Aeolian [ee-oh-lee-uh n], or wind induced vibrations, can damage conductors and overhead shield wires (OHSW) on transmission and distribution lines, reducing reliability and service life. Understanding aeolian vibration and how it can be managed or controlled is key to minimizing its possible effect on a line or network.
This tutorial summarizes the research and findings of industry experts. The references cited provide a more detailed explanation and effective recommendations.
Figure 1: Fatigue failure of conductor under armor rods due to aeolian vibration.
Almost all powerlines have some degree of aeolian vibration from time to time, usually without damage. However, if the magnitude of the vibration is high enough, abrasion or fatigue failures will generally occur over time.
Figure 2: Fatigue breakage of conductor strands due to excessive bending cycle caused by aeolian vibration.
How are the vibrations produced?
When a non-turbulent (“smooth”) stream of air passes across a conductor or overhead shield wire (OHSW), vortices (eddies) are formed on the leeward side (back side). These vortices create alternating pressures producing movement at right angles to the direction of the air flow. This is the mechanism that causes aeolian vibration1.
Ironically, turbulent air flow generally will not create the alternating vortices required to drive the associated mechanical vibrations. Because the degree of turbulence in the wind is affected both by the terrain over which it passes and the wind velocity itself, aeolian vibration is generally produced by wind velocities below 15 miles per hour (MPH). Higher speed winds usually contain a considerable amount of turbulence, except for special cases such as open bodies of water or canyons where the effect of the terrain is minimal.
The frequency at which the vortices alternate from the top to bottom surfaces of conductors and shield wires can be closely approximated by2 :
This is the frequency of the alternating forces on the powerline or shield wire. However, the complete magnitudes and frequencies of the resulting mechanical vibrations depend on other factors, such as span length, dampers, and spacers.
It’s clear from the above equation that the aeolian vibration frequency is higher for smaller diameter conductors or OHSW. For example, the vortex frequency for a 795 kcmil 26/7 ACSR (“Drake”) conductor under the influence of an 8 MPH wind is 23.5 Hertz. A 3/8” OHSW under the same 8 MPH wind will have vortices alternating at 72.4 Hertz.
Interaction with span resonance
Sustained aeolian vibrations occur when the vortex frequency approaches one of the natural vibration frequencies of the span of the conductor or OHSW. As a result, sustained vibration activity takes the form of discrete standing waves. Forced nodes occur at the support structures. Intermediate nodes occur along the span at intervals that depend on the particular natural frequency3.
Conductor tension reduces damping
The self-damping characteristics of a conductor or OHSW are dependent on the freedom of movement or “looseness” between the individual strands or layers of the overall construction. In standard conductors, the freedom of movement (self-damping) will be reduced as the tension is increased. That’s one reason why vibration activity is most severe in the coldest months of the year when the tensions are the highest.
Some conductors designed with higher self-damping performance use trapezoidal-shaped outer strands that “lock” together to create gaps between layers. Other conductors, such as ACSS (formerly SSAC), utilize fully annealed aluminum strands that become inherently looser when the conductor progresses from initial to final operating tension.
Damage caused by aeolian vibration
Abrasion is generally associated with loose connections between the conductor or OHSW and attachment hardware or other conductor fittings. This “looseness” allows the abrasion to occur and is often the result of excessive aeolian vibration.
Figure 4: Abrasion damage at spacer.
Abrasion damage can occur within the span itself at spacers (Figure 4), spacer dampers and marker spheres, or at supporting structures (Figure 5).
Maximum bending stresses occur at locations where the conductor or OHSW is being restrained from movement. such as at the edge of clamps of spacers, spacer dampers and dampers. The highest level of bending stresses generally occurs at the supporting structures. When the bending stresses in a conductor or OHSW due to aeolian vibration exceed the endurance limit, fatigue failures will occur (Figure 1). The time to failure will depend on the magnitude of the bending stresses and the number of bending cycles accumulated (frequency)4 5 6.
Figure 5: Abrasion damage at loose hand tie.
Designing for safe tension
CIGRE7 provides guidelines for safe design tensions based on the ratio of the horizontal conductor tension, H, and the conductor weight per unit length, w. The effects of terrain on the turbulence intensity of the wind were also studied and included as part of the overall recommendations.
The horizontal conductor tension used to calculate the H/w ratio is the initial, unloaded tension at the average temperature of the coldest month (AAMT) at the location of the line.
By applying the H/w ratio and the newly created terrain categories to all available field experience data, the CIGRE Task Force published the recommendations shown in Table 1 for single undamped, unarmored conductors. The Task Force also published the warning that: “Extra-long spans, spans covered with ice, rime or hoarfrost, spans equipped with aircraft warning devices, and spans using non-conventional conductors”, require special attention.
Table 1: Safe design tension for single, undamped, unarmored conductors.
CIGRE Report #273 also provides recommendations for safe design tensions for bundled (twin, tri and quad) conductors.
Figure 6: Armor Rod applied to conductor within a suspension clamp.
Effects of suspension hardware
The use of Armor Rods (Figure 6) or high-performance suspension assemblies (Figures 7 & 8) reduces the level of dynamic bending stress on a vibrating conductor.
Figure 7: ARMOR-GRIP® Suspension (AGS).
Figure 8: CUSHION-GRIP® Suspension (CGS).
As a result, high performance suspensions will allow higher safe design tensions (H/w) and increase the “protectable” span length of a damper. The amount of positive influence and additional protection provided by performance suspensions is difficult to reduce to a simple table. Contact PLP with specific line design and environmental (terrain and temperatures) data for more information.
Dampers can help
Dampers of many different types have been used since the early 1900s to reduce the level of aeolian vibration within the span and, more importantly, at the supporting structures. The most commonly used damper is the Stockbridge type, named after the original invention by G.H. Stockbridge about 1924. The original design has evolved over the years but the basic principle remains: weights are suspended from the ends of specially designed and manufactured steel strand, which is secured to the conductor with a clamp (Figure 9).
When the damper is placed on a vibrating conductor, movement of the weights will produce bending of the steel strand. The bending of the strand causes the individual wires of the strand to rub together, thus dissipating energy. The size and shape of the weights and the overall geometry of the damper influence the amount of energy that will be dissipated for specific vibration frequencies. An effective damper design must have the proper response over the range of frequencies expected for a specific conductor and span parameters.
Figure 9: VORTXTM Damper.
Some dampers, such as the VORTX™ Damper (Figure 9), utilize two different weights and an asymmetric placement on the strand to provide the broadest effective frequency range possible. Placement programs, such as those developed by Performed Line Products for the VORTX Damper, take into account span and terrain conditions, suspension types, conductor self- damping, and other factors. This methodology identifies a specific location in the span where the damper(s) will be most effective.
For smaller diameter conductors (< 0.75”), overhead shield wires, and optical ground wires (OPGW), a different type of damper is available that is generally more effective than a Stockbridge type damper. The Spiral Vibration Damper (Figure 10) has been used successfully for over 35 years to control aeolian vibration on these smaller sizes of conductors and wires.
Figure 10: Spiral Vibration Damper.
The Spiral Vibration Damper is an “impact” type damper made of a rugged non-metallic material that has a tight helix on one end that grips the conductor or wire. The remaining helixes have an inner diameter that is larger than the conductor or wire, such that they impact during aeolian vibration activity. The impact pulses from the damper disrupt and negate the motion produced by the wind. The Spiral Vibration Damper is so effective because it can be placed anywhere in the span and has no specific resonant frequencies. It responds to all frequencies, especially the high frequencies associated with smaller diameter conductors and wire.
Continuing Education regarding Aeolian Vibrations
This discussion is meant to be a quick overview of the causes, effects, and mitigation of aeolian vibrations on powerlines. A more in-depth discussion is provided in “Aeolian Vibration Basics,” part of the Preformed Line Products reference series. Preformed Line Products offers seminars covering subjects pertinent to the utility lineman and foreman, as well as the design engineer. Subjects include: galloping motion, sub conductor oscillation, conductor repair, and wildlife protection to name a few.
1Electric Power Research Institute, “Transmission Line Reference Book, Wind Induced Conductor Motion”, Research Project 795, 1978
2V. Strouhal, “On Aeolian Tones”, Annalen der Physik und Chemie, Band V, 1878, p. 216
3“Preformed Line Products reference report, “Aeolian Vibration Basics”, 2006
4P.W. Dulhunty, A. Lamprecht and J. Roughens, “The Fatigue Life of Overhead Line Conductors”, CIGRE SC22-WG04 Task Force Document, 1982
5C.B. Rawlins, “Exploratory Calculations of the Predicted Fatigue Life of Two ACSR and One AAAC”, Report CIGRE SC-22, WG11, TF4-96-5, April 1996
6O.D. Zetterholm, “Bare Conductors and Mechanical Calculation of Overhead Conductors”, CIGRE Session Report #223, 1960
7CIGRE Report #273, “Overhead Conductor Safe Design Tension with Respect to Aeolian Vibrations”, Task Force B2.11.04, June 2005