In this article we will study about the causes of vibration in overhead transmission lines. we will also studied about types of vibration conductors and also know effect of vibration in overhead transmission lines.
Causes of Vibration In Overhead Transmission Lines.
The following types of mechanical vibration are common to line conductors:
- Resonant vibration or Aeolian vibration.
- Dancing and sleet-jump.
Resonant vibration of conduction arise from the eddies produced behind the conductor by the action of low-velocity winds. Due to the formation of eddies; the velocities of the wind at the sides towards and away from conductor become unequal. The unequal velocities result in unequal pressures, the pressures at the higher velocity side being lower. Air from the lower velocity side will therefore; come to fill rarefied space behind the pressure of air. The conductor thus sets in motion in upward and downward direction depending on the existing conditions. During its motion the conduction leaves behind an empty space, which is then filled up by eddies from the higher velocity side and the direction of motion changes. Thus, oscillation of the conductor starts.
When the frequency of eddies coincides with the natural frequency of eddies of the conductor, resonant vibration of conductor results.
Self-excited vibrations are produced on line conductors by the aerodynamic forces acting upon non-circular cross-section. The conductor takes a non-circular section due to uneven coating of ice on its surface. This low frequency (1-2Hz), large amplitude oscillation of conductors is called galloping. Oscillations with amplitude of about 11m and periods of about 8 seconds have been witnessed. Galloping conditions occur rarely. Galloping has been observed with wind velocities ranging between 15-75 km/h inclined to the line at an angle between 10-90. Both torsion and transnational motions are set up in the conductor. it is causes of vibration in overhead transmission lines. Abnormal stresses may be produced at the points of attachment to damage the conductors and fittings. Galloping also introduces the possibilities of electrical contact between phase conductors or from phase conductor to earth. The supply may thus be interrupted.
3. Dancing and Sleet-jump:
Ice falling from a conductor throws it into violent oscillations of large period and long amplitude. The oscillations die out quickly if the ends of the dancing span are dead-ended, otherwise the oscillations are transmitted in the adjoin spans to considerable distance of line. As such dancing is not harmful from mechanical damage point of view, but large amplitude vibration may bring the conductors together resulting in short circuit and burning of conductors. Conductor clashing may be reduced by arranging the conductors in a horizontal configuration.
Effect of vibration on The Transmission Lines
The following adverse effects are observed in the transmission line due to conductor vibrations:
- Fatigue failure of strands or complete stranded conductor
- Bird caging of stranded conductors
- Breaking of insulator discs
- Excessive sag
- Collapse of supporting structures
these are causes of vibration in transmission lines
Prevention of vibration
Of the various methods employed for minimizing or eliminating vibration in a conductor, the two mentioned below have been found to be very effective.
- Amour rods.
- Stock bridge dampers
1. Armour Rods:
Armour rods consist of layer of wires of rods wrapped apically around the conductor for a short distance on either side of the point of support. They provide a reinforcement of conductors at suspension points and reduce amplitude of vibration from 10 to 20 percent. They relieve and distribute the tresses at the support point. They also serve as a protection against flash over on a conductor. it is main cause of vibration in overhead transmission lines. these are basic causes of vibration in overhead transmission lines.
Stock bridge damper:
the Stock bridge damper consists of two weights joined by a flexible steel wire. It is provided with its middle point to attach it to the conductor. Usually one damper is attached at each end of the span,
V-strings have also been used to minimize vibrations of conductors. The adjustment of the system prevents the conductors from vibration.
Hence these are causes of vibration in overhead transmission lines if you will find any incorrect in above article please comment below in comment box.