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Corona occurs between two transmission conductor when they
Have high potential difference
Are closely spaced
Carry DC power
Both (a) and (b)
Both (a) and (b)
Corona is an electrical discharge caused by the ionization of the surrounding air when the potential gradient exceeds the dielectric strength of air. It occurs when the operating voltage is high (increasing the potential difference) or when conductors are closely spaced (increasing the electric field intensity between them).
Corona is an electrical discharge caused by the ionization of the surrounding air when the potential gradient exceeds the dielectric strength of air. It occurs when the operating voltage is high (increasing the potential difference) or when conductors are closely spaced (increasing the electric field intensity between them).
E=rln(d/r)VтАЛ тАФ Electric field intensity at the conductor surface
PcтАЛтИЭ(VтИТVcтАЛ)2 тАФ Corona power loss proportional to the square of voltage excess
The electric field intensity E near the surface of a conductor is proportional to the potential difference and inversely proportional to the spacing. When E exceeds the breakdown strength of air (approx. 30kV/cm peak), electrons accelerate and collide with air molecules, causing ionization and a glowing discharge known as corona.
Corona is more prevalent in AC systems compared to DC systems at the same RMS voltage.
The critical disruptive voltage (VcтАЛ) is affected by air density factor and surface irregularity factor.
Higher voltage levels (Extra High Voltage lines) are more susceptible to corona discharge.
Bundled conductors are used to increase the effective radius, thereby reducing the surface potential gradient.
Power loss in the form of heat, light, and sound.
Radio and television interference due to electromagnetic radiation.
Production of ozone gas leading to corrosion of conductors.
High Voltage Transmission Lines
Electrostatic Precipitators
Ozone Generators
The dielectric strength of air is roughly 30kV/cm peak or 21.2kV/cm RMS.
Option B is correct because decreasing spacing d increases the potential gradient g=dVтАЛ, accelerating air ionization.
Option C is technically a factor as well, but 'both (a) and (b)' is the most comprehensive answer given the physics of the potential gradient.
D is correct тАФ Corona discharge is initiated when the electric field intensity at the conductor surface exceeds the dielectric strength of air, which is directly influenced by high potential difference and narrow conductor spacing.
Always remember that bundling conductors is the primary technique to mitigate corona in transmission lines by effectively increasing the radius of the conductor.