Aluminum Vs. Copper Conductors in Dry Type Cast Resin Transformers
Republish Technical Article by courtesy of Tesar SrL
Prepared by Dr Egizia Maffucci
In Europe, aluminum is the predominant choice (more than 95%) of winding material for Cast Resin Transformers. In other areas of the world, copper is the predominant winding material. The primary reason for choosing aluminum windings is its lower initial cost. The cost of copper base metal has historically proven to be much more volatile than the cost of aluminum, so that the purchase price of copper conductor generally is the most expensive choice. Also, because aluminum has greater malleability and is easier to weld, it is the lower-cost manufacturing choice.
Technical arguments about the pros and cons of aluminum vs. copper have been traded back and forth in the electrical industry for many years. Most of these arguments are inconsequential and some can be classified simply as misinformation..
A. Electrical Conductivity
Often, arguments point to inferiority of aluminum conductivity, citing the fact that aluminum has only 61% of the conductivity of copper, which causes higher energy losses in the temperature below the insulation rating, aluminum transformers are designed with aluminum conductors of larger cross-sectional area than copper. Therefore, transformers of similar design with the same temperature rise have roughly equivalent losses regardless of conductor material. As a confirmation in fact the International Standard do not make any difference between losses in copper and in aluminum transformers.
Transformer manufacturers limit the variety of conductor sizes stacked. Because of this, some design in aluminum can obtain lower losses than copper simply because the choice of wire size is limited.
Connectivity is far the most common reason for “prejudice” against the use of aluminum wound transformers. Both copper and aluminum are prone to oxidation or other chemical changes when exposed to atmosphere. The problem is that aluminum oxide is a very good insulator, whereas copper oxide, although not considered to be good conductor, is not nearly as troublesome in bolted connections. The connection between terminals of aluminum transformers to copper cables is made by means of tin-plated cable lugs.
This lugs are specifically rated (Al/Cu) for connecting copper building wire to either metal. This practice is universally accepted and has proven to be reliable throughout the more than 30 years aluminum-wound transformers have been in use.
C. Internal Connection And Welding
Another argument focuses on the different techniques used to make copper and aluminum connections. Internal transformer connections made with copper are generally brazed, whereas the same aluminum connections are welded using inert gas. Technically, the brazing technique causes the copper connection to have lower conductivity than the copper base metal. Inert gas welding of aluminum produces a continuous aluminum joint with no degradation of conductivity.
In addition, some argue that over time copper oxide continues to form, flaking off exposed copper and eventually damaging the entire conductor. On the other hand, aluminum oxide forms a tenacious, protective coat over the exposed metal, which stops the oxidation after only a few microns. However, the average user should really not be too concerned about these theoretical considerations because both copper and aluminum transformers have excellent records in long years of practical use.
The only valid engineering reason for choosing copper over aluminum appears to be space considerations. An irrefutable fact is that copper-wound transformer can be made smaller than aluminum transformers, but we are speaking of few centimeters. In spite of this a copper wound transformer is heavier of about 20 – 25 % compared to an aluminum wound transformers and this may have big economic impact on the costs of the civil works.
|Aluminum-wound transformer terminations are in compatible with copper line and load cables.||F|
|Line and load connections to copper-wound transformers are more reliable than those to aluminum-wound transformers.||F|
|Aluminum-wound transformers are lighter in weight than copper wound equivalents.||T|
|Copper-wound transformers are better for “high-impact” loads because copper has higher tensile strength than aluminum.||F|
|Aluminum-Wound transformers have higher losses because copper is a better conductor||F|
|Aluminum-wound transformers have higher hot-spot temperatures because copper is a better thermal conductor than aluminum.||F|
a. Table 1. Some of the most common questions related to installation
Choosing between aluminum and copper transformer windings comes down to personal preference. The premium price for copper often requires purchase justification. In truth, industry experience simply does not support any of the commonly stated reasons for choosing copper over aluminum. Aluminum- wound transformers will probably continue to gain increased acceptance because of their significant cost advantage over copper, having same technical characteristics, same efficiencies and without of any doubt same or higher reliability.