Electrical insulation paper

Abstract

The present disclosure relates to an electrically insulating paper comprising: a cellulose fiber content of at least 25 wt.%, based on the total weight of the electrically insulating paper; a synthetic fiber content of 8 wt.%, based on the total weight of the electrically insulating paper; and a heat stabilizer comprising nitrogen, wherein the content of the nitrogen by weight is from 1 wt.% to 4 wt.% of the content of the cellulose fiber.

Description

[0001] Related applications

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

[0003] This disclosure relates to an electrical insulating paper. This disclosure further relates to methods of manufacturing the paper and to cables, transformers, capacitors, and / or other electrical equipment equipped with such electrical insulating paper. Background Technology

[0004] This invention relates to an electrical insulating paper. The invention further relates to a method of manufacturing the paper and to cables, transformers, capacitors, and / or other electrical equipment equipped with such electrical insulating paper.

[0005] Electrical insulating paper is used in various devices (e.g., transformers, cables, and capacitors) and specifically in liquid-filled transformers, cables, and capacitors for electrical insulation.

[0006] and Compared to base paper, there is particular interest in materials with good mechanical and electrical properties that can be produced at low cost.

[0007] Cellulose-containing electrical insulating paper is well-known and plays an important role in the field of electrical insulation. Cellulose-based insulating paper combines good electrical insulation with good mechanical properties and can be produced at low cost. However, for example, in liquid-immersed transformers, the insulating paper is exposed to various thermal, chemical, and / or oxidative stresses, which can cause the cellulose to age rapidly. Aging manifests as a loss of tensile strength and can easily lead to transformer failure.

[0008] There is a need to provide smaller transformers and other electrical equipment without compromising electrical insulation, and it is known that the operating temperature and / or operating time limitations of the equipment are not always satisfactory. There is also a need to provide transformers of the same size as existing transformers but capable of operating at higher temperatures.

[0009] The purpose of this disclosure is to address at least one drawback of the prior art. Summary of the Invention

[0010] The aforementioned objectives are achieved by the electrical insulating paper according to this disclosure.

[0011] One aspect of this disclosure relates to an electrical insulating paper. The electrical insulating paper comprises: 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; and a nitrogen-containing heat stabilizer, wherein the nitrogen content, by weight, is from 1% to 4% of the cellulose fiber content. The synthetic fibers comprise aliphatic polyamide fibers and / or glass fibers.

[0012] Another aspect of this disclosure relates to a method for manufacturing an electrically insulating paper. The method comprises the steps of: providing cellulose fibers and synthetic fibers, and manufacturing a base paper on a paper machine from the cellulose fibers and the synthetic fibers, wherein the cellulose fibers comprise at least 25% by weight of the total weight of the electrically insulating paper, and the synthetic fibers comprise at least 5% by weight of the total weight of the electrically insulating paper. Another aspect of this disclosure is an electrically insulating paper comprising at least one layer formed by the method described herein.

[0013] Another aspect of this disclosure is the use of the electrical insulation disclosed herein for insulating conductors in high-voltage liquid-immersed transformers or dry-type transformers, or for insulating conductors used in traction transformers, or for low-voltage foil windings in distribution transformers. In other words, another aspect is a transformer comprising conductors insulated with the electrical insulating paper described herein, or a transformer comprising low-voltage foil windings containing the electrical insulating paper described herein. Attached Figure Description

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

[0015] Figure 2 A graph showing the initial tensile index of an example of electrical insulating paper.

[0016] Figure 3 A chart showing the retention rate of tensile index after aging of example electrical insulating paper.

[0017] Figure 4 A chart showing the retention rate of tensile index after aging of example electrical insulating paper.

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

[0019] Figure 6 A graph showing the initial tensile index of an example of electrical insulating paper.

[0020] Figure 7 A chart showing the retention rate of tensile index after aging of example electrical insulating paper. Detailed Implementation

[0021] Electrical insulating paper

[0022] 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% of cellulose fibers, based on the total weight of the electrical insulating paper. Electrical insulating paper may also contain, for example, up to 94 wt%, up to 92 wt%, up to 91 wt%, up to 86 wt%, or up to 66 wt% of cellulose fibers, based on the total weight of the electrical insulating paper.

[0023] According to some embodiments, the cellulose fiber comprises one or more of the following: kraft paper fiber, cotton fiber, flax fiber, hemp fiber, and wherein the cellulose fiber is unbleached, bleached and / or semi-bleached hardwood fiber and / or softwood fiber.

[0024] Electrically insulating paper may contain at least 5%, 5 to 55%, 7 to 35%, or 8 to 25% synthetic fibers based on the total weight of the insulating paper, said synthetic fibers comprising polyamide fibers, glass fibers, or combinations thereof. The polyamide fibers may be, for example, aliphatic polyamide fibers, or blends of polyamide fibers (such as aliphatic polyamide fibers). Synthetic fibers can promote higher tensile strength retention in electrically insulating paper. More generally, synthetic fibers provide good strength parameters for electrically insulating paper.

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

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

[0027] The RTE index of a material or system is the temperature at which the endpoint (e.g., 50% tensile retention of an insulation material) is reached after a given time required to reach the same endpoint as a reference material or system with known thermal durability (e.g., non-thermal 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 the electrical insulating paper according to this disclosure, they may be particularly suitable for liquid-immersed transformers, wherein the liquid may be mineral oil or ester.

[0029] The system's RTE can be determined according to IEC 60332-2, a standard based on accelerated aging tests in a sealed tube at different temperatures for different durations. For example, compared to a reference, the system undergoes one or three different aging tests and should have the same or higher tensile retention as the reference, but at higher temperatures (+10°C to 60°C), depending on the expected increase in thermal class. The standard IEEE C57.100 provides a clear description of the experimental portion of such accelerated testing.

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

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

[0032] Electrical insulating paper also provides mechanical properties that allow it to be wound around conductors in a technically practical manner. Therefore, electrical insulating paper can provide smaller transformers and other electrical equipment without compromising electrical insulation or operating temperature and / or runtime limitations. Electrical insulating paper can also provide transformers of the same size as existing transformers but capable of operating at higher temperatures.

[0033] In other words, the electrical insulating paper according to this disclosure can withstand high potential gradients while offering advantages over alternatives using very thin paper according to the prior art, because by reducing its thickness while keeping other properties constant, the breakdown strength (i.e., dielectric strength) is increased. In this respect, 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 feasibility of the winding process, making it not a practical solution in itself. The electrical insulating paper according to this disclosure provides a solution.

[0034] The electrical insulating paper may contain at least 50% by weight of cellulose fibers based on the total weight of the electrical insulating paper; and 7% to 35% by weight 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 blends of polyamide fibers such as aliphatic polyamide fibers with glass fibers.

[0035] The electrical insulating paper may contain at least 65% by weight of cellulose fibers based on the total weight of the electrical insulating paper; and 7% to 27% by weight 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 blends of polyamide fibers such as aliphatic polyamide fibers with glass fibers.

[0036] The electrical insulating paper may comprise at least 65% by weight of cellulose fibers based on the total weight of the electrical insulating paper; and 8% to 25% by weight 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 blends of polyamide fibers such as aliphatic polyamide fibers with glass fibers.

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

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

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

[0040] When the synthetic fiber is glass fiber, the length of these glass fibers can be from 3 mm to 32 mm, preferably from 3 mm to 20 mm, and more preferably from 3 mm to 13 mm. Furthermore, the diameter of these glass fibers can be from 3 μm to 30 μm, preferably from 6 μm to 20 μm, and more preferably from 8 μm to 15 μm.

[0041] When the synthetic fiber is polyamide fiber, the length of these polyamide fibers can be from 2 mm to 12 mm, preferably from 3 mm to 8 mm. The linear density of the polyamide fiber can be from 0.4 dtex to 7.0 dtex, and preferably from 1.2 dtex to 2.0 dtex.

[0042] The electrical insulating paper may further comprise an adhesive, wherein the amount of adhesive is from 5% to 20% by weight based on the total weight of the electrical insulating paper. The adhesive may be selected from hot melt fibers, resins, or mixtures thereof.

[0043] The adhesive can be a resin. Resin can improve the mechanical strength parameters of electrical insulating paper.

[0044] In this context, the resin may specifically be a liquid with a viscosity of less than 100 centipoise (cP) at 50°C, optionally in the range of 10 cP to 75 cP at 50°C. The resin may be pure or diluted to achieve this viscosity so that it can be impregnated or coated onto a paper substrate.

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

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

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

[0048] Adhesives may contain hot-melt fibers. For example, electrical insulating paper may contain 5% to 18% by weight, 9% to 17% by weight, or 10% to 16% by weight of hot-melt fibers based on the total weight of the insulating paper. Hot-melt fibers can provide the paper with higher tensile strength. A narrower range of indications for the presence of hot-melt fibers can largely promote high tensile strength without compromising other desired properties. The length of hot-melt fibers can be 2 mm to 12 mm or 3 mm to 8 mm. The linear density of hot-melt fibers can be 0.4 dtex to 7.0 dtex or 1.2 dtex to 2.0 dtex.

[0049] Electrically insulating paper may contain cellulose fibers, glass fibers, a nitrogen-containing heat stabilizer (containing all the foregoing components in the above range by weight%), an adhesive, and the remainder not exceeding 1%, optionally 0.5% or 0.1% of the total weight of the electrically insulating paper.

[0050] Manufacturing method.

[0051] This document also discloses a method for manufacturing electrical insulating paper, the method comprising: providing cellulose fibers and synthetic fibers; manufacturing a base paper on a paper machine from the cellulose fibers and the synthetic fibers, wherein the cellulose fibers comprise at least 25% by weight of the total weight of the electrical insulating paper, and the synthetic fibers comprise at least 5% by weight of the total weight of the electrical insulating paper; and adding a nitrogen-containing heat stabilizer, wherein the nitrogen content, by weight, is from 1% to 4% of the cellulose fiber content. The method may further comprise adding a nitrogen-containing heat stabilizer, wherein the nitrogen content, by weight, is from 1% to 4% of the cellulose fiber content.

[0052] The description and amounts of cellulose fibers, synthetic fibers, heat stabilizers, and optional adhesives are as discussed above regarding electrical insulating paper.

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

[0054] When the adhesive is a resin, the resin can be applied (e.g., coated or impregnated) onto the base paper, for example, after the addition of a nitrogen-containing heat stabilizer. As another example, the resin can be applied to the base paper immediately after the step of manufacturing the base paper and before the addition of the nitrogen-containing heat stabilizer. As yet another example, the resin may include a nitrogen-containing heat stabilizer.

[0055] When the adhesive is a resin, the resin can be applied to the base paper online or offline on the paper machine.

[0056] Adhesives (especially resin adhesives) can be applied by sizing press (e.g., corresponding to the impregnation step) or by another coating method (such as bar coating, road coating, roller coating, etc.).

[0057] When the adhesive contains hot-melt fibers, the hot-melt fibers can be mixed with the cellulose fibers and the synthetic fibers prior to the step of manufacturing the base paper.

[0058] According to some embodiments, the adhesive is a resin, which also contains the nitrogen-containing heat stabilizer, and the resin is applied to the base paper immediately after the step of manufacturing the base paper.

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

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

[0061] The linear density of hot melt fibers can range from 0.4 dtex to 7.0 dtex.

[0062] The linear density of hot melt fibers can be 1.2 dtex to 2.0 dtex.

[0063] According to one example, electrical insulating paper is manufactured using a liquid adhesive. The liquid adhesive may contain a heat stabilizer containing nitrogen.

[0064] The adhesive may contain polyvinyl alcohol as described herein, such as polyvinyl alcohol, modified polyvinyl alcohol, such as poly(vinyl alcohol-co-ethylene) or blends or mixtures thereof. Polyvinyl alcohol adhesives may have a degree of hydrolysis of at least 88 mol%.

[0065] The method may include, after the steps of adding the nitrogen-containing heat stabilizer or coating the base paper with the resin, the 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 ten Newtons (daN) to 1200 daN.

[0066] The manufacturing process may include (according to some embodiments, consisting of) the production of base paper on a paper machine from the cellulose fibers and the synthetic fibers, wherein the synthetic fibers are polyamide fibers, such as aliphatic polyamide fibers, or blends of polyamide fibers such as aliphatic polyamide fibers with glass fibers.

[0067] Based on the total weight of the electrical insulating paper, the cellulose fiber content can be at least 65% by weight.

[0068] Based on the total weight of the electrical insulating paper, the polyamide fiber content by weight can be from 7% to 27%.

[0069] Based on the total weight of the electrical insulating paper, the content of polyamide fiber by weight can be from 8% to 25%.

[0070] The manufacturing process may include (according to some embodiments, consisting of) the production of base paper on a paper machine from the cellulose fibers and the synthetic fibers, wherein the synthetic fibers are glass fibers or blends of glass fibers with polyamide fibers such as aliphatic polyamide fibers, based on the total weight of the electrical insulating paper.

[0071] Based on the total weight of the electrical insulating paper, the cellulose fiber content can be at least 65% by weight.

[0072] Based on the total weight of the electrical insulating paper, the glass fiber content can range from 7% to 42% by weight.

[0073] Based on the total weight of the electrical insulating paper, the glass fiber content can range from 8% to 25% by weight.

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

[0075] The paper can be creased, pleated and calendered, and / or printed with epoxy dots to form so-called diamond-shaped dot paper. The paper can also be stretchable paper with improved stretchability to conform to the winding, but the stretchable paper is obtained through a process different from creasing. For example, the stretchability of so-called stretchable paper is obtained on a paper machine in the presence of units consisting of a moving blanket carrying the paper, which, when wetted, passes through a pressure zone, causing it to shrink before the pressure zone and then compact (micro-crease) in the machine direction (MD) after the pressure zone.

[0076] This disclosure also relates to the use of electrical insulating paper according to any one or more aspects discussed above for insulating conductors of high-voltage liquid-immersed transformers or dry-type transformers.

[0077] This disclosure also relates to the use of electrical insulating paper according to any one or more aspects discussed above for insulating conductors used in traction transformers.

[0078] This disclosure also relates to the use of electrical insulating paper according to any one or more aspects discussed above, for use in low-voltage foil windings in distribution transformers.

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

[0080] Another aspect of this disclosure relates to a dry-type transformer comprising at least one conductor insulated by electrical insulating paper according to any one or more of the foregoing aspects.

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

[0082] Another aspect of this disclosure relates to a distribution transformer having at least one low-voltage foil winding comprising electrical insulating paper according to any one or more of the foregoing aspects.

[0083] Another aspect of this disclosure relates to insulating paper and transformer plate insulation or molded fiber insulation components used in transformers.

[0084] Further advantages and features of this disclosure will become apparent from the following description of specific embodiments, which can be implemented individually or in combination with one or more features discussed above, provided that these features do not contradict each other.

[0085] The following abbreviations will be used in the following text:

[0086] PVAb refers to polyvinyl alcohol adhesives in resin form;

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

[0088] • PVAf refers to polyvinyl alcohol in fibrous form;

[0089] PA refers to polyamide;

[0090] UKP refers to unbleached kraft paper fibers.

[0091] Example

[0092] To better understand this disclosure and to show how to implement it, reference will now be made only by way of example to embodiments according to this disclosure and experimental data relating to these embodiments.

[0093] The embodiments of the electrical insulating paper according to this disclosure are manufactured using a pilot paper machine.

[0094] The first group of electrical insulating papers is made of cellulose fibers and glass fibers. Unbleached kraft paper fibers (UKP) are used as the cellulose fibers. However, this disclosure is not limited to this. For example, bleached kraft paper fibers or other cellulose fibers can also be used. The cellulose fibers can be, for example, cotton fibers, flax fibers, hemp fibers, bleached, unbleached or semi-bleached fibers, cork or hardwood, or any mixture of the mentioned fibers, etc. Specifically, the base paper is manufactured with different cellulose and glass fiber content ratios: the first series, using 50% by weight of cellulose fibers and 50% by weight of glass fibers; the second series, using 70% by weight of cellulose fibers and 30% by weight of glass fibers; and the third series, using 90% by weight of cellulose fibers and 10% by weight of glass fibers. These three series having the above-mentioned different content ratios will hereinafter be referred to as Example Series 1, 2, and 3.

[0095] The second group of electrical insulating paper is made of cellulose fiber, glass fiber, and hot-melt fiber. In this case, hot-melt fiber with a length in the range of 2 mm to 12 mm and a linear density in the range of 0.4 dtex to 7 dtex is used. Specifically, hot-melt fiber with a length in the range of 3 mm to 8 mm and a linear density in the range of 1.2 dtex to 2.0 dtex is used. UKP is used as the cellulose fiber. However, this disclosure is not limited to this. Other cellulose fibers can be used as discussed above. Specifically, the base paper is manufactured with different ratios of cellulose, glass, and hot-melt fiber content. A series is manufactured 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 Example Series 4. In addition, a series is manufactured 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 Example Series 5. A series of materials were also manufactured using 72% by weight of cellulose fiber, 8% by weight of glass fiber, and 20% by weight of heat-fused fiber containing PVAf. This series will be referred to below as Example Series 6.

[0096] In both the first and second groups of insulating paper, 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 paper is made of cellulose fibers and polyamide (PA). UKP is used as the cellulose fiber. However, this disclosure is not limited to this. For example, other cellulose fibers can be used. Specifically, the base paper is manufactured with different ratios of cellulose and PA fibers: a series with 50% by weight of cellulose fibers and 50% by weight of PA fibers; a series with 70% by weight of cellulose fibers and 30% by weight of PA fibers; and a series with 90% by weight of cellulose fibers and 10% by weight of PA fibers. These three series are hereinafter referred to as Example Series 7, 8, and 9.

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

[0099] In the third and fourth groups of electrical insulating paper, PA fibers with 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 with a length of 3 mm to 8 mm and a linear density of 1.2 dtex to 2.0 dtex are used.

[0100] Furthermore, in Examples 4, 5, 6, 10, 11, and 12, a portion of the base paper is hot-calendered, while a portion is not calendered. In the experiments of this invention, calendering is performed at 140°C, with a pressure of 1000 daN, and calendering is performed once on each side. However, this disclosure is not limited thereto.

[0101] Furthermore, each of series 1-12 is manufactured in different variants (using three different ratios of two different polyvinyl alcohol binders (PVAb)). Some of the series are manufactured with or without a nitrogen-containing heat stabilizer, namely dicyandiamide (Dicy). Dicy is adjusted to meet a 1.9% by weight content based on cellulose fibers.

[0102] Table 1 below summarizes the manufacturing parameters for the different variants of Examples 1-6 described above. For example, for Series 1, all samples comprised 50% cellulose fiber and 50% glass fiber without calendering. The amount and type of binder, as well as the addition or absence of Dicy, were varied to provide ten types of samples. For example, the first sample used 13.5% binder 1 without adding Dicy. As another embodiment, Series 4 used 48% cellulose fiber, 48% glass fiber, and PVAf fiber as binder, with and without the addition of Dicy. Table 2 summarizes the manufacturing parameters for the different variants of Examples 7-12 described above.

[0103]

[0104] Table 1: Examples Series 1-6

[0105]

[0106] Table 2: Examples Series 7-12

[0107] The conductivity of different variants of the manufactured embodiment series was examined after manufacturing and aging. Figure 1 The results are shown in the image. Specifically, Figure 1 The conductivity (in μS / cm) of the water extracts (unimpregnated base paper, black), impregnated with 10% polyvinyl alcohol binder (PVAb) 1 (white), and impregnated with 15% PVAb 1 (shaded) is shown according to IEC 60554-2. Legend for the horizontal axis from bottom to top: cellulose%, glass%, PA%, PVAf%.

[0108] The initial tensile index (in Nm / g) of the samples was also tested using 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 the figure. The horizontal axis, from bottom to top, represents the legend: cellulose %, presence and type of PVA, PVA %. White and shaded lines do not contain heat stabilizers and use either glass or polyamide, respectively. Black and dotted filler bars use glass and polyamide, respectively, with Dicy as the heat stabilizer. Figure 2In this context, the acronym TU is used to indicate thermal upgrade paper that conforms to the IEC 60076-14 standard.

[0109] 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 examples with 50% non-cellulose fiber content and high PVAb content had very high initial tensile strength. The use of PVAf appears to increase the initial tensile index of examples containing 30% or 10% non-cellulose fibers.

[0110] Furthermore, a simplified and rapid aging test was developed to promote the primary degradation mechanism of cellulose materials in liquid-immersed transformers, namely acidic hydrolysis: paper, conditioned at 50% relative humidity (RH) and 23°C with its initial moisture content (approximately 7%), is placed in a 0.5L sealed container with air at 50% RH and then placed in an oven at a given temperature for a given time. For example, tests were conducted at 140°C until 50% tensile retention was achieved. The simplified aging test is particularly rapid due to the presence of water. The latter promotes acidic hydrolytic degradation.

[0111] Specifically, aging tests were conducted on a series of different manufactured examples. Specifically, the retention rate of tensile index (as a percentage relative to the initial tensile index) was evaluated. Accelerated aging tests were performed by exposing the manufactured electrical insulating paper to 155°C for three consecutive days in a moisture-trapping sealed tube. Figure 3 The results are shown in the figure.

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

[0113] from Figure 3 It can be seen that the following embodiments, in particular, have a particularly good tensile index retention rate (approximately 70% or higher):

[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 fiber, 10% to 15% PVAb and 1.9% N / UKP.

[0118] Further observation revealed a potential synergistic effect in terms of the higher stretch index retention rate between the PVAb and Dicy combinations.

[0119] Figure 4 The contribution to tensile index retention (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 contributions to tensile index retention are shown for Dicy, PVAb, and combinations of both (circular markers).

[0120] like Figure 4 As demonstrated, there is a synergistic effect between PVAb and Dicy. In fact, in Figure 4 In the graph, the bars represent the cumulative effect of individual Dicy and PVAb on the stretch index retention, while the points represent the stretch index retention of the Dicy and PVAb blend. It is readily apparent that these point numbers are significantly higher than those in the bars, indicating an improvement in stretch index retention when using blends of these compounds.

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

[0122] According to another embodiment, adhesive 1 (PVAb) can be wholly or partially replaced by poly(vinyl alcohol-co-ethylene) with a degree of hydrolysis of at least 88 mol%, i.e., PVAmb can be used alone, or a blend of PVAmb and PVAb can be used.

[0123] The fifth group of electrical insulating paper is made of cellulose fibers and glass fibers. UKP is used as the cellulose fiber. However, this disclosure is not limited to this. For example, other cellulose fibers can be used. Specifically, the base paper is manufactured with different ratios of cellulose and PA fibers, namely a series using 70% by weight of cellulose fibers and 30% by weight of glass fibers, and a series using 90% by weight of cellulose fibers and 10% by weight of glass fibers. These two series are hereinafter referred to as Example Series 13 and 15. 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.

[0124] The sixth group of electrical insulating paper is made of cellulose fibers and polyamide (PA). UKP is used as the cellulose fiber. However, this disclosure is not limited to this. For example, other cellulose fibers can be used. Specifically, the base paper is manufactured with different ratios of cellulose and PA fibers, namely a series using 70% by weight of cellulose fibers and 30% by weight of PA fibers, and a series using 90% by weight of cellulose fibers and 10% by weight of PA fibers. These two series are hereinafter referred to as Example Series 14 and 16. PA fibers with 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 with a length of 3 mm to 8 mm and a linear density of 1.2 dtex to 2.0 dtex are used.

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

[0126] Table 3 below summarizes the manufacturing parameters for the different variants of the above-described series of embodiments 13-16.

[0127]

[0128] Table 3: Examples Series 13-16

[0129] The different variants of the manufactured embodiment series 13-16 were inspected after manufacturing. Figure 5 The results are shown in the figure. The electrical conductivity of the water extracts of unimpregnated base paper (black), paper impregnated with PVAb and PVAmb at a ratio of 15% in a 1:1 ratio (corresponding to paper series 13 and 14) (horizontal stripes), paper impregnated with 15% PVAmb (shaded lines), paper impregnated with PVAb and PVAmb at a ratio of 10% in a 1:1 ratio (corresponding to paper series 15 and 16) (grey), and paper impregnated with 10% PVAmb (white) were measured according to IEC 60554-2.

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

[0131] Figure 6 The initial tensile index corresponding to the prototypes in series 13-16 is shown compared to unimpregnated base paper and base paper impregnated with 10 wt% or 15 wt% PVAmb. Legend for the horizontal axis from bottom to top: cellulose %; presence of resin composition. It is noteworthy that 100% fiber blends are 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 %) for prototypes corresponding to series 13-16 is shown compared to base paper that was not impregnated with resin or impregnated with 10 wt% or 15 wt% PVAmb. Legend for the horizontal axis from bottom to top: cellulose %; presence of resin composition. It is worth noting that 100% fiber blends were achieved by the synthetic fibers used (glass or PA), and the composition of the prototypes corresponds to the compositions listed in Table 3.

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

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

[0135] Aspect 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 is from 1% to 4% by weight of the cellulose fibers.

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

[0137] Aspect 3. The electrical insulating paper according to aspect 1, comprising: cellulose fibers in an amount of at least 45% by weight, optionally at least 65% by weight, based on the total weight of the electrical insulating paper; and synthetic fibers in an amount of 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, said synthetic fibers being glass fibers, or blends of glass fibers with polyamide fibers such as aliphatic polyamide fibers.

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

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

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

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

[0142] Aspect 8. The electrical insulating paper according to item 7, wherein the adhesive comprises a polyvinyl alcohol adhesive, 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 adhesive comprises polyvinyl alcohol or modified polyvinyl alcohol.

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

[0145] Aspect 11. The electrical insulating 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 adhesive 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, characterized in that the adhesive comprises hot-melt fibers, and the electrical insulating paper comprises hot-melt fibers in a content of 5% to 18% by weight, preferably 9% to 17% by weight, or more preferably 10% to 16% by 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 fiber is 2 mm to 12 mm, optionally 3 mm to 8 mm, and / or the linear density is 0.4 dtex to 7.0 dtex, optionally 1.2 dtex to 2.0 dtex.

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

[0154] Aspect 20. The electrical insulating paper according to any one of the preceding aspects, 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.

[0155] Aspect 21. The electrical insulating paper according to aspect 20, wherein the diameter of the glass fiber is 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 fiber is 2 mm to 12 mm, preferably 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 of manufacturing electrical insulating paper, the method comprising the steps of: providing cellulose fibers and synthetic fibers; - manufacturing base paper on a paper machine with the cellulose fibers and the synthetic fibers comprising at least 25% by weight of the cellulose fibers and at least 5% by weight of the synthetic fibers, based on the total weight of the electrical insulating paper; wherein the method further comprises adding a heat stabilizer comprising nitrogen, wherein the nitrogen comprises, by weight, 1% to 4% by weight of the content of the cellulose fibers.

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

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

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

[0162] Aspect 28. The method according to aspect 27, wherein the adhesive is a resin, which is coated onto the base paper after the addition of the nitrogen-containing heat stabilizer.

[0163] Aspect 29. The method according to aspect 27, wherein the adhesive is a resin, which is applied to the base paper immediately after the step of manufacturing the base paper and before the addition of the nitrogen-containing heat stabilizer.

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

[0165] Aspect 31. The method according to aspect 27, wherein the adhesive is a resin, the resin further comprising the nitrogen-containing heat stabilizer, the resin being applied to 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 of hot melt fibers at a weight content of 8 to 18%, optionally 9 to 17% or 10 to 16%.

[0167] Aspect 33. The 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 to 7.0 dtex, preferably 1.2 dtex to 2.0 dtex.

[0168] Aspect 34. The method according to any one of Aspects 27 to 33, wherein the electrical insulating paper is made of resin, preferably containing 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 according to aspect 36, wherein the modified polyvinyl alcohol adhesive comprises poly(vinyl alcohol-co-ethylene).

[0172] Aspect 38. The method according to 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 fiber comprises kraft paper fiber, cotton fiber, flax fiber, hemp fiber, and wherein the cellulose fiber is unbleached, bleached, and / or semi-bleached hardwood fiber and / or softwood fiber.

[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 diameter of the glass fiber is from 3 μm to 30 μm, preferably from 6 μm to 20 μm, and more preferably from 8 μm to 15 μm.

[0181] Aspect 47. The method according to any one of Aspects 24 to 46, wherein the length of the polyamide fiber is 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 from 0.4 dtex to 7.0 dtex, preferably from 1.2 dtex to 2.0 dtex.

[0183] Aspect 49. A method of manufacturing electrical insulating paper according to any one of Aspects 30, 32 and 33, comprising, prior to the step of adding the nitrogen-containing heat stabilizer or coating the base paper with the resin, 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. An electrical insulating paper comprising at least one layer manufactured according to any one of Aspects 24 to 49.

[0185] Aspect 51. Use of an electrical insulating paper according to any one of Aspects 1 to 23 or 50, for insulating conductors of high-voltage liquid-immersed transformers or dry-type transformers.

[0186] Aspect 52. Use of an electrical insulating paper according to any one of Aspects 1 to 23 or 50, for insulating conductors used in traction transformers.

[0187] Aspect 53. Use of an electrical insulating paper according to any one of Aspects 1 to 23 or 50, for use in low-voltage foil windings in distribution transformers.

[0188] Aspect 54. A high-voltage liquid-immersed transformer comprising at least one conductor insulated by 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 conductor insulated by 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 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 electrical insulating 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 can be made to the disclosed apparatus and systems without departing from the scope of this disclosure. Other aspects of this disclosure will become apparent to those skilled in the art upon consideration of the specification and practice of the features disclosed herein. This specification and embodiments are intended to be considered exemplary only. Many additional variations and modifications are possible and should be understood to fall within the framework of this disclosure.

Claims

1. An electrical insulating paper, said electrical insulating paper comprising the following: Based on the total weight of the electrical insulating paper, at least 25% by weight of cellulose fibers are present; Based on the total weight of the electrical insulating paper, at least 5% by weight of synthetic fibers, said synthetic fibers comprising aliphatic polyamide fibers and / or glass fibers; and A nitrogen-containing heat stabilizer, wherein the nitrogen content, by weight, is from 1% to 4% of the cellulose fiber content, and Optionally, an adhesive.

2. The electrical insulating paper according to claim 1, comprising: Based on the total weight of the electrical insulating paper, at least 50% by weight of cellulose fibers; and Based on the total weight of the electrical insulating paper, the content of synthetic fibers is from 7% to 35% by weight.

3. The electrical insulating paper according to claim 1, comprising: Based on the total weight of the electrical insulating paper, at least 45% by weight of cellulose fibers; and Based on the total weight of the electrical insulating paper, the content of synthetic fibers is 5% to 55% by weight.

4. The 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. The electrical insulating paper according to any one of the preceding claims, characterized in that, It further comprises the adhesive, the amount of which is 5% to 20% by weight based on the total weight of the electrical insulating paper, and the adhesive is selected from hot melt fibers, resins or mixtures thereof.

6. The electrical insulating paper according to claim 5, wherein the adhesive is a resin, optionally comprising the nitrogen-containing heat stabilizer.

7. The electrical insulating paper according to claim 6, wherein the resin comprises polyvinyl alcohol.

8. The electrical insulating paper according to any one of the preceding claims, wherein the heat stabilizer is substantially composed of dicyandiamide.

9. The electrical insulating paper according to claim 1, wherein the electrical insulating paper is impregnated with the heat stabilizer.

10. A method for manufacturing an electrical insulating paper, said electrical insulating paper comprising cellulose fibers, synthetic fibers, a heat stabilizer, and optionally an adhesive, said method comprising the following steps: - Provide the cellulose fiber and the synthetic fiber, wherein the synthetic fiber comprises aliphatic polyamide fiber and / or glass fiber; - A base paper is made on a paper machine from the cellulose fibers and the synthetic fibers, and optionally a binder, with a content of at least 25% by weight of the cellulose fibers based on the total weight of the electrical insulating paper and a content of at least 5% by weight of the synthetic fibers based on the total weight of the electrical insulating paper. The heat stabilizer is added during the manufacturing process of the base paper or after the manufacturing process of the base paper, wherein the heat stabilizer contains nitrogen, and the nitrogen content is from 1% to 4% by weight of the cellulose fiber content.

11. The method of claim 10, further comprising the step of adding the adhesive, wherein the amount of the adhesive is from 5% to 20% by weight based on the total weight of the electrical insulating paper, and the adhesive is selected from hot melt fibers, resins, or mixtures thereof.

12. The method of claim 11, wherein the adhesive is a resin, which is coated onto the base paper after the addition of the nitrogen-containing heat stabilizer.

13. The method of claim 11, wherein the adhesive is a resin, which is applied to the base paper immediately after the step of manufacturing the base paper and before the addition of the nitrogen-containing heat stabilizer.

14. The method of claim 11, wherein the adhesive is a hot melt fiber, which is mixed with the cellulose fiber and the synthetic fiber prior to the step of manufacturing the base paper.

15. The method of claim 11, wherein the adhesive is a resin, the resin further comprising the nitrogen-containing heat stabilizer, and the resin is applied to the base paper immediately after the step of manufacturing the base paper.

16. The method of manufacturing electrical insulating paper according to claim 14, comprising, prior to the steps of adding the nitrogen-containing heat stabilizer or coating the base paper with the resin, 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.

17. The method of claim 10, wherein the electrical insulating paper is impregnated with the heat stabilizer.

18. A transformer comprising electrical insulating paper according to any one of claims 1 to 9, wherein the transformer is selected from a high-voltage liquid-immersed transformer, a dry-type transformer, or a traction transformer.

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