E+C August 2018

round up

TRANSFORMERS + SUBSTATIONS

Copper or aluminium for dry-type transformers? While many experts argue the respective pros and cons of using copper or aluminium in dry-type transformers, it is important to understand that innovations in transformer design have ensured that either of these two materials can be applied with equal effect in most applications. Dry-type transformer specialist Hammond Power Solutions (HPS), whose products are distributed in Africa byTrafo Power Solutions, has been using both copper and aluminium in the manufacture of conductors and bus bars in its low voltage and medium voltage transformers. HPS has found no significant difference in the transformer lifespan delivered when using either copper or aluminium; from a technical point of view, the two materials are equivalent. There is clearly a cost advantage to selecting aluminium as a conductor, as the commodity’s price tends to be significantly less expensive than copper, and more stable over time. It is therefore not surprising that more than 80% of HPS Europe’s transformer production over the past 40 years incorporates aluminium windings. However, the argument is sometimes made that aluminium is inferior in terms of con- ductivity – as it has only 61% of the conductivity of copper – leading to higher energy losses. Winding temperature is, of course, always a concern, but designers at HPS have been able to maintain temperatures below the insulation rating by designing aluminium winding transformers with conductors with larger cross-sectional areas than those made of copper. Such design innovations in aluminium conductors have succeeded in reducing their energy losses to the same level as copper. This allows transformers of similar design and with the same temperature rise to exhibit roughly equivalent losses, regardless of the conductor material. It is also argued that the thermal conductivity of copper is superior to that of aluminium in reducing hot-spot temperature rise in transformer windings. But this is true only when copper and aluminium windings of identical size, geometry and design are compared. In fact the thermal conductivity characteristics of aluminium can be very close to those of copper, for any given transformer kVA size. The aluminium coil must just be designed and built 66% larger in its cross-sectional area, for the aluminium coils to achieve the same current-carrying capacity as copper. In terms of aluminium’s lower tensile and yield strength, there have been concerns expressed about its use in cyclic load applications – with aluminium demonstrating only 20% of the rupture stress capacity of copper. In applications where loads draw high peaks of current, electromagnetic forces can cause movement of conductors and coil leads. Again, the use of larger sized aluminium conductors addresses this issue, giving the aluminium windings almost the equivalent strength to copper windings. Expansion of aluminium due to changing temperatures has caused some problems when bolted connections are improperly installed, as aluminium expands nearly a third more than copper. With the right type of spring-pressure connection, however, any loosen- ing of joints can be avoided.This can be done using either cupped or split washers, to provide the necessary elasticity at the joint without compressing the aluminium.The result is that al-

uminium joints can perform equal- ly well. When it comes to connec- tivity, both copper and aluminium are prone to oxidation and other chemical changes when exposed to the atmosphere. Cleaning and brushing with a quality joint com- pound to prevent oxidation is rec- ommended for both materials but is more essential for aluminium. Enquiries: Trafo Power Solutions.

Tel. +27 (0) 11 325 4007 or email david@trafo.co.za

Electricity + Control

AUGUST 2018

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