The Application of Insulating Materials in Transformers

1. Insulating paper

Plant fiber paper is divided into wood fiber, cotton fiber and hemp fiber. Among them, pure sulphate wood pulp fiber paper is the most commonly used. Its raw material is wood. Commonly used is the pine fir wood in softwood, such as yellow pine, white pine, and red pine. Wood such as cedar and red pine, whose main component is cellulose, is a natural polymer compound. Insulating paper manufacturing methods use chemical methods, such as the sulfate method, in which the main component of the cooking liquor is sodium sulfide (Na2S), which is hydrolyzed to produce sodium hydrogen sulfide and sodium hydroxide. Sodium hydrogen sulfide can be combined with wood other than cellulose. The element reacts and dissolves it in the lye. The cooking liquor is relatively mild, so the molecular weight of the cellulose decreases very little. The commonly used plant cellulose insulating papers in transformers include: power cable paper, high-voltage cable paper and transformer turn-to-turn insulating paper.

1) Cable paper: Cable paper is made of sulphate pulp, grades DL08, DL12, DL17, thicknesses of 0.08mm, 0.12mm and 0.17mm, respectively, supplied in rolls. After the cable paper is impregnated with transformer oil, its mechanical strength and The electrical strength will be significantly improved. For example, the electrical strength of the power cable paper in the air is 6-9×103kV/m, and the electrical strength of the dry immersed transformer oil will reach 70-90×103kV/m. The operation of the cable paper in the transformer It has sufficient thermal stability and is usually used as winding insulation and interlayer insulation. Cable paper also includes high-voltage cable paper, low-voltage cable paper, high-density cable paper and insulating crepe paper. High-voltage cable paper is suitable for 110～330kV transformers and transformers, with low dielectric loss tangent value; low-voltage cable paper is used for the insulation of power cables and transformers or other electrical products of 35kV and below; insulation crepe paper is made of electrical insulation paper It is made by wrinkle processing and has wrinkles along its transverse direction. It is pulled apart when stretched. It is often used for winding insulation of oil-immersed transformers, such as insulation wrapping of coil heads, leads and electrostatic shielding devices; high-density cable paper is also insulation A type of crepe paper, 100% to 150% higher in electrical strength than ordinary crepe paper, 50% higher in mechanical strength, high electrical strength, oil resistance, good elasticity, easy to stretch, and can be used as a lead wire instead of varnished tape And insulation of wire connections and bending parts.

2) Telephone paper: Telephone paper is also made of sulphate pulp. It is commonly used with telephone cables. It has poor mechanical strength and is generally used as wire insulation, layer insulation or conductor covering insulation.

3) Capacitor paper: Capacitor paper is divided into Type A and Type B according to usage requirements. Type A capacitor paper is used for metallized paper dielectric capacitors in the electronics industry. Type B is mainly used as the inter-electrode medium of power capacitors. Capacitor paper is characterized by high tightness and thin thickness. Generally, current transformers often use capacitor paper, and transformers are used less.

4) Winding insulation paper: Winding insulation paper is used as the bottom paper of the gluing paper, and the gluing paper is used to wind the insulating tube (tube) and the capacitive sleeve. Its characteristic is that the water absorption height is higher than that of the cable paper and lower than the impregnation Paper. Glued paper is divided into single-sided or double-sided glue (phenolic or epoxy resin), which is cured at low temperature. When glued paper is used to roll paper tubes or press laminates, the glue will finally cure when heated and pressurized. , The roll generally uses single-sided tape, and double-sided tape is used for pressed cardboard. In addition, there is rhombus dispensing paper (mesh-type dispensing paper), which is used for interlayer insulation of oil-immersed foil winding coils. After curing, it can ensure the adhesion between insulation and between insulation and foil, and strengthen Strength and good oil permeability.

Cable paper, crepe paper and rhombic dispensing paper are commonly used in conventional transformer insulation paper. They are used in transformers as inter-turn insulation, inter-layer insulation, lead binding, etc. Generally, the prices of various insulation papers are not different. It will be too big, about 20 yuan/kg.

2. Electrical composite materials

Electrical thin films and electrical composite materials have excellent dielectric properties, and both belong to sheet insulating materials. Electrical films include polyester film and polyimide film, which can also be used for wire insulation and interlayer insulation in transformers. Electrical composite material is a composite product made by bonding fiber materials on one or both sides of a film. It can be used as interlayer insulation in transformers, especially in dry-changed foil-wound coils. Low-voltage coils are usually prefabricated with composite materials. After being impregnated with resin, it is used as interlayer insulation. Commonly used composite materials include DMD, GHG and so on.

GHG's full name is polyimide film pre-impregnated H-grade resin glass fiber soft composite material, which is a three-layer soft composite material made of a layer of polyimide film (H) pasted with glass fiber cloth (G) on both sides . Compared with DMD, it has better heat resistance and can be used for interlayer insulation of low-voltage foil-wound coils of H-class insulated dry-type transformers.

NHN's full name is polyimide film polyaramid fiber paper soft composite material, which is made by bonding polyamide fiber paper (Nomex) with H-grade adhesive on both sides of polyimide film. NHN is currently the most high-grade thin-layer insulating material, with excellent heat resistance, better dielectric properties, less water absorption and excellent moisture resistance. It belongs to Class H insulating material and can be used for interlayer insulation of Class H dry-type transformers.

3. Electrical composite materials

The full name of DMD is polyester film polyester fiber non-woven soft composite material, which is divided into prepreg DMD and non-pre-impregnated DMD. It is a layer of polyester film (M) pasted on both sides of polyester fiber non-woven fabric ( D) Three-layer soft composite material made. DMD has excellent electrical insulation, heat resistance, mechanical strength, and excellent impregnation. Non-pre-preg DMD can be used as interlayer insulation of oil-immersed transformers, and pre-preg DMD can be used as inter-layer insulation of low-voltage foil-wound coils of class F dry-type transformers.

4. Insulating cardboard

Insulating cardboard is made of pure sulphate wood pulp paper. It can be used for oil gap pads, oil gap stays, partitions, cardboard tubes, corrugated paper, iron yoke insulation, clip insulation and end insulation winding pressure plates for cake windings. The usual thickness is 1.0mm, 1.5mm, 2mm, 3mm, 4mm, 6mm. Insulating paperboard is divided into low-density paperboard, medium-density paperboard and high-density paperboard according to the density. Low-density paper is usually called T3 soft paperboard. Between 0.75g/cm3～0.9g/cm3, the strength is low. It is often used for bending parts or making stretched parts after wetting, such as forming corner rings, ring parts and soft paper tubes. Low-density paperboard has high oil absorption, good formability, but poor mechanical properties; medium-density board is usually called T1 paperboard, with a density of 0.95g/cm3～1.15g/cm3, used as a spacer block, etc.; high density Paperboard is usually called T4 cardboard, with a density of 1.15g/cm3 to 1.3g/cm3, used as an insulating cardboard tube, insulating pressing plate, and end ring. In the oil-board spacing structure composed of multi-layer paper tubes of high-voltage coils, corrugated cardboard can also be used instead of cardboard stays to form an oil gap, which can save materials on the basis of ensuring insulation performance.

5. Polypropylene film

Polypropylene film is made of polypropylene resin (PP) extruded thick sheet and oriented stretched. Its characteristics are: 1) The density is small, can be stretched to 0.06mm or even thinner, and its density is 0.89g/cm3～ 0.92g/cm3. 2) It has good electrical properties and chemical stability, the relative dielectric coefficient is 2～2.2, and the breakdown pressure is greater than 150MV/m; 3) It has good mechanical properties, and its tensile strength is greater than 100MPa 4) It can be used for a long time at 125°C and belongs to Class E insulation; 5) It is hydrophobic and has a strong ability to prevent water absorption, and can be used for wire insulation of oil-immersed transformers.

6. Other insulating materials

Transformer oil and insulating paper are the main insulating materials for oil-immersed transformer coils. Resin, insulating paper, and composite materials are the main insulating materials for dry-type transformer coils. In addition to these materials, the following insulating materials are commonly used in transformers: (Laminated wood, laminate, insulating paint, insulating glue, cotton tape, shrink tape, weftless tape, etc.

1) Laminates: electrical laminates are insulating materials with a layered structure made of paper, cloth and wood veneer as a substrate, impregnated (or coated) with different adhesives, and hot pressed (or rolled). According to the application requirements, laminated products can be made into products with excellent electrical and mechanical properties, heat resistance, oil resistance, mildew resistance, arc resistance and corona resistance. Laminated products mainly include laminated boards, laminated wood, laminated tubes, rods, capacitor casing cores and other special profiles. The performance of laminates depends on the nature of the substrate and the adhesive and its molding process. According to different raw materials and adhesives, laminates are divided into insulating laminates (paperboard, used for oil change), phenolic paper board (commonly known as bakelite, paper board impregnated with phenolic resin, used for oil change), and 3026 phenolic laminated cotton sheet (Cotton cloth impregnated with phenolic resin, often used for oil change), FR4 sheet (glass fiber cloth with epoxy resin as adhesive, can be used for F grade dry change or oil change), modified diphenyl ether glass cloth board (Glass fiber cloth uses modified diphenyl ether resin as the binder, which can be used for H-class dry change), bismaleimide glass cloth board (glass fiber cloth uses bismaleimide resin as the binder, Can be used for H-class dry change). Laminates usually have good mechanical strength and insulation properties, and are often used as core clamp insulation and external support in transformers.

2) Insulation tube (tube): The insulation tube in the transformer is mainly used between the inner and outer coils, between the coil and the core, and is used for the coil lining skeleton. The wire is directly wound on the insulation tube. At the same time, the insulation tube can also be used for the main Insulation, increase the number of main insulation oil gaps and enhance insulation. Insulation cylinders are divided into phenolic paper cylinders (usually used for oil transformation), epoxy glass cloth cylinders (usually used for oil transformation or F grade dry transformation), and modified diphenyl ether glass cloth cylinders (usually used for Grade H dry change), glass fiber reinforced plastic tube (usually used for H grade dry change), bismaleimide glass cloth tube (usually used for H grade dry change), etc.

3) Laminated wood: Use high-quality hardwood for electrical laminated wood, such as birch, beech, etc., and cook it twice at 70°C to 80°C to remove the wood's own wood acid and grease. The board is dried and then coated with resin adhesive. After pre-curing, it is formed by repeated assembly and stacking. It has good insulation strength and mechanical strength. It can be used as a cushion, corner ring, etc. in oil change.

4) Binding straps: lashings for transformers are equipped with cotton tape, shrink tape, mesh semi-dry weftless tape, glass cloth tape, polyester tape, etc., which are used for tying and tightening iron cores and coils.

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