A “bundle conductor” is a conductor made up of two or more ‘(sub-conductors”, and is used as one phase conductor. Bundle conductors are also called duplex, triplex, etc., conductors, referring to the number of sub-conductors and are sometimes referred to as grouped or multiple conductors.
Considerable work on bundle conductors has been done by the engineers of Siemens Schuckertwerkez who concluded that bundle conductors were not economical at 220 kv, but for rated voltages of 400 kv or more, are the best solution for overhead transmission. Rusck and Rathsman state that the increase in transmitting capacity justifies economically the use of two conductor bundles on 220-kv lines.
The advantages of bundle conductors are higher disruptive voltage with conductors of reasonable dimensions, reduced surge impedance and consequent higher power capacity, and less rapid increase of corona loss and RI with increased voltage.
These advantages must be weighed against increased circuit cost, increased charging kva if it cannot be utilized, and such other considerations as the large amount of power which would be carried by one circuit.
It is possible with a two-conductor bundle composed of conductors of practical size to obtain electrical characteristics, excepting corona, equivalent to a single conductor up to eight inches in diameter.
Theoretically there is an optimum sub-conductor separation for bundle conductors that will give minimum crest gradient on the surface of a sub-conductor and hence high- disruptive voltage. For a two-conductor bundle, the separation is not very critical, and it is advantageous to use a larger separation than the optimum which balances the reduced corona performance and slightly increased circuit cost against the advantage of reduced reactance.
Because of the effect of the sub-conductors on each other, the gradient at the surface of a sub conductor is not uniform. It varies in a cosinusoidal manner from a maximum at a point on the outside surface on the line-of-centers, to a minimum at the corresponding point on the inside surface.
This effect modifies the corona performance of a bundle conductor such that its corona starting point corresponds to the voltage that would be expected from calculations, but the rate of increase of corona with increased voltage is less than for a single conductor.