Understanding wind-induced vibration and how to manage or control wind-induced vibration is the key to minimizing the possible impact of wind-induced vibration on the line or grid. Wind or wind-induced vibration can damage conductors and overhead shields on transmission lines and distribution lines, reducing reliability and service life. Understanding wind-induced vibration and how to manage or control wind-induced vibration is the key to minimizing the possible impact of wind-induced vibration on a line or network. This tutorial summarizes the research and findings of industry experts. The citations provide more detailed explanations and effective recommendations. Figure 1: Fatigue failure of the wire due to wind vibration. Almost all transmission lines have a certain degree of wind or wind dance, usually not damaged. However, if the amplitude of the vibration is sufficiently high, wear or fatigue failure usually occurs over time. Figure 2: The bending of the wire strand is fatigued due to excessive bending caused by wind vibration. How is vibration generated? When a non-turbulent ("smooth") airflow passes through a conductor or overhead shielded wire (OHSW), eddy currents (vortex) are formed on the leeward side (leeward side). These eddy currents create alternating pressures that produce motion at right angles to the direction of the gas flow. This is the mechanism that causes wind vibration. Ironically, turbulent airflow generally does not produce the alternating eddy currents required to drive the associated mechanical vibrations. Because the degree of turbulence in the wind is affected by both the terrain it passes through and the wind speed itself, wind-induced vibration is usually produced by wind speeds of less than 15 miles per hour (MPH). High-speed winds usually contain a significant amount of turbulence, except in special circumstances, such as open water bodies or canyons, where terrain has the least impact. Vibration frequency The frequency at which the vortex alternates from the top of the conductor to the shield to the bottom surface can be assessed by: This is the frequency of the alternating force on the power line or shield line. However, the full magnitude and frequency of the resulting mechanical vibrations depend on other factors such as span length, damper and spacer. It is clear from the above equation that the wind vibration frequency is higher for smaller diameter conductors or overhead shielded wires. For example, the vortex frequency of a 795kcmil 26/7ACSR ("Drake") conductor under the influence of 8MPH wind is 23.5 Hz. A 3/8 "overhead shielded line" under the same 8 mph wind will have an alternating vortex of 72.4 Hz. Span resonance Continuous wind-induced vibration occurs when the eddy current frequency is close to one of the natural vibration frequencies of the conductor or overhead shielded line span. As a result, the continuous vibration occurs in the form of discrete standing waves. Force the node to appear on the support structure. The intermediate nodes appear on the span at intervals of a specific natural frequency. Wire tension reduction damping The self-damping characteristics of the wire or overhead shield wire depend on the freedom of movement or "looseness" between the individual strands or layers of the overall structure. In standard wires, as the tension increases, the degree of freedom of motion (self-damping) will decrease. This is one of the reasons why vibration activity is the most serious when the tension is the coldest month of the year. Some of the wires designed to have higher self-damping properties use trapezoidal outer strands that are "locked" together to create a gap between the layers. Other conductors, such as ACSS (pre-SSAC), utilize fully annealed aluminum strands that become looser as the conductor develops from the initial operating tension to the final operating tension. Wind vibration damage Wear is often associated with loose connections between wires or overhead shielded wires and accessory hardware or other conductor fittings. This "looseness" causes wear to occur and is often the result of excessive wind vibration. Figure 4: Wire wear at the gasket Wear damage can occur in the span of the spacer (Figure 4), the spacer damper and the marker ball, or at the support structure (Figure 5). The maximum bending stress occurs where the wire or overhead shield wire is prevented from moving. For example at the edge of the spacer, the spacer damper and the edge of the damper. The highest level of bending stress typically occurs on the support structure. Fatigue failure occurs when the bending stress of a wire or overhead shielded wire due to wind vibration exceeds the durability limit (Fig. 1). The time to failure will depend on the magnitude of the bending stress and the cumulative number of bending cycles (frequency). Figure 5: Wear at the fixed wire Safety tension design CIGRE7 provides guidelines for safe design tension based on the ratio of horizontal wire tension H to unit length wire weight w. The effect of terrain on wind turbulence intensity has also been studied and included as part of the overall recommendations. The horizontal wire tension used to calculate the H/W ratio is the initial, unloading tension at the average temperature of the coldest month (AAMT) at the line location. Using the H/W ratio and the newly created terrain category to apply to all available field experience data, the CIGRE Task Force published the recommendations in Table 1 for a single undamped, armor-free conductor. The Task Force also issued a warning: “Special attention needs to be paid to extra long spans, ice coating, spans equipped with aircraft warning devices, and spans using unconventional wires.†Table 1: Safety design tension for single, undamped, unarmored conductors. The CIGRE report also provides recommendations for the safe design of bundled (double, triple, and fourth) wires. Figure 6: Armored rods for wires used in suspension clamps. Suspension hardware impact The use of an armored rod (Figure 6) or a high performance suspension assembly (Figures 7 and 8) reduces the level of dynamic bending stress on the vibrating wire. Figure 7: ARMOR-GRIP ® suspension (AGS). Figure 8: CUSHION - GRIP ® suspension (CGS). Therefore, the high performance suspension will allow for a higher safety design tension (H/w) and increase the "protectable" span length of the damper. The positive impact and additional protection provided by the high performance suspension is difficult to simplify into a simple table. Contact the PLP with specific line design and environment (terrain and temperature) data for more information. Damper has effect Since the beginning of the 20th century, many different types of dampers have been used to reduce the level of wind vibration within the span, especially in the support structure. The most commonly used damper is the Stockbridge model, named after GH Stockbridge's 1924 invention. The original design has been developed for many years, but the basic principles still exist: the weight is suspended at the end of the specially designed and manufactured steel-cored aluminum stranded wire, and the steel-cored aluminum stranded wire is clamped to the conductor with a clip (Fig. 9). When the damper is placed on the vibrating wire, the movement of the weight will cause the steel core aluminum strand to bend. The bending of the steel core aluminum strand causes the individual wires of the steel core aluminum strand to rub against each other, thereby consuming energy. The size and shape of the weight and the overall geometry of the damper affect the energy that will be dissipated at a particular vibrational frequency. An effective damper design must have an appropriate response over the expected frequency range for a particular conductor and span parameter. Figure 9: VORTXTM damper. Some dampers, such as VORTX dampers (Figure 9), utilize two different weights and asymmetric positions on the strand to provide the widest possible range of effective frequencies. Installers, such as VORTX damper performance line product development programs, consider span and terrain conditions, suspension type, conductor self-damping, and other factors. This method determines a specific position in the most effective span of the damper (S). For smaller diameter conductors (<0.75"), overhead shielded wires and optical ground wires (OPGW), different types of dampers can be used, which are generally more efficient than Stockbridge dampers. ) has been successfully used to control wind-induced vibrations on these smaller sized conductors and wires for more than 35 years. Figure 10: Spiral vibration damper. The spiral vibration damper is an "impact" damper made of a sturdy non-metallic material with a compact spiral at one end for holding the wire. The remaining spirals have a larger inner diameter than the wires so that they have an effect during the aerodynamic vibrational activity. The shock pulse from the damper breaks and counteracts the motion generated by the wind. The spiral vibration damper is very efficient because it can be placed anywhere in the span and has no specific resonant frequency. It responds to all frequencies, especially the high frequencies associated with small diameter conductors and wires. references: [1] Electric Power Research Institute, "Transmission Line Reference Book, Wind Induced Conductor Motion", Research Project 795, 1978. [2] V. Strouhal, “On Aeolian Tonesâ€, Annalender Physik und Chemie, Band V, 1878, p. 216. [3] “Preformed Line Products referencereport, “Aeolian Vibration Basicsâ€, 2006. [4] PW Dulhunty, A. Lamprecht and J. Roughens, “The Fatigue Life of Overhead Line Conductorsâ€, CIGRE SC22-WG04 TaskForce Document, 1982. [5] CB Rawlins, “Exploratory Calculations of the Predicted Fatigue Life of Two ACSR and One AAACâ€, Report CIGRE SC-22, WG11, TF4-96-5, April 1996. [6] OD Zetterholm, "Bare Conductors and Mechanical Calculation of Overhead Conductors", CIGRE Session Report #223, 1960. [7] CIGRE Report #273, “Overhead Conductor SafeDesign Tension with Respect to Aeolian Vibrationsâ€, Task Force B2.11.04, June2005. 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