What is Corona effects in Electrical Transmission System?

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• Power transmission lines can produce small amounts of sound energy as a result of the corona.
• The corona is a phenomenon associated with all transmission lines. Under certain conditions, the localized electric field near the active components and conductors may produce a small electrical discharge or corona that causes the surrounding air molecules to ionize, or undergo minor local changes of electric charge.
• Utility companies try to reduce the amount of corona because the corona is a power loss in addition to low levels of results, and in extreme cases, it can damage system components over time.
• The corona occurs on all types of transmission lines, but it becomes more noticeable at higher voltages (345 kV and higher). Under fair weather conditions, audible noise from the corona is slight and rarely seen.
• During wet and moist conditions, droplets of water collect on the conductors and increase corona activity. Under these conditions, a hoarse or lukewarm sound can be heard in the vicinity of the line.
• The corona results in power loss. Electrical losses such as the corona result from operational inefficiencies and increase the cost of service for all ratepayers; A major concern in transmission line design is the lack 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 location of the conductors in relation to each other, the line above sea level, the position and hardware of the conductors, and the local weather conditions. Electric current does not affect the amount of corona produced by a transmission line.
• The electric field gradient is largest on the surface of the conductor. Larger-diameter conductors have fewer electric field gradients on the surface of the conductor and, therefore, have fewer coronas than smaller conductors, all else being the same. The calumet was chosen to line the conductors chosen for larger diameters and to use two-conductor bundles. This reduces the ability to produce audible noise.
• Irregularities (such as nicks on the surface of the conductor and sharp edges on the scraper or suspension hardware) concentrate the electric field at these locations and thus increase the electric field gradient and consequent corona in these areas. Similarly, foreign objects on the conductor surface, such as dust or insects, can create irregularities on the surface that are a source for corona.
• The corona also grows at higher altitudes where the density of the atmosphere is lower than the sea level. Audible noise will vary with height. An increase in height of 1000 feet will result in an increase in audible noise of about 1 dB (A). At a height of 5000 feet, the audible noise will be 5 dB (A) compared to the same audio noise at sea level, which will be equal to everything else. The new Calumet for the Komanch 345 kV double circuit line was built with a height of 6000 feet.
• Rain, snow, fog, hoarseness, and condensation that accumulate on the surface of the conductor 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 the wet season, the number of these sources increases (for example, due to standing raindrops on the conductor) and the corona effect is therefore greater.
• During wet or unscrupulous weather conditions, the conductor corona will produce the greatest amount of noise. However, the noise generated by the raindrops hitting the ground during heavy rains will usually be higher than the noise generated by the corona and thus mask the audible noise from the transmission line.
• The corona built on the transmission line can be minimized by the design of the transmission line and the selection of hardware and conductors used to construct the line. For example, the use of conductor hangers that are rounded instead of sharp edges and no bulging bolts with sharp edges will reduce the corona. Conductors themselves can be made and handled with larger diameters so that the conductor strands have a smooth surface without them. The transmission lines proposed here are designed to reduce corona generation.

Types of Corona:

There are three types of the corona.

• A glow discharge occurs at a gradient of about 20 kV RMS / cm. A glow discharge is light flashes from sharp points that do not produce objectionable RIV / TVI or produce any audible noise.
• At approximately 25 kV RMS / cm, a negative polarity "brush" discharge occurs. So named because the appearance is similar to the round ends of a bottle brush. Audible noise related to brushing corona is usually a continuous background type of hissing or frying noise.
• Approximately 30 kVrms / cm positive polarity plume is generated on a gradient of the corona; Hence its name is similar to a mango. When viewed in the dark, it has a concentrated stem that merges into the branches and purple, tree-like aura. The audible noise associated with the plum corona is a sharp snapping and hissing sound. The plume corona forms the important RIV / TVI.

These observations are based on fair weather conditions. In wet conditions, virtually all active electrodes will be in the corona of one form or another.

Many are under the impression that the dielectric strength of air is higher in dry conditions. That is not true. In fact, the dielectric strength of the air increases with an increase in moisture up to the dew point when moisture begins to condense on the surface of the insulator and other components of the line.

Physical parameters of corona:

• Corona is caused by ionization of the media (air) surrounding the electrode (conductor)
• The corona is a function of the starting voltage
• Corona onset is a function of relative air density
• Corona onset is a function of relative humidity

1. Corona and Electric Field

• The corona is not just a function of the electric field
• The corona is a function of the electric field on the surface of the electrode (conductor)
• The corona is also a function of the radius of curvature of the electrode (conductor).
• The corona is also a function of the rate of decay of the electric field away from the electrode (conductor).
• For the foregoing reasons, it is not correct to select a conductor with the smallest electric field on its surface.

2. Corona and 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 that are used on established lines in hilly areas

3. Corona and Humidity

• Corona has an inverse relationship with humidity at power frequencies
• Fairweather corona is more prevalent in low humidity environments

4. Corona is the dependent surface condition of conductors

• The corona is increased by irregularities on the surface of the conductor
• Irregularities include: dust, insects, burr and scratches and water droplets present on new conductors
• The corona will typically be higher on new conductors and will decrease to a steady-state value in about a year of service
• The corona rises considerably during the foul season.