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WCN
Issue N° 43
www.iwma.org25
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
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
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.
Figure 1
▲
▲
:
Cut-away and cross-sectional views of an ACSR/ACSS conductor cable with seven wire steel core