The live parts of an overhead transmission line are the conductors. These are held high in the air(dictated by the electrical and mechanical design considerations as applicable to ehv overhead lines)by insulators fixed on the earthed supports. The conductors are subject to alternating stresses due to wind induced vibrations in addition to static forces due to wind action, components of conductor pull etc. Bundling of conductors, characteristic of ehv lines of 400 kV and above, makes flow induced conductor vibration problems more complex. If the power line is in an icy area galloping oscillation of the ice coated conductor also has to be considered in the scheme of conductor protection.
Vibrations cause conductor fatigue due to alternating stresses in conductor, particularly at the span-ends where the conductor static stresses are also high as compared to those at other conductor portions. Most of the conductor failures were seen at the conductor span- ends. It is generally accepted that the fatigue stresses combined with fretting(or stress corrosion)constitute the primary cause of conductor failures at span-ends(or spacer- ends in bundled conductors).As many span-ends so many conductor portions vulnerable to fatigue failure. But, obviously, we cannot limit the number of spans to just one; the number of spans are worked out considering the transmission line economics. Therefore, engineers have to look for other methods to limit fatigue stresses, fretting and static stresses at span-ends. Multipoint suspension reduces static stresses at span-ends. Improved conductor damping(either self-damping or added damping)reduces fatigue and fretting.
Speaking about conductor self-damping, I may say, any structure has some self-damping. Stranded conductors are preferred over solid conductors from the early days of overhead transmission system, primarily due to the increase in self-damping and better endurance of the former as compared to the latter. But it is now known that the stranded conductor self- damping itself is dependent on the current carried by the conductor. Multilayer stranded conductor exhibits highly reduced self-damping at high conductor currents, due to a locking-in effect of the wires of the ultimate layer and those of the penultimate layer and the resulting reduced inter-strand motion among wires(A similar phenomenon was seen in aged conductors due to better compaction of wires).A few designs of self-damping conductors were tried with varying amounts of success, but they all suffer from one or more problems including high initial cost ,high maintenance cost, low field life etc. A lot remains to be done in developing cost effective self-damping conductors that retain the self-damping properties under varying conductor currents and state of ageing.
We need to do more to evaluate conductor self-damping under a given set of conditions both in the laboratory and in the field. We also need to do a lot more for developing self-damping ehv conductors. In addition, we need to develop suitable span-end external damper(like the well known Stockbridge damper).Developing flexible suspensions like multipoint suspension clamp is another area of work. Accelerated R & D on all these is an urgent need to improve the performance and investment on ehv transmission line conductors.
Dr T.V.Gopalan
