Bundled Conductors for Extra and Ultra High Voltage transmission lines

Last updated: January 20, 2017

Transmission of large amount of power over long distances can be accomplished most economically using Extra and Ultra High Voltage lines. An increase in transmission voltage results in reduction of electrical losses, increase in transmission efficiency, improvement in voltage regulation and reduction in conductor material requirements.

Effect of Voltage Gradient on Atmospheric Air: 

In an overhead transmission line, the atmospheric air in between the conductors behaves perfectly like an insulator when the potential difference between the conductors is small. With the increase in system voltage, there is a corresponding increase in the electric field intensity. When the electric field intensity or voltage gradient reaches a critical value of 30 kV/cm, the air in the immediate vicinity of conductors no more remains a dielectric but it ionizes and becomes conducting.

Corona phenomenon at High Voltage:

Corona phenomenon is the ionization of air surrounding the power conductors. Free electrons are normally present in the atmosphere. The free electrons will move with certain velocity depending upon the field strength.  These electrons on their movement collide with the molecules of air and liberate more electrons. The process of ionization is cumulative and ultimately forms and electron avalanche.

The electrical breakdown of surrounding air around the conductor is accompanied by-

1. a faint glow around the conductor, 
2. a hissing sound, 
3. vibration in conductors, 
4. formation of ozone and oxides of nitrogen, 
5. loss of power, and 
6. radio interference. 

Factors affecting Corona loss in an overhead Transmission line:

The important factors that affect the corona loss in an overhead transmission line are -

1. frequency and waveform of supply, 
2. spacing between conductors, 
3. condition of conductor surface, 
4. atmospheric conditions, 
5. conductor diameter and 
6. number of conductors per phase.   

Bundled Conductors

At voltages above 300 kV, corona causes a significant power loss and radio interference if a single conductor per phase is used. Instead of using a single conductor, it is preferable to use two or more conductors per phase, in close proximity, which is called bundled conductors. 

Thus, a bundled conductor is a conductor made up of two or more sub-conductors and is used as one phase conductor. The high voltage gradient is reduced considerably by the use of bundled conductors.

Number of sub-conductors:

The number of sub-conductors used per phase in an Indian transmission system are two (for 400 kV), and four (for 765 kV). The bundled conductors for the proposed 1200 kV Ultra High Voltage (UHV) transmission system in India will have 8 sub-conductors. Figure 1 shows Bundled conductors with twin, triple and quadruple conductor. Figure 2 shows a 765 kV Transmission line in India with quadruplex sub-conductors.

Fig.1: Bundled conductors with twin, triple and quadruple sub-conductors.

Fig.2: 765 kV transmission line in India with 4 sub-conductors per phase (Quadruplex bundle).

Bundle spacing:

The spacing between adjacent sub-conductors is called bundle spacing and is almost 30 cm or more. In figure 1, bundle spacing is denoted by 'B'. In almost all cases, the sub-conductors are uniformly distributed on a circle. The radius of the pitch circle on which the sub-conductors are located is called bundled radius.

The various advantages of using bundled conductors are reduced reactance, voltage gradient, corona loss, radio interference, and surge impedance.