Vibrations Of Overhead Transmission Lines Research Articles

Spectral Approach in Vibrations of Overhead Transmission Lines

In the paper, the description of the type of vibrations of overhead transmission lines and theoretical background of spectral element method is presented. The main goal of this paper is to develop a model of the transmission line via spectral element method. The methodology of analysis is presented. Due to the accuracy of the results and the efficiency of the method, it is recommended to analyse the vibrations of the systems in various configurations of boundary and initial conditions. Numerical analysis investigates the natural frequency of the model due to different parameters, such as a tension force and the area of the cross-section of the conductor.

Multi-parametric optimizations for power dissipation characteristics of Stockbridge dampers based on probability distribution of wind speed

• Random excitation model of the Stockbridge dampers. • Modeling of the mathematical expectation of power dissipation for the Stockbridge dampers. • Optimization of the multi-structure parameters for structural parameters. Power dissipation characteristics of Stockbridge dampers (SD) is one of the important indexes in anti-vibration work of transmission line. The study focuses on the optimization of the SD's power dissipation characteristics under the effect of multi-structure parameter coupling. The aeolian vibration of overhead transmission lines is uncertain and random in stochastic dynamics. According to Strouhal formula, the relationship between the vibration frequency of transmission lines and wind speed can be found out. Based on the Weibull wind speed probability distribution, the probability density function of the transmission line conductor and damper coupling system vibration frequency is derived. The SD is considered as a typical 2-dimension of stochastic dynamical system. Based on the random process generated by the power dissipation of the SD, the characteristics of power dissipation and SD's resonant frequencies are analyzed when the multi-structure parameters of the SD are coupled. And the diagrams of the power dissipation at various frequencies are obtained. Based on the probability density function of the vibration frequency of the overhead conductor and damper, the objective function, namely the mathematical expectation of power dissipation ( E ( P D)), of the optimizations for the SD's power dissipation under the coupling of multiple structural parameters is proposed for the first time according to the author's knowledge. Constraint conditions of the optimizations are built by the quantization processing. The energy dissipation characteristics of the dampers can be evaluated by E ( P D), and the power dissipation of SD with different coupled dual structure parameters is optimized based on the proposed method. The optimal values or the optimal value intervals of different coupled dual structure parameters are found, which may provide practical data.

Asynchronous parametric excitation, total instability and its occurrence in engineering structures

In mechanical engineering systems self-excited and parametrically excited vibrations are in general unwanted and sometimes dangerous. There are many systems exhibiting such vibrations which up to this day cannot be completely avoided, such as brake squeal, the galloping vibrations of overhead transmission lines, the ground resonance in helicopters and others. In general, problems of parametric excitation are studied for the case in which all the time-periodic terms are synchronous. In this case the stability behavior is well understood. However, if the time-periodic terms are asynchronous, an “atypical” behavior may occur: The linear system may then be unstable for all frequencies of the parametric excitation, and not only in the neighborhood of certain discrete frequencies (total instability). Until recently it was believed that such “atypical” behavior would not appear in mechanical systems. The present paper discusses some recent insights and results obtained for linear and nonlinear systems with asynchronous parametric excitation. The method of normal forms is used to prove total instability and to calculate limit cycles of a generalized nonlinear system. Further, a mechanical example of a minimal disk brake model featuring such out of phase parametric excitation is presented. The example outlines the importance of the observed effects from the engineering point of view, since similar terms are also expected in the equations of motion of disk brakes with disks with ventilation channels and most likely also in other physical systems.

Specific power input: Comparison among rigid and flexible models

The specific power input imparted by the blowing wind to an oscillating circular cylinder is a key topic in calculating aeolian vibrations of overhead transmission lines using the energy balance principle, i.e. the most common methodology used to design new lines and to dimension fittings and damping devices.The specific power input as function of the vibration amplitude was measured, during the last decades, by different research groups using different models. The obtained curves show a large dispersion of the results increasing the uncertainties in determining the vibration amplitudes.Aim of this paper is to describe the results obtained during experimental campaigns carried out at Politecnico di Milano Wind Tunnel using both flexible and rigid sectional model and to highlight the differences among the two cases in terms of vortex induced vibrations with particular reference to specific power input.

Self-Excited Vibrations and Damping in Circulatory Systems

Self-excited vibrations in mechanical engineering systems are in general unwanted and sometimes dangerous. There are many systems exhibiting self-excited vibrations which up to this day cannot be completely avoided, such as brake squeal, the galloping vibrations of overhead transmission lines, the ground resonance in helicopters and others. These systems have in common that in the linearized equations of motion the self-excitation terms are given by nonconservative, circulatory forces. It has been well known for some time, that such systems are very sensitive to damping. Recently, several new theorems concerning the effect of damping on the stability and on the self-excited vibrations were proved by some of the authors. The present paper discusses these new mathematical results for practical mechanical engineering systems. It turns out that the structure of the damping matrix is of utmost importance, and the common assumption, namely, representing the damping matrix as a linear combination of the mass and the stiffness matrices, may give completely misleading results for the problem of instability and the onset of self-excited vibrations. The authors give some indications on improving the description of the damping matrix in the linearized problems, in order to enhance the modeling of the self-excited vibrations. The improved models should lead to a better understanding of these unwanted phenomena and possibly also to designs oriented toward their avoidance.

Study of Breeze Vibration of Overhead Transmission Lines with Dampers Using FEM Analysis

The vertical, steady-state breeze vibration of transmission line with dampers attached are studied using FEM analysis. The lines are simulated by cable element, the dampers by mass element and beam element. The parameters of FEM emulation mode such as breeze vibration force, the conductor self-damping and dampers damping are emphasized by the energy equivalent theory. The breeze vibration force induced by vortex is educed by wind power curve, the hysteresis damping and friction damping of conductor and damper are translated into viscous damping. Results of the FEM emulation show the calculation accuracy of natural frequency of dampers, and prove that it can effectively restrain breeze vibration of transmission lines by installing dampers. The method lays foundation for further research on protecting breeze vibration of transmission lines.

Application of Monitoring Technology Based on Aeolian Vibration in Smart Grid

The aeolian vibration of overhead transmission lines frequently brings about the breaking fatigue of wires, which makes a serious threat to the security of transmission lines.Today, more and more attentions have been paid to the aeolian vibration both in research and commercial realms . As aeolian vibration monitoring can accurately monitor wire damage, it is conducive to timely maintenance and avoiding accidents. In our paper, we propose a system model adopting bending amplitude method to estimate bending amplitude, calculate dynamic bending strain of wire and obtain early warning tips by comparing with strain chart based on the research of principles of aeolian vibration. Besides, A prototype system is developed to realize the effectiveness of the proposed model.

Wind induced vibrations of long electrical overhead transmission line spans: a modified approach

For estimating the vortex excited vibrations of overhead transmission lines, the Energy Balance Principle (EBP) is well established for spans damped near the ends. Although it involves radical simplifications, the method is known to give useful estimates of the maximum vibration levels. For very long spans, there often is the need for a large number of in-span fittings, such as in-span Stockbridge dampers, aircraft warning spheres etc. This adds complexity to the problem and makes the energy balance principle in its original form unsuitable. In this paper, a modified version of EBP is described taking into account in-span damping and in particular also aircraft warning spheres. In the first step the complex transcendental eigenvalue problem is solved for the conductor with in-span fittings. With the thus determined complex eigenvalues and eigenfunctions a modified energy balance principle is then used for scaling the amplitudes of vibrations at each resonance frequency. Bending strains are then estimated at the critical points of the conductor. The approach has been used by the authors for studying the influence of in-span Stockbridge dampers and aircraft warning spheres; and for optimizing their positions in the span. The modeling of the aircraft warning sphere is also described in some detail.

Aeolian vibrations of overhead transmission lines: computation in turbulence conditions

This paper deals with an analytical approach to define the dynamic behaviour of a cable under vortex shedding excitation in turbulence conditions. The conductor is schematized by f.e.m. The time history of the velocity field is computed in any nodal point of the schematization. The vortex shedding effects are reproduced by means of equivalent non linear oscillators.

Aeolian vibrations of overhead transmission lines: computation in turbulence conditions

This paper deals with an analytical approach to define the dynamic behaviour of a cable under vortex shedding excitation in turbulence conditions. The conductor is schematized by f.e.m. The time history of the velocity field is computed in any nodal point of the schematization. The vortex shedding effects are reproduced by means of equivalent non linear oscillators.

A study on galloping in overhead transmission lines.

The present paper first theoretically describes that galloping in overhead transmission lines is a phenomenon of nonlinear self-excited vibration of a distributed continuous system, due to the variation in the angle of attack caused by rotational motion. The paper then explains empirically and theoretically that an antigalloping system can be designed as a compensating device which stabilizes the self-excited vibration of overhead transmission lines.

LITERATURE REVIEW: survey and analysis of the Shock and Vibration literature: Vibration of Overhead Transmission Lines V

LITERATURE REVIEW: survey and analysis of the Shock and Vibration literature: Vibration of Overhead Transmission Lines V

On the role of the space charge produced by hanging water drops in the mechanism of the corona-induced vibration of high-voltage conductors

Although the corona-induced vibration of overhead transmission lines, i.e. the vibration excited by the water drops hanging from the conductors, has been reported by the utilities since 1932, the mechanism responsible for the vibration is not fully understood. The present paper shows that a high-voltage conductor vibrates even when the hanging water drops are replaced by conical metal points, and an impulse voltage, its frequency being equal to the natural frequency of the system, is applied to the conductor. In addition, it was observed that even in the absenced of ionic wind, a H.V. sphere electrode suspended above a plane electrode vibrates when an intermittent space charge is injected in the space between the sphere and the grounded plane. Thus, the intermittent shielding effect of the corona space charge, produced by the water drops hanging from the conductor, would appear to be the cause of the vibration.

On the computation of damped wind-excited vibrations of overhead transmission lines

In the calculation of wind-excited overhead transmission line vibrations with Stockbridge dampers the damper behaviour is usually represented by its impedance corresponding to a vertical translatory damper clamp motion. The moments introduced by the damper clamp into the cable are normally disregarded. In this paper the dampers are characterized by means of a 2×2 complex impedance matrix which can be experimentally determined in the laboratory and which includes the effects of the rotatory motion of the clamp. The energy balance method is then adapted to this case and the bending strains in the cable are calculated at the dangerous points. It turns out that the moments introduced by the damper into the cable are of little or no importance with regard to the energy absorbed. They may however affect strongly the local bending strains in the cable at the damper clamp.

Self‐excited vibrations of overhead transmission lines

There are three principal types of vibration which occur on overhead lines and which are of engineering significance. These are classified as aeolian‐ or vortex‐shedding‐induced, wake‐induced, and the bluff‐body or galloping type of mechanical oscillation. The latter two are self‐excited and are discussed in some detail. Theoretical studies are compared with experimental results and a number of methods of control are described.

Vibration of overhead transmission lines

STUDIES of vibration of overhead conductors have been prosecuted vigorously for several years in laboratory and fields. The importance of magnitude and distribution of stresses in multiplestrand conductors led to the development of a unique method for measuring stresses in vibrating conductors in the laboratory under conditions closely simulating field conditions, and numerous schemes for effecting better stress distribution at conductor supports have been investigated. These tests have been facilitated by the development of special Machinery. Field work has been conducted on special outdoor spans, and observations have been made also upon operating lines at widely separated locations.