Copper and aluminium are the two conductors used in transformer windings. In distribution and small power transformers, aluminium–aluminium windings have been successful. For large power transformers, a copper–copper design is more common. To select the right material, the designer has to take several factors, such as weight, maximum size, transformer total cost, availability and cost of the material, into consideration.
The first transformers were built with copper conductors, since copper was more accessible and the lowest priced suitable metal at that time. As a result of the shortage of copper, which was in huge demand for the arms industry during the Second World War, some industries began to manufacture transformers with aluminium [1, 2, 3].
In the 1960s the demand for copper caused a large increase in its price. Following this, a number of technological problems were overcome and the reliability of aluminium windings in transformers was explored and confirmed. These positive results led to an intensive use of aluminium in the United States. Then, transformer manufacturers began using aluminium as strips [4, 5].
Since 1970, the technology of aluminium wound transformers has also gained widespread acceptance in European countries.
Aluminium is used in a number of industrial applications, and some 200 aluminium alloys are available on the market. World production and reserves of aluminium are substantially greater than those of copper.
World production and reserves
World bauxite resources are estimated to be 55 to 75-billion t; in Africa (32%), Oceania (23%), South America and the Caribbean (21%), Asia (18%), and elsewhere (6%).
A global assessment undertaken in 2014 showed copper deposits of about 2,1-billion t (porphyry deposits accounted for 1,8-billion t of those resources), and undiscovered resources were estimated at 3,5-billion t .
The cost difference between copper and aluminium varies as a result of the fluctuating cost of the base metals on the commodities market.The cost difference is often the deciding factor when a customer is considering aluminium conductors in equipment. All the references on the cost differences are based on the effect that the current commodities market has on the components in electrical equipment.
As shown in Table 1 , the cost of windings in distribution transformers ranges from 16 to 28% of the total cost of transformer materials. The percentage difference in the cost between aluminium and copper also varies as the percentage of the conductor is a component of the overall equipment.
In any comparison with a conventional transformer using copper windings, two questions arise:
|Cost of material in distribution transformers|
|Transformer material||Cost, %|
|Magnetic steel||32,5 ± 5,5|
(copper or aluminium)
|22 ± 6,0|
|Insulation||14 ± 5,5|
|Carbon steel||16,5 ± 8,5|
|Fabricated parts||15 ± 9,0|
These questions are interconnected with the transformer design, where four main characteristics must be considered to ensure a reliable and cost effective product:
The following comments are based on a comparison of the main physical properties of the aluminium and copper, as summarised in Table 2.
General design and electrical characteristics
For the same transformer, with the same general characteristics, it is necessary to use aluminium conductors with a cross section approximately 1,6 to 1,8 times larger than that which applies to windings with copper conductors, because of the higher resistivity of the aluminium.
On the other hand, since the ratio of specific weight of the two materials is 0,304, the mass of an equivalent aluminium winding is 0,5 to 0,55 times that of a copper winding.
In a complex piece of equipment such as a transformer, it also transpires that an increase in the volume of the winding automatically results in a volume increase of the other components, such as the magnetic core and enclosure. However, this effect is significantly reduced by the fact that the conductors only take up space of a part of core window, which also includes the insulation structure and the cooling channels.
The core space factors, used as a measure in this regard, vary between 10 to 30%, depending on the insulating class of the windings, which results in a marginal increase for the magnetic circuit of approximately 10 to 20%.
|Electrical||Resistivity at 20°C||Ω mm2 / m||0,0175||0,0285||1,65|
|Electrical conductivity at 20°C||m / Ω mm2||57,1||35,4||0,606|
|Temperature Coefficient of electrical resistance at 20°C||–||0,00427||0,0044||1,03|
|Thermal||Thermal conductivity||W / m °C||393||203||0,517|
|Specific heat||W sec / kg °C||385||920||2,43|
|Mechanical||Specific weight||kg / m3||8900||2700||0,304|
|Tensile strength||kgf / mm2||24||10||0,417|
|Elastic limit||kgf / mm2||6 – 8||2 – 5||–|
|Coefficient of linear expansion at 20°C||–||17 x 10-6||24 x 10-6||1,41|
Consequently, in order to maintain the same no-load losses in a transformer with aluminium windings, the flux density in the core must be slightly decreased, which results in an increase of the core mass when compared with transformer with copper windings.
Based on a transformer with copper conductors, it transpires that a transformer with aluminium must be entirely redesigned in order to achieve an optimal design, taking into account performance and cost of the materials.
Summarising, for the same level of electrical losses, the characteristics of the two transformers that apply aluminium or copper are as follows:
Effect of thermal properties
The melting point of aluminium (665°C) is considerably lower than that of copper, but it is still well above the real working temperature of the windings, which are determined by the insulating materials. In normal circumstances, the “hot-spot” temperature in the windings is within a range of 105 to 120°C, depending on the thermal class (oil cooled). The mechanical properties of aluminium are not affected at these temperatures.
The lower thermal conductivity of aluminium does not affect the performance in the overall temperature of the transformer and the temperature differences in the conductor are negligible in relation to the temperature rise between the ambient air and the windings (gradient). These temperature gradients between the windings and the coolant (oil or air) depend essentially on the losses to be dissipated per unit area of conductor in contact with the coolant.
Since the volume of aluminium is approximately 1.8 times the volume of copper, it follows that the surface of aluminium is around 1,3 times that of copper. Therefore for equal losses, the aluminium-oil temperature gradient will be smaller than the copper-oil gradient.
During conditions of short circuit or overload, the maximum temperature reached by the windings depends primarily on the specific heat of the conductor and its mass. Once the specific heat of aluminium is far greater (2,43 times) than that of copper, and taking into account the reduced aluminium mass (0,55 times), it can be calculated that the temperature rise in aluminium windings will be limited to 75% of that in copper. This represents is a significant advantage with aluminium windings relating to the life of the insulating materials.
Effect of resistivity on eddy losses
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the “skin” of the conductor between the outer surface and a level called the skin depth or penetration depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross section of the conductor. At high frequencies, the skin depth becomes much smaller. This is because the interior of a large conductor carries so little of the current. The penetration depth for a good conductor can be calculated from the following equation :
δ = the skin depth in metres
µr = the relative permeability of the material
µ0 = the permeability of the free space
σ = the conductivity of the conductor in [Ω.m]-1
ƒ = frequency of the current in Hz
In a good conductor, skin depth is proportional to the square root of the resistivity. This means that better conductors have a reduced skin depth.
The penetration depth of Cu at 50 Hz is 9,4 mm, while for Al is 12,3 mm.
The conductivity of Cu at 75°C is 0,4703 . 108 [oh.m]-1, while for Al is 0.28935 . 108 [oh.m]-1.
As a result of this difference in the material properties, eddy loss in aluminium windings is 38% lower than in copper windings with the same volume of conductor as shown in Table 3.
Eddy losses due to high harmonics
Harmonic currents are generated whenever a non-linear load is connected to the mains supply. The problems caused by harmonic currents include overheating of cables, especially overheating and vibration in induction motors and increased losses in transformers.
Losses in transformers are due to magnetic losses in the core, and eddy current and resistive losses in the windings. Eddy current losses are of most concern when harmonics are present because they increase with the square of the frequency.
The K-factor is the ratio of eddy current losses when driving non-linear and linear loads.
Considering a VSD transformer with equal no-load and load losses designed with copper and aluminium conductors, eddy losses are reduced by approximately 38% in case of aluminium.
Effect of mechanical properties
To complete the consideration about the use of aluminium in transformers, it is also necessary to consider the mechanical behaviour of the windings under service, particularly switching and short circuit stresses. During these events, the electromagnetic forces stress the windings in both radial and axial directions.
The conductors are normally separated from each other by vertical and horizontal oil ducts. Spacers and space holders are provided for this purpose. It follows that the conductors are stressed in a number of ways: compression, tension, and bending between coil supports.
In spite of the fact that the tensile strength and the elastic limit of aluminium are lower than those for copper, this difference is compensated for largely by the greater volume of conductor material applied.
Since the respective tensile strengths of aluminium and copper are 10 kg/mm2 and 24 kg/mm2, the design of aluminium winding must consider the ratio of 0,417 of the maximum stress level supported by copper windings.
Since stresses are particularly severe during short circuit conditions, this parameter shall be used as the base for comparison between the performance of the aluminium and copper windings.
Regarding this condition, to achieve the same safety margin, it is enough to slightly reduce the bending moment on the aluminium conductor by making the spacers closer together in a ratio of 0,935. This results as an equally supported stress in the two windings.
As described above, a few minor adjustments on windings supports and spacers is enough to achieve the same mechanical integrity of transformer with aluminium windings as in the case of copper. Even this minor modification is not always necessary, since it is possible to change the geometry of the conductor in order to obtain an adequate stress level in the aluminium conductor.
There are no significant differences in this regard between the design applying aluminium or copper windings.
In the case of oil cooled transformers, it is recognised that the catalytic action of aluminium on oil oxidation is significantly less than those with copper.
It can also be said that because of the larger surface area of aluminium conductors, there is an improvement in the series capacitance of the windings, which results in better impulse withstand.
Manufacture of aluminium windings does not represent any difficult technological problem.
Where aluminium is to be used in an outdoor atmosphere, there has been great concern about joining and terminating aluminium either to itself or to copper. Today, completely reliable aluminium joints and terminations can be made without any problem.
The basic techniques used for joints are bolting, crimping and arc welding under inert gas with or without refractory electrodes (TIG and MIG methods).
These procedures have undergone a series of very stringent tests and have been proven by many years of operation.
|Aluminium price is more stable because of its market availability (twice copper worldwide production);||X|
|Aluminium transformers are lighter than copper transformers||X|
|Just copper winding transformers bear short circuit efforts||X|
|Aluminium transformers have bigger losses||X|
|Aluminium winding transformers are not compatible with copper connectors||X|
|Copper transformers are more compact than aluminium ones||X|
As presented there are no significant differences between designing and manufacturing distribution and small to medium power transformers using with either aluminium or copper windings.
Both will give the user a transformer with the same quality of performance operation.
WEG Transformers has successfully manufactured thousands of distribution and hundreds of small power transformers with aluminium windings.
This means that given the significant economic advantages of aluminium, this electrical conductor can be considered one of the best options to apply in the manufacture of electromagnetic equipment like transformers, especially considering the possibility of producing equipment capable of an equivalent performance under service condition to that provided by transformers with copper windings.
 WW Orr: “Aluminum and its future in power transformers”, http://ieeexplore.ieee.org/document/6445796.
 EW Tipton: “Experiences with the use of aluminum in windings for dry-type power transformers”, http://ieeexplore.ieee.org/document/4499216.
 GC Wilburn: “Aluminum in small power transformers”, IEEE.
 JH Harlow: “Electric power transformer engineering”, 2004.
 T Pelikan: “Is there a future for the distribution transformer with aluminum windings?”, 1973.
 US Geological Survey, Mineral Commodity Summaries, January 2016.
 InfoMine: “Metal prices”, www.goo.gl/KzhOIO
 JC Olivares, et al: “Reducing losses in distribution transformers”, IEEE transmission, Power delivery, 2003.
 R Wangsness: “Electromagnetic Fields” (2nd ed.), 1979.
Contact Kirsten Larkan, Zest WEG group Africa, Tel 011 723-6000, firstname.lastname@example.org