The demand of electric power is increasing throughout the world and in many countries it is doubling every five to eight years. The power stations (hydro, thermal or nuclear) are usually located far away from the load centres. Thus transmission of large amounts of power over long distances, which can be accomplished most economically only by using extra high voltages (or simply EHV), is necessary. Voltages in excess of 230 kV fall in this category.
An increase in transmission voltage results in reduction of electrical losses, increase in transmission efficiency, improvement of voltage regulation and reduction in conductor material requirement. At voltages above 300 kV, corona causes a significant power loss and interference with communication circuits, if a round single conductor per phase is used. Instead of going for a hollow conductor it is preferable to use more than one conductor per phase which is called the bundling of conductors. Lines of 400 kV and higher voltages invariably use bundled conductors.
A bundled conductor is a conductor made up of two or more conductors, called the sub-conductors, per phase in close proximity compared with the spacing between phases (Fig. 4.21).
The basic difference between a composite conductor and a bundled conductor is that the sub-conductors of a bundled conductor are separated from each other by a constant distance varying from 0.2 m to 0.6 m depending upon designed voltage and surrounding conditions throughout the length of the line with the help of spacers whereas the wires of a composite conductor touch each other. The bundled conductors have filter material or air space inside so that the overall diameter is increased.
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The use of bundled conductors per phase reduces the voltage gradient in the vicinity of the line and thus, reduces the possibilities of the corona discharge.
Although the bundled conductors are used on EHV transmission lines primarily to reduce corona loss and radio interference, but they have several other advantages over single conductors such as given below:
1. The bundle conductor lines transmit bulk power with reduced losses, thereby giving increased transmission efficiency.
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2. Since the bundle conductor lines have a higher capacitance to neutral in comparison with single conductor lines, they have higher charging current, which helps in improving the power factor.
3. Since by bundling, the self GMD or GMR is increased, the inductance per phase, in comparison with single conductor lines, is reduced. As a result reactance per phase is reduced.
4. Since surge impedance of a line is given by 
and the bundle conductor lines have higher capacitance and lower inductance in comparison with single conductor lines, therefore, bundle conductor lines have comparatively lower surge impedance with a corresponding increase in the maximum power transfer capability.
The following table represents relative power transfer with number of sub-conductors forming a bundle conductor:
It is to be noted that there is a little to be gained by using more than four sub-conductors per phase, two or three sub-conductors per phase are sufficient for most of the EHV lines.
The geometric mean radius of a bundled conductor for a two- conductor (duplex), a three-conductor (triplex) and four conductor (quadruplex) arrangements shown in Figs. 4.22 (a), 4.22 (b) and 4.22 (c) respectively can be determined from the following relations,
where r’ is the geometric mean radius of each sub-conductor of bundle and s is the spacing between sub-conductors of a bundle.
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GMD of a bundled conductor line can be determined by taking the root of the product of distances from each conductor of a bundle to every other conductor of the other bundles. However, if the distances from centre of one bundle to the centres of other bundles are taken as the distances dab, dbc, dca, accuracy will not be much affected.