One of the phenomena associated with all energized electrical devices, including highvoltage transmission lines, is corona. The localized electric field near a conductor can be sufficiently concentrated to ionize air close to the conductors. This can result in a partial discharge of electrical energy called a corona discharge, or corona.
What is Corona?
Electric transmission lines can generate a small amount of sound energy as a result of corona. Corona is a phenomenon associated with all transmission lines. Under certain conditions, the localized electric field near energized components and conductors can produce a tiny electric discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight localized change of electric charge. Utility companies try to reduce the amount of corona because in addition to the low levels of noise that result, corona is a power loss, and in extreme cases, it can damage system components over time. Corona occurs on all types of transmission lines, but it becomes more noticeable at higher voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is minor and rarely noticed. During wet and humid conditions, water drops collect on the conductors and increase corona activity. Under these conditions, a crackling or humming sound may be heard in the immediate vicinity of the line. Corona results in a power loss. Power losses like corona result in operating inefficiencies and increase the cost of service for all ratepayers; a major concern in transmission line design is the reduction of losses.
Sources of Corona
The amount of corona produced by a transmission line is a function of the voltage of the line, the diameter of the conductors, the locations of the conductors in relation to each other, the elevation of the line above sea level, the condition of the conductors and hardware, and the local weather conditions. Power flow does not affect the amount of corona produced by a transmission line.
The electric field gradient is greatest at the surface of the conductor. Large-diameter conductors have lower electric field gradients at the conductor surface and, hence, lower corona than smaller conductors, everything else being equal.
Irregularities (such as nicks and scrapes on the conductor surface or sharp edges on suspension hardware) concentrate the electric field at these locations and thus increase the electric field gradient and the resulting corona at these spots. Similarly, foreign objects on the conductor surface, such as dust or insects, can cause irregularities on the surface that are a source for corona.
Corona also increases at higher elevations where the density of the atmosphere is less than at sea level. Audible noise will vary with elevation. An increase in 1000 feet of elevation will result in an increase in audible noise of approximately 1 dB (A). Audible noise at 5000 feet in elevation will 5 dB (A) higher than the same audible noise at sea level, all other things being equal.
Raindrops, snow, fog, frost, and condensation accumulated on the conductor surface are also sources of surface irregularities that can increase corona. During fair weather, the number of these condensed water droplets or ice crystals is usually small and the corona effect is also small. However, during wet weather, the number of these sources increases (for instance due to rain drops standing on the conductor) and corona effects are therefore greater. During wet or foul weather conditions, the conductor will produce the greatest amount of corona noise. However, during heavy rain the noise generated by the falling rain drops hitting the ground will typically be greater than the noise generated by corona and thus will mask the audible noise from the transmission line.
Corona produced on a transmission line can be reduced by the design of the transmission line and the selection of hardware and conductors used for the construction of the line. For instance the use of conductor hangers that have rounded rather than sharp edges and no protruding bolts with sharp edges will reduce corona. The conductors themselves can be made with larger diameters and handled so that they have smooth surfaces without nicks or burrs or scrapes in the conductor strands. The transmission lines proposed here are designed to reduce corona generation.
Types of Corona
There are three types of corona.
- A glow discharge occurs at a gradient of approximately 20 kVrms/cm. Glow discharge is a light glow off sharp points that does not generate objectionable RIV/TVI (Radio Influence Voltage/Television Interference) or cause any audible noise.
- At about 25 kVrms/cm, negative polarity “brush” discharges occur. So named because the appearance is similar to the round ends of a bottle brush. The audible noise associated with brush corona is generally a continuous background type of hissing or frying noise.
- At a gradient of around 30 kVrms/cm positive polarity plume corona is generated; so named because of its general resemblance to a plume. When viewed in the dark it has a concentrated stem that branches and merges into a violet-colored, tree-like halo. The audible noise associated with plume corona is a rather intense snapping and hissing sound. Plume corona generates significant RIV/TVI.
- These observations are based on fair weather conditions. Under wet conditions virtually all energized electrodes will be in corona of one form or another.
- Many are under the impression that the dielectric strength of air is greater under dry conditions. That is not true. In fact, the dielectric strength of air increases with increased moisture up to the dew point when moisture begins to condense on the surface of insulators and other components of the line.
Physical Parameters of Corona
- Corona is caused by the ionization of the media (air) surrounding the electrode (conductor)
- Corona onset is a function of voltage
- Corona onset is a function of relative air density
- Corona onset is a function of relative humidity
Corona and the Electric Field
- Corona is NOT solely a function of the Electric Field.
- Corona is a function of the electric field on the surface of the electrode (conductor).
- Corona is also a function of the radius of curvature of the electrode (conductor).
- Corona is also a function of the rate of decay of the electric field away from the electrode (conductor).
- For the preceding reasons, selecting the conductor with the smallest electric field at its surface is not correct.
Corona and the Relative Air Density
- Corona has an inverse relationship with air density.
- Standard line designs that perform well at sea level, may have significant corona issues if used on lines that are installed over mountainous areas
Corona and the Humidity
- Corona has an inverse relationship with humidity at power frequencies.
- Fair weather corona is more prevalent in low humidity environments.
Corona is Dependent Surface Condition of the Conductors
- Corona is enhanced by irregularities on the conductor surface.
- Irregularities include: dust, insects, burrs and scratches and water drops present on new conductors.
- Corona will generally be greater on new conductors and will decrease to a steadystate value over a period of approximately one year in-service.
- Corona is significantly increased in foul weather.
What’s The Fuss?
- Corona from conductors and hardware may cause audible noise and radio noise
- Audible noise from conductors may violate noise standards
- Radio noise from conductors may interfere with communications or navigation
- Corona loss may be significant when compared with resistive loss of conductors
- Corona can cause possible damage to polymeric insulators
Methods to reduce Corona Discharge Effect
Corona can be avoided by:
- By minimizing the voltage stress and electric field gradient: This is accomplished by using utilizing good high voltage design practices, i.e., maximizing the distance between conductors that have large voltage differentials, using conductors with large radii, and avoiding parts that have sharp points or sharp edges.
- Surface Treatments: Corona inception voltage can sometimes be increased by using a surface treatment, such as a semiconductor layer, high voltage putty or corona dope.
- Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well.
- If you are limited to using air as your insulator, then you are left with geometry as the critical parameter. Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients, which can cause corona to start.
- Using Bundled Conductors: Installing multiple conductors per phase is a common way of increasing the effective diameter of the conductor, which in turn results in less resistance, which in turn reduces losses.
- Elimination of sharp points: electric charges tend to form on sharp points; therefore when practicable we strive to eliminate sharp points on transmission line components.
- Using Corona rings: These rings have smooth round surfaces which are designed to distribute charge across a wider area, thereby reducing the electric field and the resulting corona discharges.
- Weather: Corona phenomena much worse in foul weather, high altitude.
- New Conductor: New conductors can lead to poor corona performance for a while.
- By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phase’s results in heavier metal supports. Cost and Space requirement increases.
- By increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.
Sources of Corona and Arcing in Polymer Insulators
- Loose hardware
- Contamination and surface tracking
- Missing corona rings
- Damaged or incorrectly installed corona ring
- Damaged end fittings or end fitting seal
- Exposed internal rod due to carbonized internal rod by internal discharges Split sheath due to weathering
Electro Magnetic Inductions
- EM field or radio noise field from high-voltage transmission lines are caused by corona, which is essentially due to the electrical breakdown of the air surrounding the conductors at higher voltage.
- When the conductor surface electric field exceeds the corona onset electric field, a partial electrical breakdown occurs in the surrounding air medium near the conductor surface and is called the corona discharge. The increase of conductor surface gradient takes place with increase of supply voltage. In addition, organic contamination or attachment -of water droplets also may contribute to localized field enhancement.
- When organic particles or water droplets are attached to the conductor surface, the charge accumulation at that point increases which enhances the local electric field. The intensification of surface gradient locally leads to the corona discharge.
- The streamer generated during corona discharge, transports electric charge into the surrounding air during the discharge cycle. These moving charges contribute directly to the noise fields. ‘They also cause currents to be induced on the transmission line conductors. Since the charge is moved by a time varying electric field, it is equivalent to a current pulse and this When a communication line passes near the corridor of a HV or EHV transmission line, if the frequency of the radiated EM signal due to corona matches with that of the transmitted signal on the communication line, then the communication signal may get distorted. To mitigate this effect, the communication line should pass at a safe distance away from the transmission line.
- Hence there is a need to estimate the radiated EM1 signal in dB at a given distance from the HV or EHV transmission line. In this paper, radiated EM1 in dB is computed for a single conductor high voltage over headline. This theoretical result is compared with the published experimental results available in the literature. In the computational work, earth is considered as an infinitely conducting ground.
Physical description of corona and Electro Magnetic Induction
- When alternating supply voltage energizes the conductor, the conductor surface electric field exceeds the corona on set electric field of the conductor. The corona discharge occurs in both positive and negative half cycle. So the corona is divided into positive and negative corona depending upon the polarity of the supply voltage.
- When the conductor is positive with respect to ground, an electron avalanche moves rapidly into the conductor leaving the heavy positive-ion charge cloud close to conductor, which drifts away.
- The rapid movements of electrons and motion of positive ions gives the steep front of the pulse, while the further drift of positive ions will give slow tail of the corona pulse.
- When conductor is negative with respect to the ground, an electron avalanche moves away from the energized conductor and the positive heavy ions move towards the conductor. Since the heavy positive ions are, moving towards the higher electric field, their motion is very rapid which gives rise to a much sharper pulse than the positive pulse. Due to rapid moment of the electrons from the conductor surface, the electric field regains its original value at conductor surface very quickly than in the case of positive polarity. Thus the negative corona pulses are lower in amplitude and lower in rise and fall times as compared to positive corona pulses. They have also higher repetition rates than the positive pulse
- Light Ultraviolet radiation: Corona can be visible in the form of light, typically a purple glow, as corona generally consists of micro arcs. Darkening the environment can help to visualize the corona.
- Sound (hissing, or cracking as caused by explosive gas expansions): You can often hear corona hissing or cracking Sound.
- In addition, you can sometimes smell the presence of ozone that was produced by the corona.
- Salts, sometimes seen as white powder deposits on Conductor.
- Mechanical erosion of surfaces by ion bombardment
- Heat (although generally very little, and primarily in the insulator)
- Carbon deposits, thereby creating a path for severe arcing
- The corona discharges in insulation systems result in voltage transients. These pulses are superimposed on the applied voltage and may be detected, which is precisely what corona detection equipment looks for. Commercially available corona detectors include electronic types (as above) as well as ultrasonic types.
The following corona calculations are from Dielectric Phenomena in High Voltage Engineering
1. For Concentric Cylinders in Air: Corona will not form when RO / RI < 2.718. (Arcing will occur instead when the voltage is too high.)
2. For Parallel Wires in Air: Corona will not form when X / r < 5.85. (Arcing will occur instead when the voltage is too high.)
3. For Equal Spheres in Air: Corona will not form when X / R < 2.04. (Arcing will occur instead when the voltage is too high.) Arcing difficult to avoid when X / R < 8
- RO = Radius of outer concentric sphere
- RI = Radius of inner concentric sphere
- R = Sphere radius
- r = wire radius
- X = Distance between wires or between spheres
Effects of Corona
- Audible Noise
During corona activity, transmission lines (primarily those rated at 345 kV and above) can generate a small amount of sound energy. This audible noise can increase during foul weather conditions. Water drops may collect on the surface of the conductors and increase corona activity so that a crackling or humming sound may be heard near a transmission line. Transmission line audible noise is measured in decibels using a special weighting scale, the “A” scale that responds to different sound characteristics similar to the response of the human ear. Audible noise levels on typical 230 kV lines are very low and are usually not noticeable. For example, the calculated rainy weather audible noise for a 230 kV transmission line at the right-ofway edge is about 25 dBA, which is less than ambient levels in a library and much less than background noise for wind and rain.
- Radios and Television Interference
Overhead transmission lines do not, as a general rule, interfere with radio or TV reception. There are two potential sources for interference: corona and gap discharges. As described above, corona discharges can sometimes generate unwanted electrical signals. Corona-generated electrical noise decreases with distance from a transmission line and also decreases with higher frequencies (when it is a problem, it is usually for AM radio and not the higher frequencies associated with TV signals). Corona interference to radio and television reception is usually not a design problem for transmission lines rated at 230 kV and lower. Calculated radio and TV interference levels in fair weather and in rain are extremely low at the edge of the right-of-way for a 230 kV transmission line. Gap discharges are different from corona. Gap discharges can develop on power lines at any voltage. They can take place at tiny electrical separations (gaps) that can develop between mechanically connected metal parts. A small electric spark discharges across the gap and can create unwanted electrical noise. The severity of gap discharge interference depends on the strength and quality of the transmitted radio or TV signal, the quality of the radio or TV set and antenna system, and the distance between the receiver and power line. (The large majority of interference complaints are found to be attributable to sources other than power lines: poor signal quality, poor antenna, door bells, and appliances such as heating pads, sewing machines, freezers, ignition systems, aquarium thermostats, fluorescent lights, etc.). Gap discharges can occur on broken or poorly fitting line hardware, such as insulators, clamps, or brackets. In addition, tiny electrical arcs can develop on the surface of dirty or contaminated insulators, but this interference source is less significant than gap discharge. Hardware is designed to be problem-free, but corrosion, wind motion, gunshot damage, and insufficient maintenance contribute to gap formation. Generally, interference due to gap discharges is less frequent for high-voltage transmission lines than lower-voltage lines. The reasons that transmission lines have fewer problems include: predominate use of steel structures, fewer structures, greater mechanical load on hardware, and different design and maintenance standards. Gap discharge interference can be avoided or minimized by proper design of the transmission line hardware parts, use of electrical bonding where necessary, and by careful tightening of fastenings during construction. Individual sources of gap discharge noise can be readily located and corrected. Arcing on contaminated insulators can be prevented by increasing the insulation in high contamination areas and with periodic washing of insulator strings.
- Gaseous Effluents
Corona activity in the air can produce very tiny amounts of gaseous effluents: ozone and NOX. Ozone is a naturally occurring part of the air, with typical rural ambient levels ranging from about 10 to 30 parts per billion (ppb) at night and peaks at approximately 100 ppb. In urban areas, concentrations exceeding 100 ppb are common. After a thunderstorm, the air may contain 50 to 150 ppb of ozone, and levels of several hundred ppb have been recorded in large cities and in commercial airliners. Ozone is also given off by welding equipment, copy machines, air fresheners, and many household appliances. The Ambient Air Quality Standard for Oxidants (ozone is usually 90 to 95 percent of the oxidants in the air) is 120 ppb, not to be exceeded as a peak concentration on more than one day a year. In general, the most sensitive ozone measurement instrumentation can measure about 1 ppb. Typical calculated maximum concentrations of ozone at ground level for 230 kV transmission lines during heavy rain are far below levels that the most sensitive instruments can measure and thousands of times less than ambient levels. Therefore, the proposed transmission lines would not create any significant adverse effects in the ambient air quality of the project area.
- Induced Currents
Small electric currents can be induced by electric fields in metallic objects close to transmission lines. Metallic roofs, vehicles, vineyard trellises, and fences are examples of objects that can develop a small electric charge in proximity to high voltage transmission lines. Object characteristics, degree of grounding, and electric field strength affect the amount of induced charge. An electric current can flow when an object has an induced charge and a path to ground is presented. The amount of current flow is determined by the impedance of the object to ground and the voltage induced between the object and ground. The amount of induced current that can flow is important to evaluate because of the potential for nuisance shocks to people and the possibility of other effects such as fuel ignition. The amount of induced current can be used to evaluate the potential for harmful or other effects. As an example, when an average woman or man grips an energized conductor, the threshold for perception of an electric current is 0.73 milliampere (mA) and 1.1 mA, respectively. If the current is gradually increased beyond a person’s perception threshold, it becomes bothersome and possibly startling. However, before the current flows in a shock situation, contact must be made, and in the process of establishing contact a small arc occurs. This causes a withdrawal reaction that, in some cases, may be a hazard if the involuntary nature of the reaction causes a fall or other accident. The proposed 230 kV transmission lines will have the highest electric field within the right-of-way, approximately 0.2 to 1.5 kV per meter (kV/m), and approximately 0.1 to 0.9 kV/m at the edge of the right-of-way. These fields are less than many other 230 kV transmission lines due to the use of cross-phasing on the double-circuit lines and higher clearance above ground. Induced currents have been calculated for common objects for a set of worst-case theoretical assumptions: the object is perfectly insulated from ground, located in the highest field, and touched by a perfectly grounded person. Even though the maximum electric field only occurs on a small portion of the right-of-way, and perfect insulation and grounding states are not always common, the calculated induced current values are very low therefore, in most situations, even in the highest field location, induced currents are below the threshold of perception and are far below hazardous levels. Agricultural operations can occur on or near a transmission line right-of-way. Irrigation systems often incorporate long runs of metallic pipes that can be subject to magnetic field induction when located parallel and close to transmission lines. Because the irrigation pipes contact moist soil, electric field induction is generally negligible, but annoying currents could still be experienced from magnetic field coupling to the pipe. Pipe runs laid at right angles to the transmission line will minimize magnetically induced currents, although such a layout may not always be feasible. If there are induction problems, they can be mitigated by grounding and/or insulating the pipe runs. Operation of irrigation systems beneath transmission lines presents another safety concern. If the system uses a high-pressure nozzle to project a stream of water, the water may make contact with the energized transmission line conductor. Generally, the water stream consists of solid and broken portions. If the solid stream contacts an energized conductor, an electric current could flow down the water stream to someone contacting the high-pressure nozzle. Transmission line contact by the broken-up part of the water stream is unlikely to present any hazard.
- Fuel Ignition
If a vehicle were to be refueled under a high-voltage transmission line, a possible safety concern could be the potential for accidental fuel ignition. The source of fuel ignition could be a spark discharge into fuel vapors collected in the filling tube near the top of the gas tank. The spark discharge would be due to current induced in a vehicle (insulated from ground) by the electric field of the transmission line and discharged to ground through a metallic refueling container held by a well-grounded person. Theoretical calculations show that if a number of unlikely conditions exist simultaneously, a spark could release enough energy to ignite gasoline vapors. This could not occur if a vehicle were simply driven or parked under a transmission line. Rather, several specific conditions would need to be satisfied: A large gasoline-powered vehicle would have to be parked in an electric field of about 5 kV/m or greater. A person would have to be refueling the vehicle while standing on damp earth and while the vehicle is on dry asphalt or gravel. The fuel vapors and air would have to mix in an optimum proportion. Finally, the pouring spout must be metallic. The chances of having all the conditions necessary for fuel ignition present at the same time are extremely small. Very large vehicles (necessary to collect larger amounts of electric charge) are often diesel-powered, and diesel fuel is less volatile and more difficult to ignite. The proposed 230 kV transmission line electric field levels are too low (about 0.2-1.5 kV/m on the right-of-way) for the minimum energy necessary for fuel ignition under any practical circumstances.
- Cardiac Pacemakers
One area of concern related to the electric and magnetic fields of transmission lines has been the possibility of interference with cardiac pacemakers. There are two general types of pacemakers: asynchronous and synchronous. The asynchronous pacemaker pulses at a predetermined rate. It is practically immune to interference because it has no sensing circuitry and is not exceptionally complex. The synchronous pacemaker, on the other hand, pulses only when its sensing circuitry determines that pacing is necessary. Interference resulting from the transmission line electric or magnetic field can cause a spurious signal in the pacemaker’s sensing circuitry. However, when these pacemakers detect a spurious signal, such as a 60 hertz (Hz) signal, they are programmed to revert to an asynchronous or fixed pacing mode of operation and return to synchronous operation within a specified time after the signal is no longer detected. The potential for pacer interference depends on the manufacturer, model, and implantation method, among other factors. Studies have determined thresholds for interference of the most sensitive units to be about 2,000 to 12,000 milligauss (mG) for magnetic fields and about 1.5 to 2.0 kV/m for electric fields. The electric and magnetic fields at the right-of-way edge are below these values, and on the right-of-way, only the lower bound electric field value of 1.5 kV/m is reached. Therefore, the potential impact would not be significant.
- Computer Interference
Personal computer monitors can be susceptible to 60 Hz magnetic field interference. Magnetic field interference results in disturbances to the image displayed on the monitor, often described as screen distortion, “jitter,” or other visual defects. In most cases it is annoying, and at its worst, it can prevent use of the monitor. Magnetic fields occur in the normal operation of the electric power system. This type of interference is a recognized problem by the video monitor industry. As a result, there are manufacturers who specialize in monitor interference solutions and shielding equipment. Possible solutions to this problem include: relocation of the monitor, use of magnetic shield enclosures, software programs, and replacement of cathode ray tube monitors with liquid crystal displays that are not susceptible to 60 Hz magnetic field interference. Because these solutions are widely available to computer users, potential impacts would be less than significant.
The ring, which surrounds the energized end of the transformer bushing, serves two functions. It is a corona ring that is intended to electrically shield the bushing terminal and connections. It does so by reducing the voltage gradient to a level well below the ionizing gradient of the surrounding air at the maximum transformer output voltage. It’s also a grading ring, which helps electrically grade the external voltage on the bushing from line to ground (at the bushing flange). The bushing is likely a condenser bushing, which contains a capacitance-graded core to grade the voltage radically from 100% at the central conductor to ground at the flange and, axially from ground to the top and bottom ends of the core. Grounding the test transformer following a circuit breaker test is necessary for safety but you are grounding the entire test circuit; not just the corona ring. I suspect the corona ring just happens to be a convenient attachment point for the hook on your ground stick. Die cast are usually 380, sand and permanent mold 356 or A356, and fabricated rings are usually made from 6061 thin wall tubing or pipe that is formed and welded; with appropriate brackets and other mounting provisions. Corona grading ring should be designed to reduce the critical dielectric voltage gradient (typ. 20 to 30 kVrms/cm) to prevent corona effect, internal discharge and reduce E-field in live parts and fitting that cause radio/ TV interference (RIV), audio noise and losses. Corona ring could also help to smooth the voltage profile distributing the voltage more uniform along the insulator preventing concentration of over stresses. For porcelain post insulators, some manufacturer recommends one corona ring and for 500 kV and above two rings. However, for composite insulator the corona ring is recommended for 220/230 kV. Most equipment manufacturer provide corona ring base on testing such surge arrester, switches, CT’s/PTs, etc.
Difference between Arcing Horn Gap and Corona Ring
At transmission line voltage the arcing horns, when the breaker is closed normally have nothing except corona from the tips and arc marks, the instant the breaker begins to open an arc is established across the gap between the arc horns, when the gap is long enough the arc breaks. The plan is to keep the sliding contacts from getting arc metal removal so the contacts maintain low resistance, arcing horns are sacrificial. At switchgear voltage, there are arc chutes and usually puffers to extinguish the arc during breaker opening, the arc chutes may be of a sand-crystal cast material (like space shuttle heat tiles), asbestos layers, and electrical insulating board to protect the works during an explosive event when temperatures get hotter than the sun. There is specific NFPA training for arc flash exposure. Arcing horns are also commonly used to protect insulation from impulse and other over voltages. The horn gap (distance between arcing horns) is set to ensure that flashover occurs across the gap rather than along the insulation surface thereby protecting the insulation surface and preventing arc termination and associated damage to the end terminals or line and ground end hardware. They may also be used to connect a surge arrester to protect transformers and other equipment from overvoltage surges (gapped arrester). A gapped connection is one method of preventing line lockout in the event of arrester failure. Corona rings are meant to distribute the electrical field and neither the hardware protected or the corona ring should have corona, the typical line voltage that corona rings are applied is 150KV and higher, altitude or high temperatures can reduce the voltage to 138KV lines. Properly designed corona rings do not have corona. Corona can appear to start and stop at essentially the same voltage, there are other variables. Corona produces light (from UV to visible and into the infrared), sound (all wavelengths), ozone, and nitric acid (in the presence of moisture). Arcing arrestors were used long ago, some of the old-old transmission lines. They were opposing arcing fingers mounted in parallel with the insulators; the gap determined the flash-over voltage. The intent was to protect insulators from lightening surges. To break an arc the voltage must be decreased below about 60% of the voltage an arc starts at, thus if a transmission line insulator arcing arrestor flashes over and maintains an arc the line is going to be shutdown. Thus arcing arrestors (without an arc extinguishing capability) decrease the reliability of a transmission line.