Therefore, careful design of the overhead transmission lines should be made to avoid the corona loss at 33 kV or higher voltages, otherwise highly ionized air may cause flashover in the line insulators or between the line conductors, causing considerable damage to the equipment. In practice, it has been observed that the corona effect occurs at a line voltage of 33 kV or above. Therefore, a stranded conductor has irregular surface and hence gives rise to more corona effect than a solid conductor. The rough and uneven surface will give rise more corona loss since the unevenness of the surface of the conductor reduces the value of breakdown voltage. Size of the Conductor – The corona loss is also affected by the shape and size of the conductors. It is because the large space between the conductors decreases the electrostatic stresses at the conductor which helps in avoiding the corona formation. Spacing between Conductors – If the spacing between the conductors is made very large as compared to their diameters, then there may not be any corona discharge. Although, if the line voltage has a very value such that there is the electrostatic stresses developed at the conductor surface making the surrounding air conducting, then corona effect takes place. If the line voltage is low, then there is no change in the condition of the air surrounding the conductors and hence no corona occurs. Line Voltage – The corona loss is greatly affected by the line voltage. In the stormy weather, the number of ions is more than the normal and hence the corona occurs at very low voltage as compared to the fair weather. Therefore, the physical state of the atmosphere affects the corona loss.
The factors which affect the corona loss in the overhead transmission line are given as follows −Ītmosphere – Since the corona loss is formed due to the ionization of the surrounding air of the conductors. The physical state of the atmosphere and the conditions of the transmission line affects the phenomenon of corona. the positive wire has uniform corona glow about it and the negative wire has spotty glow.
In case of DC transmission lines, there is a difference in the appearance of corona glow for the two conductors, i.e. If the transmission conductors are polished and smooth, then the violet glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. If the applied voltage attained the breakdown value, then a flashover will occur between the conductors due to breakdown of the insulation of air. The higher is the transmission voltage, the larger and higher the luminous envelope becomes and the higher are the power loss and the radio interference. In practice, the corona effect is accompanied by a hissing sound, production of ozone, power loss and radio interference. When this applied voltage exceeds a certain value (called critical disruptive voltage), then the conductors are surrounded by a faint violet glow that is known as corona effect. When a very high voltage is applied across two conductors of the overhead transmission line whose spacing is large as compared to their diameters. The phenomenon of violet glow, hissing sound and production of ozone gas in an overhead transmission line is known as corona. Corona Effect in Overhead Transmission Line