Thursday, 22 September 2016

Vibration and Vibration Dampers in Transmission Line


The overhead Transmission Line experiences vibrations in vertical plane. This vibration of Transmission Line in vertical plane can be categorized into two groups.

1) Aeoline Vibration
2) Galloping or Dancing of Conductors

Aeoline Vibration is of high frequency and low amplitude. The frequency of vibration is of the order of 5 to 40 cycle/second while the amplitude being of the order of 2 cm to 5 cm with a loop length of 1 m to 10 m. This type of vibration occurs when wind velocity is very low around 4 to 15 km/hr, that it is steady, free from eddies and spread over a wide area.



Galloping or Dancing of Conductors are of low frequency but higher amplitude. The frequency of Galloping or Dancing of Conductors is of the order of 1/4 - 3/2 cycle/second and amplitude of vibration of the order of 10 m.

Aeoline Vibration is forced vibration whereas Galloping or Dancing of Conductors is self-excited type. Galloping or Dancing of Conductors is caused due to the wind when it blows past a non-circular conductor portion. Once start, it builds with itself and the vibration may become very large and may result into flexure fatigue in conductors or there may be flash-over between the conductors of different phases which may lead to outage of system and loss of revenue. While Aeoline Vibration is itself not harmful but it may give rise to troubles at points where free vibration is restricted.

Vibration in Transmission Line is solved by using specially designed dampers. A damper is basically a device which is so designed to absorbed vibration energy and prevent vibration in Transmission Line. Stock Bridge damper is most popular damper used for prevention of vibration in Transmission Line.

Stock Bridge damper consists of two hollow weights of special shape fixed at either end of length of line and fastened to the line by aluminum clamp. Figure below shows a Stock Bridge damper.



The weights in Stock Bridge damper are of Galvanized iron or in smaller size of damper Zinc is used. The damper is enclosed in an enclosure to prevent any ingress of water or dirt.
Normally, two dampers are required for each point of suspension of the conductors, one on either side and clamped to the conductor at appropriate distance depending upon the conductor size and density of material used for the conductor. Thus each span of conductor will have two dampers to dampen out vibrations.

For damping Galloping Vibration, it is necessary to make the conductor circular in shape. Thus for stranded conductor, PVC tape is wrapped to make the surface circular. In places where ice or sleet formation is observed, I2R loss can be increased to eliminate these ice or sleet, but this is actually not the practical solution. Thus utmost care shall be taken so as to maintain the conductor surface circular.

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