Why are twisted pair cables used in electric power distribution systems?

Question

I'm an Electrical Engineer, and understand how a twisted pair of wires can be used to reduce noise in the presence of magnetic fields.

However, I recently noticed the power company replacing the cables on a high voltage transmission system with a twisted pair of cables. The top most ground cable is still a single cable, but they now use two cables twisted in a pair for each of the 3 power cables as shown here.

Why do they use these types of cables?

EDIT: After reading the answers by WhatRoughBeast and Waqar, it would seem that the most plausible explanation for using 2 cables would be current capacity versus weight, but there must be more to it than that.

The engineers that designed these transmission systems 30 years ago weren't stupid. They certainly knew about skin depth, and could easily calculate the optimal cable size and number of cables to use.

As far as I can tell, this idea of using multiple cables for each phase is relatively new. I recently noticed that in some cases, they use multiple sets of two cables for each phase.

Answer 1

After nearly 2 years, I found the answer to my question.

I recently walked past an electric utility's storage yard and noticed this reel of twisted pair cable.

The manufacturer is Southwire Company of Carrollton, Georgia and the cable is called VR2, Vibration Resistant Cable.

Their web page says VR2 uses a twist to provide resistance to Aeolian vibration and ice galloping.

Since I had never heard of Aeolian vibration, here is a quote from this article titled: "Aeolian Vibration Basics" en-ml-1007-4aeolianvibook-1.pdf

When a smooth stream of air passes across a cylindrical shape, such as a conductor or OHSW, vortices (eddies) are formed on the leeward side (back side). These vortices alternate from the top and bottom surfaces, and create alternating pressures that tend to produce movement at right angles to the direction of the air flow. This is the mechanism that causes aeolian vibration.

Here is an example of an Aeolian Vibration Failure.

Here is a YouTube clip showing these vibrations.

Answer 2

At a guess, they're doubling the current capacity of the lines. If, instead of a pair of wires, they had used a single wire 40% thicker, the wire would have been noticeably stiffer and harder to spool and unspool for transportation and installation. The reason they're twisted is not for reasons of noise susceptibility or anything like that - it's to provide mechanical support for the wires, and makes the pair stronger than they would be if they were not twisted together.

Answer 3

One possible benefit is reduction of corona discharge. This article states,

The electric field gradient is greatest at the surface of the conductor. Large-diameter conductors have lower electric field gradients at the conductor surface and, hence, lower corona than smaller conductors, everything else being equal. The conductors chosen for the Calumet to the line were selected to have large diameters and to utilize a two conductor bundle. This reduces the potential to create audible noise.

In my country it is common to see four 'bundled' conductors per phase with 'X' spacers at intervals between the pylons to maintain the desired separation. This article from India gives some details on the technique. The OP's conductors are in contact but perhaps there is still some benefit.

Answer 4

There are a lot of advantages for doing that

1. As @WhatRoughBeast said, current capacity. For same amount (and hence weight) of conductor.
2. As you know AC currents flow closer to surface in a conductor than flowing evenly in each part (Skin effect), so one thicker cable will be loss of metal on the inside.
3. Heard of solenoids? This twisted conductor pair creates a similar effect (although not exact because twists are not exact close circles) and keeps magnetic field induced at minimum outside of pair. More twists/inch are better for field cancellation but increase the current path length (and so heat losses) so a trade off is done.

Top ground wire is for protection and most of the time no currents are flowing through it. (Generally speaking, they use a delta circuit for HV lines) So no need to replace it.