Concept of Wind Induced Oscillations
Oscillations in conductors occur due the following phenomena :
1.Aeolian or Vertical Vibrations:
This is the most common type of vibration that results due to vortex shedding under laminar flow of wind. These are low amplitude medium frequency (3 – 60 Hz for wind speeds of 1-8 m/s). Sometimes, additional sinusoidal waves of different frequencies arise on the line, corresponding to a higher mode of vibrations. These vibrations are in the vertical plane and exert continuous alternating bend stresses on the conductor strands and eventually may lead fatigue failure of the conductor. Purview of this article is to cover protection of conductor from Aeolian Vibration.
It is also termed as Long wave vibrations and is characterized by low frequencies of 0.1 to 1 Hz. This phenomenon is most common when there is a sleet covering of the cable. Ice coating creates irregular edges and surfaces, which disturb the airflow, which breaks away at these points to induce a certain self-excitation. Thus the system becomes susceptible to vibrations subject to resonance. This effect is not so pronounced in India. Conditions that are conducive for galloping are – Low pressure area with High winds, temperature between 0 and -5 °C.