Conductor gallop

Conductor gallop is the high-amplitude, low-frequency oscillation of overhead power lines due to wind.[1] The movement of the wires occur most commonly in the vertical plane, although horizontal or rotational motion is also possible. The natural frequency mode tends to be around 1 Hz, leading the often graceful periodic motion to also be known as conductor dancing.[2][3] The oscillations can exhibit amplitudes in excess of a metre, and the displacement is sometimes sufficient for the phase conductors to infringe operating clearances (coming too close to other objects), and causing flashover.[4] The forceful motion also adds significantly to the loading stress on insulators and electricity pylons, raising the risk of mechanical failure of either.

The mechanisms that initiate gallop are not always clear, though it is thought to be often caused by asymmetric conductor aerodynamics due to ice build up on one side of a wire.[3] The crescent of encrusted ice approximates an aerofoil, altering the normally round profile of the wire and increasing the tendency to oscillate.[3]

Gallop can be a significant problem for transmission system operators, particularly where lines cross open, windswept country and are at risk to ice loading. If gallop is likely to be a concern, designers can employ smooth-faced conductors, whose improved icing and aerodynamic characteristics reduce the motion.[4] Additionally, anti-gallop devices may be mounted to the line to convert the lateral motion to a less damaging twisting one. Increasing the tension in the line and adopting more rigid insulator attachments have the effect of reducing galloping motion. These measures can be costly, are often impractical after the line has been constructed, and can increase the tendency for the line to exhibit high frequency oscillations.[5]

Once gallop has started on a transmission line, an operator's options are more limited. If ice loading is suspected, it may be possible to increase power transfer on the line, and so raise its temperature by Joule heating, melting the ice.[3] The sudden loss of ice from a line can result in a phenomenon called "jump", in which the catenary dramatically rebounds upwards in response to the change in weight.[1][2] If the risk of trip is high, the operator may elect to pre-emptively switch out the line in a controlled manner rather than face an unexpected fault. The risk of mechanical failure of the line remains.[6]

Conductor gallop analysis

Conductor gallop analysis overlaps several academic disciplines. Mechanical vibrations covers the laws of motion of the conductor and the long conductor acts as a mass suspended by an elastic spring obeying Hooke's law. Within the discipline of Mechanical vibration, conductor gallop is categorized as a self-excited vibration because the forces which generate conductor gallop are generated by the motion itself. One of the early leaders in modern mechanical vibrations, J. P. Den Hartog, described conductor gallop in a chapter on self-excited vibrations in his text book Mechanical Vibrations, copyrighted in 1956 and reprinted by Dover Publications, where he develops general stability criteria for conductor gallop but without a complete mathematical solution. However, conductor gallop analysis also relates to Civil engineering because the electric conductors are carried by towers and the study of wind influences on structures, including any kind of vibration, has been much studied, especially after the collapse of the Tacoma Narrows bridge due to Karman vortex shedding from the structural members. In fact, perfectly round electrical conductors experience vortex shedding in certain ranges of the Reynolds number. The underlying behavior in the conductor gallop phenomenon also applies to other civil engineering structural elements such as cables and stays on bridges. A more recent reference related to the analysis of conductor gallop is Flow-Induced Vibrations, An Engineering Guide, by Eduard Naudascher and Donald Rockwell copyrighted in 1994 and still published by Dover Publications in 2005, in which experimental data related to vortex shedding frequencies as well as the aerodynamic forces on various structure shapes including cylinder models for a conductor or cable. Another book titled Flow-Induced Vibration by Robert D. Blevins, 2nd Edition published by Van Nostrand-Reinhold in 1990, also treats conductor gallop while reporting experimental data related to vortex shedding and aerodynamic forces on various structural shapes. Both of the last mentioned references include references to scientific and engineering journal articles many of which directly relate to conductor gallop. In aeronautical engineering the term flutter is used to describe conductor gallop and analogous other phenomena involving aerodynamic forces interacting with elastic structures having inertial mass.

Flutter

A similar aeolian phenomenon is flutter, caused by vortices on the leeward side of the wire, and which is distinguished from gallop by its high-frequency (10 Hz), low-amplitude motion.[2][3] To control flutter, transmission lines may be fitted with tuned mass dampers (known as Stockbridge dampers) clamped to the wires close to the towers.[5] The use of bundle conductor spacers can also be of benefit.

See also

References

  1. ↑ 1.0 1.1 Moore, G. F. (1997), BICC Electric Cables Handbook, Blackwell Publishing, p. 724, ISBN 0-632-04075-0
  2. ↑ 2.0 2.1 2.2 Guile A. & Paterson W. (1978), Electrical Power Systems, volume I, Pergamon, p. 138, ISBN 0-08-021729-X
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Pansini, Anthony J. (2004), Power Transmission and Distribution, Fairmont Press, pp. 204–205, ISBN 0-88173-503-5
  4. ↑ 4.0 4.1 Ryan, Hugh (2001), High Voltage Engineering and Testing, IET, p. 192, ISBN 0-85296-775-6
  5. ↑ 5.0 5.1 McCombe, John; Haigh, F.R. (1966), Overhead Line Practice (3rd ed.), Macdonald, pp. 216–219
  6. ↑ "Delen van Diksmuide en Kortemark zonder stroom (In Dutch, mechanical failure due to galloping effect)".

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