Electronic – Why weights on cables between utility poles

transmission line

Yesterday I saw what look like weights hanging from a cable between utility poles. This was in northeast Arizona on Route 60 between Show Low and Springerville. Here is a picture of a whole span:

The white blobs are the weights. Here is a closeup of a single weight:

They appear to be just concrete blocks with no dashpot or any other apparent way to dissipate power. These weights seem to be at 1/4 and 3/4 of each span, which would be the nodes of the first harmonic standing wave. They would add inertia and affect the frequency, but is that really their purpose? I can guess some possibilities, but I'd like to hear what these things are for from someone that actually knows.

Due to being a single cable, it must be for communications, not power, but I'm guessing that doesn't matter.

Added

Sorry, I added this note earier, but apparently somehow aborted the editor so that it wasn't posted.

To answer PlasmaHH's question, yes, these were on just about every span. The bottom of the cable was quite high, and a little lower didn't look like it would have been any problem. The bushes you see in the first picture are well in front of the cable. There is a lot of clearance below the actual cable. Even if there was a tall bush there, there would still be a lot of clearance.

Best Answer

There are several modes of vibration on conductors between poles. Different devices damp different vibrations. These weights are intended to primarily dampen torsional vibration.

Torsional vibration is more closely linked to low frequency high amplitude oscillation - conductor gallop - versus the high frequency low amplitude flutter, which is more commonly tamed with tuned mass dampers.

These weights, in other words, are more effective for the low frequency vibrations linked particularly to torsional vibration expected at this specific location than stockbridge dampers would be. Stockbridge dampers are more useful for high frequency vibrations (flutter, 10Hz or so).

As such I expect these are pendulum detuners:

Conductor with attached weight

Pendulum detuners.

These anti-galloping devices are based on the fact that the torsional movement of the bundle interacts dynamically with the vertical motion. Wind energy is injected to the vertical motion through torsional movement. The control of torsion can control the vertical movement. This occurs only when the torsional movement is close to the frequency of the vertical motion, which is valid for bundle conductor lines. To avoid frequency coalescence between torsion and vertical movement, which is at the basis of the instability, it is necessary to separate the frequencies one from each other (so called detuning). So, the principle of detuning is the avoidance of such frequency coalescence due to torsional stiffness increase.

Thus, while the despacering technique relies on changing the ice shape from a potentially unstable shape to a stable one, detuning accepts the ice shape but modifies the conductor dynamics in order to prevent the potential aerodynamic instability.

Some of the testing realized with detuning pendulum on bundle conductors gave satisfactory results. Their negatives impacts on the lines are quite small: some tests showed that the mass of the pendulums can lead to high values of conductor dynamic bending strain at the fixing clamps from aeolian vibration. An appropriate design (weight, arm length, location) is imperative.

(source, emphasis added)

There are newer devices that better control torsional vibration, each with distinct advantages (usually less weight) but they are also more expensive, and require new engineering work to determine the correct parameters, so you'll still see a lot of simple weights such as those you've pictured.

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