ON VIBRATIONS OF EHV OVERHEAD TRANSMISSION LINE CONDUCTOR



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

On Extra High Voltage Overhead Power Transmission Lines

Extra High Voltage(EHV) power transmission lines have to be overhead on considerations of economy and available technology. A lot of research and development has gone into the design of EHV line components; but a lot more need to be done to improve the reliability and reduce maintenance expenditure. Remember, the overhead lines, being aerial lines, are subject to the vagaries of weather like changing wind action, temperature variation, rain, snow, lightning etc. Also a transmission line is composed of heterogeneous materials of varying characteristics which respond differently to the forces of nature. All these make the field behavior of overhead lines complex.

We need to remember that whenever there is high wind the early structures to succumb to the fury of nature and become dysfunctional are the overhead power transmission lines, reminding us the need to carry out more R&D into the design and performance of EHV supports.

The EHV conductors are normally stranded for improved fatigue performance under vibratory forces of atmospheric origin. But they still require external damping devices. Research into conductor damping is relevant in this context since an ideal overhead conductor should be such as to kill vibration at the point of origin; evolving a conductor that has distributed damping to combat vibratory forces has to be given adequate consideration in EHV lines design.

Insulators which are interfaced between the earthed supports and live conductors are  not only subject to transmitted forces but are exposed to huge direct forces due to  lightning. About 30% of line failures are known to be due to insulator failures of lightning origin. The difficulty to provide continuity to the earth for the line supports contribute to the majority of lightning failures of insulators. Earthing of EHV lines also constitutes an important topic of R & D in EHV line practice.

An overhead line system with supports, insulators, conductors and accessories comes in the way of scenic beauty and is often an eyesore to the viewing public. Therefore, developing aesthetic design is a subject in itself. The best an overhead line designer can do is to make his line merge into the nature say, design supports which look like tall trees, avoid sky-lining of conductors and take the lines away from populated areas. Considerable R&D needs to be done in developing aesthetic EHV   line design. It may be desirable to have a multidisciplinary approach in design of aesthetic EHV overhead power lines.

The requisite data for a comprehensive line design is available in relevant technical literature. But I hasten to add that the ancient works like Rigveda also contain a lot of information which will supplement the data. It may be interesting to note that even the temple architecture in India are educative in this respect. I hope to add more in subsequent  articles.