What is Corona Effect?
Corona is the phenomenon in which hissing sound, violet glow, and production of ozone take place in the transmission line. It is not necessary that the violet glow appears, but the hissing sound is always present in Corona. The effect which is the cause of corona is known as Corona Effect.
Formation of Corona:
In overhead lines, there is a potential difference applied to the conductors. This potential difference creates a potential gradient in the air. This potential gradient increases the velocity of the ionized particles of the air such as the free electron. These free electrons move in the air and strike other molecules. The speed of strike depends on the potential gradient. When the potential gradient is equal to the breakdown strength then the speed of electrons is enough to eject one or more electron from the neutral molecules present in the air. This ionization produces corona. This process continues unless the potential gradient is lowered.
The voltage gradient can be found as,
Where,
V = Phase to Neutral Voltage
r = Radius of conductors in cm
d = Distance between the conductors in cm
Critical Disruptive Voltage:
It is the minimum phase to the neutral voltage at which the corona occurs. This voltage creates enough potential gradient to carry out continuous ionization. The formula can be given as,
Where,
VC = Critical Disruptive Voltage
r = Radius of conductors
d = Distance between the conductors
g˳ = Dielectric Strength/ Breakdown Strength
The Breakdown strength (g˳) is 30kV/cm (Peak Value or Maximum Value) at atmospheric pressure 76cm of Hg and temperature of 25˚C.
The r.m.s value of breakdown strength is 21.2kV/cm.
Consideration Factors:
Air Density Factor (δ):
Corona depends on the molecules of the surrounding atmosphere. If the air density is changed, then it will affect corona. If there are more molecules present, then the required voltage for corona will be less. The effect of Air Density factor can be included as,
Where δ is the air density factor, its value can be found as,
b = atmospheric pressure in cm of Hg
t = temperature in ˚C
Irregularity Factor (m˳):
This is the factor due to the unevenness of the surface of the conductor due to dirt, strands or roughness. Unevenness reduces the minimum voltage for corona. Its typical values are,
The final equation including both the factors can be given as,
Visual Critical Voltage:
This is the minimum phase to the neutral voltage at which the violet glow appears. It can be given as,
Corona Power Loss:
Due to corona, power is lost. The power loss depends on the frequency, applied voltage, the radius of conductor and the distance between the conductors. The formula can be given as,
Where,
f = Applied frequency
V = Applied phase to neutral voltage (r.m.s)
VC = Critical Disruptive Voltage
If the Critical Disruptive Voltage is decreased due to increase density of molecules in stormy weather, the corona loss is increased.
Factors Affecting Corona:
Applied Voltage:
If the applied voltage is less, at which the potential gradient is not equal or greater than the breakdown strength, then there will be no corona.
Size of Conductor:
As we know from the power loss equation that the power loss is directly proportional to the square root of conductor radius. This means that by increasing the conductor radius the power loss will increase but this will not happen. The reason is that by increasing the radius, the Critical Disruptive Voltage will increase (eq3). This will decrease (V− VC) which will decrease the power loss. The effect of (V− VC) is more dominant than the effect of the square root of radius.
Hence by increasing the conductor size, the corona loss will decrease.
Spacing between the Conductors:
If the spacing between the conductor is large, then there will be no corona as due to increasing in the distance the Critical Disruptive Voltage will increase (eq3).
Surface of Conductor:
As discussed above, the unevenness will create the corona to occur at less voltage. Hence due to unevenness corona loss will increase.
Weather Condition:
Due to change in weather, the Air Density is changed. If there are more molecules, then the corona will occur at less voltage. For stormy weather, the value Critical Disruptive Voltage is reduced by a factor of 0.8.
Advantages of Corona:
- Due to corona, the effective diameter of the conductor is increased which reduces the electrostatic stress between the conductors.
- Due to lightning and switching, surges are produced. Their transients are reduced by corona.
Disadvantages of Corona:
- Power is lost due to corona.
- There can be deterioration of the conductor due to the production of Ozone.
- It causes radio interference.
Radio Interference:
Corona produces radiations that create disturbance in the radio waves, this is known as Radio Interference. The signal affected by it can be television, radio or communication signal. It affects the area of few kilometers. The interference is significant if the voltage is above 200kV. It is more significant in lower frequency signals.
How to Reduce Corona Effect?
The methods used to reduce corona are,
- Increase the size of the conductor.
- Increase the spacing between the conductors.
- By using bundled and hollow conductors.
Corona in HVDC:
Corona is also present in HVDC. The difference is that both the positive and negative lines have a difference in their glow. There is a uniform glow in the positive line and spotty glow in the negative line.
