Sylvain Goudreau

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.

Effect of lubricant in electrical conductor fretting fatigue

Bending fatigue tests have been performed on the lubricated electrical conductor ZEBRA held with two different spacer clamps. A constant axial load of 25% RTS (rated tensile strength) was imposed on all specimens. After each test, fretting patterns have been studied for each aluminium layer. OM and SEM metallographic examinations have been carried out to study fretting cracking behaviour. Compared with a similar unlubricated conductor, results show that the lubricant plays an important role in preventing contact wear and in retarding fretting cracking. Further tests on single lubricated and unlubricated aluminium wires show that influence of lubricant on fretting fatigue strongly depends upon the contact conditions (normal load and slip amplitude).

Single wire fretting fatigue tests for electrical conductor bending fatigue evaluation

Abstract 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.

Strain Measurements on ACSR Conductors During Fatigue Tests I—Experimental Method and Data

Electrical transmission lines are subjected to aeolian vibrations which may lead to the failure of overhead conductors by fretting fatigue at suspension clamps. In order to obtain a better understanding of the damaging mechanism of these conductors, strain measurements have been taken on wires near critical sites. The strain gauges were glued in order to separate the bending and the traction strain modes, which gives major insights into conductor mechanics. While the static loading is a combination of traction and bending loadings, the alternating loading is mainly an alternating traction. Alternating stresses used as fatigue indicators do not correlate well with the experimental results due to crude assumptions.

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 understaruling of the mechanical behavior at those critical points.

Strain Measurements on ACSR Conductors During Fatigue Tests II—Stress Fatigue Indicators

Fatigue strength of overhead conductors of different types is often presented on the same diagram, a so-called S-N diagram, the vertical axis being generally taken as the alternating stress amplitude. In this paper, it is shown that these stress levels are merely fatigue indicators with inherent limits, and should be used accordingly. Their practical interest is to allow a regrouping of the fatigue results of different types of conductors. It will be shown that these indicators have some inherent limits in their definition and can only be used according to their intended purpose.

Strain Measurements on ACSR Conductors During Fatigue Tests III—Strains Related to Support Geometry

A simple conductor bending model taking into account the geometry of the suspension clamp has been used to interpret the strains measured on top and bottom wires of two ACSR conductors outer layer. Taut Drake and Bersfort ACSR conductor specimens were fitted with a short commercial metallic suspension clamp and forced to vibrate at one of their normal modes. On each gauged wire, five equally-spaced cross-sections were monitored near the last point of contact of the conductor with the bed of the clamp or with the bolted keeper. Three strain gauges were used at each selected wire cross-section making it possible to obtain the wire traction strain as well as the binormal and normal bending strains. This more complex conductor-clamp model was found to yield theoretical strain values closer to measured values than the usual models where clamp geometry is ignored. Hence, this could be a step to a more precise estimate of a given conductor-clamp fatigue limit when undergoing Aeolian vibrations.

Use of Eddy Current Technology to Assist in the Evaluation of the Fatigue Damage of Electrical Conductors

The article deals with the evaluation of fatigue damage on overhead conductors which have been in service for a period exceeding 50 years. Under wind excitation, electrical transmission lines are subjected to aeolian vibrations that may induce wire fatigue of conductor at the exit of supporting equipment. This fatigue phenomenon is a fretting fatigue problem which is located at contact points between wires belonging to adjacent wire layers or between external layer wire and the supporting equipment. This article reports the methodology followed: 1) in selecting wires with the more severe damaged contact point using eddy current technology and 2) in comparing, through a metallographic examination, the contact points of those selected wires coming from in situ worn conductors and from laboratory fatigue tested virgin conductors used as reference.

Experimental Study of Dynamic Bending Stiffness of ACSR Overhead Conductors

Aeolian vibrations of transmission-line conductors may cause fretting fatigue failure at or near the location of clamped devices. At these locations, the bending stiffness variation of the conductor has a large influence on its deformed shape and, hence, on its fatigue mechanics. Variable bending stiffness models could be integrated in nonlinear finite-element programs to obtain better mechanical behavior predictions. However, there is very little data available in the literature to validate such numerical models. The objective of this paper is to present experimental data for the deformed shape of two types of ACSR conductors undergoing vibrations. The tests were performed on a 5.83-m test bench for various tensions, displacement amplitudes, and frequencies. The displacement amplitude was measured at the vibration anti-node and at five locations near the square-faced bushing. The results suggest a large stiffness variation near the bushing. This experimental study provides valuable data to compare with a numerical model of a vibrating conductor that includes variable bending stiffness.