Number of wires per power cable. "single" (1), "double" (2), "triple" (3) or "quad" (4).
High voltage transmission lines often use conductor bundles (cables) each consisting of two or more individual sub-conductors (wires) separated by spacers. This has two main purposes:
- Reduction of corona discharges. The strong electric field around a conductor can cause ionisation of the surrounding air. This may result in signicant power losses and audible noise and it is therefore important to limit such coronal discharges. The electric field can be reduced by increasing the effective diameter of the conductor. A conductor bundle will behave similarly to a single conductor having a diameter roughly corresponding to the diameter of the bundle.
- Mitigation of skin effect. AC current tend to flow at the surface of conductors. At 50 Hz the current cannot penetrate more than approximately 1 cm into the conductor. It is therefore inefficient to increase the diameter of thick conductors to reduce the electric resistance. A better way to increase the current capacity is to use bundled conductors. A conductor bundle having multiple thin sub-conductors will have a lower resistance than a single solid conductor having an equivalent cross section.
Conductor bundles are normally used at voltages above 200 kilovolt to limit corona losses but may also be used at lower voltages to increase the current capacity of the line.
Possible values are:
- One wire (no bundle): wires=single
- Two wires: wires=double
- Tree wires: wires=triple
- Four wires: wires=quad
Other values are created by appending the phrase 'fold' after the numeral:
- Five wires: wires=fivefold
- Eight wires: wires=eightfold
The photo on the right shows an example of wires=double. You see three cables each with two wires.
The number of cables of a line is tagged by cables=* (here lightning arresters do not count). To distinguish wires and cables use this simple rules:
- Wires are not isolated one another, but isolated to the tower.
- Cables are isolated one another and to the tower.
- Lightning arresters are not isolated to the tower.
A note on terminology
The names of the keys wires=* and cables=* may have been chosen a little bit unfortunate. The more common term for "wire" is sub-conductor (IEV ref 466-10-21) of a conductor bundle (IEV ref 466-10-20). A single conductor (IEV ref 466-10-19) corresponds to wires=single. Similarly twin bundle, triple bundle and quad bundle correspond to wires=double, wires=triple and wires=quad, respectively. See also Wikipedia on conductors.