Tuned mass damper used to suppress wind-induced vibrations

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A Stockbridge damper is a tuned mass damper used to suppress wind-induced vibrations on slender structures such as overhead power lines,[1] long cantilevered signs[2] and cable-stayed bridges. The dumbbell-shaped device consists of two masses at the ends of a short length of cable or flexible rod, which is clamped at its middle to the main cable. The damper is designed to dissipate the energy of oscillations in the main cable to an acceptable level.[3]

Stockbridge dampers installed on high voltage power lines

Wind-induced oscillation

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Wind can generate three major modes of oscillation in suspended cables:[4]

• Gallop has an amplitude measured in metres and a frequency range of 0.08 to 3 Hz
• Aeolian vibration (sometimes termed flutter) has an amplitude of millimetres to centimetres and a frequency of 3 to 150 Hz
• Wake-induced vibration has an amplitude of centimetres and a frequency of 0.15 to 10 Hz

The Stockbridge damper targets oscillations due to aeolian vibration; it is less effective outside this amplitude and frequency range. Aeolian vibration occurs in the vertical plane and is caused by alternating shedding of vortices on the leeward side of the cable. A steady but moderate wind can induce a standing wave pattern on the line consisting of several wavelengths per span.[4] Aeolian vibration causes damaging stress fatigue to the cable[5] and represents the principal cause of failure of conductor strands.[4] The ends of a power line span, where it is clamped to the transmission towers, are at most risk.[5] The effect becomes more pronounced with increased cable tension,[5] as its natural self-damping is reduced.

Description

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Stockbridge's original concrete block design

The Stockbridge damper was invented in the s by George H. Stockbridge, who was an engineer for Southern California Edison. Stockbridge obtained US patent on 3 July for a "vibration damper".[6] His patent described three means of damping vibrations on lines: a sack of metal punchings tied to the line; a short length of cable clamped parallel to the main cable; and a short (30 in, 75 cm) cable with a concrete mass fixed at each end.[6] This last device developed into the widely used Stockbridge damper.

Vibrations in the main cable were passed down through the clamp and into the shorter damper, or "messenger", cable. This would flex and cause the symmetrically-placed concrete blocks at its ends to oscillate. Careful choice of the mass of the blocks, and the stiffness and length of the damper cable would match the mechanical impedance of the damper to that of the line, and greatly attenuate oscillation of the main cable. Since Stockbridge dampers were economical, effective and easy to install, they became used routinely on overhead lines.[5] Live-line working using hot stick tools meant it was possible to retrofit dampers to lines while energised.[7]

Modern designs

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A modern design with metal weights

Modern designs use metal bell-shaped weights rather than Stockbridge's concrete blocks. The bell is hollow and the damper cable is fixed internally to the distal end, which permits relative motion between the cable and damping weights.[7] To provide for greater freedom of motion, the weights may be partially slotted in the vertical plane, allowing the cable to travel outside the confines of the bell. Some more complex designs use weights with asymmetric mass distribution, which enables the damper to oscillate in several different frequency modes and ranges.[8][9]

Dogbone dampers on the road-support cables of the Severn Bridge

Another modern design is the Dogbone invented by Philip Dulhunty in [dubious &#; discuss] is so called due to its configuration, a larger metal sphere attached to the end of the damper, with a smaller sphere projecting sideways from it, resembling a dog's bone. The damper offsets the weights sideways in order to introduce a third degree of freedom, twisting the damper cable in addition to bending it up and down. Additional intra-strand friction was created in the damper cable, dissipating significantly more energy.[2][10][verification needed]

The most vulnerable section of the cable is where it is clamped to the end of an insulator string, so dampers are typically installed at the nearest anti-nodes (points of maximum displacement) either side of the clamp.[8] There are thus normally two dampers per span, though more can be installed if necessary on longer spans.[5][7]

Overhead transmission lines form a catenary for which vibration is predominantly in the vertical plane. When more than one plane of vibration is anticipated, Stockbridge dampers may be mounted at right angles to each other. This is common when the cable runs in a vertical or off-horizontal plane, for example in cable-stayed bridges or radio mast guy-wires.

See also

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• Conductor gallop
• Tacoma Narrows Bridge - destruction once thought to be a case of resonance

References

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An Introduction to Vibration control

8 May,

An Introduction to Vibration control
Electrical utilities use vibration dampers on their overhead transmission lines to reduce the effects of wind-induced vibration on the lines. When wind blows across overhead transmission lines, it can cause them to vibrate or oscillate, which can lead to fatigue and even failure of the conductors over time. This can be especially problematic in areas with high wind speeds or where the lines are subject to other sources of vibration, such as nearby highways or railways.

Vibration dampers are typically installed along the length of the transmission line at regular intervals. They work by absorbing some of the energy of the oscillations, which reduces the amplitude of the vibrations and the stress on the conductors. This helps to prevent damage to the conductors and ensures the safe and reliable operation of the transmission line.

In addition to improving the safety and reliability of the transmission line, vibration dampers can also help to reduce the noise generated by the conductors as they vibrate in the wind. This can be especially important in urban areas where transmission lines are located close to homes and businesses.

Selecting the correct size and quantity of dampers for a transmission line section depends on several factors, including the length of the conductor between two supports (Tower or Pole), the type of conductors used, the anticipated wind conditions, and the desired level of vibration reduction.

Guide to Damper Selection
Here are some general guidelines that can help in selecting the correct size and quantity of dampers required for a line between two supporting structures:

Determine the length of the transmission line section:
The length of the section will determine the number of dampers needed. As a rule, a damper should be installed every one or two spans (the distance between two supporting structures).

Determine the type and size of conductors used:
The type and size of conductors used will determine the appropriate size of damper needed. The damper should be sized to fit the diameter of the conductor and should be rated for the appropriate load capacity.

Determine the anticipated wind conditions:
Wind conditions can vary greatly depending on location and time of year. The wind speed, direction, and frequency of gusts should be considered when selecting the appropriate damper. Some dampers are designed to work better in specific wind conditions, such as high wind speeds or gusty winds.

Determine the desired level of vibration reduction:
The amount of vibration reduction needed will depend on the specific transmission line and its operating conditions. The desired level of vibration reduction should be balanced with other factors, such as cost and ease of installation.

In general, it is recommended to consult with a vibration damper manufacturer or a professional engineer with experience in transmission line design to determine the correct size and quantity of dampers needed for a specific transmission line section. They consider all the factors listed above and provide a customised solution for your specific needs.

Maclean Power has developed special recommendation software for the type and installation of 4D series vibration dampers. The software is preloaded with over 400 hundred conductors with standard line tension calculated at 20% CBL. As manufacturers of the dampers, they have collected information from a multitude of tests made on the dampers and from field tests they carried-out on lines where the dampers are installed. Vibration analysis has been performed in consequence of these tests and the effectiveness of damping has been determined. Wind power, damper power and self-damping of the conductor are all considered.

For Damper selection, installation and placement recommendations, please contact TEN for a Damping Proposal Form or download a copy here.

Article PDF available here.

The MacLean Power 4D range of Vibration Dampers have been designed to provide the best possible protection from Aeolian vibration for conductors and earth wires (ground or shield wires) used for electric power transmission and distribution.
The 4D dampers are designed to dissipate the maximum amount of vibration energy to prevent fatigue damage to the strands of the conductors.
The 4D damper has an asymmetric design that features 4 resonant frequencies, allowing the dampers to be effective across a much wider frequency range than standard Stockbridge dampers.
4D dampers are suitable for use on all conductor and earth wire constructions including ACSR, AAC, AAAC, AAAC , HDC and Galvanized and Aluminium Clad Steel (SC/GZ and SC/AC) construction, and sizes are available to cover the range of sizes from 7.5mm to 45mm.
For installation on bare Copper conductor, MacLean Power 4D dampers are supplied with a Brass clamp.

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