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The size of conductor on modern EHV lines is obtained based on
Voltage drop
Current density
Corona
Skin effect
Corona
In Extra High Voltage (EHV) transmission lines, the conductor size is primarily determined by the corona effect rather than just the current-carrying capacity. Corona discharge causes significant power loss, audible noise, and radio interference, necessitating a larger surface area to keep the electric field intensity below the dielectric strength of air.
In Extra High Voltage (EHV) transmission lines, the conductor size is primarily determined by the corona effect rather than just the current-carrying capacity. Corona discharge causes significant power loss, audible noise, and radio interference, necessitating a larger surface area to keep the electric field intensity below the dielectric strength of air.
gmaxтАЛ=rln(D/r)VтАЛ тАФ Electric field gradient at the conductor surface
PcтАЛтИЭ(f+25)DrтАЛтАЛ(VтИТVdтАЛ)2 тАФ Empirical formula for Corona power loss
Corona is an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged. The critical disruptive voltage is directly proportional to the conductor radius. By increasing the conductor size (or using bundled conductors), the surface potential gradient is reduced, preventing the air around the conductor from reaching its breakdown voltage (EmaxтАЛтЙИ30kV/cm peak).
Corona leads to power loss, interference, and ozone production.
For lines above 220 kV, conductor size is dictated by corona and RI (Radio Interference) limits.
Bundled conductors effectively increase the radius (GMR), further controlling corona.
Voltage drop is the criteria for low-voltage or medium-voltage short lines.
Reduces power loss due to corona discharge.
Minimizes radio and television interference.
Prevents audible noise pollution near EHV corridors.
Increased conductor cost due to larger diameter.
Higher tower weight and mechanical loading due to larger conductor weight.
400 kV transmission lines
765 kV and 1200 kV UHV transmission systems
Current density is the design factor for lower voltage distribution systems where thermal limits are dominant.
Skin effect affects conductor resistance but is not the primary sizing criterion for EHV lines.
For EHV lines, the geometric mean radius (GMR) is manipulated via bundling to satisfy corona limits.
C is correct тАФ The size of conductors in EHV lines is primarily governed by the need to minimize corona discharge and its associated losses/interference.
Remember: For distribution lines, focus on Current Density (thermal limit); for Transmission lines, focus on Voltage Drop (stability); for EHV/UHV lines, always focus on Corona.