Reasons for stranded and bundled conductors

Stranded conductors

A stranded conductor is made of many thin wires twisted together in layers (e.g. 1, 7, 19, 37 ... strands), each layer twisted in the opposite direction.

Reasons:

1. Flexibility: A solid conductor of large area is stiff and hard to handle, wind on drums and string. Stranding makes it flexible.
2. Less breakage: Solid wires can crack due to vibration and bending; in a stranded conductor a broken strand does not break the whole conductor.
3. Higher strength with composite strands: Steel core strands can be combined with aluminium strands (ACSR) to give high tensile strength and good conductivity.
4. Easy manufacture and transport of long lengths.
5. Slightly larger diameter for the same area, helping to reduce surface stress.

   Stranded (7 strands)    ACSR
       o o                ooo
      o O o              o###o   ### = steel core
       o o                ooo

Bundled conductors

In EHV lines (220 kV and above), each phase uses two or more sub-conductors spaced 30–45 cm apart by spacers.

Reasons:

1. Reduced corona: The bundle behaves like a conductor of larger equivalent radius, so the surface electric field falls, raising the critical disruptive voltage and reducing corona loss, radio and audible noise.
2. Lower inductive reactance: The geometric mean radius (self-GMD) increases, so $L = 2\times10^{-7}\ln(D_m/D_s)$ falls.
3. Higher capacitance and surge impedance loading: Lower $Z_c=\sqrt{L/C}$ gives higher SIL and power capacity.
4. Better stability: Lower reactance raises $P_{max}=V_sV_r/X$.
5. Better cooling and current capacity: More surface area dissipates heat.

 2-bundle    3-bundle      4-bundle
  o---o        o          o----o
              / \         |    |
             o---o        o----o