetic field may energize the disk head when it should be quiescent. To you, the user, such behavior will be viewed as the "disk crashing." Some computer equipment may have magnetic relays. The same aberrant magnetic fields which cause disk crashes may activate relays when they shouldn't be activated, causing unpredictable, unacceptable performance. Finally, there is the effect of lightning on program logic controllers (PLCS) which are found in the manufacturing environment. Many of these PLCs use programs stored in ROMS. A lightning-induced surge can alter the contents of the ROM causing aberrant operation by the PLC.

So these are some of the unhappy things which happen when a computer experiences lightning. But you may say, "Come on, equipment hit by lightning, that's like winning the lottery. It has never happened and I doubt that it ever will." This is a typical reaction and unfortunately it is based on ignorance. True, people may never, or rarely, experience, direct lightning strikes on exposed, in-building cable feeding into their equipment. However, it is not uncommon to find computer equipment being fed by buried cable. In this environment, a lightning strike, even several miles away, can induce voltage/current surges which travel through the ground and induce surges along the cable, ultimately causing equipment failure. The equipment user is undoubtedly aware of these failures but usually does not relate them to the occurrence of lightning during thunderstorm activity since the user does not experience a direct strike. In a way, such induced surges are analogous to chronic high blood pressure in a person; they are "silent killers." In the manufacturing environment, long cable runs are often found connecting sensors, PLCs and computers. These cables are particularly vulnerable to induced surges.

6.2 SHOULD YOU WORRY ABOUT LIGHTNING? This question primarily relates to the geographical location of computer equipment end-users. When other interfering phenomena which can cause a deterioration of performance is considered, it matters little where the equipment is geographically located. When do you have to worry during a thunderstorm? Typically, thunderstorms are characterized as intense individual rain cells or showers embedded in a broad area of light rain. These intense cells are only over a fixed location for a few minutes. They are on average, several miles in each direction. In the continental United States thunderstorm thunderstorm cells move from west to east along a squall line as shown in Figure 17. This squall line is about 12 30 miles in width and up to 1,250 miles long. The speed at which the thunderstorm cell moves is generally 30 knots (approximately 34.4 statute miles per hour).

6.4 EQUIPMENT PROTECTION Coming right down to it, a lot can be done as far as protection is concerned. However, it is best to begin by describing the magnitude of the threat from which you need protection. The first stroke of lightning during a thunderstorm can produce peak currents ranging from 1,000 to 100,000 Amperes with rise times of 1 microsecond. It is hard to conceive of, let alone protect against, such enormous magnitudes. Fortunately, such threats only apply to direct hits on overhead lines. Hopefully, this is a rare phenomenon. More common is the induced surge on a buried cable. In one test, lightning-induced voltages caused by strokes in ground flashes at distances of about 5 km were measured at both ends of a 448 meter long, lon g, unenergized power distribution line.

Typical test results are illustrated in Figure 19. Notice that the maximum-induced surge exceeds 80 Volts peak-to-peak. This is more than enough to destroy semiconductor devices and computer related equipment. Yet, 80 Volts is well within the range of affordable protection. Conceptually, lightning protection devices are switches to ground. Once a threatening surge is detected, a lightning protection device grounds the incoming signal connection point of  the equipment being protected. Thus, redirecting the threatening surge on a path-of-least resistance (impedance) to ground where it is absorbed. Any lightning protection device must be composed of two "subsystems," a switch which is essentially some type of switching circuitry and a good ground connection-to allow dissipation of the surge energy. The switch, of course, dominates the design and the cost. Yet, the need for a good ground connection can not be emphasized enough. Computer equipment has been damaged by lightning, not because of the absence of a protection device, but because inadequate attention was paid to grounding the device properly. The basic elements used as protective switches are: gas tubes, metal oxide varistors and silicon avalanche diodes (transorbs). Each has certain advantages and disadvantages. Because they can withstand many kilovolts and hundreds of Amperes, gas tubes have traditionally been used to suppress lightning surges on telecommunications lines. This is just what is needed to protect against a direct strike. Because gas tubes have a relatively slow response time, this slowness lets enough energy to pass to destroy typical solid state circuits. Metal oxide varistors (MOVS) provide an improvement over the response time problem of  gas tubes. But, operational life is a drawback. MOVs protection characteristic decays and fails completely when subjected to prolonged over voltages. Silicon avalanche diodes have proven to be the most effective means of protecting computer equipment against over voltage transients. Silicon avalanche diodes are able to withstand thousands of high voltage, high current and transient surges without failure. While they can not deal with the surge peaks that gas tubes can, silicon avalanche diodes do provide the fastest response time. Thus, depending upon the principal threat being protected against, devices can be found employing gas tubes, MOVS, or silicon avalanche diodes. This may be awkward, since the threat is never really known in advance. Ideally, the protection device selected should be robust, using all three basic circuit breaker elements. The architecture of such as device is illustrated in Figure 20. This indicates triple stage protection and incorporates gas tubes, MOVs and silicon avalanche diodes as well as various coupling components and a good ground. With the architecture shown in Figure 20 a lightning strike surge will travel, along the line until it reaches a gas tube. The gas tube dumps ex tremely high amounts of surge energy directly to earth ground. However, the surge rises very rapidly and the gas tube needs several microseconds to fire.

As a consequence, a delay element is used to slow the propagation of the leading edge wavefront, thereby maximizing the effect of the gas tube. For a 90 Volt gas tube, the rapid rise of the surge will result in its firing at about 650 Volts. The delayed surge pulse, now of  reduced amplitude, is impressed on the avalanche diode which responds in about one nanosecond or less and can dissipate 1,500 Watts while limiting the voltage to 18 Volts for EIA-232 circuits. This 18 Volt level is then resistively coupled to the MOV which clamps to 27 Volts. The MOV is additional protection if the avalanche diode capability is exceeded. As previously mentioned, the connection to earth ground can not be over emphasized. The best earth ground is undoubtedly a cold water pipe. However, other pipes and building power grounds can also be used. While cold water pipes are good candidates you should even be careful here. A plumber may replace sections of corroded metal pipe with plastic. This would render the pipe useless as a ground.

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LIGHTNING FLASHOVERS ON OVERHEAD TRANSMISSION LINES Lightning flashovers are segregated into three main types, for stroke locations on a phase conductor, on an overhead shield wire, or to nearby ground.

SHIELDING FAILURE FLASHOVERS Shielding failure flashovers events result from a lightning stro ke terminating directly on a phase conductor. For shielded lines, these events should be very infrequent and of very low stroke current magnitude.

For unshielded lines (i.e., “static less” l ines), these events will be much more common and wi ll

involve the full distribution of lightning stroke current magnitudes. Arresters can be used to address shielding failure flashovers by applying the arresters on the exposed phases.

The arresters must be installed at every tower o r pole to be effective at preventing shielding failure flashovers. For unshielded line applications, arrester energy requirements must be adequately addressed since the stroke currents and durations they will be exposed to are harsher than in shielded line applications.

BACK FLASHOVERS Back flashovers, events result from a lightning stroke terminating on the ground system (i.e., shield wires, tower tops, and pole tops) causing a potential across the insulation that causes a flashover to occur.

The surge traveling on the shield wire will cause surge voltages to be induced in the phase conductors. The magnitude of the induced vo ltage is a function of the current magnitude, resistance, and geometry.

Stroke currents exceeding a critical current value will develop sufficient voltage between the structure and the phase conductor to cause an insulator flashover. The phase with the poorest coupling to the shield wire will be the most highly stressed and therefore most likely to flash over. Local grounding conditions have a major impact on back flashover performance.

Arresters can be used to address these types of outages by placing them o n the least coupled phases (e.g., bottom phases) or in high footing resistance areas. For applications in high footing resistance areas, it is important to apply the arresters not only in the areas of high footing resistances, but also one or two structures away from the high footing resistance areas.

INDUCED VOLTAGE FLASHOVERS Induced voltage flashovers events result from nearby lightning strokes inducing voltages on line conductors. Because the induced overvoltages measured on distribution lines rarely exceed 300 kV, it is common belief that this phenomenon has little effect at transmission voltage levels. However, the induced voltages tend to increase with line height.

There may be some structures used at 34. 5 kV through 69 kV (sometimes referred to as “sub transmission” voltages) that could be susceptible to i nduced voltage flashovers from nearby

lightning strokes.

For lines that are susceptible to induced voltage flashovers, arresters at relatively wide spacing m ay be used to minimize the effects of these events.

SURGE ARRESTERS APPLICATION IN TRANSMISSION and DISTRIBUTION LINES SURGE ARRESTER DEFINITION

Surge Arresters are used to limit voltage surges on an electrical system to level that it can be controlled. It is designed and connected between a conductor of an electrical system and ground to limit the magnitude of transient over voltages on equipment.

Surge Arresters Surge Arresters are the most commonly used add-on equipment for over voltage protection. It very helpful in limiting over voltage on equipment by discharging or bypassing surge current, prevent continued flow of follow current to ground, and is capable of repeating these functions as specified.

WHAT EXACTLY DOES A SURGE ARRESTER DO?

1. Surge Arresters does not absorb the lightning. 2. Surge Arresters does not stop the lightning. 3. Surge Arresters divert the lightning to ground. 4. Surge Arresters clamp (limit) the voltage produced by lightning. 5. Surge Arresters equipment electrically in parallel with it.

HOW DOES SURGE ARRESTERS WORK?

At the heart of all arresters is Metal Oxide Varistors (MOV). The MOV disk is a semiconductor that is sensitive to voltage. At normal voltage, the MOV disk is an insulator and will not conduct current. But at higher (extreme) voltage caused by lightning or any surges, it becomes a conductor.

The usual construction of a typical surge arrester consists of disks of zinc oxide material sized in cross-sectional area to provide desired energy discharge capability, and in axial length proportional to the voltage capability. The disks are then placed in porcelain enclosures to provide physical support and heat removal, and sealed for isolation from contamination in the electrical environment.

TYPES OF SURGE ARRESTERS

There are four basic types of surge arresters defined by industry standards. The surge arrester type selected for the application depends on t he equipment being protected and what level of pr otection is required.

Secondary Type – Available in ratings up to 650 Volts, and are used to protect equipment at the utilization voltage level.

Distribution Type – Typically used for the protection of equipment on power distribution circuits. They are available in ratings up to 42 kV. This type of surge arrester is further defined by normal and heavy duty.

Intermediate Type  – Available in ratings up to 144 kV. This type of surge arrester offers improved protective characteristics and durability. They are generally used for protection of smaller substations, or medium class power equipment.

Station Type Surge Arresters

TRANSMISSION LINES DISTANCE RELAY BASIC INFORMATION What Is Transmission Lines Distance Relay?  Distance relays respond to the voltage and current, i.e., the impedance, at the relay location. The impedance per mile is fairly constant so these relays respond to the distance between the relay location and the fault location. As the power systems become more complex and the fault current varies with changes in generation and system configuration, directional overcurrent relays become difficult to apply and to set for all contingencies, whereas the distance relay setting is constant for a wide variety of changes external to the protected line. There are three general distance relay types as shown in Fig. 9.32. Each is distinguished by its application and its operating characteristic.

Impedance Relay The impedance relay has a circular characteristic centered at the origin of the R-X d iagram. It is nondirectional and is used primarily as a fault detector. Admittance Relay The admittance relay is the most commonly used distance relay. It is the tripping relay in pilot schemes and as the backup relay in step distance schemes. Its characteristic passes through the origin of the R-X diagram and is therefore directional. In the electromechanical design it is circular, and in the solid state design, it can be shaped to correspond to the transmission line impedance.

Reactance Relay The reactance relay is a straight-line characteristic that responds only to the reactance of  the protected line. It is nondirectional and is used to supplement the admittance relay as a tripping relay to make the overall protection independent of resistance. It is particularly useful on short lines where the fault arc resistance is the same order of  magnitude as the line length. Figure 9.33 shows a three-zone step distance relaying scheme that provides instantaneous protection over 80 –90% of the protected line section (Zone 1) and time-delayed protection over the remainder of the line (Zone 2) plus backup protection

over the adjacent line section. Zone 3 also provides backup protection for adjacent lines sections.

In a three-phase power system, 10 types of faults are possible: three single phase-toground, three phase-to-phase, three double phase-to-ground, and one three-phase fault. It is essential that the relays provided have the same setting regardless of the type of fault. This is possible if the relays are connected to respond to delta voltages and currents. The delta quantities are defined as the difference between any two phase quantities, for example, Ea – Eb is the delta quantity between phases a and b. In general, for a multiphase fault between phases x and y, Ex-Ey/ Ix-Iy = IZ

where x and y can be a, b, or c and Z1 is the positive sequence impedance between the relay location and the fault. For ground distance relays, the faulted phase voltage, and a compensated faulted phase current must be used. Ex / ( Ix+mI0) = Z1

where m is a constant depending on the line impedances, and I0 is the zero sequence current in the transmission line. A full complement of relays consists of three phase distance relays and three ground distance relays. This is the preferred protective scheme for high voltage and extra high voltage systems.

Pilot Protection As can be seen from Fig. 9.33, step distance protection does not offer instantaneous clearing of faults over 100% of the line segment. In most cases this is unacceptable due to system stability considerations. To cover the 10 –20% of the line not covered by Zone 1, the

information regarding the location of the fault is transmitted from each terminal to the other terminal(s). A communication channel is used for this transmission. These pilot channels can be over power line carrier, microwave, fiberoptic, or wire pilot. Although the underlying principles are the same regardless of the pilot channel, there are specific design details that are imposed by this choice. Power line carrier uses the protected line itself as the channel, superimposing a high frequency signal on top of the 60 Hz power frequency. Since the line being protected is also the medium used to actuate the protective devices, a blocking signal is used. This means that a trip will occur at both ends of the line unless a signal is received from the remote end. Microwave or fiberoptic channels are independent of the transmission line being protected so a tripping signal can be used. Wire pilot channels are limited by the impedance of the copper wire and are used at lower voltages where the distance between the terminals is not great, usually less than 10 miles.

Directional Comparison The most common pilot relaying scheme in the U.S. is the directional comparison blocking scheme, using power line carrier. The fundamental principle upon which this scheme is based utilizes the fact that, at a given terminal, the direction of a fault either forward or backward is easily determined by a directional relay. By transmitting this information to the remote end, and by applying appropriate logic, both ends can determine whether a fault is within the protected line or external to it. Since the power line itself is used as the communication medium, a blocking signal is used.

Transfer Tripping If the communication channel is independent of the power line, a tripping scheme is a viable protection scheme. Using the same directional relay logic to determine the location of a fault, a tripping signal is sent to the remote end. To increase security, there are several variations possible. A direct tripping signal can be sent, or additional underreaching or overreaching directional relays can be used to supervise the tripping function and increase security. An underreaching relay sees less than 100% of the protected line, i.e., Zone 1. An overreaching relay sees beyond the protected line such as Zone 2 or 3.

Phase Comparison Phase comparison is a differential scheme that compares the phase angle between the currents at the ends of the line. If the currents are essentially in phase, there is no fault in

stress in each direction. Selecting the Anchor Anchors come in many types and sizes, each designed for certain soil and guying conditions. While each will do its specific job better than another design of anchor, most find use under more than one set of conditions.

SIZES OF TRANSMISSION LINE CONDUCTORS BASIC INFORMATION What Are The Sizes of Transmission Line Conductors?  Conductor Sizes Since it is impractical to manufacture an infinite number of wire sizes, standards have been adopted for an orderly and simple arrangement of such sizes for manufacturers and users. The American Wire Gauge (AWG), formerly known as the Browne and Sharpe Gauge (B&S), is the standard generally employed in this country and where American practices prevail. In defining conductor sizes, the circular mil (cmil) is usually used as the unit of  measurement. It is the area of a circle having a diameter of 0.001 in, which works out to be 0.7854 × 10 –6 in2. In the metric system, these figures are a diameter of 0.0254 mm and an area of 506.71 × 10 –6 mm2. Wire sizes are given in gauge numbers, which, for distribution system purposes, range from a minimum of no. 12 to a maximum of no. 0000 (or 4/0) for solid-type conductors. Solid wire is not usually made in sizes larger than 4/0, and stranded wire for sizes larger than no. 2 is generally used. Above the 4/0 size, conductors are generally given in circular mils (cmil) or in thousands of  circular mils (cmil × 103); stranded conductors for distribution purposes usually range from a minimum of no. 6 to a maximum of 1,000,000 cmil (or 1000 cmil × 10 3) and may consist of  two classes of strandings. These wire sizes and their dimensions are given in Table 9-2.

Gauge numbers may be determined from the formula: 0.3249 Diameter, in = ———

1.123n or 105,500 Cross-sectional area, cmil = ———— 1,261n where n is the gauge number (no. 0 = 0; no. 00 =  – 1; no. 000 = – 2; no. 0000 = – 3).

It will be noted that the diameter of the wire doubles approximately every sixth size (e.g., no. 2 has twice the diameter of no. 8), and the cross-sectional area therefore doubles every third size and is 4 times as great every sixth size (e.g., no. 2 has twice the area of no. 5 and 4 times that of no. 8). The diameter of stranded wire is approximately 15 percent greater than the diameter of a solid wire of the same cross-sectional area. The gauge numbers and wire designations apply to conductors of all materials. Usually, however, the equivalent wire sizes are denoted for the several materials in comparison to copper (e.g., 4/0 aluminum is equivalent to 2/0 copper). These are indicated in the tables for such conductors.

TRANSMISSION LINES CONDUCTOR TENSION LIMITS BASIC What Are The Tension Limits Of Transmission Lines Conductors?  Conductor Tension Limits The NESC recommends limits on the tension of bare overhead conductors as a percentage of the conductor’s rated breaking strength. The tension limits are: 60% under maximum i ce and wind load, 33.3% initial unloaded (when installed) at 60°F, and 25% final unloaded (after maximum loading has occurred) at 60°F. It is common, however, for lower unloaded tension limits to be u sed. Except in areas experiencing severe ice loading, it is not unusual to find tension limits of 60% maximum, 25% unloaded initial, and 15% unloaded final. This set of specifications could easily result in an actual maximum tension on the order of  only 35 to 40%, an initial tension of 20% and a final unloaded tension level of 15%. In this case, the 15% tension limit is said to govern. Transmission-line conductors are normally not covered with ice, and winds on the conductor are usually much lower than those used in maximum load calculations. Under such everyday conditions, tension limits are specified to limit aeolian vibration to safe levels. Even with everyday lower tension levels of 15 to 20%, it is assumed that vibration control devices will be used in those sections of the line that are subject to severe vibration. Aeolian vibration levels, and thus appropriate unloaded tension limits, vary with the type of  conductor, the terrain, span length, and the use of dampers. Special conductors, such as ACSS, SDC, and VR, exhibit high self-damping properties and may be installed to the full code limits, if desired.

INSULATOR WASHING OF HIGH VOLTAGE TRANSMISSION LINES How To Wash Insulators Of High Voltage Transmission Lines?  Insulator Washing Another common practice is to utilize helicopters for insulator washing. Again, this is a method that allows for the line to remain energized during the process. The helicopter carries a water tank that is refilled at a staging area near the work locatio n. A hose and nozzle are attached to a structure on the helicopter and are op erated by a qualified line worker who directs the water spray and adequately cleans the insulator string. Again, with the ease of access afforded by the helicopter, the speed of this operation can result in a typical three-phase tower being cleaned in a few minutes.

Inspections Helicopters are invaluable for tower line and structure inspections. Due to the ease of the practice and the large number of inspections that can be accomplished, utilities have increased the amount of maintenance inspections being done, thus promoting system reliability. Helicopters typically carry qualified line workers who utilize stabilizing binoculars to visually inspect the transmission tower for signs of rusting or weakness and the transmission hardware and conductor for damage and potential failure. Infrared inspections and photographic imaging can also be accomplished from the helicopter, either by mounting the cameras on the helicopter or through direct use by the crew. During these inspections, the helicopter provides a comfortable situation for accomplishing the necessary recording of specific information, tower locations, etc. In addition, inspections from helicopters are required following a catastrophic event or system failure. It is the only logical method of quickly inspecting a transmission system for the exact location and extent of damage.

Helicopter Method Considerations The ability to safely position a helicopter and worker at the actual work site is the most critical consideration when deciding if a helicopter method can be utilized for construction or maintenance. The terrain and weather conditions are obvious factors, as well as the physical spacing needed to position the helicopter and worker in the proximity required for the work method. If live-line work methods are to be utilized, the minimum approach distance required for energized line work must be calculated very carefully for every situation. The geometry of  each work structure, the geometry of the individual helicopter, and the positioning of the helicopter and worker for the specific work method must be analyzed. There are calculations that are available to analyze the approach distances (IEEE Task Force 15.07.05.05, 1999).

When choosing between construction and maintenance work methods, the safety of the line workers is the first consideration. Depending on circumstances, a helicopter method may be the safest work method. Terrain has always been a primary reason for choosing helicopters to assist with projects since the ability to drive to each work site may not be possible. However, helicopters may still be the easiest and most economic alternative when the terrain is open and flat, especially when there are many individual tower locations that will be contacted. Although helicopters may seem to be expensive on a per person basis, the ability to quickly position workers and easily move material can drastically reduce costs. When live-line methods can be utilized, the positioning of workers, material, and equipment becomes comparatively easier. Finally, if the safe use of the helicopter allows the transmission systems to remain energized throughout the project, the helicopter may be the only possible alternative. Since the transmission system is a major link in the competitive energy markets, transmission operation will have reliability performance measures which must be achieved. Purchasing replacement energy through alternate transmission paths, as was done in the regulated world, is no longer an option. Transmission system managers are required to keep systems operational and will be fined if high levels of performance are not attained. The option of deenergizing systems for maintenance practices may be too costly in the deregulated world.

FACTORS AFFECTING POWER TRANSMISSION LINE STRUCTURE SELECTION What Are The Factors Affecting The Selection Of Power Transmission Lines Structures?  There are usually many factors that impact on the selection of the structure type for use in an OHTL. Some of the more significant are briefly identified below.

Erection Technique: It is obvious that different structure types require different erection techniques. As an example, steel lattice towers consist of hundreds of individual members that must be bolted together, assembled, and erected onto the four previously installed foundations. A tapered steel pole, on the other hand, is likely to be produced in a single piece and erected directly on its previously installed foundation in one hoist. The lattice tower requires a large amount of labor to accomplish the considerable number of bolted joints, whereas the pole requires the installation of a f ew nuts applied to the foundation anchor bolts plus a few to install the crossarms. The steel pole requires a large-capacity crane with a high reach which would probably not be needed for the tower. Therefore, labor needs to be balanced against the need for large, special equipment and the site’s accessibility for such equipment.

Public Concerns: Probably the most difficult factors to deal with arise as a result of the concerns of the general public living, working, or coming in proximity to the line. It is common practice to hold public hearings as part of the approval process for a new line. Such public hearings offer a platform for neighbors to express individual concerns that generally must be satisfactorily addressed before the required permit will be issued. A few comments demonstrate this problem. The general public usually perceives transmission structures as ‘‘eyesores’’ and distractions in the local landscape. To combat this, an industry study was made in the late 1960s (Dreyfuss, 1968) sponsored by the Edison Electric Institute and accomplished by Henry Dreyfuss, the internationally recognized industrial designer.

While the guidelines did not overcome all the objections, they did provide a means of  satisfying certain very highly controversial installations (Pohlman and Harris, 1971). Parents of small children and safety engineers often raise the issue of lattice masts, towers, and guys, constituting an ‘‘attractive challenge’’ to determined climbers, particularly youngsters.

Inspection, Assessment, and Maintenance: Depending on the owning utility, it is likely their in-house practices will influence the selection of the structure type for use in a specific line location. Inspections and assessment are usually made by human inspectors who use diagnostic technologies to augment their personal senses of sight and touch. The nature and location of the symptoms of critical interest are such that they can be most effectively examined from specific perspectives. Inspectors must work from the most advantageous location when making inspections.

Methods can include observations from ground or fly-by patrol, climbing, bucket trucks, or helicopters. Likewise, there are certain maintenance activities that are known or believed to be required for particular structure types. The equipment necessary to maintain the structure should be taken into consideration during the structure type selection process to assure there will be no unexpected conflict between maintenance needs and r-o-w restrictions.

Future Upgrading or Uprating: Because of the difficulty of procuring r-o- w’s and obtaining the necessary permits to build new lines, many utilities improve their future options by selecting structure types for current line projects that will permit future upgrading and=or uprating initiatives.

HIGH VOLTAGE POWER TRANSMISSION LINES CORONA DISCHARGE EFFECTS What Are The Effects of Corona Discharge on High Voltage Power Transmission Lines?  Impact of corona discharges on the design of high-voltage lines has been recognized since the early days of electric power transmission when the corona losses were the limiting factor. Even today, corona losses remain critical for HV lines below 300 kV. With the development of EHV lines operating at voltages between 300 and 800 kV, electromagnetic interferences become the designing parameters. For UHV lines operating at voltages above 800 kV, the audible noise appears to gain in importance over the other two parameters. The physical mechanisms of these effects —corona losses, electromagnetic interference, and audible noise—and their current evaluation methods are discussed below.

Corona Losses The movement of ions of both polarities generated by corona discharges, and subjected to the applied field around the line conductors, is the main source of energy loss. For AC lines, the movement of the ion space charges is limited to the immediate vicinity of the line conductors, corresponding to their maximum displacement during one half -cycle, typically a few tens of centimeters, before the voltage changes polarity and reverses the ionic movement. For direct current (DC) lines, the ion displacement covers the whole distance separating the line conductors, and between the conductors and the ground. Corona losses are generally described in terms of the energy losses per kilometer of the line. They are generally negligible under fair-weather conditions but can reach values of several hundreds of kilowatts per kilometer of line during foul weather. Direct measurement of  corona losses is relatively complex, but foul-weather losses can be readily evaluated in test cages under artificial rain conditions, which yield the highest energy loss. The results are expressed in terms of the generated loss W, a characteristic of the conductor to produce corona losses under given operating conditions.

Electromagnetic Interference Electromagnetic interference is associated with streamer discharges that inject current pulses into the conductor. These pulses of steep front and short duration have a high harmonic content, reaching the tens of megahertz range. A tremendous research effort was devoted to the subject during the years 1950 –1980 in an effort to evaluate the electromagnetic interference from HV lines. The most comprehensive contributions were made by Moreau and Gary (1972a,b) of E ´ lectricite´ de France, who introduced the concept of the excitation function, G(v), which characterizes the ability of a line conductor to generate electromagnetic interference under the given operating conditions.

TRANSMISSION LINE ARRESTER STANDARD GUIDELINES Line Arrester Standards Line arresters are not specifically addressed in C62.11-1999, although the arresters used in these applications are part of the standard. Most of the test requirements that apply to line arresters are based on station requirements or distribution c lass requirements.

When specifying line arresters, it should be noted that the following points are inherent to C62.111999.

1. Lightning energy handling capability can be a major factor in selecting line arresters depending on their application. The requirement of lightning related energy is typically much more significant for lines than stations. Although present standards do contain some lightning-related tests, there is no t presently an accepted test to quantify the lightning energy handling capability of surge arresters. Published energy handling capability of arresters is typically based on switching-related tests.

2. Heavy-duty distribution arresters may be subjected to more severe lightning-related tests than station class or intermediate class arresters. Although it is common belief that arrester lightning energy capabilities increase from heavy-duty distribution to intermediate to station, the present standards do not necessarily prove this through testing.

3. The 100-kA test for heavy-duty distribution arresters should not be confused with an arrester surviving a 100-kA lightning stroke. First, the 100-kA test is a 4 x 10 ms wave that has much less energy than a typical 100-kA lightning stroke. Second, the 100 -kA tests allow up to 5 minutes before the arrester is connected to MCOV to prove thermal stability.

4. Short-circuit tests permit polymer arresters to fall apart as long as the pieces fall within specific areas. The tests allow 2 minutes before the arrester must self-extinguish. These allowances in the present standards may not be acceptable for certain areas on a line right-of-way.