TRANSMISSION LINE BI-METAL CONNECTION AND CORROSION BASICS AND TUTORIALS

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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