Considerable attention has been given to conductor vibration of suspended transmission lines during the past 25 years. Even so, the engineers of the public utilities feel that they have only started to get a solution to the vibration problems. Extensive research programs are being conducted at the present time by manufacturers of conductor cable and ground wires and by numerous public utilities themselves.

Aeolian vibrations are the chief concern because they occur whenever a steady wind, of from one mile per hour to hurricane velocity, blows across the suspended line. Thus, millions of flexures, however small or large in amplitude, may be produced in a relatively short period of time. Each flexure will contribute to fatigue, especially if its affects are concentrated at a suspension point.

Stockbridge dampers and armor rods are two popular means of combating aeolian vibrations. Heretofore most analysists have considered these means of reducing conductor vibrations as being energy absorbers or dissipaters. By introducing the electrical analog it is easily shown that Stockbridge dampers, armor rods and similar dampers do not function as energy absorbers. They are mechanical impedance matching devices, which introduce reflected waves of the proper amplitude to cancel the unwanted waves. Dampers of the Stockbridge type are tuned mechanical impedance matching devices, and armor rods and "festoons" are untuned impedance matching devices. Proper design and installation of these dampers (impedance matching devices) can make them much more effective.

Ten public utilities and manufacturers of cable recently reported vibration data and experiences. Each of the reports contributes considerable to the analysis of conductor vibration, although only two of them have been given publication as technical papers. The valuable data and experiences of these ten reports are reviewed and correlated in this paper, Analysis of Conductor Vibration.

The limitations for stressing overhead high-strength shield or grounding cable should be lowered to 15% of ultimate strength at 600F. as compared with the present National Electrical Safety Code of 25% of ultimate at 60°F. Flexible suspension clamps should be used.

The conductors of high voltage transmission lines should be sagged to carry a tension of not to exceed 20% of ultimate strength at 60°F. Stockbridge or similar dampers should best be installed at all supports.

It has been found desirable in many cases to also install armor rods at each flexible suspension support and dead end. It is known that armor rods effectively suppress vibration and give added protection. However, the collected data indicated that properly installed dampers of the Stockbridge type, and others, should sufficiently reduce aeolian vibrations.


Electrical and Computer Engineering

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