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Overhead Conductor Vibration
The two types of wind-induced conductor vibration that are of interest to overhead transmission line
engineers and designers are aeolian and galloping. The most prevalent is aeolian (the subject of this
discussion) because it is caused by wind velocities ranging from about 2 – 15 miles per hour. Below 2
mph, the wind input energy is insufficient to sustain vibration, in most cases, and above 15 mph the winds
tend to be turbulent.
Aeolian vibration is resonant vibration caused by low velocity laminar wind blowing across tensioned
overhead conductors. This low amplitude (less than one conductor diameter peak-to-peak), high
frequency (typically 5 – 100 Hz) vibration can result in excessive bending strains and conductor strand
damage at attachment points and dead ends.
The exciting mechanism causing conductor vibration is rhythmic aerodynamic forces created by wind
eddy currents on the leeward side called Karman vortices. These vortices result in vertical forces causing
alternating upward and downward conductor motion (lift and drag).
The frequency of this motion is directly proportional to the wind velocity and inversely proportional to the
conductor diameter according to the formula: F = 3.26V/d, where F = frequency in Hz, V = wind velocity
in miles per hour, d = conductor diameter in inches and 3.26 is an empirical aerodynamic constant.
As the frequency approaches one of the natural resonance frequencies of the conductor span, traveling
waves are initiated. These waves travel away from the injection point in both directions towards the span
ends at about 450 – 550 ft/sec. At the span ends some traveling wave energy is absorbed, some passes
through to the next span, and the majority is reflected back from the termination points with the same
amplitude and frequency. As these traveling waves meet and superimpose, they create standing waves
with frequencies that are multiples or harmonics of the fundamental frequency of the conductor span.
The wavelength of these standing waves is directly proportional to the square root of the conductor
tension and inversely proportional to the square root of its mass, expressed by the relationship: L =
1/2f T/M where L = wavelength in feet, f = frequency in Hz, T = conductor tension in pounds, and M =
conductor mass expressed by the conductor weight in pounds per foot divided by the acceleration due to
gravity (32.2 ft per second²).
Part of the wind input energy is stored in the wave as increased amplitude. The balance is dissipated by
the conductor strands as they flex with the sinusoidal wave. The energy dissipated by the conductor
strands occurs primarily in the outer strands and increased tension reduces its self-damping ability (and
increases its tendency to vibrate) by increasing the radial forces between strands and strand layers
thereby reducing inter-strand friction. It should also be noted that self-damping of new stranded
conductors is significantly greater than that of aged and oxidized stranded conductors. Therefore,
increasing tension on aged lines and conductors may increase the tendency to vibrate.
Ref: Fargo Technical Bulletin TB-107R2