What is Corona?
Corona is a self-electric discharge of atmospheric air. When the operating voltage of the transmission line is much higher than the dielectric strength of the air, molecules get ionized and a conducting path is created between the conductors, which is called Corona. Ozone gas is released if Corona occurs.
Critical Disruptive Voltage:
The voltage at which Corona initiates is called Critical Disruptive Voltage [Vc]. At this voltage, Corona is not visible but noise can be heard.
Vc = mₒgrδ㏑(d/r) kV/ph (RMS)
Where,
mₒ = Surface or irregularity factor = 0.8-0.9 for ACSR/Rough Conductor
g = Dielectric Strength of the air = 21.1kV/cm (RMS)
r = Radius of the Conductor in cm
δ = Air Density Factor = [3.92b/(273+T)]
b = Baromatic atmospheric pressure in cm of Hg
T = Temperature in °C
Critical Visual Voltage:
The voltage at which Corona becomes visible with a violet or blue color is called Critical Visual Voltage [Vv].
Vv = mᵥgrδ[1 + 0.3/√(rδ)]㏑(d/r) kV/ph (RMS)
| Corona in Transmission Lines |
Corona Power Loss[Pₗ]:
If per phase operating voltage of the transmission line [Vph] becomes more than Critical Disruptive Voltage [Vc], then Corona occurs which produces Power Loss. It depends upon the frequency of transmission line voltage and is proportional to the frequency of the transmission line.
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.
Pₗ = 241×10-5 {(f+25)/δ}{√(r/d)}{Vph-Vc}² kW/km/ph
How can we avoid Corona?
- 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.
- 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.
- Homogenous Insulators: Use a good, homogeneous insulator. Void-free solids, such as properly prepared silicone and epoxy potting materials work well.
- 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 the corona to start.
- Using Bundled Conductors: Using bundled conductors effective diameter of the conductor is increased which increases the value of critical disruptive voltage and hence reduces the power loss associated with corona.
- 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.
- 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.
- Weather: Corona phenomena are much worse in foul weather, high altitude
- New Conductor: New conductors can lead to poor corona performance for a while.
- 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, an increase in the spacing results in heavier metal supports. Cost and Space requirement increases.
- By increasing the conductor's diameter: can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.
Disadvantages of Corona:
- It causes Power Loss.
- Radio Interference Increased
- Charging Current Increased
Advantages of Corona:
- If Corona is initiated by the lightning surge then the steepness of the surge is reduced.