Vibrations of Conductors and Damper

Vibrations of Conductors and Damper

The conductors are supported on the string insulators at each tower. Under widely varying atmospheric conditions like strong wind velocities, the conductor can start vibrating mechanically in the vertical plane. Such vibrations can have different frequencies, amplitudes and modes. There are two types of vibrations in the vertical plane,

1. Aeoline vibrations and 2. Galloping or dancing of conductors.

1. Aeoline Vibrations

The Aeoline vibrations have the frequency range of 8 to 40 Hz with amplitudes varying between 2 to 5 cm. Thus these vibrations are high frequency low amplitude vibrations. The wind velocities of about 2 to 40 kmph can generate such type of vibrations.

The frequency of Aeoline vibrations is given by,

fα = 2 (V_P / d) × 10³

where          V_p = Wind velocity in kmph perpendicular to conductor

d = Diameter of conductor in m 

These vibrations are not much harmful. But if the frequency of vibrations coincides with the natural frequency of the conductors then resonant condition can be achieved. This may result in breaking of strands and mechanical failure of the conductors.

The Aeoline vibrations are harmful from suspension points of view. The Aeoline vibrations travel along the length of the conductors. Now where the clamps are used and the free vibrations are restricted, the Aeoline wave reflects back causing a momentary bending of conductor. And this may damage the conductors. The conductors are vibrated back and forth continuously at the suspensions due to Aeoline vibrations, such a stress is called the metallurgical fatigue.

Use of bundled conductors, proper design and locations of spacers and clamps and use of dampers can minimise the effects of Aeoline vibrations.

2. Galloping of Conductors

The galloping means dancing of conductors at the low frequency and high amplitude. These are the oscillations of complete span of conductors. It causes the longitudinal movement of conductors through the conductor clamps. The frequencies of galloping are about 0.25 to 1.5 Hz with the amplitudes of about 6 m. The Fig. 4.10.1 shows the three modes of galloping.

When the wind blows past the noncircular conductor, the galloping initiates. The oscillations are build on their own and galloping becomes very large. The path of conductor is elliptical in galloping. This can cause flexure fatigue and there is a possibility that conductors may ripped off from pin insulators. If the amplitude is large more than the sag then clashing of conductor with earth occurs and flashover may result.

The galloping is rare but possible mainly in rainy and snowfall conditions. Apart from the icing conditions, the galloping is rare. But when occurs it can cause damage to conductors, spacers, clamps and towers too.

There are no methods by which galloping can be restricted. But in icing conditions, height of the conductors can be designed properly considering the amplitude of possible galloping. As galloping vibrations are longitudinal, horizontal configuration of conductors can avoid clashing of conductors during galloping.

3. Stock-Bridge Type Vibration Damper

The galloping cannot be restricted but the Aeoline vibrations can be restricted using vibration damper. A damper used to restrict Aeoline vibrations is called stock bridge type damper.

It consists of a steel cable. At the two ends of the cable, two hollow specific shaped weights are fixed. At the midpoint the damper can be fastened to the conductor by means of aluminium clamp. The weights used are of galvanized iron. The main feature of the damper is the grip of the damper with conductor uses only one bolt. Another feature is that these dampers can be fastened to the conductors even though the line is alive, using special long handled tools. The arrangement of stock bridge type damper is shown in the Fig. 4.10.2.

The special shaped weights actually act as dampers. The damper acts as a concentrated mass at a single point on the conductor. The vibrational energy due to hysteresis and inter-strand friction gets absorbed by the damper. Due to absorption of vibration wave energy, the first vibration itself gets damped before the amplitude of vibrations becomes significant. Thus Aeoline vibrations are reduced by the use of dampers. Depending upon the span and the size of the conductors, the number of dampers and their locations are selected.

4. Factors Affecting Conductor Vibrations

The various factors affecting the vibrations of conductors are,

1. Atmospheric conditions like icing, rain and high wind velocities.

2. Span of conductor.

3. Tension in conductor.

4. Conductor configuration.

5. Types of clamps used.

6. Height of the towers.

7. Number of dampers and spacers used and their locations.

8. Some other factors depending on electrical and electromagnetic phenomenon like corona. 

Review Question

1. Explain the vibrations of conductors and methods to minimise them.