Why are Bundle Conductors used in Transmission Lines?

A bundle conductor is a conductor made up of two or more sub-conductors and is used as one phase conductor. 

Bundled conductors are those conductors which form from two or more stranded conductors, bundled together to get more current carrying capacity.

For voltages greater than 220 kV it is preferable to use more than one conductor per phase which is known as Bundle conductor.

By using bundle conductors instead of the single conductor in the transmission line increases the GMR of the conductors.

by using bundle conductors GMR is increased, so inductance L decreases, and capacitance C increases.

A bundled conductor reduces the reactance of the electric transmission line. It also reduces voltage gradient, corona loss, radio interference, surge impedance of the transmission lines. By making bundle conductor, the geometric mean radius (GMR) of the conductor increased

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.

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.

Advantages of Bundled Conductors:
Bundling of conductors leads to a reduction in line inductance.
Bundling of conductors leads to increases in capacitance.
An important advantage of bundled conductors is its ability to reduce corona discharge
Reduction in the formation of corona discharge leads to less power loss and hence improved transmission efficiency of the line.
Reduction in communication line interference.

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