Bi-metal
connectors
Where an
aluminium conductor is terminated on a copper terminal of, say, an isolator a
special copper/aluminium joint is necessary to prevent the formation of a
corrosion cell.
A termination of this type usually comprises of an aluminium sleeve
compressed onto a copper stalk with an insulating disc separating the two
surfaces which are exposed to the atmosphere.
The two
dissimilar materials are generally welded together by friction welding as this
process ensures a better corrosion resistance at the interface. An additional protection
is afforded by the use of an anticorrosion varnish.
When using such fittings it
is always recommended that the aluminium component is above the copper one.
Even slight traces of copper on aluminium have a disastrous effect on the
aluminium material.
Corrosion
Since
overhead lines are erected in different climatic conditions throughout the
world knowledge of their performance has been built up over the years.
Aluminium
conductors have good corrosion behaviour essentially resulting from the
formation of an undisturbed protective surface oxide layer which prevents
further corrosion attack.
ACSR is
known to suffer from bi-metallic corrosion which is noticeable as an increase
in conductor diameter due to corrosion products in the steel core known as
‘bulge corrosion'.
Early
problems associated with deterioration of the steel cores used in ACSR
conductors have been resolved over the years by the use of high temperature greases.
These greases prevent the onset of any galvanic corrosion between the
galvanized steel core and the outer aluminium wires.
They have a high
drop point which allows continuous operation of the conductor at 75°C and full
service life protection. AAAC will obviously offer superior corrosion resistance
than ungreased ACSR.
Conductors that are not fully greased are not recommended
for corrosive areas.
The resistant properties of ACSR also depend
upon the number of layers of aluminium surrounding the steel core. The
conclusions of research carried out in the late 1960s showed that:
• Pure
aluminium had the best corrosion resistance under the majority of environmental
conditions.
• Smooth
body conductors were the most corrosion resistant, especially if the inner
layers were greased.
• Small
diameter wires were most susceptible to corrosion damage and to failure. Thus
for a given conductor area it is preferable to have fewer larger diameter
strands.
• The
overall corrosion performance of aluminium alloy conductors depends upon the
type of alloy used.
For very
aggressive environments the following order of preference is suggested:
• Aluminium
conductor fully greased.
• Aluminium
conductor with alumoweld core fully greased.
• ACSR fully
greased.
• Aluminium
alloy conductor fully greased.
• Aluminium
conductor with alumoweld core ungreased.
• ACSR with
greased core.
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