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The size of conductor on modern EHV lines is obtained based on
Voltage drop
Current density
Corona
and (b)
Corona
In Extra High Voltage (EHV) lines, the conductor size is primarily determined by the corona effect rather than current-carrying capacity. For lines operating at voltages typically above 230 kV, the electric field strength at the conductor surface must be kept below the critical disruptive breakdown voltage of air to prevent significant power loss and interference.
In Extra High Voltage (EHV) lines, the conductor size is primarily determined by the corona effect rather than current-carrying capacity. For lines operating at voltages typically above 230 kV, the electric field strength at the conductor surface must be kept below the critical disruptive breakdown voltage of air to prevent significant power loss and interference.
VcтАЛ=goтАЛтЛЕrтЛЕln(rdтАЛ) тАФ Disruptive critical voltage formula
PlossтАЛтИЭ(VтИТVcтАЛ)2 тАФ Power loss due to corona
Corona is an electrical discharge caused by the ionization of fluid such as air surrounding a conductor that is electrically charged. The critical disruptive voltage is directly proportional to the radius of the conductor. By increasing the conductor diameter (often using bundled conductors), the surface voltage gradient is reduced, thereby suppressing corona discharge, reducing power loss, and minimizing radio interference.
Corona results in power loss, radio interference, and audible noise.
Bundled conductors increase the effective radius, effectively reducing the surface electric field gradient.
Current density determines conductor size for lower voltage transmission lines (LT/HT).
For EHV lines, corona loss is a critical design constraint that often mandates a larger diameter than required by current density.
Reduced power loss (I┬▓с┤┐ and corona loss)
Lower radio and television interference
Minimized audible noise from transmission lines
Higher material cost due to increased conductor diameter
Increased wind load on towers
Complexity in installation for bundled configurations
400 kVTransmission Lines
765 kVUHVTransmission Systems
Bundled conductors are a standard solution for EHV lines to manage the surface electric field.
Option B (Current density) is the primary criterion for low voltage (distribution) lines where corona is negligible.
C is correct тАФ The conductor size for EHV lines is determined by the corona effect, which requires maintaining a low surface voltage gradient to prevent air ionization.
Always remember: Distribution lines are designed for thermal limits (current density/voltage drop), whereas EHV/UHV lines are constrained by corona/dielectric stress.