Bundled Conductor’

At 400 KV & above, P.G. at conductor surface becomes very high. Results in   Corona, causing losses & interference with communication circuits.

P.G. = -D/k = Electric flux density/permittivity.

To reduce P.G. & avoid corona, ‘D’ is reduced by increasing surface area of conductor.

Effected by employing more than one conductor per phase viz. two, three or four placed in close proximity. This is called ‘bundled conductor’.

Bundling increases ‘GMR’ of the conductor, causing reduction in inductance per phase.

For higher amounts of current, bundle conductors are used for several reasons. Due to the skin effect, for larger conductors, the current capacity does not increase proportional to the cross-sectional area; instead, it is only with the linear dimension. Also, reactance decreases only slowly with size. But the cost and weight do increase with area. Due to this, several conductors in parallel become more economical.

The combination of more than one conductor per phase in parallel suitably spaced from each other used in overhead Transmission Line is defined as conductor bundle. The individual conductor in a bundle is defined as Sub-conductor.

1. At Extra High Voltage (EHV), i.e. voltage above 220 KV corona with its resultant power loss and particularly its interference with communication is excessive if the circuit has only one conductor per phase. The High-Voltage Gradient at the conductor in the EHV range is reduced considerably by having two or more conductors per phase in close proximity compared with the spacing between conductor-bundle spaced 450 mm is used in India.

2. The three conductor bundle usually has the conductors at the vertices of an equilateral triangle and four conductors bundle usually has its conductors at the corners of a square.

3. The current will not divide exactly between the conductor of the bundle unless there is a transposition of the conductors within the bundle, but the difference is of no practical importance.

4. Reduced reactance is the other equally important advantage of bundling. Increasing the number of conductor in a bundle reduces the effects of corona and reduces the reactance. The reduction of reactance results from the increased Geometric Mean Radius (GMR) of the bundle.