PSI - Issue 19

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

690

3

2. Fatigue of overhead cables

2.1. Cable material

The overhead cable is the only component of the power line transmission which carries electricity. It is designed in order to sustain some electrical, mechanical and environmental loads in order to achieve the projected life expectancy. The cables used for this publication are made of aluminium 1350 and 1120, both pure aluminium belonging to series 1xxxx. The usage of 1120 aluminium for cables was recently initiated and currently, increasing numbers of transmission power companies around the world are starting to upgrade or construct their lines using cables made of this type of pure aluminium. Aluminium1120 has an electrical conductivity equal to almost 60% of the conductivity mean, while aluminium1350 has almost 62%. Although the aluminium1120 has a conductivity slightly lower than that of the pure aluminium 1350, it presents good mechanical properties such as a low strength to weight ratio and high mechanical strength. Beside the mechanical properties, the cable made of aluminium 1120 presents low costs as one can use low structure (tower) and long span for the transmission power line. The chemical composition of both pure aluminium types is presented in Table 1 by considering the maximum value of the component.

Table 1. Chemical composition of Aluminium 1350 and 1120.

B

Cr

Cu

Ga

Fe

Mn

Mg

Si

V+Ti Zn

Aluminium Al

Bal. 0.05 0.01 0.05 0.03 0.40 0.01 ----

0.10 0.02

0.05

1350

Bal. 0.05 0.01 0.01 0.03 0.40 0.01 0.20 0.10 0.02

0.01

1120

2.2. Bending stress of cable Overhead cables are exposed to nature’s dynamic forces which could originate from snow, wind, rain or earthquakes. These forces lead overhead cables to vibration and could lead to damage and failure after extensive periods of time. One of the main causes of damage or failure of cables is fatigue mainly due to aeolian vibration, especially at points where the cyclic motion of the cable is restrained, for example at support locations, dead-ends, suspension clamp and other clamp types (Chan 2006, Loredo-Souza and Davenport 1998, Kiessling et al. 2003). The cyclic bending of a cable causes fatigue of cable strands near the devices which restrain the cable motion. As part of the cyclic bending, this increases stress because the stretching load of the cable and the pressure load due to the suspension clamp contribute to the fatigue of the strand cable. In the case of usage of the system suspension clamp/cable, most of the strand fatigue occurs inside the suspension clamps. Accurate measurement of bending stress at the strand fatigue points is quite difficult, therefore the implementation of some assumptions is vital. The strain gauges and the Poffenberger-Swart (P-S) formula allow for the measurement of the strain of wire and the confirmation of this measurement using the P-S calculation. Poffenberger and Swart (1965) considered that the dynamic behaviour of the cable is similar to an Euler beam close to the suspension. Figure 1 illustrates the schematic montage of the system cable and suspension clamp and the point and specification used by P-S to established the formula.