The wind tunnel experiment also allowed studying the effect of suspension angle of insulators being tested. When the angle was less than 30°, the widest sheds deformed, as seen in Fig. 6, but did not vibrate as in Fig. 4 – even at very high wind speed. When the suspension angle was 81°, the widest sheds would vibrate at a lower frequency than in Fig. 4b. If wind speed was higher, vibration frequency and amplitude would not increase and be lower than shown in Figs. 4b or 4c. This finding suggests that suspension angle influences vibration and deformation of sheds. In the case of the affected 750 kV lines, for example, composite insulator strings were designed and fixed in a ‘V’ pattern, meaning their suspension angle was poor to be able to better resist high winds.
To study stress and wind distribution on insulators sheds, a composite insulator model was set up in the Ansys software, with size and geometry parameters for the model being the same as shown in Fig. 2. While a composite insulator core is normally an FRP rod, to assist calculations in this case the core was made of aluminum. The shed and housing material was HTV silicone rubber with density set as 1420 kg/m3. The elastic modulus and Poisson’s ratio of the rubber were measured by the tensile test. The computation domain was set as cubic and the air was gas of density was 1.18 kg/m3. Input fluid velocity was set as 40 m/s.
Figs. 7 and 8 show fluid distribution around the insulator as well as stress distribution at the sheds. When the wind fluid blows around the insulator, vortexes appear at the edge of sheds or near the core. These vortexes vibrate at some frequency associated with wind speed, shed structure and material modulus. When vibration frequency of the vortexes was near the natural frequency of shed vibration, resonance would appear and amplitude of vibration would increase. This resonance would be the main reason for shed vibration at high amplitude.
As seen in Fig. 8, when the widest shed vibrated periodically at very high amplitude, stress at the shed surface would distribute differently. Obviously, stress was greatest near the shed base and the maximum value was concentrated at the point where shed and housing meet. This maximum stress value is lower than the tensile strength of the HTV silicone rubber. As such, if sheds begin to vibrate at high amplitude, their base would not immediately tear. Tears at the shed surface were therefore due to stress fatigue, i.e. the stress is greater than the fatigue limit of the material. When wind speed increased from 20 to 60 m/s, maximum stress would increase from 0.1 to 0.6 MPa.
• When used in high wind service areas, sheds of composite insulators studied would vibrate at high frequency and amplitude whenever wind speed was greater than 30 m/s. Vibration of sheds leads to stress being concentrated at their base.
• While the maximum value of this stress is less than the tensile strength of the HTV silicone rubber material, it is greater than its fatigue limit. Periodic stress at the shed surface would cause ‘micro tearing’ and this then expands into full tears.
• To minimize resonance between a fluid (i.e. wind) and sheds, there may be a need to further improve shed structure parameters as well as material.