The present invention relates to a process for manufacturing electrical conductors for power transformers with high electrical and thermal conductivity, enhanced through the deposition of copper and graphene. In particular, the present invention relates to an industrial process for the production of conductors having a rectangular section in copper and/or aluminium coated with GNP graphene (Graphene NanoPlatelets) with different thicknesses, in order to increase thermal and electrical conductivity of said conductors, which are then used in all the windings for electrical machines, transformers, generators, motors and, in particular, the power transformers.

BACKGROUND OF THE INVENTION The transformer i a static electric machine powered by alternating current, based on the phenomenon of electromagnetic induction, intended to transform, between the primary circuit (input) and the secondary circuit (output) of the transformer, the voltage and current factors of the electric power.

The transformer therefore transfers the electric power from an electric circuit to another, which has a different voltage, coupling them inductively, without the transformer windings coming into contact with each other.

An electric current transformer in the primary winding generates a variable magnetic flux in the core of the transformer and consequently a variable magnetic field across the secondary winding (Faraday's law and Lenz's law). This variable magnetic field induces an electromotive force, or voltage, in the secondary winding. This effect is called mutual induction.

The transformer is, therefore, a machine capable of operating essentially in alternating current, because it generally exploits the principles of electromagnetism linked to variable flows. The efficiency of a transformer is very high and the losses are very low (in iron, due to the hysteresis and eddy currents, and in copper, due to the Joule effect).

To reduce the decrease in performance, a transformer must have the following characteristics: · Windings: o as few coils as possible o minimum length: the winding wire must be as short as possible; generally this depends on the section and shape of the core, square or (better still, where possible) circular. · Core: o a suitable section; o a length as short as possible; o in ferromagnetic material so as to have an electrical resistance as high as possible, to minimize losses due to the Joule effect and a coercive force as low as possible, in order to have a hysteresis cycle as tight as possible

(and therefore magnetic losses as low as possible).

• Cooling: the cooling is necessary to prevent the overheating of the transformer due to the dissipated power. It is particularly important in the transformers operating at high powers. The cooling can be: o with air - typical solution of normal civil transformers and for small powers; o in oil bath - the transformers work in an oil bath in suitably shaped metal casings to facilitate heat dispersion; o in forced oil bath - compared to the oil bath system, there are also pumps for the forced circulation of the oil and a system of external fans to increase heat removal; o in resin - compared to the other systems, the windings are immersed in a special resin, which makes them dissipate the heat. This resin takes the place of oil. Size benefits can be gained from the resin transformer, since they are slightly smaller than an oil transformer.

For this purpose, various types of transformers are recognized according to their cooling method. The type of cooling is recognized by a code consisting of two or four letters (two, if the transformer has a single cooling; four, if the cooling is double). Specifically, in the code the first letter shows what type of substance is being used for cooling (e.g. oil, resin, air), the second letter shows the type of fluid circulation inside the transformer (natural or forced circulation). For this purpose, the following transformer types are recognized:

• NA: with natural air circulation (not forced);

• NO: with natural oil circulation;

• FA: with forced air circulation;

• NANO: with natural air and oil circulation;

• FAFO: with forced air and oil circulation.

WO20 14/ 141071 reports a method for the preparation of coated metal foams (through an electrodeposition process) with a metal matrix and graphene. EP0859381A1 mentions an electrical conductor for the low voltage windings (and high currents), which allows greater heat exchange, maintaining the same performance as a transformer, which has, however, a much greater encumbrance. This allows the manufacturing of a power transformer having reduced dimensions, thus saving large quantities of copper during the construction of said transformer.

DESCRIPTION OF THE INVENTION

The present invention relates to a process for manufacturing electrical conductors for power transformers with high electrical and thermal conductivity, enhanced through the deposition of copper and graphene.

In particular, the present invention relates to an industrial process for the production of conductors having a rectangular section in copper and/or aluminium coated with GNP graphene (Graphene NanoPlatelets) with different thicknesses, in order to increase thermal and electrical conductivity of said conductors which are then used in all the windings for electrical machines, transformers, generators, motors and in particular the power transformers.

Applying graphene on the surface of such conductor having a rectangular section, it is possible to increase the electrical conductivity and thus allowing the same current flow in a smaller section and, at the same time, improving the thermal conductivity. This means, being able to make a conductor with the same electrical performance, but much smaller in size and therefore much less expensive. In addition, the improved thermal conductivity allows for better heat dissipation and further prevents damage from hot spots and thermal aging. In particular, for the power transformers it is extremely important to be able to design and build machines more and more efficient and less expensive, in our case because the total volume of transformers have a significantly reduced total volume. It is therefore object of the present invention the manufacturing of metal conductors having a rectangular section for transformers (metal strips) electrodeposited with graphene, obtained through the following processing steps. Phase 1 - Drawing the metal wire rod

The metal wire rod is drawn with special synthetic diamond and/or tungsten carbide dies, using an emulsion bath of water and oil. The drawing step is necessary to reduce the diameter of said metal wire rod until a circular section having a diameter of 1.8 mm is reached. According to the present invention, a metal wire rod is defined as a round object made of metallic material, which usually has a cylindrical shape. Said metal wire rod:

• generally has a circular section, with a diameter between 8 mm and 20 mm;

• is made of a metal selected from the group comprising: aluminium, silver, nickel, gold, copper and/or their alloys; preferred are copper and aluminium.

Phase 2 - Laminating and edging The metal wire rod obtained at the end of phase 1 is shaped through a laminating and edging process to obtain a metal strip (having a rectangular section) that complies with the IEC (International Electrotechnical Commission) tolerance standards and has the following characteristics:

• width between 2 mm and 25 mm; length between 2 mm and 25 mm; • thickness between 0.9 mm and 8 mm.

Phase 3 - Static cooking in an inert atmosphere (nitrogen)

The metal strip obtained at the end of phase 2 is subjected to a thermal cooking cycle, optionally having the following temperatures and the following time intervals: cooking at a temperature of 550°C for a period of time of 4 hours (parameters usually used for copper strips), using an electric oven for static cooking in an inert atmosphere (nitrogen), to restore the metal to a soft and malleable physical state. Phase 4 - Electrodepositing on the metal strips

The metal strip obtained at the end of phase 3 is coated through the subsequent steps:

Step 1 - the metal strip is coated with a “first metal layer”, deposited using a physical deposition technique (PVD/physical vapor deposition) or with a chemical deposition technique (CVD/ chemical vapor deposition), as described in Adv. Mater. 2000, 12, No. 9; Step 2 - on the first metal layer of step 1 a “second metal layer (or its alloys) and graphene” is deposited/ stratified, using the electrodeposition described in W02014141071, wherein the metal associated with the graphene can be the same or different from the one used in step 1 ;

Step 3 - on the second “metal and graphene” layer of step 2 a “third metal layer” (or its alloys) is deposited/ stratified through a further electrodeposition process as described W02014141071, or a chemical or physical vapor deposition process using the PVD or CVD procedure as described in http:/ / www. mag- data. com/ dettaqli-tecnici/ introduzione-ai-film-polimerici/ ; Journal of Materials Chemistry C Volume 4 Number 37, 7 October 2016, Pages 8585-8830; and/or Adv. Mater. 2000, 12, No. 9. The metal material of the first, the second and third layer is selected from the group comprising: aluminium, silver, nickel, gold, copper and/or their alloys; the metal of the first layer can be the same of different from the second layer, which can in turn be the same of different from the metal of the third layer. Phase 5 - Insulating the electrodeposited strips through taping

The electrodeposited metal strip obtained at the end of phase 4 is insulated through a taping process, using a material selected from the group comprising: pure cellulose paper for oil impregnation, aramid paper, mica tape and/or polyamide tape. Phase 6 - Coupling the metal strips thus obtaining the power transformer.

The strips obtained at the end of phase 5 are grouped or coupled (depending on the transformer to be made) in helical windings, disc windings or other types of windings, thus obtaining the desired power transformer.

Said power transformer can be used in the industrial, industrial, energy, naval and / or aerospace field and having better characteristics of electrical and thermal conductivity, dissipated power and efficiency compared to current transformers on the market.

The increase in electrical conductivity of the metal strips and of the power transformer, obtained at the end of the phases 5 and 6 of the present invention, led to a reduction of the resistivity of the initial material (copper) between 5% and 9%. This reduction in resistivity is associated, for the same section, with a decrease in the heat generated by the Joule effect, also between 5% and 9%. Furthermore, an appropriate sizing and optimization of the shapes of the contacts and the geometry of the surrounding areas has contributed, in addition to a further reduction of at least 5% of the thermal load, also to a homogenization of temperatures with a reduction of hot spots.

It is a further object of the present invention the metal strip obtained at the end of phase 5, which can be used for the construction of windings for electrical machines, transformers, generators and / or motors.