Why bundled conductors are preferred for EHV transmission line?

Bundled Conductors are used:
  • It reduces the reactance of the electric transmission line
  • It also reduces corona loss, which improves the transmission efficiency as loss due to corona effet is countered (Corona effect can be observed when the air medium present between the phases charged up and start to ionize and acts as a conducting medium)
  • Bundled conductor lines do have the higher capacitance to neutral, when compared to single lines. This results in higher charging currents which helps in improving the power factor
  • In bundled conductor, as the GMR (geometric mean radius) of the conductor increases, the inductance per phase of the conductor decreases.
  • The optimum spacing (8 to 10 times of the diameter of conductor) between sub conductors in bundled conductors gives a minimum voltage gradient on the surface of bundle conductor
  • As the voltage gradient is reduced, the radio interference is also reduced or minimized
  • With decrease in inductance of bundled conductors, the surge impedance of the line is also reduced
  • Decrease in surge impedance due to bundling the conductor, surge impedance loading (SIL) of the conductor increases. This makes the transmission capacity of the system more.

GMD(Geometric mean distance): Geometrical Mean of distances between the strands of two Transmission Lines
GMR(Geometric mean radius): Geometrical Mean of distances between the strands of a single composite conductor are calculated.

Effect of bundle conductor-
In a bundle conductor the spacing between the conductors of the same phase increases.
As the GMR increses, the line inductance decreases.
If the inductance of the line decreases then the voltage gradient of the conductor increases. Hence corona effects reduce.
Hence The bundled conductor in the EHV transmission line system provides a decrease in inductance and an increase in voltage gradient.
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