Overhead Electrical Conductors Research Articles

Hysteretic Behaviour of Stockbridge Dampers: Modelling and Parameter Identification

Stockbridge dampers are widely used to mitigate the vortex-induced vibrations of overhead electrical line conductors and of many other cable or cable-like structures exposed to the action of wind. Aim of this work is to develop a simple, but accurate, mechanical model of a Stockbridge damper to use in the assessment of such structures vibrations. The model is based on a beam-like description of the messenger cable and on a nonlinear formulation of the cross sections cyclic bending behaviour. At the cross-sectional level, the mechanical behaviour of the messenger cable is reproduced with the classic Bouc-Wen hysteretic model, which has been recognized as adequate to represent the local behaviour mainly controlled by interwire sliding processes. The Bouc-Wen model parameters are identified from experimental results available in literature and used at the local (cross-sectional) level to characterize the mechanical behaviour of the messenger cable. The descending global behaviour of the Stockbridge damper compares very favourably with the experimental results in terms of the impedance function at the clamp and allows for the confident use of the proposed model inside the assessing process of full power lines. The important role of the end zones of the messenger cable is highlighted. In these zones, a boundary layer like transition is found to occur for the bending stiffness of the messenger cable cross section. This largely affects the global behaviour of the Stockbridge damper. The length of these zones complements the model parameters.

A novel ice-shedding model for overhead power line conductors with the consideration of adhesive/cohesive forces

An ice-detachment failure model is proposed for iced overhead electrical conductors to predict the transient response of line systems subjected to shock loads. The model considers the bending strains in the ice deposit, the adhesive force at the ice–cable interface and the cohesive force within the ice. It is implemented into finite element analysis by setting a critical vertical acceleration corresponding to inertia forces overcoming the adhesive/cohesive forces on the ice chunk. The model is validated by results from reduced and real scale de-icing tests of glaze ice but the approach can apply to different types of atmospheric icing.

Temperature profile of progressive damaged overhead electrical conductors

The Brazilian transmission system is characterized by installations that average 20years of operation. Competitiveness of the energy sector, lack of resources for new investments and the aging of the system lead to the necessity of not only to reform but also to optimize it in order to face the increasing demand of the Brazilian energy market. The overhead electrical conductor is naturally the most important element on the transport of electrical energy and requires careful predictive and preventive maintenance programs. Therefore, it is of fundamental importance to predict the operational life of overhead electrical conductors using mathematical models so that their energy transport capacity can be optimized. However, it is necessary to validate such models by means of test programs so that their predictions can be trusted. One of the most important factors on the structural decay of overhead electrical conductors is the variation of the temperature profile in the event of the rupture of one or more of its Aluminum wires by fatigue or fretting. Therefore, models that include this temperature variation must be validated through test programs that can experimentally establish a mathematical relationship which considers effects such as contact resistance, material thermo-mechanical properties modification, annealing of the material and the presence of defects. The objective of this work is to evaluate the effect of progressive Aluminum wire rupture on the temperature profile of an ACSR (Aluminum Conductor Steel Reinforced) overhead electrical conductor, not only on the section of the damage but also along the length of the conductor. A test procedure was developed and performed to establish this effect. Results were compared to a mathematical model and showed that the temperature profile dependency of the increasing rupture of Aluminum wires is not linear and includes effects other than the cross section loss.

Elastic-Plastic Microcontact Model for Elliptical Contact Areas and Its Application to a Treillis Point in Overhead Electrical Conductors

Aeolian vibrations represent a threat to the integrity of electrical transmission lines. The fretting fatigue of conductors is thus a major concern. The modelization of the contact conditions at critical points is an important tool in assessing the life of conductors. Treillis points around the last point of contact between the conductor and the pieces of equipment are such critical points. We observe a fully plastic contact condition at these points. Finite element results for the contact between an ellipsoid and a rigid plane and between two wires at different angles are compared with an elastic-plastic microcontact model for elliptical contact areas. These numerical results are then compared with experimental ones for the contact between two wires of a conductor (ACSR Bersfort), showing a very similar relationship between the contact force and the observed contact area. We have a good correlation between the microcontact model and the finite elements ones in the fully plastic contact regime on both the contact area and the contact force for a given interference between bodies. The use of the elastic-plastic microcontact model for elliptical contacts presented in this paper proves to be a strong tool in getting a better understanding of the mechanical behavior at those critical points.

The Influence of the Steel and Aluminium Components Separation at the Conductors Locking for Stress-Strain Tests

At the stress-strain tests for the overhead electrical conductors stranded in alternate directions (aluminum conductor steel reinforced - ACSR), the locking mode of the specimen ends on the tensile machine represents a difficult problem which if is not correctly solved may seriously influence the results obtained. The stress-strain tests are performed according to standards as the European Standard EN 50182:2001. The specimen must have a length imposed by the conductor diameter, but not less than 10m. During the stress-strain tests the specimen is loaded in steps at successive cycles of loading- unloading (30%, 50%, 70%, 85% of Rated Tensile Stress –RTS). Finally, in order to determine the breaking forces, the conductor is loaded up to total breaking. In these conditions, the test success is decisively influenced by the correct locking of the conductor in the grips. The locking of the conductor ends is usually performed by casting the conductor ends in the gripping devices, after the wires ends reflection. In this paper there is presented a new locking method of the conductor ends by separation of the steel wires from the aluminum ones and their separate winding on two drums with helical groove. The tests performed have confirmed that this method is cheaper, and not influencing the real behavior of the conductor.

Flexural self-damping in overhead electrical transmission conductors

Internal damping of tensioned cables during flexure by transverse vibration is analyzed. The flexure causes relative movements between the wires or strands of the cable, movements which are constrained by friction between them. Under conditions common to vibration of overhead transmission line conductors the friction is great enough to prevent gross sliding. However, there is microslip at the edges of the interstrand contacts, so there is frictional dissipation. In addition, the frictional forces cause shear strains at the contacts with resulting material damping. An analysis is presented that connects the bodily flexure of the conductor with the internal interstrand movements and forces, and with the amounts of dissipation that occur—self-damping. Comparison of estimates based on the analysis with measured data on self-damping reveals reasonable agreement, for a limited range. Cases lying outside that range appear to be associated with treatments applied to cable samples involved in the measurements prior to testing. Possible mechanisms activated by these treatments are discussed.

Use of Satellite Technologies for Power System Measurements, Command, and Control

This paper analyzes the use of wide-area measurement technologies including satellite-based methods for the command and control of power systems. The methods studied include the global positioning system (GPS) and low earth orbit satellites (LEOS). Satellite technologies have been used in a variety of applications requiring precise timing between geographically diverse locations. The deregulation of the electric power industry is placing increased demands on power transmission system utilization. Because deregulated power systems utilize long-distance high-power exchanges, satellite-based communication systems are useful in control of geographically large interconnected power systems. In this paper, the satellite-based measurements and commands are introduced for wide-area control. With the support of wide-area signals, a multiagent supervisory-level power system stabilizer is proposed here as a potential wide-area control structure. Increased loading of transmission facilities is an impetus for accurate dynamic thermal overhead electrical conductor ratings. The application of satellite-based measurement for improving the dynamic thermal rating of overhead transmission circuits is suggested.

Methodology for Exact Solution of Catenary

Computational methods are presented for facilitating a mathematically exact determination of the geometric and mechanical elements of a catenary, including cases where the supports are at different elevations. Application of the methodology to the design of overhead electrical conductors is illustrated.

Mechanical Models of Helical Strands

Cables, ropes, and overhead electrical conductors are complex helical wire assemblies. Several analytical models are currently available for predicting their mechanical behavior or, at least, some technically interesting aspects of it. In these models, the underlying theory is generally based on a common elementary system, which can be termed a helical wire strand, generally consisting of a core surrounded by one or several wire layers. Depending on strand geometry, loading type (axial or bending), and also, on the required specifications (stiffness, strength, damping), a wide variety of hypotheses can be made. To address this problem, the following tentative classification is presented: fiber models, taking into account only axial wire forces; curved rod models, which include wire bending stiffness and in the most general case, twisting stiffness; and semi-continuous models, in which each wire layer is replaced with an equivalent elastic orthotropic medium. The different models are compared, together with the corresponding experimental data when available. The review is restricted to the elastic behavior under small deformations, which may include contact conditions, with or without friction, and possible stick-slip behavior. Studies related to more complex situations (multi-strand cables, contact with mechanical elements such as pulleys or clamps, viscoelasticity, fatigue, etc) are not included. This review article contains 107 references.

Fundamental investigations of electrical conductor fretting fatigue

Fretting is known to be the main factor leading to conductor individual wire breaks under aeolian vibration in the vicinity of a clamp. In this paper, previous studies on overhead electrical conductor bending fatigue are summarized. Results obtained with several conductor types and clamps are compared. A general fretting analysis as well as testing procedure are suggested. Influence of the main mechanical parameters on the occurrence of several types of degradation processes is discussed.

Single wire fretting fatigue tests for electrical conductor bending fatigue evaluation

Overhead electrical conductors are often subjected to aeolian vibrations which may induce fretting fatigue damage of individual aluminium wires in suspension clamp regions. Many bending fatigue tests have been performed on electrical conductors. Depending on the test conditions, wire fracture may be found to occur in the external as well as internal layers. Individual wire fretting fatigue is very difficult to predict due to a conductor complex structure and dynamic mechanical behaviour. The main objective of this work is to present experimental results obtained from tests on single wires under conditions simulating a typical conductor-clamp contact. A fretting fatigue test bench specifically designed for such simulation has been used on single H19 aluminium wires. They have been subjected to an initial minimal axial stress of 59 MPa. At the fretting point, a transverse compressive load of 130 N to 4000 N has been imposed, as well as an alternating displacement of 100 to 900 μm displacement amplitude. Cycling frequency has been kept at 10 Hz and test duration went up to 1.6 × 10 7 cycles. All tests were performed in the stick-slip regime occurring in the plastically deformed contact zone. No global slip was allowed. Subsequent examination of the fretting scars at the contact surface and through the cross-section have been carried out by optical microscopy and scanning electron microscopy. The mechanical parameters influence is studied and comparison with results from complete conductor fatigue tests is discussed.

AUTOMATED WIND TUNNEL MEASUREMENTS

It is common practice in power transmission to use bundled overhead electrical conductors because the lower rate of corona discharge, compared with single conductors, makes the bundle attractive for high voltages. However, secondary problems may arise from the vibrations induced by the aerodynamic forces which act on the leeward conductors as a result of their immersion in the wake of a windward conductor. Thus, in addition to possible aeolian and galloping oscillations observed for a single conductor, bundle conductors may also be subjected to wake-induced oscillations. 1,2 Although extensive research has been done on the aeroelastic behavior of single and bundled conductors which are bare, little is known about the galloping or wake oscillation characteristics of given bundle conductors which, depending upon the environmental conditions during the formation of ice, may have five or so representative shaped cross-sections. One important task involves the aerodynamic quasi-static force and moment measurements in a wind tunnel using full scale conductor cross-sections each of which must be rotated through many angles at different spacings. Such measurements are repetitive as well as tediously time consuming and, hence, are beneficially automated. A reliable, accurate and inexpensive automated apparatus is described in this paper which permits the aerodynamic measurements to be performed efficiently and without intervention. Most importantly, it relieves the operator of the usual responsibility for manually controlling the separation and orientation of the sections as well as the need to manually compensate fluctuations in the free stream air speed. For simplicity, the apparatus is detailed in the context of a twin bundle conductor.

Fretting fatigue in electrical transmission lines

One of the worst problems in overhead electrical transmission line fatigue is fretting, which can lead to wire failure or even conductor breakage at suspension clamps. However, fretting behaviour is extremely complicated owing to its synthetic geometry and loading condition. The tests concern bending fatigue on the overhead electrical conductor DRAKE. The imposed amplitude ranges from 0.60 to 0.80 mm. The static axial load was set at 25% RTS (Rated Tensile Strength). The tests were performed (including the static test) with a frequency of 10 Hz for any number of cycles. Post-test studies of the fretting scars were carried out by OM and SEM. Plastic flow, local wear and fretting cracking were observed at different conductor locations. Fretting cracking depends strongly on fretting regime, and the later propagation on the external load. Fretting regime and external load are determined by numerous factors such as contact type and position. Through metallographic examinations, it has been demonstrated that fretting is most likely to lead to cracks in the vicinity of the keeper contact edges, and probable propagation occurs at the upper half-section for the outer contact. The wire failure is found precisely in this zone and it fits our analysis well.

FREE CONSTANT CURVATURE BENDING OF AXIALLY PRELOADED STRANDS

A model is developed to predict bending stiffness for helical strands such as wire ropes and overhead electrical conductors subjected simultaneously to tension and quasi-static bending when friction is present. The analysis combines and extends earlier works to investigate internal slippage and its effect on the bending stiffness of strands. It consists on a quantitative treatment of the transition from no-slip to full slip where a criterion is established to estimate relative displacements and to locate the sliding zones. Yet, this model allows prediction of the contact forces distribution, energy dissipation and bending stiffness when the cable is subjected to constant curvature. An experimental set up is used to compare calculated and measured bending stiffness. Theoretical and experimental bending stiffness lie well within theoretical bounds. Encouraging agreement between theory and experiment is found.

Suspension Clamp and ACSR Electrical Conductor Contact Conditions

A one-dimensional model is presented for the analysis of contact between a typical suspension clamp and an overhead electrical conductor. Besides span parameters (sag angle, static tensile load), conductor curvature at the last point of contact is shown to depend on clamp profile. Strain measurements are performed on the bottom free surface of a typical commercially available suspension clamp, and of a simpler, yet similar, generic clamp, with a Bersimis ACSR conductor specimen. Various sag angle and tensile load combinations are imposed. Using an inverse method, measurements made on the generic clamp allow, through finite-element method modeling, the calculation of an approximate contact load distribution between clamp and conductor. With the usual assumption of minimum conductor bending stiffness, good agreement is found between experimental and theoretical results. Most notably, an experimentally observed tensile strain zone at the clamp bottom surface is shown to correlate well, both qualitatively and quantitatively, with a calculated localized contact region. Such tensile zone could be used to study contact conditions in other clamp-conductor systems.

Heat transfer in an overhead electrical conductor

A heat transfer model is presented to describe the thermal responses of an overhead electric conductor in a high voltage power transmission line when subjected to changes in operating and ambient conditions. The thermal model takes into account all major heat transfer mechanisms happening to the conductor in a natural environment. The original heat transfer and simplified macroscopic models are employed to predict the conductor temperature distributions under various indoor and outdoor operating conditions. Good agreement between the theoretical predictions and the observed data justifies the various assumptions involved in the development of the simplified macroscopic model.

Accelerated Atmospheric Corrosion Testing Using a Cyclic Wet/Dry Exposure Test:Galvanic Couples of Aluminum with Graphite Neoprene Rubber, Galvanized Steel, and Steel

Abstract Aluminum corrosion is important in overhead electrical conductors constructed from aluminum wire centrally reinforced by galvanized steel strands. Inspection of conductor after long service has implicated rubber bushing material, on the outside, and the galvanized strands, on the inside, as providing potential galvanic sites for the initiation of rapid aluminum corrosion. Therefore, the galvanic corrosion of aluminum in contact with graphite-loaded neoprene rubber, hot-dip galvanized steel and steel was assessed in a cyclic wet/dry exposure test using mixed-salts spray solutions containing appropriate ratios of sulfate and chloride ion. Aluminum was found to corrode at between 3 to 6 times its uncoupled rate when associated with the rubber material. While the eta-phase, relatively pure Zn, galvanized layer remained intact, galvanic corrosion of aluminum was slow. However, on exposure of the zeta-phase, Zn/Fe intermetallic layer, aluminum corroded about 35 times faster than expected in a solution ...

A study of helically reinforced cylinders under axially symmetric loads and application to strand mathematical modelling

Locally transversely isotropic elastic material can be used in a cylinder configuration in which the material principal direction is helically wound. Stress-strain equations are obtained for such a cylinder under axially symmetric loading: axial force, twisting moment and uniform radial internal and external pressure forces. The equations are generalized to a system of n coaxial tubes with an isotropic core. Finally, it is shown how this can be used to represent the global mechanical behaviour of strand-like systems such as overhead electrical conductors and cables.

Corrosion testing and appearance of grease coated galvanised steel

The effectiveness of grease, used as a corrosion preventative in galvanised steel reinforced overhead electrical power conductors, was evaluated by a variety of methods. The ability of ions to penetrate grease was assessed by an electrochemical technique and by solution analysis. The structure of free grease films was observed by scanning laser optical microscopy to give insight into the nature of ion motion. Changes in the grease both after exposure to the outdoor environment and after laboratory testing were evaluated by optical microscopy and infrared spectroscopy. In addition, the corrosion performance of grease coated galvanised steel wire in a mixed salts spray, wet-dry cycle test was evaluated by electrochemical potential measurements. Morphological examination of free grease films indicated the presence of features at the surface and within the body of the films. These appeared to be microscopic ‘defects’, which could allow the penetration of aggressive species to the substrate metal. ‘Degraded’ grease, observed in conductors after extended periods of natural weathering, could be reproduced in the test and was found to be primarily a physical mixture of grease and corrosion product. Solutions of low pH, containing AlCl3, penetrated grease more rapidly than near neutral solutions of NaCl. The corrosion morphology of grease· coated wires exposed in the accelerated test resembled that of corroded galvanised steel core wires in naturally weathered conductors. Corrosion was found to commence under thin grease films which were apparently free from macroscopic defects as well as beneath thicker grease films which had had defects introduced using a hot needle. Grease was thus found to play a major role in restricting the initiation of corrosion in conductors, eventual attack probably initiating at ‘holidays’ in the coating.

The electrical conductivity and tensile properties of light magnesium-aluminium alloys as affected by atmospheric exposure

The problem of finding a light alloy of aluminium which will satisfactorily eliminate the steel core for overhead electrical conductors is of importance at the present time. The ½ per cent magnesium alloy, under the name of Aldrey, is being used on the Continent without steel core, and the present paper is concerned with its exposure to London atmosphere. The conductors are in the form of wires and have been exposed on the roof of King's College since October,1928. The variations in their electrical and mechanical properties have been studied with special reference to type of corrosion and mechanical stress has proved beneficial so far as variations in tensile properties are concerned, it has had but a small effect upon electrical conductivity. An account is given of experiments which have been made to explain the reason for the beneficial results obtained. The paper refers to published work on the atmospheric exposure of light nickel alloys, duralumin, and high-purity aluminium, and comparisons with the present experiments have been made and discussed.