Vibration and dance of wires of overhead power lines: what is it, methods of struggle

Various mechanical loads and voltages arise on the wires and lightning protection cables of high-voltage power lines. For example, when there is wind, such a phenomenon as vibration or dancing of wires is observed. What it is, what the consequences and methods of struggle can be, you will learn from this article.

Definition

Vibration of wires is called periodic vibrations of a wire or cable in the span between the power transmission line supports. Oscillations occur with a frequency of 3 to 150 Hz in the vertical plane under the influence of a laminar air flow. As a result, standing waves are formed, the double amplitude of which can be greater than the diameter of the wire or cable, but at the same time does not exceed 0.005 of the wavelength.

A dance is called stable periodic oscillations, with a greater amplitude than in the previous case and a lower frequency - from 0.2 to 2 Hz. Thus, standing waves are formed with an amplitude of 0.3 to 5 meters, and in some cases even more.

The phenomenon is observed on power lines, overhead wires and lightning protection cables. The concept of "self-oscillation" is also applied to the contact network, although in essence it is the same thing. Another name is Aeolian vibrations.
So the main difference between vibration and dance is frequency. Vibration is hardly noticeable to the eye due to the high frequency, lower amplitude and number of half-waves, and dancing is strong vibrations with a longer wavelength and amplitude.

Causes of occurrence

Vibration of wires and cables of air power lines occurs with a laminar air flow (with a wind speed of 0.5-7 m / s, with higher velocity, the flow becomes turbulent), the direction of which is perpendicular or at a certain angle to him.

Then the air streams flow around the cylindrical surface of the wire and a circular flow arises, while in its upper part (point A in the figure below) the speed of this flow is greater than in the lower one (point B). This happens due to the disruption of air vortices from the upper and lower sides, as a result of which a pressure imbalance appears.
Hence, not only the horizontal, but also the vertical component of the pressure of air flows (wind) arises. If the frequency of vortex formation coincides with the frequency of (one of) the natural vibrations of the wire, then its vibrations in the vertical plane will begin.

Oscillations that arise in the system in the absence of variable external influences, as a result of the initial deviation, are called proper oscillations. As happens with a guitar string.

Antinodes of waves will appear at certain points, at which the amplitude will be maximum. Those points that will remain stationary are called nodes. Angular movements of the wire will occur in them, in simple terms - it will bend and rotate. Standing waves arise when the wavelength is equal to or a multiple of the distance between the supports (span length).

The vibration frequency is directly proportional to the wind speed and can be calculated using the formula:

f = (0.185V) / d,

where f is the oscillation frequency, V is the wind speed, d is the diameter, 0.185 is the Strouhal number characteristic in this case.

The formula also shows that the thinner the wire, the more frequency it vibrates. At the same time, wind speeds of 0.6-0.8 m / s are especially dangerous, since at a wind speed of more than 5-8 m / s the amplitudes are small and not dangerous. As a rule, the phenomenon occurs in spans longer than 120 meters, with increasing distance it only intensifies. This is especially important when the length of the intersection of overhead lines is more than 500 m, for example, through rivers and reservoirs.

The difference between dancing and vibration is, first of all, the amplitude - it is larger and can reach 12-14 meters, as well as a longer wavelength. The character and trajectory of movement when dancing repeats the shape of an elongated ellipse, with the axis deflected by 10-20 degrees from the vertical line.

With ice (ice and icing on the line), the wire diameter increases based on the formula given above - the vibration frequency decreases and the vibration wavelength increases.

Ice does not appear evenly, but from the leeward side. As a result, wires and cables become irregular rather than cylindrical. With this shape, a lifting force occurs during the wind, in the figure below Vy.

It also causes the dance. On the left, there are dancing waves in the span between the supports, and on the right, an icy cable and an air stream enveloping it.

The dance occurs at a higher wind speed than vibration, namely 5-20 m / s, at an angle to the line of 30-70 degrees. Oscillations occur with less frequency and greater amplitude.

You can see the external differences between the phenomena of these two phenomena by comparing the following two videos:

Danger

Let's see what the danger of dancing and vibration on overhead power lines. The dance is dangerous because the wires do not vibrate synchronously, and the amplitude can reach such a value that an overlap with the lightning protection cable, or with each other, can occur. Because of what electrical discharges occur, with all the ensuing consequences. To prevent collisions, in some cases, insulating spacers are installed between the conductive parts of the lines.

Vibration, in turn, has a destructive effect on the conductor cores, and line breaks are also possible at connections and clamps or outputs from clamps.

Control methods

Since the danger of vibration and dancing lies in the failure of the overhead line, breaks and short circuits, we will consider the main method of protection against it.

Installation of vibration dampers is the main method for eliminating the considered phenomena. They are of various types. A common feature is that they are made in the form of a rod with weights at the ends, which is suspended from the middle part on ropes and wires. The type of vibration damper is selected in accordance with the span length and conductor diameter, in accordance with Table 2.5.9. PUE, p. 2.5.85 (Chapter 2.5 PUE).

To determine climatic conditions and calculate the load of mechanical stresses during vibrations, they also use the information set forth in points of PUE 2.5.38-2.5.74, they show wind pressure, ice wall thickness, average annual duration of thunderstorms and others data. If you want to know more, you can read RD 34.20.182-90 “Guidelines for typical protection against vibrations and sub-vibrations of wires and lightning protection cables of overhead power lines with a voltage of 35-750 kV ".

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• Power transmission over long distances without wires
• What to do if a tree falls on the wires
• Protection zones of power lines