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 today be applied with equal effect in most applications, according to David Claassen, managing director of Trafo Power Solutions.

Dry-type transformer specialists Hammond Power Solutions (HPS) – whose products are distributed in Africa by Trafo 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 been a global leader in dry-type transformers for more than a century, and both materials have been used by HPS Europe in its production process for more than forty years.
The company 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 – across all markets and applications – incorporates aluminium windings.

However, the argument is sometimes made that aluminium is inferior in terms of conductivity – 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.

Claassen notes that, 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.

HPS designs the cooling surface area, the coil geometry, the air ducting and the conductor shape to produce acceptable hot-spot gradients, regardless of the winding material used.

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.
HPS has found that the ability of its transformers to withstand the long-term mechanical effects of high impact loads or short circuits really depends more on adequate coil balance and lead support than on the choice of conductor material.

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 loosening 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 aluminium joints can perform equally as well as copper joints.

When it comes to connectivity, both copper and aluminium are prone to oxidation and other chemical changes when exposed to the atmosphere.

Cleaning and brushing with a quality joint compound to prevent oxidation is recommended for both materials but is more essential for aluminium.
HPS have a solution for aluminium and copper connectors that involves applying silver or tin-plating to either or both of the conductors in the bolted connection, or by putting a cupal transition plate between the materials.

Claassen emphasises that HPS has developed dry-type transformers for a range of industry applications – from distribution and marine vessels to railway, mining and off-shore rigs.

“Both aluminium and copper have been used in various applications for many years without any problem.
With our innovative design and production procedures, it is possible to avoid the technical limitations of aluminium conductors while retaining the advantages like cost and weight,” he concludes.

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