Thermal expansion is the tendency of matter to change in volume in response to a change in temperature, through heat transfer. This property is measured by the thermal linear expansion coefficient; defined as the fractional change in length of a particular material for each degree of temperature change. The thermal linear coefficients of expansion for aluminium and copper per degree centigrade are:
- Copper 17∙10E-6
- Aluminium 23∙10E-6
In other words, the coefficient of thermal expansion for aluminium is 35% greater than that of copper. This characteristic is of concern when we study the expansion and contraction of conductors in electrical connections during thermal cycling.
This is a regular phenomenon in the electricity supply system, and involves the cable conductor expanding and contracting according to the temperature variation. In general, a system can be installed in such a way that the expansion and contraction are under control – for instance by installing a cable according to a snaking arrangement. However, if the system contains discontinuities such as joints, mechanical forces will occur at the joints.
These forces depend not only on the coefficient of expansion but also on Young’s Modulus and the cross-section of the conductor. Taking into account the different values of the relevant parameters, the mechanical forces due to temperature variation are about 20% higher for aluminium compared to copper. If these forces are high enough they may become destructive and result eventually in joint failure.
Although a similar situation may happen in the joint itself if the conductor and connector are not of the same material, this will probably not cause serious problems as the connector has small dimensions; much smaller than the length of the cable. A more serious point of concern with joints of dissimilar materials is contact corrosion.
In the case of conductors in overhead lines, the expansion due to temperature increase results in increased sag. If an overloaded line sags into a tree, the current can be discharged to the ground, causing a short circuit and sometimes triggering a major power outage. Increased sag was a cause of two major U.S. blackouts in 1996 and 2003.
Which material to choose?
The tendency to expand and contract due to temperature variation will inevitably result in some sort of problem for conductors. The most significant consequences of the higher coefficient of thermal expansion of aluminium are destructive forces in the joint and increased sag. With regard to these problems, copper is certainly a better choice than aluminium, on account of its significantly lower coefficient of thermal linear expansion.
The effect of high current loads on joints in MV cable systems: 21st International Conference on Electricity Distribution Frankfurt, 6-9 June 2011
The thermo-mechanical behavior of joints in MV cable systems exposed to high current loads: 8th International Conference on Insulated Power Cables, 2011
Measurement of the force induced by thermal expansion of conductor of MV cables and impact on MV joints: 22nd International Conference on Electricity Distribution, 2013
Towards In-Field Current Rating Verification of Underground Power Cable Systems: Bernd C. van Maanen, Eindhoven University of Technology, Department of Electrical Engineering