For transmission of more power for long distances to load centers Extra High Voltage (EHV) transmission is employed. Implementing Extra High Voltage has advantage of reduction in the copper losses and improves efficiency. However transmission of voltage beyond 300kV will poses some problems such as Corona effect which causes significant power loss and interference with communication circuits if round single conductor per phase is used.
In order to reduce corona effect hollow round conductors are used. Keeping economical constraints other option is instead of using hallow round conductor it is preferable to use more than one conductor per phase which is called Bundled Conductors. For transmission of power beyond 400kV bundled conductors are employed.
How Bundled Conductors Work
For extra high voltage transmission lines corona causes a large problem if the conductor has only one conductor per phase. Corona occurs when the surface potential gradient of a conductor exceeds the dielectric strength of the surrounding air. This causes ionization of the area near the conductor.
Corona produces power loss. It also causes interference with communication channels. Corona manifests itself with a hissing sound and ozone discharge. Since most long distance power lines are either 220 kV or 400 kV, avoidance of the occurrence of corona is desirable.
The high voltage surface gradient is reduced considerably by having two or more conductors per phase in close proximity. This is called conductor bundling . The conductors are bundled in groups of two, three or four . The conductors of a bundle are separated at regular intervals with spacer dampers that prevent clashing of the conductors and prevent them from swaying in the wind. They also connect the conductors in parallel.
Advantages of Bundled Conductors
Bundled conductors are primarily employed to reduce the corona loss and radio interference in transmission lines. However they have several advantages as mentioned on this page.
Bundled conductors per phase reduces the voltage gradient in the vicinity of the line. Thus reduces the possibility of the corona discharge. (Corona effect will be observed when the air medium present between the phases charged up and start to ionize and acts as a conducting medium. This is avoided by employing bundled conductors).
Improvement in the transmission efficiency as loss due to corona effect is countered.
Bundled conductor lines will have higher capacitance to neutral in comparison with single lines. Thus they will have higher charging currents which helps in improving the power factor.
Bundled conductor lines will have higher capacitance and lower inductance than ordinary lines they will have higher Surge Impedance Loading (Z=(L/C)1/2). Higher Surge Impedance Loading (SIL) will have higher maximum power transfer ability.
With increase in self GMD or GMR inductance per phase will be reduced compared to single conductor line. This results in lesser reactance per phase compared to ordinary single line. Hence lesser loss due to reactance drop.