What Is Corona Effect
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What is Corona Effect One of the phenomena associated with all energized electrical devices, including high- voltage 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. Source 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. The conductors chosen for the Calumet to the line were selected to have large diameters and to utilize a two conductor bundle. This reduces the potential to create audible noise. • 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. The new Calumet to Comanche 345 kV double circuit line was modeled with an elevation of 6000 feet. • Raindrops, snow, fog, hoarfrost, 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 kV rms/cm. Glow discharge is a light glow off sharp points that does not generate objectionable RIV/TVI or cause any audible noise. • At about 25 kV rms/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 1. 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. 2. 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 3. Corona and the Humidity • Corona has an inverse relationship with humidity at power frequencies • Fair weather corona is more prevalent in low humidity environments 4. 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 steady- state 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 1. 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. 2. 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. 3. Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well. 4. 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. 5. Using Bundled Conductors: on our 345 kV lines, we have installed multiple conductors per phase. This is a common way of increasing the effective diameter of the conductor, which in turn results in less resistance, which in turn reduces losses. 6. 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. 7. Using Corona rings: On certain new 345 kV structures, we are now installing 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. 8. Whether: Corona phenomena much worse in foul weather, high altitude 9. New Conductor: New conductors can lead to poor corona performance for a while. 10. 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.