WCN Autumn 2010

Evaluation of high- temperature behaviour of Zn and ZnAl coatings on core wires and strands for ACSR, ACSS, and alike overhead power conductors

By Charles Coffin, Hans Depamelaere, Dale King and Walther Van Raemdonck, NV Bekaert SA, Belgium

Abstract: Because of the growing importance of power trade (which is a result of the power market liberalisation in many parts of the world), there is a need to increase transmission conductor capacities. The most straightforward and most often used method to increase a conductor’s current carrying capacity is to allow it to operate at higher temperatures. For ACSS (Aluminium Conductor Steel Supported) designs with average working temperatures of 80°C, this tendency results in emergency peaks up to 250°C. Galvanised (zinc) coatings are gener- ally limited to a maximum service temperature for extended times of 180- 200°C, depending on steel composition. Galvanised wire exposed at higher temperatures risks failing wrapping tests. This paper describes how Bezinal ® coatings (Bezinal is a deposited trademark of Bekaert’s ZnAl coatings) are resistant to temperatures as high as 350ºC with no significant negative effects on either coating ductility or salt

Figure 1 ▲ ▲ : Cut-away and cross-sectional views of an ACSR/ACSS conductor cable with seven wire steel core

provide mechanical strength. As such cables are located outdoors exposed to the elements; corrosion resistance is also needed and is commonly provided through a zinc (Zn) or zinc-aluminium (ZnAl) coating that has been applied to the steel wires via the hot dip coating process. A byproduct of the transmission of electrical energy is the creation of heat due to the internal resistance within the wires. Thus the temperature of the conductor cable is observed to rise as a function of the electrical current the cable is carrying.

spray corrosion resistance and explains the mechanism. It is also shown that the typical Ce and La mischmetal additions do not influence the high temperature or the corrosion performance. Introduction ACSR (aluminium core steel reinforced) and ACSS (aluminium core steel supported) cables facilitate overhead electrical power transmission across distances. Because of the relatively low strength of the high purity aluminium conductors, steel wires are needed to

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