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Corona can be reduced by
Increasing operating voltage
Reducing the spacing between conductors
Increasing the effective conductor diameter
Any of above
Increasing the effective conductor diameter
Corona discharge is an electrical phenomenon occurring when the voltage gradient exceeds the dielectric strength of air ┬╖ By increasing the effective conductor diameter, the surface voltage gradient is reduced, thereby suppressing the onset of ionization.
Corona discharge is an electrical phenomenon occurring when the voltage gradient exceeds the dielectric strength of air ┬╖ By increasing the effective conductor diameter, the surface voltage gradient is reduced, thereby suppressing the onset of ionization.
g=rln(D/r)VтАЛ тАФ Electric field intensity at conductor surface
VcтАЛ=gcтАЛтЛЕ╬┤тЛЕrln(D/r) тАФ Disruptive critical voltage formula
Corona occurs when the electric field at the conductor surface exceeds the critical disruptive voltage (gcтАЛ) ┬╖ Since the voltage gradient g is inversely proportional to the radius of the conductor (r), increasing the radius lowers the surface stress ┬╖ Bundle conductors are commonly used in EHV lines to increase the effective diameter, effectively minimizing power loss and radio interference due to corona.
Corona results in power loss, audible noise, and radio interference.
Higher line voltages increase the likelihood of corona discharge.
Bundle conductors effectively increase the radius r, reducing the surface gradient.
Surface roughness and atmospheric conditions (humidity/rain) also significantly affect corona.
Reduced power loss in transmission lines
Lower electromagnetic interference (EMI) with communication lines
Increased cost due to the requirement for bundle conductors
Higher mechanical loading on transmission towers
Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines
High-altitude transmission design
Option A (Increasing voltage) actually increases the electric field strength, exacerbating corona.
Option B (Reducing spacing) increases the capacitance and voltage gradient between phases, which increases corona.
Standard practice uses bundle conductors (2, 3, or 4 conductors per phase) to increase effective radius.
C is correct тАФ Increasing the effective conductor diameter decreases the surface electric field intensity, preventing the dielectric breakdown of surrounding air.
Always remember that corona is a surface effect; anything that reduces the local electric field (like increasing radius or smoothing the surface) reduces corona losses.