Electrical insulation paper

Abstract

The present disclosure relates to an electrical insulation paper comprising: cellulose fibers in an amount of at least 25 wt%, based on the total weight of the electrical insulation paper; synthetic fibers in an amount of 8 wt% based on the total weight of the electrical insulation paper; and a heat stabilizer comprising nitrogen, wherein the content of the nitrogen is between 1 wt% and 4 wt% of the content of the cellulosic fibers by weight.

Description

[0001] Related Applications

[0002] This application is an international application claiming priority to European patent application No. 22306242.3 filed on August 19, 2022, which is incorporated herein in its entirety. Technical Field

[0003] The present disclosure relates to an electrical insulating paper, and further relates to a method for producing the same, as well as cables, transformers, capacitors and / or other electrical equipment items equipped with such electrical insulating paper. Background Art

[0004] The present invention relates to an electrical insulating paper and further relates to a method for its manufacture and to cables, transformers, capacitors and / or other electrical equipment items equipped with such electrical insulating paper.

[0005] Electrical insulating papers are used for electrical insulation in various devices such as transformers, cables and capacitors and in particular liquid filled transformers, cables and capacitors.

[0006] and There is particular interest in materials that can be produced at low cost and have good mechanical and electrical properties compared to base papers.

[0007] Electrical insulating papers containing cellulose are already known and play an important role in the field of electrical insulation. Cellulose-based insulating papers combine good electrical insulation with good mechanical properties and can be produced at low cost. However, for example in liquid-immersed transformers, the insulating papers are exposed to various thermal, chemical and / or oxidative stresses, which can cause the cellulose to age rapidly. Aging is manifested in the form of a loss in tensile strength and can easily lead to transformer failures.

[0008] It is desirable to be able to provide smaller transformers and other electrical equipment without compromising electrical insulation, and the operating temperature and / or operating time limitations of known devices are not always satisfactory. It is also desirable to provide a transformer that is the same size as an existing transformer but can operate at a higher temperature.

[0009] The present disclosure aims to address at least one of the disadvantages of the prior art. Summary of the invention

[0010] Various aspects of the above objects are achieved by an electrical insulating paper according to the present disclosure.

[0011] One aspect of the present disclosure relates to an electrical insulating paper. The electrical insulating paper comprises: cellulose fibers in an amount of at least 25% by weight based on the total weight of the electrical insulating paper; synthetic fibers in an amount of at least 5% by weight based on the total weight of the electrical insulating paper; and a heat stabilizer comprising nitrogen, wherein the nitrogen content by weight accounts for 1% to 4% by weight of the cellulose fibers. The synthetic fibers comprise aliphatic polyamide fibers and / or glass fibers.

[0012] Another aspect of the present disclosure relates to a method for making electrical insulating paper. The method comprises the steps of providing cellulose fibers and synthetic fibers, and making base paper from the cellulose fibers and the synthetic fibers on a paper machine, with at least 25% by weight of the cellulose fibers based on the total weight of the electrical insulating paper, and at least 5% by weight of the synthetic fibers based on the total weight of the electrical insulating paper. Another aspect of the present disclosure is an electrical insulating paper comprising at least one layer made by the method described herein.

[0013] Another aspect of the present disclosure is the use of the electrical insulation disclosed herein for insulating the conductors of a high voltage liquid-immersed transformer or a dry-type transformer, or for insulating the conductors used in a traction transformer or for low voltage foil windings in a distribution transformer. In other words, another aspect is a transformer comprising a conductor insulated with the electrical insulation paper described herein or a transformer comprising a low voltage foil winding comprising the electrical insulation paper described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A graph showing the electrical conductivity of an example of electrical insulating paper.

[0015] Figure 2 A graph of initial tensile index for examples of electrical insulating papers.

[0016] Figure 3 A graph showing the retention of tensile index after aging for an example of electrical insulation paper.

[0017] Figure 4 A graph showing the retention of tensile index after aging for an example of electrical insulation paper.

[0018] Figure 5 A graph showing the electrical conductivity of an example of electrical insulating paper.

[0019] Figure 6 A graph of initial tensile index for examples of electrical insulating papers.

[0020] Figure 7 A graph showing the retention of tensile index after aging for an example of electrical insulation paper. DETAILED DESCRIPTION

[0021] Electrical insulation paper

[0022] The electrical insulating paper may contain at least 25 wt.-%, at least 39 wt.-%, at least 41 wt.-%, at least 45 wt.-%, at least 48 wt.-%, at least 50 wt.-%, at least 65 wt.-%, or at least 71 wt.-% cellulose fibers, based on the total weight of the electrical insulating paper. The electrical insulating paper may contain, for example, at most 94 wt.-%, at most 92 wt.-%, at most 91 wt.-%, at most 86 wt.-%, or at most 66 wt.-% cellulose fibers, based on the total weight of the electrical insulating paper.

[0023] According to some embodiments, the cellulose fibers comprise one or more of: kraft fibers, cotton fibers, flax fibers, hemp fibers, and wherein the cellulose fibers are unbleached, bleached and / or semi-bleached hardwood fibers and / or softwood fibers.

[0024] The electrical insulating paper may contain at least 5%, 5 to 55%, 7 to 35% or 8 to 25% of synthetic fibers, based on the total weight of the electrical insulating paper, the synthetic fibers comprising polyamide fibers, glass fibers or combinations thereof. The polyamide fibers may be, for example, aliphatic polyamide fibers, or a blend of polyamide fibers such as aliphatic polyamide fibers. The synthetic fibers may contribute to a higher tensile strength retention of the electrical insulating paper. More generally, the synthetic fibers provide good strength parameters for the electrical insulating paper.

[0025] The electrical insulation may comprise a heat stabilizer comprising nitrogen, wherein the nitrogen content by weight is 1 to 4 wt %, 1.2 to 2.3 wt % or 1.2 to 1.6 wt % of the content of the cellulose fibers. The heat stabilizer may promote good anti-aging stability, that is, the heat stabilizer may extend the life of the insulating material. The heat stabilizer may be, for example, a heat stabilizer comprising nitrogen, the heat stabilizer comprising nitrogen being selected from dicyandiamide, urea, melamine, polyacrylamide or mixtures thereof.

[0026] The electrical insulating paper according to the present disclosure may have a relative thermal resistance index of, for example, 140°C or higher. In the context of the present disclosure, the thermal resistance class of an insulating material or an insulating system is considered to be defined by the IEC 60085 standard, i.e., "a designation of a numerical value equal to the recommended maximum continuous use temperature in degrees Celsius". According to IEC 60085, a thermal class is assigned to a material or system based on its relative thermal endurance (RTE) index. The insulating material may be solid (e.g., paper) or fluid (e.g., mineral oil). In a power transformer, a combination of various insulating materials forms an insulation system.

[0027] The RTE index of a material or system is the temperature at which an endpoint (e.g., 50% tensile retention of an insulating material) is reached after a given time required to reach the same endpoint of a reference material or system with known thermal durability (e.g., non-thermally upgraded (non-TU) paper and mineral oil). The thermal durability of non-TU paper in mineral oil is 105°C.

[0028] Due to the high thermal rating of electrical insulating papers according to the present disclosure, they may be particularly suitable for liquid immersed transformers, where the liquid may be a mineral oil or an ester.

[0029] The RTE of a system can be determined following IEC 60332-2, which is based on accelerated aging tests in sealed tubes at different temperatures for different durations. For example, the system undergoes 1 or 3 different aging tests compared to the reference and should have the same or higher tensile retention than the reference, but at a higher temperature (+10°C to 60°C), depending on the expected increase in thermal grade. Standard IEEE C57.100 provides a clear description of the experimental part for conducting such accelerated tests.

[0030] The electrical insulating paper according to the present disclosure may have a higher RTE (+10° C. to 60° C.) than a comparable paper according to the prior art, but with a relatively higher tensile retention.

[0031] Electrical insulating paper can have good mechanical strength. This facilitates processing, such as winding wires and conductors.

[0032] Electrical insulation papers can also provide mechanical properties that enable them to be wrapped around conductors in a technologically practical manner. As a result, electrical insulation papers can provide smaller transformers and other electrical equipment without compromising electrical insulation and operating temperature and / or operating time limitations. Electrical insulation papers can also provide transformers that are the same size as existing transformers but are capable of operating at higher temperatures.

[0033] In other words, the electrical insulating paper according to the present disclosure can withstand high potential gradients while providing benefits over the alternative of using very thin paper according to the prior art, since the breakdown strength (i.e. the dielectric strength) is increased by reducing its thickness while other properties remain constant. In this regard, when the insulating paper is very thin, the mechanical properties related to the strength of the insulating paper may be compromised, which in turn impairs the industrial viability of the winding process, so that this does not represent a practical solution in itself. The electrical insulating paper according to the present disclosure may provide a solution.

[0034] The electrical insulating paper may contain at least 50 wt% of cellulose fibers, based on the total weight of the electrical insulating paper; and 7 to 35 wt% of synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or a blend of polyamide fibers such as aliphatic polyamide fibers and glass fibers.

[0035] The electrical insulating paper may contain at least 65 wt% of cellulose fibers, based on the total weight of the electrical insulating paper; and 7 to 27 wt% of synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or a blend of polyamide fibers such as aliphatic polyamide fibers and glass fibers.

[0036] The electrical insulating paper may contain at least 65 wt% of cellulose fibers, based on the total weight of the electrical insulating paper; and 8 wt% to 25 wt% of synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or a blend of polyamide fibers such as aliphatic polyamide fibers and glass fibers.

[0037] The electrical insulating paper may contain at least 45 wt% of cellulose fibers, based on the total weight of the electrical insulating paper; and 5 to 55 wt% of synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are glass fibers, or a blend of glass fibers and polyamide fibers such as aliphatic polyamide fibers.

[0038] The electrical insulating paper may contain at least 50 wt% of cellulose fibers, based on the total weight of the electrical insulating paper; and 7 to 42 wt% of synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are glass fibers, or a blend of glass fibers and polyamide fibers such as aliphatic polyamide fibers.

[0039] The electrical insulating paper may contain at least 50 wt% of cellulose fibers, based on the total weight of the electrical insulating paper; and 8 to 25 wt% of synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are glass fibers, or a blend of glass fibers and polyamide fibers such as aliphatic polyamide fibers.

[0040] When the synthetic fibers are glass fibers, the length of these glass fibers may be 3 mm to 32 mm, preferably 3 mm to 20 mm, more preferably 3 mm to 13 mm. In addition, the diameter of these glass fibers may be 3 μm to 30 μm, preferably 6 μm to 20 μm, more preferably 8 μm to 15 μm.

[0041] When the synthetic fibers are polyamide fibers, the length of these polyamide fibers may be 2 mm to 12 mm, preferably 3 mm to 8 mm. The linear density of the polyamide fibers may be 0.4 dtex (dtex) to 7.0 dtex, and preferably 1.2 dtex to 2.0 dtex.

[0042] The electrical insulating paper may further include a binder, wherein the binder is present in an amount of 5 to 20 wt % based on the total weight of the electrical insulating paper. The binder may be selected from hot-melt fibers, resins or mixtures thereof.

[0043] The binder may be a resin. The resin may improve the mechanical strength parameters of the electrical insulating paper.

[0044] In this context, the resin may specifically be a liquid having a viscosity below 100 centipoise (cP) at 50° C., optionally in the range of 10 cP-75 cP at 50° C. The resin may be pure or diluted to achieve this viscosity to enable it to be impregnated or coated on a paper substrate.

[0045] The resin may contain a heat stabilizer comprising nitrogen.

[0046] According to some embodiments, the adhesive comprises polyvinyl alcohol. The polyvinyl alcohol can be a homopolymer. As another example, the polyvinyl alcohol can be a modified polyvinyl alcohol, such as a copolymer of vinyl alcohol and another ethylenically unsaturated monomer (such as ethylene). As a specific example, the modified polyvinyl alcohol can include poly (vinyl alcohol-co-ethylene). In one example, the adhesive comprises a mixture of polyvinyl alcohol and modified polyvinyl alcohol (such as poly (vinyl alcohol-co-ethylene)). The polyvinyl alcohol and / or modified polyvinyl alcohol adhesive can have a degree of hydrolysis of at least 88mol%. Polyvinyl alcohol (e.g., PVA or modified polyvinyl alcohol) can be a low ash content polymer.

[0047] When the binder is a mixture of polyvinyl alcohol and modified polyvinyl alcohol, the polyvinyl alcohol and the modified polyvinyl alcohol may be blended in a weight ratio of 1:5 to 5:1, 1:3 to 3:1, or 1:1.

[0048] Adhesive can comprise hot-melt fiber.For example, electrical insulating paper can comprise the hot-melt fiber of 5 wt % to 18 wt %, 9 wt % to 17 wt % or 10 wt % to 16 wt % content based on the gross weight of electrical insulating paper.Hot-melt fiber can provide higher tensile strength for paper.The indication range that hot-melt fiber exists is more and more narrow and can promote high tensile strength to a great extent, and can not therefore damage other desired characteristics.The length of hot-melt fiber can be 2mm to 12mm or 3mm to 8mm.The linear density of hot-melt fiber can be 0.4dtex (dtex)-7.0dtex or 1.2dtex to 2.0dtex.

[0049] The electrical insulating paper may comprise cellulose fiber, glass fiber, a nitrogen-containing heat stabilizer (comprising weight % of all the aforementioned components within the above ranges), a binder, and the remainder not exceeding 1%, optionally 0.5% or 0.1%, based on the total weight of the electrical insulating paper.

[0050] Manufacturing method.

[0051] Also disclosed herein is a method for manufacturing electrical insulating paper, the method comprising: providing cellulose fibers and synthetic fibers; manufacturing base paper on a paper machine from the cellulose fibers and the synthetic fibers, with at least 25% by weight of the cellulose fibers based on the total weight of the electrical insulating paper, and at least 5% by weight of the synthetic fibers based on the total weight of the electrical insulating paper; and adding a heat stabilizer containing nitrogen, wherein the content of the nitrogen accounts for 1% to 4% by weight of the content of the cellulose fibers. The method may further include adding a heat stabilizer containing nitrogen, wherein the content of the nitrogen accounts for 1% to 4% by weight of the content of the cellulose fibers.

[0052] The descriptions and amounts of the cellulose fibers, synthetic fibers, heat stabilizer, and optional binder are as discussed above with respect to the electrical insulation paper.

[0053] The method may include the step of adding a binder in an amount of 5 to 20 weight % based on the total weight of the electrical insulating paper. The binder may be a hot melt fiber, a resin or a mixture thereof.

[0054] When the binder is a resin, the resin may be applied (e.g., coated or impregnated) on the base paper, for example, after adding the heat stabilizer containing nitrogen. As another example, the resin may be applied on the base paper immediately after the step of manufacturing the base paper and before adding the heat stabilizer containing nitrogen. As another example, the resin may include a heat stabilizer containing nitrogen.

[0055] When the binder is a resin, the resin can be applied to the base paper either on-line or off-line on the paper machine.

[0056] The adhesive, in particular a resin adhesive, may be applied by a size press (eg, corresponding to a dipping step) or by another coating method such as rod coating, road coating, roller coating, etc.

[0057] When the binder includes hot-melt fibers, the hot-melt fibers may be mixed with the cellulose fibers and the synthetic fibers before the step of manufacturing the base paper.

[0058] According to some embodiments, the binder is a resin, the resin further comprising the heat stabilizer comprising nitrogen, and the resin is coated on the base paper immediately after the step of manufacturing the base paper.

[0059] The base paper can be made of 8% to 18% by weight of the hot-melt fiber. The base paper can be made of 9% to 17% by weight of the hot-melt fiber. The base paper can be made of 10% to 16% by weight of the hot-melt fiber.

[0060] The length of the hot-melt fiber can be 2 mm to 12 mm. The length of the hot-melt fiber can be 3 mm to 8 mm.

[0061] The linear density of the hot-melt fiber can be 0.4 dtex (dtex)-7.0 dtex.

[0062] The linear density of the hot-melt fiber can be 1.2 dtex (dtex)-2.0 dtex.

[0063] According to an example, the electrical insulating paper is manufactured with a liquid binder. The liquid binder may contain a heat stabilizer containing nitrogen.

[0064] The binder may comprise a polyvinyl alcohol as described herein, for example polyvinyl alcohol, a modified polyvinyl alcohol such as poly(vinyl alcohol-co-ethylene) or a blend or mixture thereof. The polyvinyl alcohol binder may have a degree of hydrolysis of at least 88 mol%.

[0065] The method may include, after the step of adding the nitrogen-containing heat stabilizer or coating the base paper with the resin, a step of heat calendering the base paper at a temperature in the range of 120° C. to 160° C. and at a pressure in the range of 800 degenerate Newtons (daN) to 1200 daN.

[0066] The manufacturing may comprise (according to some embodiments: consist of) manufacturing base paper on a paper machine from the cellulose fibers and the synthetic fibers, with the cellulose fibers having a content of at least 50 wt. % based on the total weight of the electrical insulating paper, and the synthetic fibers having a content of 7 wt. % to 35 wt. % based on the total weight of the electrical insulating paper, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or a blend of polyamide fibers such as aliphatic polyamide fibers and glass fibers.

[0067] The content of cellulose fibers may be at least 65% by weight, based on the total weight of the electrical insulating paper.

[0068] The polyamide fiber may be included in an amount of 7 to 27% by weight, based on the total weight of the electrical insulating paper.

[0069] The polyamide fiber may be included in an amount of 8 to 25% by weight, based on the total weight of the electrical insulating paper.

[0070] The manufacture may comprise (according to some embodiments: consist of) manufacturing base paper on a paper machine from the cellulose fibers and the synthetic fibers, with the cellulose fibers having a content of at least 45 wt. % based on the total weight of the electrical insulating paper, and the synthetic fibers having a content of 5 wt. % to 55 wt. % based on the total weight of the electrical insulating paper, wherein the synthetic fibers are glass fibers, or a blend of glass fibers and polyamide fibers such as aliphatic polyamide fibers.

[0071] The content of cellulose fibers may be at least 65% by weight, based on the total weight of the electrical insulating paper.

[0072] The glass fiber may be included in an amount of 7 to 42% by weight based on the total weight of the electrical insulating paper.

[0073] The glass fiber may be included in an amount of 8 to 25% by weight based on the total weight of the electrical insulating paper.

[0074] Another aspect of the present disclosure relates to an insulating paper comprising at least one layer manufactured according to any one or more aspects of the above method.

[0075] The paper may be creped, creped and calendered and / or printed with epoxy blocks to form so-called diamond point papers. The paper may also be a ductile paper having improved stretchability to conform to the windings but obtained by a process other than the creping process. For example, the ductility of the so-called ductile paper is obtained on a paper machine in the presence of a unit consisting of a moving blanket carrying the paper, which passes through a nip when wetted, thereby causing contraction before the nip and then compacting (micro-creping) in the machine direction (MD) after the nip.

[0076] The present disclosure also relates to the use of the electrical insulating paper according to any one or more of the aspects discussed above for insulating conductors of a high voltage liquid immersed transformer or a dry transformer.

[0077] The present disclosure also relates to the use of an electrical insulating paper according to any one or more of the aspects discussed above for insulating a conductor used in a traction transformer.

[0078] The present disclosure also relates to the use of an electrical insulating paper according to any one or more of the aspects discussed above for a low voltage foil winding in a distribution transformer.

[0079] Another aspect of the present disclosure relates to a high voltage liquid immersed transformer comprising at least one conductor insulated by the electrical insulation paper according to any one or more aspects above.

[0080] Another aspect of the present disclosure relates to a dry-type transformer comprising at least one wire insulated by the electrical insulation paper according to any one or more aspects above.

[0081] Another aspect of the present disclosure relates to a traction transformer comprising at least one conductor insulated by the electrical insulation paper according to any one or more aspects above.

[0082] Another aspect of the present disclosure relates to a distribution transformer having at least one low voltage foil winding comprising an electrical insulation paper according to any one or more of the above aspects.

[0083] Another aspect of the present disclosure relates to insulating presspaper and transformer board insulation or molded fiber insulation components used in transformers.

[0084] Additional advantages and features of the present disclosure will become apparent from the following description of specific embodiments, which advantages and features may be achieved alone or in combination with one or more of the features discussed above, as long as such features are not mutually inconsistent.

[0085] In the following, the following abbreviations will be used:

[0086] PVAb refers to polyvinyl alcohol adhesive in the form of resin;

[0087] PVAmb refers to modified polyvinyl alcohol in the form of resin;

[0088] PVAf refers to polyvinyl alcohol in fiber form;

[0089] PA refers to polyamide;

[0090] UKP refers to unbleached kraft fiber

[0091] Example

[0092] In order to better understand the present disclosure and to show how to implement the present disclosure, reference will now be made, by way of example only, to embodiments according to the present disclosure and experimental data related to these embodiments.

[0093] Embodiments of electrical insulating papers according to the present disclosure were produced using a pilot paper machine.

[0094] The first group of electrical insulating papers is made of cellulose fibers and glass fibers. As cellulose fibers, unbleached kraft fibers (UKP) are used. However, the present disclosure is not limited thereto. For example, bleached kraft fibers or other cellulose fibers may also be used. The cellulose fibers may be, for example, cotton fibers, flax fibers, hemp fibers, bleached, unbleached or semi-bleached fibers, softwood or hardwood or any mixture of the mentioned fibers, etc. Specifically, base paper is made with different cellulose and glass fiber content ratios, i.e., a first series, with 50% by weight of cellulose fibers and 50% by weight of glass fibers; a second series, with 70% by weight of cellulose fibers and 30% by weight of glass fibers; and a third series, with 90% by weight of cellulose fibers and 10% by weight of glass fibers. The three series with the above-mentioned different content ratios are referred to as embodiment series 1, 2 and 3 below.

[0095] The second group of electrical insulating papers is made of cellulose fiber, glass fiber and hot melt fiber. In this case, hot melt fibers with a length in the range of 2mm to 12mm and a linear density in the range of 0.4dtex (dtex) to 7dtex are used. Specifically, hot melt fibers with a length in the range of 3mm to 8mm and a linear density in the range of 1.2dtex (dtex) to 2.0dtex are used. As cellulose fiber, UKP is used. However, the present disclosure is not limited to this. As discussed above, other cellulose fibers can be used. Specifically, base paper is made with different cellulose, glass and hot melt fiber content ratios. A series is made with 47.5% by weight of cellulose fiber, 47.5% by weight of glass fiber and 5% by weight of hot melt fiber containing polyvinyl alcohol fiber (PVAf). Hereinafter, this series is referred to as embodiment series 4. In addition, a series is made with 63% by weight of cellulose fiber, 27% by weight of glass fiber and 10% by weight of hot melt fiber containing PVAf. Hereinafter, this series is referred to as embodiment series 5. A series was also made with 72 wt% of cellulose fibers, 8 wt% of glass fibers and 20 wt% of hot melt fibers containing PVAf. This series is referred to as Example Series 6 hereinafter.

[0096] In the first and second groups of insulating papers, glass fibers with a length of 3 mm to 32 mm and a diameter of 3 μm to 30 μm are used. Specifically, glass fibers with a length of 3 mm to 20 mm and a diameter of 6 μm to 20 μm are used. More specifically, glass fibers with a length of 3 mm to 13 mm and a diameter of 8 μm to 15 μm are used.

[0097] The third group of electrical insulating papers is made of cellulose fiber and polyamide (PA). As the cellulose fiber, UKP is used. However, the present disclosure is not limited thereto. For example, other cellulose fibers may be used. Specifically, base paper is made with different cellulose and PA fiber content ratios, i.e., a series with 50 wt% of cellulose fiber and 50 wt% of PA fiber; a series with 70 wt% of cellulose fiber and 30 wt% of PA fiber; and a series with 90 wt% of cellulose fiber and 10 wt% of PA fiber. These three series are hereinafter referred to as Example Series 7, 8, and 9.

[0098] The fourth group of electrical insulating papers is made of cellulose fiber, PA fiber and hot melt fiber. In this case, hot melt fibers with a length in the range of 2mm to 12mm and a linear density in the range of 0.4dtex (dtex) to 7dtex are used. Specifically, hot melt fibers with a length in the range of 3mm to 8mm and a linear density in the range of 1.2dtex (dtex) to 2.0dtex are used. As cellulose fiber, UKP is used. However, the present disclosure is not limited thereto. For example, other cellulose fibers can be used. Specifically, base paper is prepared with different cellulose, PA and hot melt fiber content ratios, i.e., a series of cellulose fibers with a content of 47.5% by weight, a series of PA fibers with a content of 47.5% by weight, and a series of hot melt fibers containing PVAf with a content of 5% by weight. Hereinafter, this series is referred to as Example Series 10. In addition, a series of hot melt fibers containing PVAf with a content of 63% by weight, a content of PA fibers with a content of 27% by weight, and a content of 10% by weight is manufactured. Hereinafter, this series is referred to as Example Series 11. A series was also produced with 72 wt% of cellulose fibers, 8 wt% of PA fibers and 20 wt% of hot melt fibers containing PVAf. This series is referred to as Example Series 12 hereinafter.

[0099] In the third and fourth groups of electrical insulating papers, PA fibers having a length of 2 to 12 mm and a linear density of 0.4 to 7.0 dtex are used. Specifically, PA fibers having a length of 3 to 8 mm and a linear density of 1.2 to 2.0 dtex are used.

[0100] In addition, in Example Series 4, 5, 6, 10, 11 and 12, a portion of the base paper was hot calendered, and a portion was not calendered. In the experiment of the present invention, calendering was performed at 140° C., with a pressure of 1000 daN, and calendering was performed once on each side. However, the present disclosure is not limited thereto.

[0101] In addition, each of the series 1-12 was made in different variants (three different ratios of two different polyvinyl alcohol binders (PVAb) were used). Some of the series were made with or without a nitrogen-containing heat stabilizer, namely dicyandiamide (Dicy). Dicy was adjusted to meet a 1.9 wt% content based on cellulose fibers.

[0102] Table 1 below summarizes the manufacturing parameters of different variants of the above-mentioned embodiment series 1-6 manufactured. For example, for series 1, all samples include 50% cellulose fiber and 50% glass fiber without calendering. The amount and type of binder and the addition or non-addition of Dicy are changed to provide ten types of samples. For example, the first sample uses 13.5% binder 1 without adding Dicy. As another example, series 4 uses 48% cellulose fiber, 48% glass fiber and PVAf fiber as binders with and without the addition of Dicy. Table 2 summarizes the manufacturing parameters of different variants of the above-mentioned embodiment series 7-12 manufactured.

[0103]

[0104] Table 1: Example Series 1-6

[0105]

[0106] Table 2: Example Series 7-12

[0107] The electrical conductivity of the different variants of the manufactured example series was checked after production and after ageing. Figure 1 The results are shown in . Specifically, Figure 1 The conductivity (in μS / cm) of the aqueous extracts of unimpregnated base paper (black), impregnated with 10% polyvinyl alcohol binder (PVAb) 1 (white) and impregnated with 15% PVAb 1 (hatched lines) measured according to IEC 60554-2 is shown. Legend from bottom to top on the horizontal axis: cellulose%, glass%, PA%, PVAf%.

[0108] The initial tensile index (in Nm / g) of the samples was also tested using the ISO 1924 standard with and without Dicy and with and without polyvinyl alcohol resin (in the form of PAVb or PVAf), Figure 2 The results are shown in Figure 1. Legend from bottom to top: Cellulose%, PVA presence and type, PVA%. White and shaded bars do not include a heat stabilizer and use glass or polyamide, respectively. Black and dot-filled bars use glass and polyamide, respectively, with Dicy as a heat stabilizer. Figure 2In the IEC 60076-14 standard, the acronym TU is used to indicate thermally upgradeable papers.

[0109] Can be obtained from Figure 2 It was observed that the use of PVAf slightly increased the initial tensile strength. The use of PVAb further increased the initial tensile strength. For example, the embodiments with 50% non-cellulose fiber content and high PVAb content had very high initial tensile strength. The use of PVAf seemed to increase the initial tensile index of the embodiments containing 30% or 10% non-cellulose fibers.

[0110] In addition, a simplified and rapid aging test was developed to promote the main degradation mechanism of cellulosic materials in liquid-immersed transformers, namely acid hydrolysis: the paper was placed in a sealed container of 0.5 L of air at 50% RH and 23°C at its initial moisture (approximately 7%) and placed in an oven at a given temperature for a given time. For example, the test was carried out at 140°C until 50% tensile retention was reached. The simplified aging test is particularly fast due to the presence of water. The latter promotes acid hydrolysis degradation.

[0111] Specifically, aging tests were performed on different manufactured example series. Specifically, the tensile index retention (as a percentage relative to the initial tensile index) was evaluated. Accelerated aging tests were performed by exposing the manufactured electrical insulation paper to 155° C. for three days in a sealed tube that trapped moisture. Figure 3 The results are shown in .

[0112] exist Figure 5 , tensile index retention (in %) of prototypes with / without Dicy and with / without PVA. Legend from bottom to top on horizontal axis: cellulose%, presence and type of polyvinyl alcohol (PVAf or PVAb), PVA%. White and shaded bars do not include heat stabilizer and use glass or polyamide, respectively. Black and dot filled bars use glass and polyamide, respectively, with Dicy as heat stabilizer.

[0113] from Figure 3 It can be seen that the following examples in particular have particularly good tensile index retention (about 70% or more):

[0114] -2 contains 50% glass fiber, 7% to 15% PVAb and 1.9% N / UKP,

[0115] -1 contains 30% PA fiber, 15% PVAb and 1.9% N / UKP

[0116] -1 contains 10% glass fiber, 10% PVAb and 1.9% N / UKP.

[0117] -2 contains 10% PA fibers, 10% to 15% PVAb and 1.9% N / UKP.

[0118] It was further observed that there might be a synergistic effect in terms of higher tensile index retention between the PVAb and Dicy combination.

[0119] Figure 4 The contribution to the retention of the tensile index (expressed as a percentage relative to the initial tensile index, PVAb alone, Dicy alone, and the combination of PVAb and Dicy) is shown. For PA fibers containing the prototype, the contribution of Dicy, PVAb, and the combination of the two (circular marks) to the retention of the tensile index.

[0120] like Figure 4 As shown in the results, there is a synergistic effect between PVAb and Dicy. Figure 4 In the graph, the bar graph represents the cumulative effect of individual Dicy and PVAb on the tensile index retention, while the points represent the tensile index retention of the blend of Dicy and PVAb. It can be easily seen that the points are much higher than the bar graph, indicating that the tensile index retention is improved when using a blend of these compounds.

[0121] The results show that a non-cellulose fiber content of up to 50% for glass fibers and up to 30% for PA fibers, based on the base paper, and a PVAb content equal to or higher than 10%, optionally at least 15%, can particularly contribute to a high tensile index and a good tensile index retention.

[0122] According to another embodiment, binder 1 (PVAb) can be completely or partially replaced by poly(vinyl alcohol-co-ethylene) having a hydrolysis degree of at least 88 mol %, that is, only PVAmb can be used, or a blend of PVAmb and PVAb can be used.

[0123] The fifth group of electrical insulating papers is made of cellulose fiber and glass fiber. As the cellulose fiber, UKP is used. However, the present disclosure is not limited thereto. For example, other cellulose fibers may be used. Specifically, base paper is made with different cellulose and PA fiber content ratios, i.e., a series with 70 wt % of cellulose fiber and 30 wt % of glass fiber, and a series with 90 wt % of cellulose fiber and 10 wt % of glass fiber. These two series are referred to as embodiment series 13 and 15 below. Glass fibers having a length of 3 mm to 32 mm and a diameter of 3 μm to 30 μm are used. Specifically, glass fibers having a length of 3 mm to 20 mm and a diameter of 6 μm to 20 μm are used. More specifically, glass fibers having a length of 3 mm to 13 mm and a diameter of 8 μm to 15 μm are used.

[0124] The sixth group of electrical insulating papers is made of cellulose fiber and polyamide (PA). As the cellulose fiber, UKP is used. However, the present disclosure is not limited thereto. For example, other cellulose fibers can be used. Specifically, base paper is manufactured with different cellulose and PA fiber content ratios, i.e., a series with 70 wt. % cellulose fiber and 30 wt. % PA fiber, and a series with 90 wt. % cellulose fiber weight and 10 wt. % PA fiber. These two series are referred to as embodiment series 14 and 16 below. PA fibers having a length of 2 mm to 12 mm and a linear density of 0.4 dtex to 7.0 dtex are used. Specifically, PA fibers having a length of 3 mm to 8 mm and a linear density of 1.2 dtex to 2.0 dtex are used.

[0125] In addition, each of series 13-16 is manufactured with different variants of the blend containing polyvinyl alcohol (PVAb) and modified polyvinyl alcohol (PVAmb). According to this specific embodiment, the PVAmb adhesive is poly (vinyl alcohol-co-ethylene). According to these specific series, PVAb has a degree of hydrolysis of at least 88mol%, and PVAmb has a degree of hydrolysis of at least 88mol%. More specifically, in these series, a blend of PVAb and PVAmb of 1:1 by weight is used. It should also be further noted that all these series 13-16 contain a heat stabilizer containing nitrogen, and more specifically dicyandiamide (Dicy). Dicy is adjusted to meet 1.9%N weight content based on cellulose fiber. More accurately, series 13 and 14 have been manufactured with 15wt% adhesive, and series 15 and 16 have been manufactured with 10wt% adhesive.

[0126] Table 3 below summarizes the manufacturing parameters for the different variations of the above Example Series 13-16 that were manufactured.

[0127]

[0128] Table 3: Example Series 13-16

[0129] The different variants of the manufactured example series 13-16 were inspected after manufacture. Figure 5 The results are shown in. The conductivity of the aqueous extracts of unimpregnated base paper (black), paper impregnated with 15% PVAb and PVAmb in a ratio of 1:1 (corresponding to paper series 13 and 14) (horizontal stripes), paper impregnated with 15% PVAmb (hatched lines), paper impregnated with 10% PVAb and PVAmb in a ratio of 1:1 (corresponding to paper series 15 and 16) (grey) and paper impregnated with 10% PVAmb (white) were measured according to IEC60554-2.

[0130] The tensile index of the different variations of the manufactured Example Series 13-16 was examined (both initial stretch after manufacturing (in Nm / g) and retention of tensile index after aging (in %)). Figure 6 and 7 The results are shown in .

[0131] Figure 6 The initial tensile index of the prototypes corresponding to series 13-16 is shown compared to the unimpregnated base paper and the base paper impregnated with 10 wt% or 15 wt% PVAmb. Legend from bottom to top of the horizontal axis: % cellulose; presence of resin composition. It is worth noting that the fiber mixture reaching 100% is achieved by the synthetic fibers used (glass or PA), and the composition of the prototypes corresponds to the composition listed in Table 3.

[0132] Figure 7 The tensile index retention (in %) of the prototypes corresponding to series 13-16 is shown compared to base papers not impregnated with resin or impregnated with 10 wt% or 15 wt% PVAmb. Legend from bottom to top of the horizontal axis: % cellulose; presence of resin composition. It is worth noting that the fiber mixture reaching 100% is achieved by the synthetic fibers used (glass or PA), and the composition of the prototypes corresponds to the composition listed in Table 3.

[0133] from Figure 5 It can be seen that impregnating base paper with PVAmb or a 1:1 blend of PVAb and PVAmb can improve the tensile index retention of paper.

[0134] The following aspects form part of this disclosure:

[0135] Aspect 1: An electrical insulating paper, comprising: at least 25 weight % cellulose fibers, based on the total weight of the electrical insulating paper; at least 5 weight % synthetic fibers, based on the total weight of the electrical insulating paper, the synthetic fibers comprising aliphatic polyamide fibers and / or glass fibers; and a heat stabilizer comprising nitrogen, wherein the nitrogen content by weight accounts for 1 weight % to 4 weight % of the content of the cellulose fibers.

[0136] Aspect 2. The electrical insulating paper according to aspect 1, comprising at least 50 wt%, optionally at least 65 wt% cellulose fibers, based on the total weight of the electrical insulating paper; and 7 wt% to 35 wt%, optionally 7 wt% to 27 wt%, or 8 wt% to 25 wt% synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or blends of polyamide fibers such as aliphatic polyamide fibers and glass fibers.

[0137] Aspect 3. The electrical insulating paper according to aspect 1, comprising: at least 45 wt%, optionally at least 65 wt% cellulose fibers, based on the total weight of the electrical insulating paper; and 5 wt% to 55 wt%, optionally 7 wt% to 42 wt%, or 8 wt% to 25 wt% synthetic fibers, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are glass fibers, or a blend of glass fibers and polyamide fibers such as aliphatic polyamide fibers.

[0138] Aspect 4. The electrical insulating paper according to any one of the preceding aspects, wherein the heat stabilizer containing nitrogen is selected from dicyandiamide, urea, melamine, polyacrylamide or a mixture thereof.

[0139] Aspect 5. The electrical insulating paper according to any one of the preceding aspects, wherein the content of nitrogen in the heat stabilizer is 1 wt % to 4 wt %, optionally 1.2 wt % to 2.3 wt % based on the content of the cellulose fibers.

[0140] Aspect 6. The electrical insulating paper according to any one of the preceding aspects is characterized in that it further comprises a binder, wherein the amount of the binder is 5 wt% to 20 wt% based on the total weight of the electrical insulating paper, and the binder is selected from hot-melt fibers, resins or mixtures thereof.

[0141] Aspect 7. The electrical insulating paper according to item 6, wherein the binder is a resin, optionally comprising the nitrogen-containing heat stabilizer.

[0142] Aspect 8. Electrical insulating paper according to item 7, wherein the binder comprises a polyvinyl alcohol binder, preferably having a degree of hydrolysis of at least 88 mol %.

[0143] Aspect 9. The electrical insulating paper according to aspect 8, wherein the polyvinyl alcohol binder comprises polyvinyl alcohol or modified polyvinyl alcohol.

[0144] Aspect 10. The electrical insulating paper according to aspect 9, wherein the modified polyvinyl alcohol binder comprises poly(vinyl alcohol-co-ethylene).

[0145] Aspect 11. The electrical insulation paper according to aspect 10, wherein the poly(vinyl alcohol-co-ethylene) has a degree of hydrolysis of at least 88 mol%.

[0146] Aspect 12. The electrical insulating paper according to aspect 7, wherein the binder is a mixture of polyvinyl alcohol and modified polyvinyl alcohol.

[0147] Aspect 13. The electrical insulating paper according to aspect 12, wherein the modified polyvinyl alcohol is poly(vinyl alcohol-co-ethylene).

[0148] Aspect 14. The electrical insulating paper according to any one of aspects 12 or 13, wherein the polyvinyl alcohol has a degree of hydrolysis of at least 88 mol%.

[0149] Aspect 15. The electrical insulating paper according to any one of aspects 12 to 14, wherein the modified polyvinyl alcohol has a degree of hydrolysis of at least 88 mol%.

[0150] Aspect 16. The electrical insulating paper according to any one of aspects 12 to 15, wherein the polyvinyl alcohol and the modified polyvinyl alcohol are blended in a ratio of 1:5 to 5:1 by weight, preferably in a ratio of 1:3 to 3:1 by weight, and more preferably in a ratio of 1:1 by weight.

[0151] Aspect 17. The electrical insulating paper according to Aspect 6 is characterized in that the adhesive contains hot-melt fibers, and the electrical insulating paper contains the hot-melt fibers in an amount of 5 weight % to 18 weight %, preferably 9 weight % to 17 weight %, or more preferably 10 weight % to 16 weight %, based on the total weight of the electrical insulating paper.

[0152] Aspect 18. The electrical insulating paper according to aspect 17, wherein the length of the hot-melt fibers is 2 mm to 12 mm, optionally 3 mm to 8 mm, and / or the linear density is 0.4 dtex (dtex) to 7.0 dtex, optionally 1.2 dtex (dtex) to 2.0 dtex.

[0153] Aspect 19. The electrical insulating paper according to any one of the preceding aspects, wherein the cellulose fibers comprise kraft fibers, cotton fibers, linen fibers, hemp fibers, and wherein the cellulose fibers are unbleached, bleached and / or semi-bleached hardwood fibers and / or softwood fibers.

[0154] Aspect 20. The electrical insulating paper according to any one of the preceding aspects, wherein the length of the glass fibers is 3 mm to 32 mm, preferably 3 mm to 20 mm, and more preferably 3 mm to 13 mm.

[0155] Aspect 21. The electrical insulating paper according to aspect 20, wherein the glass fibers have a diameter of 3 μm to 30 μm, preferably 6 μm to 20 μm, and more preferably 8 μm to 15 μm.

[0156] Aspect 22. The electrical insulating paper according to any one of the preceding aspects, wherein the length of the polyamide fibers is from 2 mm to 12 mm, preferably from 3 mm to 8 mm.

[0157] Aspect 23. The electrical insulating paper according to aspect 22, wherein the linear density of the polyamide fiber is 0.4 dtex to 7.0 dtex, preferably 1.2 dtex to 2.0 dtex.

[0158] Aspect 24. A method for manufacturing electrical insulating paper, the method comprising the following steps: providing cellulose fibers and synthetic fibers; - manufacturing base paper from the cellulose fibers and the synthetic fibers on a papermaking machine, with the cellulose fibers comprising at least 25 weight % based on the total weight of the electrical insulating paper, and the synthetic fibers comprising at least 5 weight % based on the total weight of the electrical insulating paper; wherein the method further comprises adding a heat stabilizer comprising nitrogen, wherein the content of the nitrogen by weight accounts for 1 weight % to 4 weight % of the content of the cellulose fibers.

[0159] Aspect 25. A method according to Aspect 24, wherein the manufacturing comprises manufacturing base paper from the cellulose fibers and the synthetic fibers on a papermaking machine, with the cellulose fibers containing at least 50 wt%, preferably at least 65 wt%, based on the total weight of the electrical insulating paper, and the synthetic fibers containing at least 7 wt% to 35 wt%, preferably 7 wt% to 27 wt%, or more preferably 8 wt% to 25 wt%, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or a blend of polyamide fibers such as aliphatic polyamide fibers and glass fibers.

[0160] Aspect 26. A method according to aspect 24, wherein the manufacturing comprises manufacturing base paper from the cellulose fibers and the synthetic fibers on a papermaking machine, with the cellulose fibers containing at least 45% by weight, preferably at least 65% by weight, based on the total weight of the electrical insulating paper, and the synthetic fibers containing 5% to 55% by weight, optionally 7% to 42% by weight, or 8% to 25% by weight, based on the total weight of the electrical insulating paper, wherein the synthetic fibers are glass fibers, or a blend of glass fibers and polyamide fibers such as aliphatic polyamide fibers.

[0161] Aspect 27. The method according to any one of Aspects 24 to 26, further comprising the step of adding a binder, wherein the amount of the binder is 5 wt% to 20 wt% based on the total weight of the electrical insulating paper, and the binder is selected from hot-melt fibers, resins or mixtures thereof.

[0162] Aspect 28. The method according to aspect 27, wherein the binder is a resin, and the resin is coated on the base paper after the adding of the heat stabilizer containing nitrogen.

[0163] Aspect 29. The method according to aspect 27, wherein the binder is a resin, and the resin is coated on the base paper immediately after the step of manufacturing the base paper and before the step of adding the heat stabilizer containing nitrogen.

[0164] Aspect 30. The method according to aspect 27, wherein the binder is a hot-melt fiber, and the hot-melt fiber is mixed with the cellulose fiber and the synthetic fiber before the step of manufacturing the base paper.

[0165] Aspect 31. The method according to aspect 27, wherein the binder is a resin, the resin further comprising the nitrogen-containing heat stabilizer, and the resin is coated on the base paper immediately after the step of manufacturing the base paper.

[0166] Aspect 32. The method according to any one of aspects 30 to 31, wherein the base paper is made with a weight content of hot-melt fibers of 8 to 18%, optionally 9 to 17% or 10 to 16%.

[0167] Aspect 33. A method according to any one of Aspects 30 to 32, wherein the length of the hot-melt fiber is 2 mm to 12 mm, preferably 3 mm to 8 mm, and / or the linear density is 0.4 dtex (dtex)-7.0 dtex, preferably 1.2 dtex (dtex)-2.0 dtex.

[0168] Aspect 34. The method according to any one of aspects 27 to 33, wherein the electrical insulation paper is made of resin, preferably comprising the nitrogen-containing heat stabilizer.

[0169] Aspect 35. The method according to aspect 34, wherein the resin comprises a polyvinyl alcohol binder, preferably having a degree of hydrolysis of at least 88 mol%.

[0170] Aspect 36. The method according to aspect 34, wherein the resin comprises modified polyvinyl alcohol.

[0171] Aspect 37. The method of aspect 36, wherein the modified polyvinyl alcohol binder comprises poly(vinyl alcohol-co-ethylene).

[0172] Aspect 38. The method of aspect 37, wherein the poly(vinyl alcohol-co-ethylene) has a degree of hydrolysis of at least 88 mol%.

[0173] Aspect 39. The method according to aspect 34, wherein the resin comprises a mixture of polyvinyl alcohol and modified polyvinyl alcohol.

[0174] Aspect 40. The method according to aspect 39, wherein the modified polyvinyl alcohol is poly(vinyl alcohol-co-ethylene).

[0175] Aspect 41. The method according to any one of aspects 39 or 40, wherein the polyvinyl alcohol has a degree of hydrolysis of at least 88 mol%.

[0176] Aspect 42. The method according to any one of aspects 39 to 41, wherein the modified polyvinyl alcohol has a degree of hydrolysis of at least 88 mol%.

[0177] Aspect 43. The method according to any one of aspects 39 to 42, wherein the polyvinyl alcohol and the modified polyvinyl alcohol are blended in a ratio of 1:5 to 5:1 by weight, preferably in a ratio of 1:3 to 3:1 by weight, and more preferably in a ratio of 1:1 by weight.

[0178] Aspect 44. The method according to any one of Aspects 24 to 43, wherein the cellulose fibers comprise kraft fibers, cotton fibers, flax fibers, hemp fibers, and wherein the cellulose fibers are unbleached, bleached and / or semi-bleached hardwood fibers and / or softwood fibers.

[0179] Aspect 45. The method according to any one of aspects 24 to 44, wherein the length of the glass fiber is 3 mm to 32 mm, preferably 3 mm to 20 mm, and more preferably 3 mm to 13 mm.

[0180] Aspect 46. The method according to aspect 45, wherein the glass fiber has a diameter of 3 μm to 30 μm, preferably 6 μm to 20 μm, and more preferably 8 μm to 15 μm.

[0181] Aspect 47. The method according to any one of aspects 24 to 46, wherein the polyamide fiber has a length of 2 mm to 12 mm, preferably 3 mm to 8 mm.

[0182] Aspect 48. The method according to aspect 47, wherein the linear density of the polyamide fiber is 0.4 dtex to 7.0 dtex, preferably 1.2 dtex to 2.0 dtex.

[0183] Aspect 49. The method for manufacturing electrical insulating paper according to any one of Aspects 30, 32 and 33, comprising, before the step of adding the heat stabilizer containing nitrogen or coating the base paper with the resin, a step of hot calendering the base paper at a temperature in the range of 120° C. to 160° C. and at a pressure in the range of 800 daN to 1200 daN.

[0184] Aspect 50. Electrical insulating paper comprising at least one layer manufactured according to the method of any one of aspects 24 to 49.

[0185] Aspect 51. Use of the electrical insulating paper according to any one of aspects 1 to 23 or 50 for insulating conductors of a high-voltage liquid-immersed transformer or a dry-type transformer.

[0186] Aspect 52. Use of the electrical insulating paper according to any one of aspects 1 to 23 or 50 for insulating a conductor used in a traction transformer.

[0187] Aspect 53. Use of the electrical insulating paper according to any one of aspects 1 to 23 or 50 for low voltage foil windings in distribution transformers.

[0188] Aspect 54. A high voltage liquid immersed transformer comprising at least one conductor insulated by the electrical insulating paper according to any one of aspects 1 to 23 or 50.

[0189] Aspect 55. A dry-type transformer comprising at least one wire insulated by the electrical insulating paper according to any one of aspects 1 to 23 or 50.

[0190] Aspect 56. A traction transformer comprising at least one conductor insulated by the electrical insulating paper according to any one of aspects 1 to 23 or 50.

[0191] Aspect 57. A distribution transformer having at least one low voltage foil winding comprising the electrical insulation paper according to any one of aspects 1 to 23 or 50.

[0192] It will be apparent to those skilled in the art that various modifications and variations may be made to the disclosed apparatus and system without departing from the scope of the present disclosure. Other aspects of the present disclosure will be apparent to those skilled in the art by considering the specification and practicing the features disclosed herein. The present specification and embodiments are intended to be considered exemplary only. Many additional variations and modifications are possible and are understood to fall within the framework of the present disclosure.

Claims

1. An electrical insulating paper, comprising: At least 25% by weight of cellulose fibers, based on the total weight of the electrical insulating paper; At least 5% by weight of synthetic fibers, based on the total weight of the electrical insulating paper, the synthetic fibers comprising aliphatic polyamide fibers and / or glass fibers; and A heat stabilizer comprising nitrogen, wherein the nitrogen content by weight accounts for 1 wt % to 4 wt % of the content of the cellulose fiber.

2. The electrical insulating paper according to claim 1, comprising: at least 50 wt%, optionally at least 65 wt% cellulose fibers in a content based on the total weight of the electrical insulating paper; and Based on the total weight of the electrical insulating paper, the synthetic fiber content is 7 wt % to 35 wt %, optionally 7 wt % to 27 wt %, or 8 wt % to 25 wt %, wherein the synthetic fiber is polyamide fiber, such as aliphatic polyamide fiber, or a blend of polyamide fiber such as aliphatic polyamide fiber and glass fiber.

3. The electrical insulating paper according to claim 1, comprising: at least 45 wt%, optionally at least 65 wt% cellulose fibers in a content based on the total weight of the electrical insulating paper; and Based on the total weight of the electrical insulating paper, the synthetic fiber content is 5 to 55 wt%, optionally 7 to 42 wt%, or 8 to 25 wt%, wherein the synthetic fiber is glass fiber, or a blend of glass fiber and polyamide fiber such as aliphatic polyamide fiber.

4. Electrical insulating paper according to any one of the preceding claims, wherein the nitrogen-containing heat stabilizer is selected from dicyandiamide, urea, melamine, polyacrylamide or mixtures thereof.

5. Electrical insulating paper according to any one of the preceding claims, characterized in that It further comprises a binder, wherein the amount of the binder is 5 to 20 weight % based on the total weight of the electrical insulating paper, and the binder is selected from hot-melt fiber, resin or a mixture thereof.

6. Electrical insulating paper according to claim 5, wherein the binder is a resin, optionally comprising the nitrogen-containing heat stabilizer. The electrical insulating paper according to claim 6 , wherein the resin comprises polyvinyl alcohol.

8. A method for producing electrical insulating paper, the method comprising the following steps: - Provide cellulose fibers and synthetic fibers; - manufacturing base paper from said cellulose fibers and said synthetic fibers on a paper machine with a content of at least 25% by weight of said cellulose fibers, based on the total weight of said electrical insulating paper, and a content of at least 5% by weight of said synthetic fibers, based on the total weight of said electrical insulating paper; The method further comprises adding a heat stabilizer comprising nitrogen, wherein the nitrogen content by weight accounts for 1 wt % to 4 wt % of the content of the cellulose fibers.

9. The method according to claim 8, further comprising the step of adding a binder, wherein the amount of the binder is 5 to 20 weight % based on the total weight of the electrical insulating paper, and the binder is selected from hot-melt fibers, resins or mixtures thereof.

10. The method according to claim 9, wherein the binder is a resin, and the resin is coated on the base paper after the adding of the heat stabilizer containing nitrogen.

11. The method according to claim 9, wherein the binder is a resin which is coated on the base paper immediately after the step of manufacturing the base paper and before the step of adding the heat stabilizer containing nitrogen.

12. The method according to claim 9, wherein the binder is a hot-melt fiber which is mixed with the cellulose fiber and the synthetic fiber before the step of manufacturing the base paper.

13. The method according to claim 9, wherein the binder is a resin, the resin further comprising the heat stabilizer containing nitrogen, the resin being coated on the base paper immediately after the step of manufacturing the base paper.

14. The method for manufacturing electrical insulating paper according to claim 12, comprising, before the step of adding the heat stabilizer containing nitrogen or coating the base paper with the resin, a step of heat calendering the base paper at a temperature in the range of 120°C to 160°C and at a pressure in the range of 800 daN to 1200 daN.

15. A transformer comprising the electrical insulating paper according to any one of claims 1 to 7, wherein the transformer is selected from a high voltage liquid immersed transformer, a dry type transformer, a traction transformer or a traction transformer.

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