Why are Bundle Conductors used in Transmission Lines?

What is Bundle Conductor?

A bundle conductor is a conductor made up of two or more sub-conductors and is used as one phase conductor. For voltages greater than 220 kV it is preferable to use more than one conductor per phase which is known as Bundle conductor.

Bundle Conductor

Bundle conductors are used for transmission purpose as it helps in obtaining better voltage regulation and efficiency by reducing the inductance and skin effect present in the power lines. These causes considerable losses in transmission line.

There are many advantages of using bundled conductors in transmission lines. I am listing some important factors:

Reduced Reactance: 

Due to bundling of conductors the self GMD of the conductors is increased, so reactance of conductors decreases, so reduced voltage drop in conductors, so voltage regulation improves (decreases).

Reduced Corona Loss: 

There is a optimum spacing between sub conductors of a bundled conductor which give minimum gradient on the surface of sub conductors and hence highest critical disruptive voltage and hence reduced corona loss (see formula of corona loss in terms of critical disruptive voltage)

Reduced Surge Impedance: 

Surge Impedance of line is given as square root of (L/C) .As inductance is reduced and capacitance is increased (due to self GMD) so surge impedance reduces and hence maximum power that can be transmitted is increased.

Thereby to transmit maximum power, Bundle conductor are used in Transmission line.

Thank you!