PSI - Issue 19

R.B. Kalombo et al. / Procedia Structural Integrity 19 (2019) 688–697 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Power line cables are one of the most important elements in the transmission of electrical energy and its effect on the rate of total installation time of a transmission line is significant. Due to wind, the cable vibrates and accumulates a number of cycles which leads to fatigue failure, especially at the element which restrains its vibration, such as the suspension clamp (Chan 2006). In addition to the force due to the wind, which generates the bending displacement, this power line element is also suggested to create many loads. Among these are the stretching load and the pressure load, caused by the bolt tighten torque of the suspension clamp (IEEE 2007, Fadel et al. 2012). Aeolian vibration and the stretching load are the main causes of power line cable fatigue (Cardou 1992, Volker et al. 2014). One of the main concerns of the transmission power line project against aeolian fatigue is to control the stretching load of the cable, as this tension should not exceed the allowable tension during the severe climate period. In addition, the stretching tension must be respected in order to protect the cable against aeolian vibrations (vibrations increase with stretching), and also to restrict the violation of the line clearance. Therefore, some organisations related to power line cables, such as the International Council on Large Electric Systems ( Conseil International de Grand Reseaux Électriques , or CIGRÉ) have fixed some limits and defined the stretching tension as the Every Day Stress (EDS). These limits were created by the EDS panel commissioned by CIGRÉ, which established the upper limit for cable stretching load that can prevail for a long period of time without fatigue of the overhead cable (CIGRÉ 1979, Barrett and Motlis 2001, IEEE 2006). The safe stretching load was established for different types of cables after an in situ investigation of some power line transmission cables around the world, in combination with experimental data. Thus, CIGRÉ refer to the safe tension as the EDS, which has been fixed according to the cable type and to the configuration of the transmission power line cable (Quad, triple, twice bundles, etc.). The value of EDS is expressed as a percentage of the Ultimate Tensile Strength (UTS) of the cable and defined as the maximum tension load to which the cable can be subjected, at the temperature which will occur for the longest period of time in one year, without any risk of fatigue damage caused by aeolian vibrations. Recently, after in situ observation of power line transmission based on a questionnaire prepared by CIGRÉ's working group, it was shown that power line cable failure by fatigue due to aeolian vibration exists, in spite of the use of the recommended EDS value. For instance, for a power transmission lines that have been in service for five to ten years stretched with an EDS value of less than 18% UTS, almost 20% of the cables showed failure by fatigue. Certainly, the need for another parameter for the safe design tension is evident. CIGRÉ suggested the use of the H/w parameter for the safe design tension of transmission power lines to provide protection against fatigue and to better understand the fatigue damage occurring on the overhead cable due to aeolian vibration. The H/w parameter can be defined as the ratio between the initial horizontal tensile load ( H ) and the cable weight ( w ) per unit length. The tensile load ( H ) would be the initial horizontal tension before any significant wind and ice loading and before creep at the average temperature of the coldest month at the site of the power line (CIGRÉ 2005). The tenets of the H/w parameter suggest that all overhead cables stretched with the same value of H/w will have a similar fatigue life, although no experimentals laboratory work on overhead cables were performed to corroborate this idea (Barrett and Motlis 2001, CIGRÉ 2005). After a thorough investigation of current literature, some studies have been identified regarding fatigue of power lines related to the EDS parameter (Chan 2006, Fadel et al. 2012, Volker et al. 2014). However, there is a limited number of studies investigating the effect of the H/w parameter on the fatigue life of a cable made of pure aluminium, especially 1120 aluminium, which was recently applied to power line transmission worldwide. This study investigates the fatigue performance of two types of cable, namely AAC Orchid and 823 MCM, made from two pure types of aluminium: 1350 and 1120, respectively. Eighteen fatigue tests were carried out on the two types of cable. Furthermore, a failure map was made to determine the morphology of wire breaks that occurred. Microscopic analysis showed that the cracks were nucleated in the fretted marks. Data presented in this study will be helpful for the fatigue design and investigation of overhead cables against aeolian vibration.