Component of a transformer and transformer comprising said component

Abstract

The invention relates to a component (20, 24, 26, 28) of a transformer (10), wherein the component (20, 24, 26, 28) is a current-carrying component and comprises a graphene-reinforced aluminium matrix composite. Furthermore, the invention relates to a transformer (10) comprising the above component (20, 24, 26, 28).

Description

[0001] Description

[0002] Component of a transformer and transformer comprising said component

[0003] Technical Field

[0004] The invention relates to a current-carrying component of a transformer.

[0005] Furthermore, the invention relates to a transformer comprising the above component.

[0006] Background Art

[0007] Transformers as passive electrical device for energy conversion in power system are well known and are used in electric power systems where the safety and stability have the highest priority. The transformer transfers electrical energy from one electrical circuit to another, or multiple circuits. A varying current in any winding of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force across any other winding wound around the same core. Thus, electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits. Transformers are most commonly used for increasing low AC voltages at high current (a step-up transformer) or decreasing high AC voltages at low current (a step-down transformer) in electric power applications, and for coupling the stages of signal processing circuits.

[0008] During the operation life, the transformer can be subjected to various short-circuit faults, which can cause the current in the transformer windings to increase above the rated current. The resulting electromagnetic forces can cause damages to the components of the transformer, in particular to the current-carrying components, such as the transformer windings. The damages can include deformation, fracture, and can thus lead to malfunctioning of the transformer. For the current-carrying components of the transformer materials having a high electrical conductivity are used, such as copper or aluminum. Compared to copper, aluminum offers the advantage of less weight, as it is up to 70 % more lightweight than copper and lower costs.

[0009] However, aluminium has poor mechanical properties with an ultimate tensile strength of only 90 MPa and a yield stress of 15 MPa, thus for transformers using aluminium for the transformer windings, transformer winding deformation faults caused by short-circuit electromagnetic forces are a common transformer fault. Damaged transformer windings are very difficult to repair on-site, thus the damage is associated with high costs.

[0010] Thus, there is a need to provide more robust transformers, in particular transformers having better mechanical properties to withstand short-circuit conditions.

[0011] Summary of invention

[0012] It is an object of the invention to increase the mechanical properties of transformers, in particularly for withstanding short-circuit conditions. It is a further object of the invention to increase the mechanical properties of transformers, without impairing the electrical properties of the transformer. In particular, it is an object of the present invention to increase the mechanical and electrical properties of transformers.

[0013] The object of the invention is at least solved in part by the features of the independent claims. Modified embodiments are detailed in the dependent claims.

[0014] Thus, the object is at least solved in part by a component of a transformer, wherein the component is a current-carrying component and comprises a graphene-reinforced aluminium matrix composite.

[0015] A current-carrying component is a component of the transformer, in which electrical current flows during the operation of the transformer. In case the component of the transformer or the transformer itself are not connected to a closed electrical circuit there may be no electrical current flowing in the current-carrying component. The current-carrying component comprises the graphene-reinforced aluminium matrix composite. A graphene-reinforced aluminium matrix composite is a type of metal matrix composite, where the metal matrix comprises aluminium, and where graphene is dispersed in the metallic matrix.

[0016] It was found that the current-carrying component that comprises the graphene-reinforced aluminium matrix composite has enhanced mechanical properties compared to a component comprising aluminium without graphene without impairing the electrical properties.

[0017] It is possible that the component of the transformer comprises in addition to the graphene-reinforced aluminium matrix composite further materials. According to a preferred embodiment of the invention, it is however preferred that the component consists of the graphene-reinforced aluminium matrix composite. In other words, it is preferred that the component does not comprise additional materials other than the graphene-reinforced aluminium matrix composite.

[0018] Regarding the graphene-reinforced aluminium matrix composite it is possible that the graphene-reinforced aluminium matrix composite comprises in addition to aluminium and graphene further materials. When at least three materials are present, the metal matrix composite would be called a hybrid composite. According to a further preferred embodiment it is however preferred that the graphene-reinforced aluminium matrix composite consists essentially of aluminium and graphene. Thus, the amount of aluminium and graphene expressed in percentages preferably add up to 100%. It is further possible that the graphene-reinforced aluminium matrix composite comprises aluminium carbide (AI4C3) generated by the chemical reaction of the aluminium with the graphene during manufacturing. It was found that the graphene-reinforced aluminium matrix composite that consists essentially of aluminium and graphene has increased mechanical properties such as a high modulus of elasticity and good electrical conductivity.

[0019] Regarding the lower amount of graphene and according to a preferred embodiment of the invention, the graphene amount in the graphene-reinforced aluminium matrix composite is at least 0.1 wt%, preferably at least 0.15 wt%, and more preferably at least 0.2 wt% based on the total weight of the graphene-reinforced aluminium matrix composite.

[0020] Regarding the upper amount of graphene and according to another preferred embodiment of the invention, the graphene amount in the graphene-reinforced aluminium matrix composite is not more than 1 wt%, preferably not more than 0.5 wt%, and more preferably not more than 0.3 wt% based on the total weight of the graphene-reinforced aluminium matrix composite.

[0021] Thus, preferably rather low amounts of graphene in between 0.1 wt% to 1 wt%, preferably in between 0.15 wt% to 0.5 wt%, based on the total weight of the graphene- reinforced aluminium matrix composite, are used for the graphene-reinforced aluminium matrix composite. It was found that higher amounts of graphene negatively affect the mechanical and electrical properties of the materials.

[0022] According to a further preferred embodiment of the invention, the graphene amount in the graphene-reinforced aluminium matrix composite is 0.25 wt% ± 0.05 wt%, preferably 0.25 wt% ± 0.03 wt%, and even more preferably 0.25 wt% ± 0.01 wt% based on the total weight of the graphene-reinforced aluminium matrix composite. It was found that with a graphene amount in between 0.20 wt% to 0.30 wt%, preferably 0.22 wt% to 0.28 wt%, and further preferably in between 0.26 wt% to 0.27 wt% the mechanical properties and in particular the resistance to yield stress is increased. It was further found that with this grapheme amount also the electrical properties, in particular the electrical conductivity, were increased.

[0023] Graphene is an essentially two-dimensional carbon nanomaterial consisting of carbon atoms arranged in a hexagonal honeycomb lattice. Within the graphene-reinforced aluminium matrix composite the graphene is preferably present as particulate.

[0024] In connection to this and according to a preferred embodiment of the invention a number of graphene layers per graphene particle in the graphene-reinforced aluminium matrix composite is equal to or less than ten. Such type of graphene is also called few layer graphene (FLG). Typically, the number of layers ranges from around two to ten per particle. Thus, in other words, the graphene preferably consists of short stacks of graphene layers that are in a planar form.

[0025] Depending on the number of layers and according to another preferred embodiment of the invention an average out of plane thickness of a graphene particle in the graphene-reinforced aluminium matrix composite is 0.5 nm to 5 nm. The average out of plane thickness is preferably the arithmetic mean of the thickness of the particles. The thickness of the particles is preferably determined by scanning electron microscopy.

[0026] In connection to this and according to a preferred embodiment of the invention an average in plane extension of a graphene particle in the graphene-reinforced aluminium matrix composite is 1 pm to 10 pm. Preferably the in-plane extension refers to the dimension of the particle within the plane of the hexagonal honeycomb lattice of the graphene. The in-plane extension is also called the lateral size of the particle. Further preferably the average in plane extension refers to the arithmetic mean of the in-plane extension of the particles. The in-plane extension of the particles is preferably determined by atomic force microscopy and / or scanning electron microscopy.

[0027] There are several methods to produce the graphene-reinforced aluminium matrix composite. According to a preferred embodiment of the invention, the component and / or the graphene-reinforced aluminium matrix composite is produced by a liquid state process, by a solid-state process, and / or by a composite state process.

[0028] In connection to this and according to a further preferred embodiment of the invention the liquid state process comprises stir casting, squeeze casting, and / or 3D printing, the solid-state process comprises powder metallurgy, friction stir welding, and / or selective laser melting, and / or the composite state process comprises rolling and / or accumulative roll bonding.

[0029] In stir casting the graphene is stirred into the molten aluminium, which is afterwards allowed to solidify. In squeeze casting the molten aluminium is injected into a form with the reinforcement pre-placed inside the form.

[0030] In powder metallurgy powdered aluminium and graphene are mixed together and bonded through a process of compaction, degassing, and thermo-mechanical treatment.

[0031] As already mentioned, the component is a current-carrying component. In connection to this and according to a further preferred embodiment of the invention, the component is a transformer winding, a busbar, a terminal, a lead-out, a y-connec- tion, or a conductor of a bushing. Particularly preferably, the component is a transformer winding. As the transformer winding makes up a major part of the transformer, providing less expensive alternatives to copper windings makes the transformer less expensive. The lead-out or busbar preferably provides the connection between the winding and the conductor of the bushing and / or the terminal.

[0032] Regarding the windings and according to another preferred embodiment of the invention, the winding is preferably configured as helical winding, double layered winding, cross-over winding, sandwich winding, or disc-type winding. Particularly preferably the winding is a disc-type winding. Preferably for the disc-type winding several conductor discs are connected in series. Initially, a disc can be formed by winding different insulated conductor turns, which are afterwards connected in series to create the disc winding. Every disc is preferably separated from an adjacent disc with spacers.

[0033] Further embodiments and advantages of the component of the transformer are directly and unambiguously derived by the person skilled in the art from the description of the transformer.

[0034] The object of the invention is also solved at least in part by a transformer comprising the above-described component.

[0035] The transformer can be an oil-type transformer or a dry-type transformer. The oiltype transformer uses insulation oil as insulation material, while the dry-type transformer is a transformer in which the transformer core and windings are not immersed in a liquid. Furthermore, the transformer can be a core-type or a shell-type transformer. In case the windings surround the core, the transformer is a core type transformer. In case the windings are surrounded by the core the transformer is a shell type transformer. Preferably the transformer is a dry-type transformer of the coretype.

[0036] According to a further preferred embodiment, the transformer is configured as a 1 .5 MVA transformer and / or as an open wound transformer. An open wound transformer is a kind of dry-type transformer. Preferably the open wound transformer is manufactured by a dip and bake method, wherein at first the windings of the transformer are heated, then dipped in varnish and finally baked. Furthermore, transformer may be configured as vacuum pressure encapsulated transformer or as vacuum pressure impregnated transformer. Preferably for the pressure encapsulated transformer resin is used in a sealing process, thus providing high dielectric strength and life expectancy. A vacuum pressure impregnated transformer is a transformer that is vacuum pressure impregnated preferably with a high temperature polyester varnish.

[0037] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0038] In the drawings:

[0039] Fig. 1 schematically shows a transformer, according to a preferred embodiment of the invention,

[0040] Fig. 2 schematically shows a model of a transformer used in a FEM-sim- ulation, having a component according to another preferred embodiment of the invention,

[0041] Figs. 3 to 4 schematically show simulation results of the FEM-simulation using the model of figure 2, and Fig. 5 schematically shows simulation results of a FEM-simulation for a comparative component.

[0042] Description of embodiments

[0043] Fig. 1 schematically shows a transformer 10 according to a preferred embodiment of the invention. As shown in figure 1 , the transformer 10 is a three-phase transformer with a U-phase 12, a V-phase 14, and a W-phase 16, which uses three groups of conductors to generate three-phase alternating current. Each of the three groups comprises a core 18, a low voltage (LV) winding (not shown in figure 1 , but visible in figure 2), and a high voltage (HV) winding 20.

[0044] For the LV winding and HV winding 20, a varying current in one winding produces a varying magnetic field, which in turn induces a voltage in another winding. Accordingly, power can be transferred between the windings through the magnetic field without a metallic connection between the windings. The transformer 10 further comprises a yoke 22 and a plurality of terminals 24 that are arranged at the yoke 22. In this preferred embodiment of the invention, the windings 20 and the terminals 24 of the transformer 10 comprise a graphene-reinforced aluminium matrix composite.

[0045] In another example a simulation using the finite element method (FEM simulation) of a current-carrying transformer component according to a second preferred embodiment of the invention was performed.

[0046] Figure 2 schematically shows a transformer 10 used as model in the FEM simulation. The transformer 10 is a 1.5 MVA Bland transformer with a primary voltage of 13.8 kV with applied voltage test of 35 kV and a lightning impulse test of 95 kV. The secondary voltage is 0.48 kV with applied voltage test of 4 kV and a lightning impulse test of 10 kV. For the simulation the transformer 10 comprises high voltage windings 20 consisting of a graphene-reinforced aluminium matrix composite. In Figure 2 it is further visible that the transformer 10 comprises lead-outs 26 also called busbars 26 as connection of the windings 20 to the terminals 24 via a y- connection 28. It is possible that the lead-outs / busbars 26, as well as the terminals 24 and the y-connection 28 also consist of a graphene-reinforced aluminium matrix composite.

[0047] The applied excitation currents for the FEM simulation are summarized in Table 1 . The high voltage windings 20 were simulated as monolithic discs 30, the contact between supports and discs were modeled as “Frictional” connection for simulating the most critical condition - i.e. no adhesion between the discs 30 and their supports. The Lorentz’s forces, inducing on the U-phase 12 and being generated on the windings 20, were analysed by Maxwell 2022r2 software embedded into Ansys.

[0048] For comparison the simulation results were compared to simulations results of a transformer 10’ with the same design having high voltage windings 20’ consisting of pure aluminium.

[0049] Figures 3 and 4 show the simulation results for the transformer 10 having windings 20 consisting of different types of graphene-reinforced aluminium matrix composite in a von-Mises stress contour plot, while figure 5 show the simulation results for a transformer 10’ having windings 20’ consisting of pure aluminium as comparative example in a von-Mises stress contour plot.

[0050] Referring to figures 3 and 4, the content of the graphene in the graphene-reinforced aluminium matrix composite is 0.25 wt% for figure 3a), 0.5 wt% for figure 3b), and 0.5 wt% for figure 4. Table 2 summarizes the mechanical and electrical properties of the different materials.

[0051] As can be seen in figure 3a) with an addition of 0.25 wt% graphene, the Von-Mises (VM) stress is below the offset yield Rp0.2 of the graphene-reinforced aluminium matrix composite in all the discs 30. The offset yield Rp0.2 is the tensile stress in a uniaxial tensile test, at which the plastic elongation corresponds to a percentage of 0.2 % of the extensometer gauge length.

[0052] When the content of graphene is increased to 0.5 wt% (Figure 3b) and 1 wt% (Figure 4), in some localized regions 32 in the top discs 30 the calculated linear elastic VM stress is slightly above the Rp0.2 of the graphene-reinforced aluminium matrix composite.

[0053] The parameters of the simulation for figures 3a), 3b) and 4 are the following: B: HV

[0054] Equivalent Stress 2

[0055] Type: Equivalent (von-Mises) Stress

[0056] Unit: MPa

[0057] Time: 1 s Max: 43.564

[0058] Min: 0.00010855

[0059] Referring to figure 5 showing the comparative results, the simulations show that the calculated linear elastic VM stress is above the offset yield Rp0.2 of the pure aluminium in localized regions 32’ in the top and central discs 30’ of the windings 20’.

[0060] The parameters of the simulation for figure 5 are the following:

[0061] B: HV

[0062] Equivalent Stress 2

[0063] Type: Equivalent (von-Mises) Stress

[0064] Unit: MPa

[0065] Time: 1 s

[0066] Max: 43.564

[0067] Min: 0.00010855

[0068] In summary the simulation results show that a small amount of graphene in the graphene-reinforced aluminium matrix composite leads to improved electrical and mechanical properties, resulting in an enhancement of the short-circuit ability of the transformer windings 20.

[0069] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosed, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope. Reference signs list

[0070] 10 transformer

[0071] 12 U-phase

[0072] 14 V-phase

[0073] 16 W-phase

[0074] 18 core

[0075] 20 HV winding

[0076] 22 yoke

[0077] 24 terminal

[0078] 26 lead-out, busbar

[0079] 28 y-connection

[0080] 30 disc

[0081] 32 localized region

[0082] 20’ HV winding (comparative example)

[0083] 30’ disc (comparative example)

[0084] 32’ localized region (comparative example)

Claims

Claims1 . Component (20, 24, 26, 28) of a transformer (10), wherein the component (20, 24, 26, 28) is a current-carrying component (20, 24, 26, 28) and comprises a graphene-reinforced aluminium matrix composite.

2. Component (20, 24, 26, 28) according to claim 1 , wherein the component (20, 24, 26, 28) consists of the graphene-reinforced aluminium matrix composite.

3. Component (20, 24, 26, 28) according to any of the previous claims, wherein the graphene-reinforced aluminium matrix composite consists essentially of aluminium and graphene.

4. Component (20, 24, 26, 28) according to any of the previous claims, wherein a graphene amount in the graphene-reinforced aluminium matrix composite is at least 0.1 wt%, preferably at least 0.15 wt%, and more preferably at least 0.2 wt% based on the total weight of the graphene-reinforced aluminium matrix composite.

5. Component (20, 24, 26, 28) according to any of the previous claims, wherein a graphene amount in the graphene-reinforced aluminium matrix composite is not more than 1 wt%, preferably not more than 0.5 wt%, and more preferably not more than 0.3 wt% based on the total weight of the graphene-reinforced aluminium matrix composite.

6. Component (20, 24, 26, 28) according to any of the previous claims, wherein a graphene amount in the graphene-reinforced aluminium matrix composite is 0.25 wt% ± 0.05 wt%, preferably 0.25 wt% ± 0.03 wt%, and even more preferably 0.25 wt% ± 0.01 wt% based on the total weight of the graphene-reinforced aluminium matrix composite.

7. Component (20, 24, 26, 28) according to any of the previous claims, wherein a number of graphene layers per graphene particle in the graphene-reinforcedaluminium matrix composite is equal to or less than ten.

8. Component (20, 24, 26, 28) according to any of the previous claims, wherein an average out of plane thickness of a graphene particle in the graphene-reinforced aluminium matrix composite is 0.5 nm to 5 nm.

9. Component (20, 24, 26, 28) according to any of the previous claims, wherein an average in-plane extension of a graphene particle in the graphene-reinforced aluminium matrix composite is 1 pm to 10 pm.

10. Component (20, 24, 26, 28) according to any of the previous claims, wherein the component (20, 24, 26, 28) and / or the graphene-reinforced aluminium matrix composite is produced by a liquid state process, by a solid-state process, and / or by a composite state process.11 . Component (20, 24, 26, 28) according to the previous claim, wherein the liquid state process comprises stir casting, squeeze casting, and / or 3D printing, the solid-state process comprises powder metallurgy, friction stir welding, and / or selective laser melting, and / or the composite state process comprises rolling and / or accumulative roll bonding.

12. Component (20, 24, 26, 28) according to any of the previous claims, wherein the component (20, 24, 26, 28) is a transformer winding (20), a terminal (24), a lead-out (26), a busbar (26), a y-connection (28), or a conductor of a bushing.

13. Component (20, 24, 26, 28) according to the previous claim, wherein the winding (20) is configured as helical winding, double layered winding, cross-over winding, sandwich winding, or disc-type winding (30), and preferably as disctype winding (30).

14. Transformer (10) comprising a component (20, 24, 26, 28) according to any of the previous claims.

15. Transformer (10) according to the previous claim, wherein the transformer (10)is configured as a 1 .5 MVA transformer and / or as an open wound transformer.

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