Investigation of conductor oscillation on the 275 kV crossing over the rivers Severn and Wye

Details are given of an investigation into the phenomenon of wind-induced oscillations under non-icing conditions that have occurred on the 275 kV crossing over the Rivers Severn and Wye since commissioning. In many instances the oscillations were of sufficient amplitude to cause electrical flashover between conductors and consequent outage of the circuits. Instruments installed on site to record the form of the oscillations and the meteorological conditions under which they occur showed that only the lowest-frequency modes were involved, and that at all times of flashover the wind speed and direction were confined within narrow limits. Wind-tunnel tests carried out on models of the stranded conductor revealed that for these bands of wind speed and direction a lift force of up to 2% of the cable weight is experienced. The gradients of this force with wind speed and direction are large in the region corresponding to flashover conditions so that small periodic changes in these quantities produce a relatively large fluctuating vertical force. Calculations indicate that, with the various normal modes present on the crossing, gusts and/or drag instability can produce vertical oscillations of clashing magnitude. Smoothing of conductors on the crossing by wrapping with p.v.c. tape to eliminate lift has proved successful in completely suppressing vertical oscillations.