Stranded and bundled conductors; bundling in EHV

Stranded conductor

A stranded conductor is made of many thin wires twisted together in concentric layers. Each layer is laid in the opposite direction to the one below it. For $n$ layers around a central wire, the number of strands is $3n^2+3n+1$ (7, 19, 37, ...). ACSR is the common example: aluminium strands around a steel core for strength.

• It is flexible and easy to wind on drums, string and sag.
• It does not break easily from vibration.
• A broken strand does not cause the whole conductor to fail.

Bundled conductor

A bundled conductor uses two, three or four sub-conductors per phase, held a short distance apart (30–45 cm) by spacers. Together they act as one phase conductor.

 twin       triple       quad
 o   o        o         o   o
             o o        o   o

Reasons for bundling in EHV lines

• Reduced corona: The effective radius is larger, so the surface voltage gradient falls. Corona loss, radio interference and audible noise are lower.
• Lower reactance: The GMR of the bundle is large ($D_s^b = \sqrt{D_s d}$ for twin, $\sqrt[3]{D_s d^2}$ for triple, $1.09\sqrt[4]{D_s d^3}$ for quad), so the inductive reactance is reduced.
• Higher power capacity: Lower $X$ and higher $C$ reduce the surge impedance, so the SIL and the steady-state stability limit $P_{max} = V_SV_R/X$ increase.
• Better current rating: Several conductors have a larger surface area for cooling and less skin effect.
• Less voltage drop and better regulation, because of the lower reactance.

Typical practice is a twin bundle at 220–400 kV and a quad bundle at 400–765 kV.