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Contents 1. Abstract 2. Introduction 3. Mechanism of corona formation 4. Types of corona 4.1. Positive corona 4.2. Negative corona 5. Voltage parameters of corona 5.1. Disruptive critical voltage 5.2. Visual critical voltage 6. Factors affecting corona 7. Waveform of corona current 8. Disadvantages 9. Ways to reduce corona 10. Advantages and Applications Abstract Corona is a phenomenon associated with all energized transmission lines. Under certain conditions, the localized electric field near an energized conductor can be sufficiently concentrated to produce a tiny electric discharge that can ionize air close to the conductors (Electric Power Research Institute (EPRI), 1982). This partial discharge of electrical energy is called corona discharge, or corona. Corona Discharge discharge results from electrical discharge and indicates ionization of oxygen and the formation of ozone in the surrounding air. It can eat through a Polymer insulator until it breaks! It produces extremely corrosive Nitric Acid when in a humid atmosphere! Several factors, including conductor voltage, shape and diameter, and surface irregularities such as scratches, nicks, dust, or water drops can affect a conductor’s electrical surface gradient and its corona performance. Corona is the physical manifestation of energy loss, and can transform discharge energy into very small amounts of sound, radio noise, heat, and chemical reactions of the air components. Because power loss is uneconomical and noise is undesirable, corona on transmission lines has been studied by engineers since the early part of this century. Many excellent references exist on the subject of transmission line corona (e.g., EPRI, 1982). Consequently, corona is well understood by engineers and steps to minimize it are one of the major factors in transmission line design for extra high voltage transmission lines (345 to 765 kilovolts (kV)). Corona is usually not a design issue for power lines rated at 230 kV and lower. The conductor size selected for the project’s transmission line is of sufficient diameter to lower the localized electrical stress on the air at the conductor surface and would further reduce already low conductor surface gradients so that little or no corona activity would exist under most operating conditions. To prevent the formation of corona, the working voltage under fair working conditions should be kept atleast 10% less than the disruptive critical voltage. Corona formation may be reduced by increasing the effective radius. Thus steel cored aluminium has the advantage over hard drawn copper conductors on account of the large diameter, other conditions remaining the same. The effective conductor diameter can also be increased by the use of bundled conductors. The corona discharges emit radiations which may introduce noise signals in the communication channels, radio and television receivers in the vicinity. This is called radio interference (RI). Radio noise from overhead power lines is caused by corona on conductors and fittings ,surface discharges on insulators and poor contacts in fitting and insulator strings. The importance of radio interference problem in the present age can hardly be over-emphasized. Great attention is paid to reduce the level of radio interference from EHV lines to tolerable limits. Corona Discharge discharge results from electrical discharge and indicates ionization of oxygen and the formation of ozone in the surrounding air. Corona reduces the magnitude of high voltage steep fronted waves due to lightning or switching by partially dissipating as a corona loss. Corona act as a safety valve for lightning surges, by causing a short circuit. In a positive corona the electron avalanche is initiated by an exogenous ionisation event in a region of high potential gradient. The electrons resulting from the ionisation collision are attracted toward the positive electrode, and the positive ions are repelled from it. The secondary electrons, required to seed further avalanches, are generated at the boundary of ionisation region by photons of light released during the ionisation process. When these photons strike neutral gas molecules they liberate electrons (through to the photoelectric effect), which are then drawn to the positive electrode. It is these electrons which seed and sustain further avalanches. While the electrons travel to the positive electrode the positive ions drift away from it towards the earthed electrode. A feature of negative coronas is that they can only be sustained in fluids which contain electronegative molecules, such as O2, H20 and CO2. These gases have molecules which readily scavenge free electrons. Without electronegative molecules to capture free electrons, small negative ions cannot form, with the result that a simple path of electron flow of ionised gas will form between the two electrodes and an arc will develop. Introduction The term corona has been derived from the glow surrounding the conductor when the operating voltage is sufficiently high. A corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient (the change in the strength of the electric field) exceeds a certain value, but conditions are insufficient to cause complete electrical breakdown or arcing. If the electric field is uniform, a gradual increase in voltage across the gap produces a breakdown of the gap in the form of a spark without any preliminary discharges. But if the field is non-uniform, an increase in voltage will first cause a localized discharge in the gas to appear at the points with the highest electric field intensity, namely at sharp points. Corona discharge usually involves two asymmetric electrodes; one highly curved (such as the tip of a needle, or a small diameter wire) and one of low curvature (such as a plate, or the ground). The high curvature ensures a high potential gradient around one electrode, for the generation of plasma. Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive with respect to the flat electrode we say we have a positive corona, if negative we say we have a negative corona. (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionising inelastic collision at common temperatures and pressures. The phenomenon of corona is accompanied by the following processes: 1. A faint glow appears around the conductor s which is visible in the dark. 2. There is an acoustical noise. 3. There is a tendency in conductors to vibrate. 4. Ozone and oxides of nitrogen are produced. 5. There is a loss of power. 6. There is radio interference. Mechanism of corona formation For an overhead transmission system the atmospheric air, which is the dielectric medium, behaves practically like a perfect insulator when the potential difference between the conductors is small. However the electrons and ions are always present to a small extent in the atmospheric air due to the random action of the ionizing sources such as cosmic rays, ultra violet radiations from the sun, radioactivity of the soil etc. If the voltage impressed between the conductors is of alternating nature, sustained charging current will flow due to the capacitance of the line. With the increase of the voltage, there is a corresponding increase in the electric field intensity. As long as the air is subjected to a uniform electric field intensity of peak value is less than 3×106 volt/m (3000 kV/m or 30kV/cm), the flow of current between the two conductor of the line is negligibly small for practical purposes. But when the electric field intensity (voltage gradient) reaches this critical value of 3×106 volt/m, the air in the immediate vicinity of conductors no more remains a dielectric but it ionizes and becomes conducting this electric break down is accompanied by a faint glow which appears around the conductor and is visible in dark. If the voltage gradient is increased further , the size and the brightness of the luminous envelope goes on increasing until finally a spark or arc is established between the conductor because of the partial breakdown of the insulating property of air between them. The effect of corona is more pronounced at the protruding points of the conductor due to local higher field intensity there. Steps of corona discharge are as follows: Corona discharge of both the positive and negative variety have certain mechanisms in common. 1. A neutral atom or molecule of the medium, in a region of strong electric field (such as the high potential gradient near the curved electrode) is ionized by an exogenous environmental event (for example, as the result of a photon interaction), to create a positive ion and a free electron. 2. The electric field then operates on these charged particles, separating them, and preventing their recombination, and also accelerating them, imparting each of them with kinetic energy. 3. As a result of the energisation of the electrons (which have a much higher charge/mass ratio and so are accelerated to a higher velocity), further electron/positive-ion pairs may be created by collision with neutral atoms. These then undergo the same separating process creating an electron avalanche. Both positive and negative coronas rely on electron avalanches. 4. In processes which differ between positive and negative coronas, the energy of these plasma processes is converted into further initial electron dissociations to seed further avalanches. 5. An ion species created in this series of avalanches (which differs between positive and negative coronas) is attracted to the uncurved electrode, completing the circuit, and sustaining the current flow. The formation mechanism of the surface corona on dielectric plates under negative impulse voltages has been investigated with a high-speed gated image intensifier in atmospheric air.

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