What is corona?

In substations and lines above 110 kV, a \"whooshing\" discharge sound and a pale blue halo can often be heard; this is corona. For a long time, it has been assumed that corona is \"permanent\" – but is it really permanent? Corona occurs because uneven conductors generate an uneven electric field; near electrodes with a small radius of curvature in such an uneven electric field, when the voltage rises to a certain level, discharge takes place due to air ionization, thus forming corona. Because the electric field at the periphery of the corona is weak, collisional ionization does not occur; the charged particles in this peripheral area are mainly electric ions, and it is these ions that constitute the corona discharge current. Simply put, when a conductor electrode with a small radius of curvature discharges into the air, corona is generated. In high-voltage motor stator windings, the electric field is concentrated at the ventilation slots and at the straight exits of these slots, as well as at the ends of the windings. When the field strength in these local areas reaches a certain value, the gas undergoes local ionization, and blue fluorescence appears at those sites; this is the phenomenon of corona. The corona effect generates heat, as well as oxides of ozone and helium, which raises the local temperature inside the coil. This leads to the deterioration and carbonization of the adhesive, as well as the whitening of the wire insulation and mica, thereby causing the wires to become loose, resulting in short circuits and insulation aging. --- In high-voltage motors, where the windings are located at the ventilation openings and outlet openings, the electric field distribution on their insulated surfaces is highly uneven. When the local field strength reaches a certain value, the gas undergoes local ionization, resulting in a blue halo at the electric corona site, thereby generating a corona. The occurrence of corona is accompanied by the generation of oxides of heat, oxygen, and nitrogen, all of which are extremely harmful to motor insulation. Furthermore, when the thermosetting insulating surface has poor or unstable contact with the slot wall, electromagnetic vibrations can cause spark discharge in the gap within the slot. The local temperature rise caused by this spark discharge will lead to severe erosion of the insulating surface. All of this will cause severe damage to the motor insulation. To effectively eliminate this corona phenomenon, it is very important to correctly determine the parameters of the corona protection structure and select suitable corona protection materials.

Corona phenomenon is the phenomenon of local discharge at the surface of a charged body in a gas or liquid medium, and it often occurs in areas with high electric field strength within an uneven electric field (such as around high-voltage wires or near the tips of charged bodies). Its characteristics include the appearance of a light layer similar to a solar halo, the emission of hissing sounds, and the production of ozone, nitrogen oxides, and other substances. In a uniform electric field, since the electric field strength is the same at all points, when a steady-state voltage (DC or power-frequency AC) is applied and the electric field strength reaches the breakdown strength of air, the gap breaks down. But uniform electric fields are hard to find in everyday life. Slightly non-uniform electric fields are very common in everyday life. Such as ball-ball gap, ball-plate gap, etc. Taking the ball-ball gap as an example, when the gap distance is less than 1/4D, the electric field is essentially a uniform electric field; when D/4 ≤ S ≤ D/2, the electric field is slightly non-uniform. There are roughly three areas in power systems where corona discharge is likely to occur. The first is at the strain clamps at both ends of the substation busbars; corona discharge in this area is mainly caused by uneven cutting at the ends of the busbars, resulting in burrs, as well as by the sharp split pins used to connect the strain clamps to the insulators, which also contribute to corona discharge. Secondly, at the strain towers along the line, corona can occur because the ends of the jumpers at these strain towers are not cut smoothly; moreover, corona can also arise from the split pins used in the strain clamps and insulator bowls. Thirdly, on straight pole towers, it is mainly because the ends of the split pins used to connect the suspension clamps to the hanging plates are quite sharp, which can also lead to corona discharge. According to incomplete estimates, the amount of electrical energy lost due to corona losses across the country amounts to 2.05 billion kW·h per year.

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Applying a high voltage between the positive and negative electrodes (a distance of 5 mm between the electrodes corresponds to approximately 10 kV) results in corona discharge; essentially, the air is ionized into a plasma state, producing a blue-white flash.

Corona phenomenon is the phenomenon of local discharge at the surface of a charged body in a gas or liquid medium, and it often occurs in areas with high electric field strength within an uneven electric field (such as around high-voltage wires or near the tips of charged bodies). Its characteristics include the appearance of a light layer similar to a solar halo, the emission of hissing sounds, and the production of ozone, nitrogen oxides, and other substances. In a uniform electric field, since the electric field strength is the same at all points, when a steady-state voltage (DC or power-frequency AC) is applied and the electric field strength reaches the breakdown strength of air, the gap breaks down. But uniform electric fields are hard to find in everyday life. Slightly non-uniform electric fields are very common in everyday life. Such as ball-ball gap, ball-plate gap, etc. Taking the ball-ball gap as an example, when the gap distance is less than 1/4D, the electric field is essentially a uniform electric field; when D/4 ≤ S ≤ D/2, the electric field is slightly non-uniform. The discharge voltage for a uniform electric field can also be calculated using a formula, as follows (units in kV): δ—relative density of air ； s—gap distance in cm ； Application note: The difference in non-uniform electric fields lies in the fact that the electric field strength varies at different points within the air gap. Near the electrodes where the electric field lines are more concentrated, the electric field strength is highest, while in areas where the field lines are sparser, the electric field strength is very low. In the case of a rod-rod gap, it constitutes a symmetric non-uniform electric field; the electric field lines are most concentrated at the tips of the electrodes, where the electric field strength is also greatest. When high voltage is applied, local discharge of air occurs near the electrodes – corona discharge. As the voltage increases further, the corona discharge becomes more intense, leading to brush-like discharges within the gap, and eventually breakdown occurs (arc discharge). As in the rod-plate gap, the electric field strength is highest near the sharp electrode; when a high voltage is applied, corona discharge occurs first near the electrode, whereas the power lines on the plate are spaced far apart and do not produce corona. When the voltage is high enough, the rod poles will also exhibit brush-like, spark discharges, which ultimately lead to arc discharge (breakdown). Corona discharge occurs most frequently at places on the conductor shell where the radius of curvature is small, as the charge density there is very high, especially at sharp points. Near a charged surface, the electric field E is proportional to the charge density σ; therefore, the field strength is very high at the tips of conductors (that is, both σ and E are extremely large). Therefore, when the potential of the conductor surrounding the air increases, corona discharge can occur at these tips. Air is generally considered a non-conductor, but it contains a small number of ions produced by cosmic radiation; a positively charged conductor will attract the negative ions in the surrounding air, thereby gradually neutralizing itself. If a charged conductor has a sharp tip, the electric field strength E in the air near that point can become very high. When ions are drawn toward the conductor, they acquire a large acceleration. When these ions collide with the air, they generate a large number of additional ions, making the air highly conductive; at the same time, corona discharge accelerates the discharge from the conductor. Since air molecules emit light when they collide, a bright light can be seen at the tip of the conductor during corona discharge. Application (1): Corona causes power loss and interferes with communications and broadcasting. For example, during thunderstorms, corona discharge at the tips is used; lightning rods employ this method to neutralize charged clouds and prevent lightning strikes. (2) The charging process of an electrostatic copier involves placing the photoconductive drum in a dark environment within an electric field of a certain polarity, thereby causing its surface to be uniformly charged with electricity of that polarity and acquire a specific surface potential. This process is actually a sensitization process for the drum, enabling the drum, which originally lacked photosensitivity, to acquire better photosensitivity. It usually employs the corona discharge method, which involves applying high voltage to an electrode wire at a certain distance from the drum, thereby causing corona discharge and charging the surface of the photoconductor with static electricity; this process is known as \"charging\".