The main materials of the transformer body include magnetic circuit materials, circuit materials, insulating material , structural materials, etc. The specific material uses and categories are:
1. silicon steel sheet
In transformers, the requirements for the performance of silicon steel are mainly:
① Low iron loss, which is the most important indicator of the quality of silicon steel sheets. All countries divide the grades according to the iron loss value. The lower the iron loss, the higher the grades.
②The magnetic induction strength (magnetic induction) is high under a strong magnetic field, which reduces the volume and weight of the motor and transformer, saving silicon steel sheets, copper wires and insulating materials.
③The surface is smooth, flat and uniform in thickness, which can improve the filling coefficient of the iron core.
④ Good punching performance and easy to process.
⑤ The surface insulating film has good adhesion and welding properties, which can prevent corrosion and improve the punching properties.
⑥ basically has no magnetic aging. Classification and grade definition of
silicon steel sheets
transformers usually use cold rolled oriented silicon steel sheets to ensure their no-load energy efficiency level. According to performance and processing methods, cold-rolled oriented silicon steel sheets can be divided into ordinary cold-rolled oriented silicon steel sheets, high-magnetic silicon steel sheets (or high-magnetic inductive silicon steel sheets), and laser-score silicon steel sheets. Generally, silicon steel sheets with the minimum magnetic polarization strength B800A=1.78T~1.85T achieved by the core under 50Hz, 800A alternating magnetic field (peak value) are called ordinary silicon steel sheets and are denoted as "CGO", while silicon steel sheets with B800A=1.85T or above are denoted as high magnetic permeability silicon steel sheets (high magnetic inductance silicon steel sheets), and are denoted as "Hi-B steel". The main difference between Hi-B steel and conventional silicon steel sheets is: the Gaussian square of Hi-B steel. The texture degree is very high, that is, the grain arrangement and alignment of silicon steel in the easy magnetization direction is very high. In the industrial process, the secondary recrystallization process is used to manufacture silicon steel sheets with a silicon content of 3%. The average deviation of the grain position direction of Hi-B steel from the rolling direction is 3°, while ordinary silicon steel sheets are 7°, which makes Hi-B steel have higher magnetic permeability. Usually its B800A can reach more than 1.88T, which improves the Gaussian azimuth texture and magnetic permeability can reduce iron loss. Another feature of Hi-B steel is that the elastic tension of the glass film and insulating coating attached to the surface of the steel sheet is 3~5N/mm2, which is better than the 1~2N/mm2 of ordinary oriented silicon steel sheets. The high tension layer on the surface of the steel strip can reduce the magnetic domain width and reduce abnormal eddy current losses. Therefore, Hi-B steel has a lower iron loss value than conventionally oriented silicon steel sheets.
laser scoring silicon steel sheet is based on Hi-B steel. Through laser beam irradiation technology, the surface of its surface is slight strain, further refine the magnetic axis, and achieve lower iron loss. Laser-scored silicon steel sheets cannot be annealed because the temperature is increased, the laser treatment effect will disappear.
The physical characteristics of silicon steel sheets of different grades are basically the same, and the density is basically 7.65g/cm3. For the same type of silicon steel sheet, the main difference in its performance quality is the influence of the silicon content and the process of the production process.
2. Amorphous Alloy Iron Core
Amorphous Alloy material is a new alloy material that was launched in the 1970s. It uses internationally advanced ultra-quenching technology to directly cool the liquid metal at a cooling rate of 106℃/S to form a solid thin strip of 0.02-0.03mm, which has solidified before it has time to crystallize. The alloy material is arranged in irregular atomic arrangement similar to glass, and has no metal-characterized crystal structure. Its basic elements are iron (Fe), nickel (Ni), cobalt (Co), silicon (Si), boron (B), carbon (C), etc. Its material has the following advantages:
a) Amorphous alloy material does not have a crystal structure and is an isotropic soft magnetic material; it has small magnetization power and has good temperature stability. Since the amorphous alloy is an unoriented material, direct seams can be used to make the process of manufacturing the core relatively simple;
b) There are no structural defects that hinder the movement of magnetic domains, and the hysteresis loss is smaller than that of silicon steel sheets;
c) The thickness of the strip is extremely thin, only 0.02~0.03mm, which is about 1/10 of that of silicon steel sheets.
d) has a high resistivity, about 3 times that of oriented silicon steel sheets; the eddy current loss of amorphous alloy materials is greatly reduced, so the unit loss is about 20% to 30% of oriented silicon steel sheets;
e) has a low annealing temperature, about 1/2 of oriented silicon steel sheets;
amorphous alloy core has excellent no-load performance. The transformer made of amorphous alloy core has a no-load loss of 70 to 80% lower than that of conventional transformers, and the no-load current is reduced by more than 50%, and the energy-saving effect is outstanding. At present, under the purpose of national energy conservation, emission reduction and reduction of network line losses, the power grid company has vigorously increased the procurement ratio of amorphous alloy transformer since 2012. At present, the proportion of amorphous alloy distribution and procurement has basically reached more than 50%.
amorphous alloy transformer also has the following disadvantages:
1) The saturation magnetic density is low, the saturation magnetic density of amorphous alloy core
Figure 1 Amorphous alloy transformer
is usually around 1.56T, which is about 20% different from the saturation magnetic density of 1.9T of conventional silicon steel sheets. Therefore, the transformer design magnetic density also needs to be reduced by 20%. The design magnetic density of amorphous alloy oil transformation is usually below 1.35T, and the design magnetic density of amorphous alloy dry transformation is usually below 1.2T.
2) Amorphous total core strip is sensitive to stress. After the core strip is stressed, the no-load performance is prone to deterioration. Therefore, special attention should be paid to the structure. The core should be placed on the support frame and coil in suspension. The overall weight is only subject to its own gravity. At the same time, special attention should be paid during assembly. The core cannot be subjected to stress, and knocking should be reduced.
3) magnetostriction is about 10% larger than conventional silicon steel sheets, so its noise is difficult to control, which is also one of the main reasons for limiting the widespread promotion of amorphous alloy transformers. At present, both South Grid and State Grid bids have put forward high requirements for the noise of amorphous alloy transformers, which are divided into sensitive zones and non-sensitive zones, and have targeted sound level requirements, which requires further reduction of the magnetic density of the core design.
4) Amorphous alloy strip is thinner and has a thickness of only 0.03mm, so it cannot be made into stacked form like conventional silicon steel sheets, and can only be made into coiled cores. Therefore, conventional transformer manufacturers cannot process their core structures themselves and usually need to be purchased as a whole. Corresponding to the rectangular cross-section of the coiled core strip, the coils of amorphous alloy transformer are usually also made into rectangular structures;
5) The degree of domestic production is not enough. At present, it is mainly imported amorphous alloy strips of Hitachi Metal, and are gradually realizing domestic production. There are already Antai Technology and Qingdao Yunlu and other amorphous alloy broadbands (213mm, 170mm and 142mm), and their performance is still a certain gap in stability compared with imported strips.
6) Maximum band length limit. The maximum peripheral band length of the amorphous alloy strip in the early stage is also greatly limited due to the limitation of the size of the annealing furnace. However, it has been basically solved at present. An amorphous alloy core frame with a maximum peripheral band length of 10m can be made, which can be used to manufacture dry transformation of amorphous alloys of 3150kVA and below and amorphous alloy oil transformation of amorphous alloys below 10000kVA and below.
is based on the excellent energy-saving effect of amorphous alloy transformers, coupled with the promotion of national energy conservation and emission reduction and a series of policies, the market share of amorphous alloy transformers is becoming increasingly large. Moreover, considering that the price of amorphous alloy strips (currently 26.5 yuan/kg) is about twice that of conventional silicon steel sheets (30Q120 or 30Q130), the gap with copper is relatively small, and considering the quality and bidding requirements of power grid products, amorphous alloy transformers usually use copper conductors. Compared with conventional silicon steel sheets, the main cost gap between amorphous alloy transformers is as follows:
1) Since the coiled core structure is adopted, the transformer core type should adopt a three-phase five-column structure, which can reduce the weight of a single frame core and reduce the assembly difficulty. The three-phase five-column structure and the three-phase three-column structure have their own advantages and disadvantages in terms of cost. Currently, most manufacturers use three-phase five-column structures. The purchased single-frame core and assembly are shown in Figure 2:
Figure 2 Procurement of amorphous alloy strip single-frame raw material
Figure 3 Three-phase five-column amorphous alloy core assembly diagram
2) Since the cross-section of the core column is rectangular, in order to maintain consistency in the insulation distance, the high and low voltage coils are also manufactured into a rectangular structure.
3) Since the core design magnetic density is about 25% lower than that of conventional silicon steel sheet transformers, and the core lamination coefficient is about 0.87, which is much lower than that of conventional silicon steel sheets. Therefore, its design cross-sectional area needs to be more than 25% larger than that of conventional silicon steel sheet transformers, and the corresponding high and low voltage coil circumference should also increase accordingly. At the same time, it is also necessary to consider the increase in the turn length of the high and low voltage coils. If the load loss of the coil does not change, the wire cross-sectional area needs to be increased accordingly. Therefore, the copper consumption of amorphous alloy transformers is about 20% higher than that of conventional transformers.
3. Circuit Materials
Overview
The internal circuit of the transformer is mainly composed of winding (also known as coils). It is directly connected to the external power grid and is the core component of the transformer. The internal circuit of the transformer is usually composed of wire windings. The conductors (electromagnetic wires) are divided into copper wires and aluminum wires according to the material. According to the cross-sectional shape of the conductor, it is divided into circular wires, flat wires (can also be divided into single wires, combined wires and transposition wires), foil conductors, etc. The conductors and wires are covered with different types of insulation layers, and finally form an integral coil. Therefore, the main conductor materials of the transformer circuit are copper and aluminum.
3.1 Characteristics of copper and aluminum
Both copper and aluminum are metal materials with good conductivity and are commonly used conductors for making transformer coil . The differences in physical properties are shown in the table below:
Table 1 Physical properties of copper and aluminum
3.2 Performance of copper and aluminum conductors in transformer windings
0 The differences of copper and aluminum transformers are also determined by the differences in materials, which are specifically reflected in the following aspects:
1) The resistivity of copper conductors is only about 60% of that of aluminum conductors. In order to achieve the same loss and temperature rise requirements, the cross-sectional area of aluminum conductors must be more than 60% larger than that of copper conductors. Therefore, under the same capacity and the same parameters, the volume of aluminum conductor transformer is usually larger than that of copper conductors. However, the heat dissipation area of the transformer also increases at this time, so its oil temperature rise is lower;
2) The density of aluminum is only about 30% of copper, so the aluminum conductor distribution transformer is lighter than that of copper conductors;
3) The melting point of aluminum conductors is much lower than that of copper conductors, so its temperature rise limit for short-circuit current is 250℃, which is lower than that of copper conductors, so its design electrical density is lower than that of copper conductors, and the voltage transformation is The cross-sectional area of the conductor wire is larger, so the volume is larger than that of the copper conductor transformer;
4) The hardness of the aluminum conductor is lower, so its surface burrs are easier to eliminate. Therefore, after making a transformer, the probability of short circuit between turns or between layers caused by the burrs is reduced;
5) Due to the low tensile and compressive strength of the aluminum conductor and poor mechanical strength, the aluminum conductor transformer has a short circuit capability not as good as that of the copper conductor transformer. When performing dynamic stability calculations, the stress of the aluminum conductor should be less than 450kg/cm2, while the stress limit of the copper conductor is 1600kg/cm2, which greatly improves the bearing capacity;
6) The welding process between the aluminum conductor and the copper conductor is poor, and the welding quality of the joint is not easy to ensure, which affects the reliability of the aluminum conductor to a certain extent.
7) The specific heat of an aluminum conductor is 239% of that of a copper conductor. However, considering the difference in density and design electrical density between the two, the actual difference in thermal time constants of the two is not as large as that of the specific heat difference, so it does not have much impact on the short-term overload capability of the dry transformer .
4. Insulation material
Overview The reliability and service life of the transformer
is largely determined by the insulating material it uses. Insulating materials, also known as dielectrics, are substances with high resistance coefficient and low conductivity. Insulating materials can be used to isolate conductors with charged or different potentials, so that current flows in a certain direction. In transformer products, the insulating material also plays the role of heat dissipation, cooling, supporting, fixing, arc extinguishing, improving potential gradient, moisture-proof, mildew-proof and protecting conductors. Under the action of DC voltage, only extremely small current passes through the insulating material. Its resistance coefficient (referring to the volume resistance coefficient in air) is relatively high, generally between 108~1020Ω·cm (the resistance coefficient of the conductor is 10-6~10-3Ω·cm, and the resistance coefficient of the semiconductor is 10-3~108Ω·cm).
insulating material has a very large resistance to DC current. Because its resistivity is very high, under the action of DC voltage, in addition to extremely small surface leakage current, it is actually almost non-conductive; and for AC current, capacitive current passes through, which is generally considered non-conductive. The greater the resistivity of an insulating material, the better its insulation performance.
insulating material is used in transformers to insulate the conductive parts between the conductive parts from each other to ground (zero potential). When used in various support parts, it should also have good mechanical properties. In addition, the insulating material also plays other roles, such as cooling, fixing, energy storage, arc extinguishing, improving potential gradient, moisture-proof, mildew-proof and protective conductors.
Normally, insulating materials are divided into three categories:
1) Gas insulation materials: Under normal temperature and pressure, general dry gases have good insulation properties, such as air, nitrogen, hydrogen, carbon dioxide, sulfur hexafluoride, etc. Among them, air and sulfur hexafluoride are widely used in transformers;
2) Liquid insulation materials: Liquid insulation materials usually exist in oil, also known as insulating oil. Such as mineral oil, vegetable oil, synthetic esters, etc.;
3) Solid insulating materials: such as insulating paint , insulating glue, insulating paper , insulating cardboard, corrugated cardboard, electrical plastics and films, electrical laminates (rods, tubes), cast-formed epoxy resin, electrical porcelain, rubber, mica products, etc.
4.1 Insulation oil
Insulation oil is characterized by high electrical strength, high lightning, low freezing point, performance temperature under the action of oxygen, high temperature and strong electric field, non-toxic, non-corrosive, small viscosity, good fluidity, etc. It is widely used in electrical products such as transformers, oil switches, capacitors and cables. It plays an insulating, cooling, impregnating and filling role. In addition, it also plays an arc extinguishing role in oil switches and also plays an energy storage role in capacitors.
insulating oil plays a dual role in insulation and cooling in the transformer;
insulating oil is currently generally divided into the following categories:
1) Mineral oil: such as transformer oil , switching oil, capacitor oil, cable oil;
2) Synthetic oil: such as dodecyl benzene, silicone oil, synthetic ester, etc.;
3) vegetable oil;
4.2 Epoxy resin
Epoxy resin is a polymer compound. Resin is characterized by a solid, semi-solid or quasi-solid organic material with uncertain molecular weight (usually high), tends to flow when stressed, usually has a softening or melting range, and the solid section often has a shell-shaped shape, which has the following basic characteristics:
1) The molecular chain is very long, each chain contains hundreds or even tens of thousands of atoms, which are covalently bonded to each other;
2) The long molecular chain consists of the smallest repeating unit, i.e. links, and the number of links in a molecule is called the degree of polymerization;
3) The total intermolecular force of a large molecule often exceeds the chemical bond force between atoms in the molecule, thus causing a series of characteristics of polymer compounds: for example, there is no gaseous polymer, the polymer dissolution process is very slow, etc. If there is crosslinking between molecules, this characteristic is more special.
Epoxy resin refers to an oligomer containing epoxy functional groups. Epoxy resin began to appear in 1891. After 1947, many companies in the United States and Switzerland successfully synthesized bisphenol A epoxy resins in industrialization. my country began production in 1956.
The electrical insulation performance of epoxy materials is particularly outstanding. When there is no filler, the EB of the cured substance is higher than 16MV/m, the pV is higher than 1011Ω·m, and the εr is 3 to 4. Tanδ is about 0.002 at the power frequency. Therefore, 20% epoxy resin is used for electrical and electronic insulation. For example, epoxy impregnated paint is used as B-grade insulating paint, impregnated with small and medium-sized motor stator windings; epoxy solvent-free paint is used for vacuum impregnation of large motor stator windings; laminated products (plates, tubes, rods) are used as slot wedges and pads of motors, high-voltage switch operating rods; adhesives are used for bonding of high-voltage ceramic sleeves; castables are used for disc-shaped isolators, transformers and high-voltage ceramic capacitors in sulfur hexafluoride fully enclosed combined electrical appliances (GIS). At present, the brand names of epoxy resins or modified epoxy resins produced in China are still not very uniform. The naming of different epoxy resin manufacturers around the world is also different and needs to be identified according to the trademark.
Epoxy resin is just an oligomer and can only be used after curing.The curing agent can react with the epoxy resin to cross-link the resin molecules from the linear structure into a bulk structure. A promoter/catalyst can reduce reaction activation energy and can promote/adjust the reaction history of the castable gel. The curing agent uses the active hydrogen contained in it to undergo a ring-opening addition reaction with the active epoxy group in the resin to achieve curing. Active hydrogen is the hydrogen in -NH2, -NH-, -C00H, -OH and -SH in the curing agent or promoter. Commonly used curing agents are amines and acid anhydrides. Some of the curing agents need accelerators/catalysts, some require high temperature conditions, and some can react violently at low temperatures. Different curing agents will also lead to huge differences in performance of cured products, which will have a significant impact on the final performance of the product. Therefore, it is very important to design and select curing agents in epoxy resin formulation systems.
epoxy insulation used in dry transformers is a new development in the past 40 years. The design life of the transformer coil is required to reach 30 years, the heat resistance level reaches F, and it is difficult for general materials to meet the requirements.
For this reason, if the materials used and their formula systems and processes must be designed, selected, tested and verified in order to achieve ideal results. In resin-insulated dry transformers, the epoxy resin system is formed by casting or impregnation, and then heat curing to form coil insulation (i.e. longitudinal insulation). During the entire operation of the dry transformer, the epoxy resin insulation must ensure the electrical insulation and mechanical strength of the coil at the same time, and dissipate heat inside the coil through heat conduction.
's biggest weakness is the irrecovery and irreparable nature of resin insulation defects and damage (generally defects occur during manufacturing and damage during operation). Therefore, it is particularly important to avoid solid insulation cracking, avoid casting defects, and avoid partial discharge (i.e., local discharge). It has become the key to solid insulation manufacturing technology and is the focus of competition among manufacturers.
Due to the high temperature rise caused by the loss during the operation of the transformer, the resin insulation operates at high temperature for a long time (such as F-class transformer, the designed maximum working temperature is generally around 140℃), and the transformer may be at low temperatures before operation and during maintenance (such as -30℃). Moreover, the transformer will be subjected to high-voltage impacts of lightning or huge electric power impacts during short circuit at any time. The resin-insulated coil should be able to adapt to these changes and can withstand or withstand the impact of short-circuit electric power at extremely high and low temperatures, so it puts forward extremely demanding requirements on the thermal, mechanical and electrical properties of the epoxy insulation system. There are currently two types of insulating material systems for
resin casting transformers. One is "pure resin casting + high-filling rate glass fiber reinforcement", and the other is "resin quartz powder casting + pre-precipitated glass mesh locally strengthened".
The insulating system (commonly referred to as insulating structure) contains a wider field than the insulating material system. It refers to the insulation whole of electrical equipment (or its independent components). It not only includes insulating materials and their combination methods, but also considers the relationship between insulation and conductors or magnets, the relationship with electric field, the relationship between insulation and the surrounding environment (gas or liquids and their conditions, surface filth, heat dissipation conditions, mechanical force or radiation effects, etc.). The adaptability between it and the operating parameters of the power system is insulation coordination. The airflow and heat dissipation conditions, insulation stress conditions, etc. in dry transformers are all within the scope of consideration of the insulation system.
4.3 Insulating paper
Plant fiber paper is divided into wood fiber, cotton fiber and hemp fiber. The most commonly used is pure sulfate wood pulp fiber paper. Its raw material is wood. Commonly used is the pine family wood of cork, such as yellow pine, white pine , erma and red pine. The main component is cellulose, which is a natural polymer compound. The insulating paper manufacturing method adopts chemical methods, such as the sulfate method. In this method, the main components of the cooking liquid are sodium sulfide (Na2S), and sodium sulfide hydrolyzes to produce sodium hydrogen sulfide and sodium hydroxide. The sodium hydrogen sulfide can react with lignin other than cellulose and dissolve it in alkaline liquid. The cooking liquid is relatively mild, so the molecular weight of cellulose drops very little. The commonly used plant cellulose insulating papers in transformers include: power cable paper, high-voltage cable paper and transformer interturn insulating paper, etc.
1) Cable paper: Cable paper is made of sulfate pulp, with grades DL08, DL12, DL17, with thicknesses of 0.08mm, 0.12mm and 0.17mm respectively, and is supplied in a roll. After the cable paper is impregnated by transformer oil, its mechanical strength and electrical strength will be significantly improved. For example, in the air, the electrical strength of power cable paper is 6~9×103kV/m, and after drying and impregnating the transformer oil, its electrical strength reaches 70~90×103kV/m. Cable paper has sufficient thermal stability during the operation of the transformer 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 insulated crumpled 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 insulation of power cables with 35kV and below and transformers or other electrical products; insulating crumpled paper is made of electric insulating paper through wrinkles, with wrinkles along its transverse direction and is pulled apart when stretched. It is often used for wrapping insulation of oil-immersed transformer , such as insulating bandage of coil outlets, leads and electrostatic shielding devices; high-density cable paper is also a type of insulating crumpled paper, which is 100% to 150% higher than ordinary crumpled paper, with 50% higher electrical strength, good oil resistance, good elasticity, and easy to stretch. It can replace lacquered fabric tape as insulation for leads, wire connections and bending parts.
2) Telephone paper: Telephone paper is also made of sulfate pulp and is commonly used with telephone cables. Its mechanical strength is poor and is generally used as a turn insulation of a conductor, layer insulation or cover insulation of a conductor.
3) Capacitor paper: Capacitor paper is divided into Class A and Class B according to the usage requirements. Class A capacitor paper is used on dielectric capacitors of metallized paper in electronic industry. Class B is mainly used as the interpole inter-pole medium of power capacitors. The characteristics of capacitor paper are large tightness and thin thickness. Generally, capacitor paper is often used for current transformers, and transformers are used less.
4) Roll-wrapped insulating paper: Roll-wrapped insulating paper is used as the base paper for adhesive paper, and adhesive paper is used to wind the insulating barrel (tube) and capacitive sleeve. Its characteristic is that the water absorption height is higher than that of cable paper and lower than that of impregnated paper. The adhesive paper is divided into single-sided or double-sided adhesive (phenolic or epoxy resin), and is cured at low temperature. When the adhesive paper is used to roll the paper tube or press the laminated product, the adhesive finally cures after heating and pressing. The reel is generally made of single-sided adhesive paper, and double-sided adhesive paper is used to press the adhesive paperboard. In addition, there is also diamond-dyeing paper (mesh-format dot-coated paper), which is used for interlayer insulation of oil-immersed foil winding coils. After curing, it ensures the bond between the insulation and between the insulation and the foil, which enhances strength and has good oil permeability.
Most commonly used transformer insulating paper are cable paper, crumpled paper and diamond glue paper, which are used in transformers as interturn insulation, interlayer insulation, lead binding, etc. Generally, the price difference between various insulating papers is not too large, and they are about 20 yuan/kg.
4.4 Electrical composite materials
Electrical films and electrical composite materials have excellent dielectric properties and are both thin insulating materials. Electrical films include polyester film and polyimide film , which can also be used as wire insulation, interlayer insulation, etc. in transformers. Electrical composite materials are composite products made of one or two-sided bonded fiber materials of thin films. They can be used as interlayer insulation in transformers, especially in dry-changed foil winding coils. Low-voltage coils are usually pre-soaked with composite materials and used as interlayer insulation. Commonly used composite materials include DMD, GHG, etc.
DMD is the full name of polyester film polyester fiber nonwoven fabric soft composite material, which is divided into prepreg resin DMD and non-prepreg DMD. It is a three-layer soft composite material made by pasting polyester fiber nonwoven fabric (D) on both sides of a layer of polyester film (M). DMD has excellent electrical insulation, heat resistance and mechanical strength, as well as excellent impregnation properties. Non-precipitation DMD can be used as interlayer insulation for oil-immersed transformers, and pre-precipitation DMD can be used as interlayer insulation for low-voltage foil winding coils for F-class dry transformers. Its specific performance indicators are shown in the table below:
Table 2 DMD performance parameters
GHG is a polyimide film prepreg H-grade resin glass fiber soft composite material. It is a three-layer soft composite material made by pasting glass fiber cloth (G) on both sides of a layer of polyimide film (H).Compared with DMD, it has better heat resistance and can be used for interlayer insulation of low-voltage foil winding coils of H-level insulated dry transformers.
NHN is the full name of polyimide film polyaramide fiber paper soft composite material. It is made by bonding polyaramide fiber paper (Nomex) with H-grade adhesive on both sides of the polyimide film. NHN is currently the highest-end thin-layer insulating material, with excellent heat resistance, good dielectric properties, less water absorption and excellent moisture resistance. It belongs to H-class insulating material and can be used for interlayer insulation of H-class dry transformers. Its specific performance parameters are shown in the table below:
Table 3 NHN performance parameters
4.5 Insulated cardboard
Insulated cardboard is made of pure sulfate wood pulp papermaking. It can be used in oil gap pads, oil gap straps, partitions, cardboard barrels, corrugated paper, iron yoke insulation, clamp insulation and end-insulated winding pressure plates, etc. of cake windings. Its commonly used thickness is 1.0mm, 1.5mm, 2mm, 3mm, 4mm, and 6mm. Insulated cardboard is divided into low-density cardboard, medium-density cardboard and high-density cardboard according to its density. Low-density paper is usually called T3 soft cardboard, with a density between 0.75g/cm3 and 0.9g/cm3 and has a low strength. It is often used to make stretched parts after being wetted, such as molded corner rings, ring parts and soft paper barrels. Low-density cardboard has high oil absorption rate and good moldability, but poor mechanical properties; medium-density board is usually called T1 cardboard, with a density between 0.95g/cm3 and 1.15g/cm3 and is used as a strap pad, etc.; high-density cardboard is usually called T4 cardboard, with a density between 1.15g/cm3 and 1.3g/cm3 and is used as an insulating cardboard barrel, an insulating pressure plate and end ring. In the oil-plate spacing structure composed of multi-layer paper barrels with high-voltage coils, corrugated cardboard can also be used instead of cardboard straps to form an oil gap, which can save materials on the basis of ensuring insulation performance.
4.6 Polypropylene film
Polypropylene film is made of extruded thick sheets of polypropylene resin (PP) and is stretched by directionally. Its characteristics are: 1) Small density, stretchable 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, relative dielectric coefficient is 2~2.2, 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℃ and belongs to Class E insulation; 5) It has hydrophobicity and strong water absorption ability, and can be used for wire insulation of oil-immersed transformers.
4.7 Other insulating materials
Transformer oil and insulating paper are the main insulating materials of oil-immersed transformer coils. Resin, insulating paper, and composite materials are the main insulating materials of dry 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, tightening tape, weftless tape, etc.
1) Laminate: Electrical laminated products are insulating materials with layered structures made of paper, cloth and wooden veneer as base materials, immersed (or coated) with different adhesives, and hot pressing (or rolled). According to the requirements of use, laminates can be made into products with excellent electrical and mechanical properties and heat resistance, oil resistance, mold resistance, arc resistance and corona resistance. Laminated products mainly include laminates, laminated wood, laminated tubes, rods, capacitor sleeve dies and other special profiles. The performance of the laminate depends on the properties of the substrate and adhesive and its forming process. According to the different raw materials and adhesives, laminates are divided into insulating laminates (paperboard, used for oil transformation), phenolic laminated paperboard (commonly known as bakery board, cardboard is impregnated with phenolic resin, used for oil transformation), phenolic laminated cloth board (cotton cloth is impregnated with phenolic resin, commonly used for oil transformation), epoxy glass cloth board (glass fiber cloth is used with epoxy resin as the adhesive, can be used for F-grade dry transformation or oil transformation), modified diphenyl ether glass cloth board (glass fiber cloth is used with modified diphenyl ether resin as the adhesive, can be used for H-grade dry transformation), and bismaleimide glass cloth board (glass fiber cloth is used with bismaleimide resin as the adhesive, can be used for H-grade dry transformation). Laminates usually have good mechanical strength and insulation properties, and are often used as core clamp insulation, external support, etc. in transformers.
2) Insulated cylinder (tube): The insulated cylinder 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 wires are directly wound on the insulated cylinder. At the same time, the insulated cylinder can also be used for main insulation, increasing the number of main insulation oil gaps and enhancing insulation. According to the different materials, insulating cylinders are divided into phenolic paper cylinders (commonly used for oil transformation), epoxy glass cloth cylinders (commonly used for oil transformation or F grade dry transformation), modified diphenyl ether glass cloth cylinders (commonly used for H grade dry transformation), fiberglass cylinders (commonly used for H grade dry transformation), bismaleimide glass cloth cylinders (commonly used for H grade dry transformation), etc.
3) Laminated wood: Electrical laminated wood uses high-quality hardwood, such as birch, beech, etc., and is steamed twice at 70℃~80℃. After removing the lucid acid and grease of the wood itself, it is cut into a 1-3mm veneer, dried and applied with resin adhesive. After pre-curing, it is stacked repeatedly by stacking the blanks. It has good insulation strength and mechanical strength. It can be used as a pad, corner ring, etc. in oil transformation.
4) Binding belt: The transformer is used to tie belts with cotton cloth tapes, compaction belts, mesh semi-dry weftless belts, glass cloth tapes, polyester belts, etc., which are used for binding and tightening of cores and coils.
5. Material structure and accessories
In the transformer, there are structural materials and accessories. The structural materials mainly play the functions of transformer support, magnetic circuit, circuit reinforcement, transformer insulation liquid packaging, etc., including clamps, oil tanks, radiators, oil storage cabinets, etc. The main material is Q235 steel. Magnetic steel is often used for the outlet casing position of the oil tank cover to reduce eddy current. In addition, there are sometimes magnetic steel or high-grade steel inside the transformer body.
transformer accessories mainly have performance monitoring and protection functions. Dry transformation includes thermostats, fans, transformers, etc., and oil transformation includes gas relays, thermostats, pressure relief valves, tap switches, etc. Some accessories requirements are proposed by customers.
