Vibration Damper in Transmission Line

Vibration Damper in Transmission Line:

Wind-induced vibration of overhead conductors is common worldwide and can cause conductor fatigue Near a hardware attachment.

As the need for transmission of communication signals increase, many Optical Ground Wires(OPWG) are replacing traditional ground wires.

In the last twenty years All Aluminum Alloy Conductors (AAAC) have been a popular choice for overhead conductors due to advantages in both electrical and mechanical characteristics. Unfortunately AAAC is known to be prone to Aeolian vibration.

Vibration dampers are widely used to control Aeolian vibration of the conductors and earth wires including Optical Ground Wires (OPGW).

In recent years, AAAC conductor has been a popular choice for transmission lines due to its high electrical carrying capacity and high mechanical tension to mass ratio. The high tension to mass ratio allows AAAC conductors to be strung at a higher tension and longer spans than traditional ACSR (Aluminum Conductor Steel Reinforced) conductors.

Unfortunately the self-damping of conductor decreases as tension increases. The wind power into the conductor increases with span length. Hence AAAC conductors are likely to experience more severe vibration than ACSR.

What is Aeolian Vibration?

• Wind-induced vibration or Aeolian vibration of transmission line conductors is a common phenomenon undersmooth wind conditions. The cause of vibration is that the vortexes shed alternatively from the top and bottom of the conductor at the leeward side of the conductor.
• The vortex shedding action creates an alternating pressure imbalance, inducing the conductor to move up and down at right angles to the direction of airflow.
• The conductor vibration results in cyclic bending of the conductor near hardware attachments, such as suspension clamps and consequently causes conductor fatigue and strand breakage.
• When a “smooth” stream of air passes across a cylindrical shape, such as a conductor or OHSW, vortices (eddies) are formed on the back side. These vortices alternate from the top and bottom surfaces, and create alternating pressures that tend to produce movement at right angles to the direction of the air flow. This is the mechanism that causes Aeolian vibration.
• The term “smooth” was used in the above description because unsmooth air (i.e., air with turbulence) will not generate the vortices and associated pressures. The degree of turbulence in the wind is affected both by the terrain over which it passes and the wind velocity itself.
• It is for these reasons that Aeolian vibration is generally produced by wind velocities below 15 miles per hour (MPH). Winds higher than 15 MPH 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 by the following relationship that is based on the Strouhal Number [2].
• Vortex Frequency (Hertz) = 3.26 V / d
• Where: V is the wind velocity component normal to the conductor or OHSW in miles per hour
• d is the conductor or OHSW diameter in inches
• 3.26 is an empirical aerodynamic constant.
• One thing that is clear from the above equation is that the frequency at which the vortices alternate is inversely proportional to the diameter of the conductor or OHSW.
• The self damping characteristics of a conductor or OHSW are basically related to 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. It is for this reason that vibration activity is most severe in the coldest months of the year when the tensions are the highest.
• Aeolian vibrations mostly occur at steady wind velocities from 1 to 7 m/s. With increasing wind turbulence the wind power input to the conductor will decrease. The intensity to induce vibrations depends on several parameters such as type of conductors and clamps, tension, span length, topography in the surrounding, height and direction of the line as well as the frequency of occurrence of the vibration induced wind streams.
• Hence the smaller the conductor, the higher the frequency ranges of vibration of the conductor. The vibration damper should meet the requirement of frequency or wind velocity range and also have mechanical impedance closely matched to that of the conductor. The vibration dampers also need to be installed at suitable positions to ensure effectiveness across the frequency range.

Effect of Aeolian Vibration:

• It should be understood that the existence of Aeolian vibration on a transmission or distribution line doesn’t necessarily constitute a problem. However, if the magnitude of the vibration is high enough, damage in the form of abrasion or fatigue failures will generally occur over a period of time.
• Abrasion is the wearing away of the surface of a conductor or OHSW and is generally associated with loose connections between the conductor or OHSW and attachment hardware or other conductor fittings.
• Abrasion damage can occur within the span itself at spacers Fatigue failures are the direct result of bending a material back and forth a sufficient amount over a sufficient number of cycles.
• In the case of a conductor or OHSW being subjected to Aeolian vibration, the maximum bending stresses occur at locations where the conductor or OHSW is being restrained from movement. Such restraint can occur in the span at the edge of clamps of spacers, spacer dampers and Stock bridge type dampers.
• However, the level of restraint, and therefore the level of bending stresses, is generally highest 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.
• In a circular cross-section, such as a conductor or OHSW, the bending stress is zero at the center and increases to the maximum at the top and bottom surfaces (assuming the bending is about the horizontal axis). This means that the strands in the outer layer will be subjected to the highest level of bending stress and will logically be the first to fail in fatigue.

working of Vibration Damper

• 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.
• Since, as presented earlier, a span of tensioned conductor will vibrate at a number of different resonant frequencies under the influence of a range of wind velocities, an effective damper design must have the proper response over the range of frequencies expected for a specific conductor and span parameters.

(1) VORTX/ Stock bridge Type:

• Some dampers, such as the VORTX Damper utilize two different weights and an asymmetric placement on the strand to provide the broadest effective frequency range possible.
  
• The “Stockbridge” type vibration damper is commonly used to control vibration of overhead conductors and OPGW. The vibration damper has a length of steel messenger cable. Two metallic weights are attached to the ends of the messenger cable. The centre clamp, which is attached to the messenger cable, is used to install the vibration damper onto the overhead conductor.
• Placement programs, such as those developed by PLP for the VORTX Damper, take into account span and terrain conditions, suspension types, conductor self-damping, and other factors to provide a specific location in the span where the damper or dampers will be most effective.
• The asymmetrical vibration damper is multi resonance system with inherent damping. The vibration energy is dissipated through inter-strand friction of the messenger cable around the resonance frequencies of the vibration damper. By increasing the number of resonances of the damper using asymmetrical design and increasing the damping capacity of the messenger cable the vibration damper is effective in reducing vibration over a wide frequency or wind velocity range.

(2) Spiral Vibration Damper:

• 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 15) has been used successfully for over 35 years to control Aeolian vibration on these smaller sizes of conductors and wires.
• 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.