Aerodynamic instability of long-span transmission lines

The oscillatory behaviour of long-span stranded transmission lines is outlined qualitatively, and the consequences of the inevitable presence of corresponding lateral and in-plane modes of oscillation with nearly equal frequencies are discussed. The desirability of designing to eliminate as many of these modal pairs as possible is emphasised. The principle of virtual work is employed to set up approximate equations of motion of a single-span transmission line under steady aerodynamic forcing in a yawed wind. (Matrix terminology is used to afford immediate extension of these equations to the multispan case.) Routh´s method is applied to the equations of motion to obtain a set of criteria for incipiently unstable oscillation of the line. These criteria may be used to assess the regions of likely instability of any long-span transmission line if the aerodynamic properties of the conductor are known. It is shown that (in the absence of ice) the interphase-clashing phenomena experienced on certain stranded transmission lines, notably the CEGB Severn¿Wye crossing, may be ascribed to a combined oscillation involving lateral and in-phase antisymmetric motions in the offending span.