[0001] The present disclosure relates to a pressboard for use as an insulation barrier and/ or supporting structure in high-voltage (HV) devices, e.g. power transformers.

BACKGROUND

[0002] Modern power transformers normally comprise an oil-immersed core surrounded by winding conductors, which are covered by insulation material.

[0003] As electrical insulation, oil-impregnated pressboard is frequently used in power transformers. One of the main functions of the pressboard is to create barriers in the insulation system that prevents electrical flashovers between potential and ground or between different potentials. A limiting property of the pressboard is its electrical withstand emerging from poor matching between pressboard and oil with respect to dielectric constants; large differences result in that the oil is highly stressed under capacitive voltage. It is known that the ratio between dielectric constants of oil and pressboard can be decreased if the density of the pressboard, regularly being about 1.0-1.3 g/cms, is decreased. One explanation is that a lower density allows the oil to distribute more evenly in the porous structure of the pressboard. However, to merely reduce the density is not an option since a decreased density will also reduce the electrical withstand as well as the mechanical strength of the pressboard. Furthermore, a reduced density will increase the mechanical compressibility that also has negative influence on the design of power transformers. In other words, there is a trade-off between mechanical and electrical properties, and one problem is thus to decrease the electrical stress of the oil without compromising electrical withstand or mechanical strength.

SUMMARY

[0004] An objective of the present disclosure is to provide an improved pressboard that may be used as insulation barriers and supporting structures in power transformers.

[0005] According to a first aspect of the present disclosure there is provided a pressboard for high-voltage devices, the pressboard having a density of 0.6-1.3 g/cms according to IEC60641-2, and comprising 1-15% microfibrillated cellulose (MFC) based on the total dry weight of the pressboard.

[0006] By inclusion of MFC in the pressboard and at the same time maintaining or decreasing the density of the pressboard compared with a regular pressboard free of MFC, a pressboard with a low density which has satisfactory mechanical properties is provided that balances electrical withstand with mechanical strength. The electrical withstand includes various voltage shapes, such as direct current (DC), alternating current (AC), lightning impulse (LI) and switching impulse (SI).

[0007] According to a second aspect of the present disclosure there is provided an insulation element comprising a pressboard having a density of 0.6-1.3 g/cms according to IEC60641-2, wherein the pressboard comprises 1-15 % MFC based on the total dry weight of the pressboard, and wherein the pressboard is impregnated with an electrically insulating compound, e.g. a liquid or a resin, such as a cured resin.

[0008] According to an aspect of the present disclosure there is provided an insulation element (101) comprising a pressboard (204) for high-voltage devices, the pressboard (204) having a density of 0.6-1.3 g/cms, comprising 1-15 % microfibrillated cellulose, MFC, based on the total dry weight of the pressboard having a thickness of 0.8-10 mm, such as 1.0-9. o mm, such as 4.0-9.0 mm; and wherein the MFC is homogenously distributed in the pressboard wherein the pressboard is impregnated with an electrically insulating compound (104), e.g. a liquid or a resin, such as a cured resin, wherein a ratio of a permittivity of the pressboard (204) in mineral oil (cboard, mineral) and a permittivity of the mineral oil (cu _q, mineral) is within a range of: aboard, mineral/ cuq, mineral = 1-1-9, preferably 1-1.6, more preferably 1-1.4 and/or a ratio of a permittivity of the pressboard (204) in ester oil (aboard, ester) and a permittivity of the ester oil (cii _q , ester) is within a range of: aboard, ester/ siiq, ester =1-1-35, preferably 1-1.2, more preferably 1-1.1.

[0009] Addition of MFC in the pressboard while keeping the density low in comparison with a regular pressboard free of MFC provides for reduced electrical constants of the pressboard, which reduces the difference in electrical constants between the pressboard and the insulating liquid or resin that the pressboard is impregnated with. As a consequence, the electrical stress in the liquid or resin volumes next to the solid insulation element is reduced and impregnated pressboards with tailored dielectric matching are provided.

[0010] As a third aspect of the present disclosure, there is provided a use of an insulation element in a high-voltage device, wherein the insulation element comprises a pressboard having a density of o.6-1.3 g/cms according to IEC60641-2, and wherein the pressboard comprises 1-15 % MFC based on the total dry weight of the pressboard, and wherein the pressboard is impregnated with an electrically insulating compound, e.g. a liquid or a resin, such as a cured resin.

[0011] As a fourth aspect of the present disclosure there is provided a high- voltage device comprising an insulation element, wherein the insulation element comprises a pressboard having a density of o.6-1.3 g/cms according to IEC60641-2, and wherein the pressboard comprises 1-15 % MFC based on the total dry weight of the pressboard, and wherein the pressboard is impregnated with an electrically insulating compound, e.g. a liquid or a resin, such as a cured resin. The HV device may be a power transformer, e.g. a liquid-filled power transformer. The HV device may be a liquid-filled HV device, e.g. a liquid-filled power transformer.

[0012] What is described above with respect to the second aspect applies to the third and fourth aspects mutatis mutandis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 illustrates an exemplary embodiment of a HV device (100) according to the present disclosure comprising insulation elements (101) comprising MFC, a HV element (102), a housing (103), and an insulating compound (104). [0014] Figure 2 illustrates an exemplary embodiment of an insulation element (101) according to the present disclosure comprising a pressboard (204) constituting of a first outer layer (201), and inner layer (202) and a second outer layer (203).

DETAILED DESCRIPTION

[0015] The pressboard (204) for high-voltage devices according to the present disclosure is having a density of o.6-1.3 g/cms according to IEC60641-2, preferably o.8-1.2 g/cms, such as 0.8-1.1 g/cms, such as 0.8-1.0 g/cms. The pressboard (204) comprises 1-15% MFC, preferably 1-10% MFC, such as 1-7% MFC, such as 1-5% MFC, such as 2-5% MFC, such as 3-5% MFC based on the total dry weight of the pressboard (204).

[0016] Generally, when MFC is added to cellulose-based paper or board, the MFC fills up the voids causing the density to increase. It has now surprisingly been found that providing a low density of the pressboard upon addition of MFC, high mechanical strength as well as electrical withstand are both achieved.

[0017] In preferred embodiments of the present disclosure the MFC is homogenously distributed throughout the pressboard (204). Homogeneity is in the context of the present disclosure referring to that the MFC is well-distributed and not concentrated to certain regions of the pressboard. In one embodiment, the pressboard consists of only one pressboard layer, and in an alternative embodiment the pressboard comprises multiple pressboard layers (201, 202, 203), and in such an embodiment, preferably all layers comprise MFC, preferably homogenously distributed.

[0018] The thickness of the pressboard (204) according to IEC60641-2 may be 0.8-10 mm, such as 1.0-9. o mm, such as 4.0-9.0 mm. Pressboard is many times thicker than general purpose insulation paper and presspaper. General insulation paper that is used e.g. as a winded insulation has according to IEC60554-1 a thickness of 15-250 pm, while presspaper has a thickness of 0.075-0.80 mm according to IEC60641-1. The production method of pressboard is also different from the production method of insulating paper. Insulating paper is produced according to standard paper-making methods on a paper machine, whereas pressboard is produced through a process of building up the specified thickness by varying pressing and feeding of furnish in specified cycles. [0019] In the context of the present disclosure, MFC means nano-scale cellulose particle fibres or fibrils with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibres. The liberated fibrils have a diameter of less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods. The smallest fibril is called elementary fibril and may have a diameter of approximately 2-4 nm, while it is common that the aggregated form of the elementary fibrils, also defined as microfibril is the main product that is obtained when making MFC e.g. by using an extended refining process or a pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils may vary from around 1 to more than 10 micrometers.

[0020] There are different synonyms for MFC which are, sometimes confusingly, used in the literature, such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose (NFC), fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibres, cellulose nanofibrils (CNF), cellulose microfibres (CMF), cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC may also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids content (1-5 wt.%) when dispersed in water. The cellulose fibre is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 200 m ² /g, or more preferably 50-200 m ² /g when determined for a freeze-dried material with the BET method (Brunauer, Stephen, Paul Hugh Emmett, and Edward Teller. "Adsorption of gases in multimolecular layers." Journal of the American chemical society 60.2 (1938): 309-319.). Nitrogen (N2) gas adsorption isotherms are recorded using an ASAP 2020 (Micromeritics, USA) instrument. Measurements are performed at liquid nitrogen temperatures (i.e., 77 K), and the specific surface areas of the samples were obtained from the isotherms using the BET method.

[0021] Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment step(s) may be conducted in order to make MFC manufacturing energy efficient including enzymatic or chemical pre-treatment.

[0022] The nanofibrillar cellulose may contain some hemicelluloses; the amount may be dependent on factors such as plant source and pulping process. Mechanical fibres may be carried out with suitable equipment such as a refiner, grinder, homogenizer, collider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines or e.g. other chemicals present in wood fibres or in papermaking process. The product might also contain various amounts of micron size fibre particles that have not been efficiently fibrillated. MFC may be produced from wood cellulose fibres, both from hardwood or softwood fibres. It may alternatively be made from agricultural fibres such as wheat straw pulp, bamboo, bagasse, or other non-wood fibre sources. It is preferably made from pulp of virgin fibre, e.g. mechanical, chemical and/or thermomechanical pulps, preferably never-dried fibres.

[0023] The above described definition of MFC includes, but is not limited to, the proposed TAPPI standard W13021 on cellulose nanofibril (CNF) defining a cellulose nanofibre material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

[0024] There is further provided an insulation element (101) according to the present disclosure comprising the pressboard (204), wherein the pressboard is impregnated with an electrically insulating compound (104), e.g. a liquid or a resin, such as a cured resin.

The inclusion of MFC provides for a low ratio between dielectric constants of electrically insulating liquid and pressboard while at the same time the pressboard has good mechanical properties.

In some embodiments, the insulating element comprises a pressboard which comprises 1-10% MFC, such as 1-7% MFC, such as 1-5% MFC, such as 2-5% MFC, such as 3-5% MFC based on the total dry weight of the pressboard.

In further embodiments the insulating element comprises a pressboard which has the density o.8-1.2 g/cnD, such as 0.8-1.1 g/cnD, such as o.8-1.0 g/cnD.

In some embodiments the insulating element comprises a pressboard which comprises a plurality of pressboard layers 201, 202, 203.

In some embodiments of the insulating element all layers comprise MFC. [0025] In some embodiments, the electrically insulating compound is or comprises an oil, such as mineral oil, or an ester-based liquid. The mineral oil is derived from crude oil or natural gas, while the ester-based liquid is an ester fluid, preferably an ester fluid derived from plants or crops, such as from rapeseed, canola or soybean.

[0026] In further embodiments, the electrically insulating compound (104) is or comprises a cured resin, such as an epoxy-based resin, a polyester-based resins or a phenolic resin, or a combination thereof.

[0027] The insulation element (204) may have a direct current (DC) and/or alternating current (AC) and/or lightning impulse (LI) and/or switching impulse (SI) electrical breakdown Weibull a-value that is 5-50 %, preferably 15-50 %, even more preferably 30-50 % higher compared with a corresponding insulation element being free of MFC. The insulation element is having an increased breakdown Weibull a- value for at least some of the voltage shapes DC, AC, LI and SI due to the density of the insulation element. That the corresponding insulation element is free of MFC is in the context of the present disclosure referring to an insulation element without MFC but otherwise identical. The Weibull a-value is a measure of breakdown strength.

[0028] The DC and/ or AC and/ or LI and/ or SI electrical breakdown Weibull b- value of the pressboard (204) maybe above 7, preferably above 9, more preferably above 12. The Weibull b-value is a measure of the statistical spread of the breakdown strength around the alpha value. A higher beta value implies a low statistical spread. In some embodiments, the permittivity determined according to IEC62631-2-1 of the pressboard in mineral oil (cboard, mineral) and permittivity of the mineral oil (cu _q, mineral) have a ratio of: Cboard, mmerai/cuq, mineral = 1-1-9, preferably 1-1.6, more preferably 1-1.4.

[0029] Alternatively, the permittivity of the pressboard (204) in ester oil (c _board, ester) and permittivity of the ester oil (cu _q, ester) may have a ratio of:

Cboard, ester/ciiq, ester =1-1.35, preferably 1-1.2, more preferably 1-1.1.

[0030] The permittivity-ratio between pressboard and insulating compound may beneficially be close to 1. A permittivity-ratio close to 1, or equal to 1, means that the insulating compound is under low stress under capacitive voltage. The permittivity- ratio is dependent on what type of insulating compound that is used. [0031] There is further provided the use of an insulation element (101) in accordance with an embodiment of the present disclosure in a high-voltage device (100), wherein the pressboard (204) is impregnated with an electrically insulating compound (104), e.g. a liquid or a resin, such as a cured resin.

[0032] There is further provided a high-voltage device (100) comprising an insulation element (204) in accordance with an embodiment of the present disclosure, wherein the pressboard (204) is impregnated with an electrically insulating compound (104), e.g. a liquid or a resin, such as a cured resin. The insulating compound is the same as the one that is impregnating the board.

[0033] Embodiments of the present invention may be described in any one of the following points.

1. A pressboard (204) for high-voltage devices, the pressboard (204) having a density of 0.6-1.3 g/cms, and comprising 1-15 % microfibrillated cellulose, MFC, based on the total dry weight of the pressboard.

2. The pressboard (204) according to point 1, comprising 1-10% MFC, such as 1-7% MFC, such as 1-5% MFC, such as 2-5% MFC, such as 3-5% MFC based on the total dry weight of the pressboard.

3. The pressboard (204) according to any one of the preceding points, wherein the density is 0.8-1.2 g/cms, such as o.8-1.1 g/cms, such as 0.8-1.0 g/cms.

4. The pressboard (204) according to any one of the preceding points, having a thickness of 0.8-10 mm, such as 1.0-9. o mm, such as 4.0-9.0 mm.

5. The pressboard (204) according to any one of the preceding points, wherein the pressboard comprises a plurality of pressboard layers (201, 202, 203).

6. The pressboard (204) according to point 5, wherein all layers comprise MFC.

7. The pressboard (204) according to any one of the preceding points, wherein the MFC is homogenously distributed in the pressboard.

8. An insulation element (101) comprising a pressboard (204) according to any one of the points 1-7, wherein the pressboard is impregnated with an electrically insulating compound (104), e.g. a liquid or a resin, such as a cured resin.

9. The insulation element (101) according to point 8, wherein the electrically insulating compound (104) is or comprises an oil, such as mineral oil, or an ester-based liquid.

10. The insulation element (101) according to point 8, wherein the electrically insulating compound (104) is or comprises a cured resin, such as an epoxy-based resin, a polyester-based resin or a phenolic resin, or a combination thereof.

11. The insulation element (101) according to any one of points 8-10, wherein the direct current, DC, and/ or alternating current, AC, and/ or lightning impulse, LI, and/ or switching impulse, SI, electrical breakdown Weibull a-value of the pressboard (204) is 5-50%, preferably 15-50%, even more preferably 30-50% higher compared with a corresponding insulation element being free of MFC.

12. The insulation element (101) according to any one of points 8-11, wherein the direct current, DC, and/or alternating current, AC, and/or lightning impulse, LI, and/ or switching impulse, SI, electrical breakdown Weibull b-value of the pressboard (204) is above 7, preferably above 9, more preferably above 12.

13. The insulation element (101) according to any one of the points 8-12, wherein a ratio of a permittivity of the pressboard (204) in mineral oil (cboard, mineral) and a permittivity of the mineral oil (eu _q, mineral) is within a range of: aboard, mineral/ cuq, mineral = 1-1-9, preferably 1-1.6, more preferably 1-1.4 and/or a ratio of a permittivity of the pressboard (204) in ester oil (cboard, ester) and a permittivity of the ester oil (cu _q, ester) is within a range of:

Cboard, ester/ciiq, ester =1-1-35, preferably 1-1.2, more preferably 1-1.1.

14. Use of an insulation element according to any one of the points 8-13 in a high- voltage device (100). 15. A high-voltage device (100) comprising the insulation element (101) of any points 8-13, wherein the high-voltage device is a power transformer, e.g. a liquid- filled power transformer.