Overview
The operational reliability and service life of a transformer, however, depend to a large extent on the insulating materials used. Insulating materials, also known as dielectrics, are substances with high resistivity and low conductivity. Insulating materials can be used to isolate conductors that are charged or at different potentials and allow current to flow in a certain direction. In transformer products, insulating materials also play the roles of heat dissipation, cooling, support, fixation, arc extinguishing, improving potential gradient, moisture-proof, mildew-proof and protecting conductors. Under the action of DC voltage, only a very small amount of current passes through the insulating material. Its resistivity (referring to the volume resistivity in air) is relatively high, generally 108~1020Ω·cm (the resistivity of conductors is 10-6~10-3Ω·cm, and the resistivity of semiconductors is 10-3~108Ω ·cm).
Insulating materials have very high resistance to DC current. Due to their high resistivity, under the action of DC voltage, except for a very small surface leakage current, they are actually almost non-conductive; while for AC current, they have capacitance. Electric current passes through it, and it is generally considered to be non-conductive. The greater the resistivity of the insulating material, the better its insulating properties are.
Insulating materials are used in transformers to insulate conductive parts from each other and from the ground (zero potential). When used in various supports, they should also have good mechanical properties. In addition, insulating materials also play other roles, such as heat dissipation, cooling, fixation, energy storage, arc extinguishing, improving potential gradient, moisture-proof, mildew-proof and conductor protection.
Typically, insulation materials are divided into three categories:
1) Gas insulation materials: Under normal temperatures and pressures, general dry gases have good insulation properties, such as air, nitrogen, hydrogen, carbon dioxide, sulfur hexafluoride, etc. Among them, air and sulfur hexafluoride are used in transformers. widely;
2) Liquid insulation material: Liquid insulation material usually exists in oil form, also known as insulating oil. Such as mineral oil, vegetable oil, synthetic ester, 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 epoxy resin, electrical porcelain, rubber, mica products, etc. .
Insulating oil
Insulating oil is characterized by high electrical strength, high lightning, low freezing point, high performance under the action of oxygen, high temperature, and strong electric field, non-toxic, non-corrosive, low viscosity, and good fluidity. It is widely used in electrical products such as transformers, oil switches, capacitors and cables. It plays the role of insulation, cooling, impregnation and filling. In addition, it also plays the role of arc extinguishing in oil switches and energy storage in capacitors.
Insulating oil plays the dual roles of insulation and cooling in transformers;
Insulating oils are currently generally divided into the following categories:
1) Mineral oil: such as transformer oil, switch oil, capacitor oil, cable oil;
2) Synthetic oil: such as dodecylbenzene, silicone oil, synthetic ester, etc.;
3) Vegetable oil;
Epoxy resin
Epoxy resin is a polymer compound. Resin is characterized by a solid, semi-solid or quasi-solid organic material with an uncertain molecular mass (usually higher), a tendency to flow when subjected to stress, and usually a softening or melting range, and the solid cross-section is often shell-shaped. 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) A long molecular chain is composed of the smallest repeating unit, that is, a chain link. The number of chain links in a molecule is called the degree of polymerization;
3) The total intermolecular force of macromolecules often exceeds the chemical bond force between atoms in the molecule, which causes the polymer compound to exhibit a series of characteristics: for example, there is no gaseous polymer, the polymer dissolution process is very slow, etc. If there are cross-links between molecules, This feature is even more outstanding.
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 industrialized and synthesized bisphenol A epoxy resin. Our country started production in 1956.
The electrical insulation properties of epoxy materials are particularly outstanding. Without fillers, the EB of the cured product is higher than 16MV/m, pV is higher than 1011Ω·m, εr is 3 to 4, and tanδ is about 0.002 at power frequency. Therefore, 20% epoxy material Oxygen resins are used for electrical and electronic insulation. For example, epoxy impregnating paint is used as Class B insulating paint to impregnate the stator windings of small and medium-sized motors; epoxy solvent-free paint is used for vacuum impregnation of stator windings of large motors; laminates (plates, tubes, Rods) are used as slot wedges and pads for motors and high-voltage switch operating rods; adhesives are used for bonding high-voltage porcelain bushings; castables are used for disc isolation in sulfur hexafluoride fully enclosed combined electrical appliances (GIS) Components such as insulators, transformers and high voltage ceramic capacitors. At present, the brand names of domestically produced epoxy resins or modified epoxy resins are still not uniform. Different epoxy resin manufacturers around the world are also named differently and need to be identified by trademark.
Epoxy resins are simply oligomers and must be cured before use. The curing agent can react with the epoxy resin to cross-link the resin molecules from a linear structure into a linear structure. Accelerator/catalyst is an additive that can reduce the activation energy of the reaction and can promote/adjust the gel reaction process of the castable. The curing agent uses the active hydrogen it contains to perform a ring-opening addition reaction with the active epoxy group in the resin to achieve curing. The active hydrogen is -NH2, -NH-, -C00H, -OH and -SH in the curing agent or accelerator. of hydrogen. Commonly used curing agents include amines and acid anhydrides. Some curing agents require accelerators/catalysts, some require high temperature conditions, and some can react violently at low temperatures. Differences in curing agents will also lead to vast differences in the properties 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 is used in dry-type transformers and is a new development in the past 40 years. The design life of the transformer coil is required to reach 30 years, and the heat resistance level reaches F level. It is difficult for ordinary materials to meet the requirements.
To this end, the materials used, their formula systems and processes must be designed, optimized, tested and verified in order to obtain the ideal results. In resin-insulated dry-type transformers, the epoxy resin system is molded by casting or dipping, and then heat-cured to form coil insulation (i.e., longitudinal insulation). During the entire operation of the dry-type transformer, the epoxy resin insulation must also ensure the electrical insulation of the coils. and mechanical strength, and dissipates the heat inside the coil through thermal conduction.
Its biggest weakness is the irrecoverability and irrepairability of resin insulation defects and damage (generally defects occur during the manufacturing process and damage occurs during operation). Therefore, it is extremely important to avoid cracking of solid insulation, avoid casting defects, and avoid partial discharge (i.e., partial discharge). It has become the key to solid insulation manufacturing technology and the focus of competition among manufacturers.
Due to the high temperature rise caused by losses during the operation of the transformer, the resin insulation works at high temperatures for a long time (for example, the maximum designed operating temperature of F-class transformers is generally around 140°C), and the transformer may be at high temperatures before being put into operation and during maintenance. Low temperature (such as -30℃), and the transformer will be subject to high voltage lightning impact or huge electric power impact during short circuit at any time. Resin-insulated coils should be able to adapt to these changes and withstand or withstand short-circuit electrodynamic impacts under extreme high and low temperatures. Therefore, extremely stringent requirements are placed on the thermal, mechanical, and electrical properties of the epoxy insulation system.
There are currently two insulation material systems for resin cast transformers, one is "pure resin casting + high filling rate glass fiber reinforcement", the other is "resin quartz powder casting + pre-impregnated glass grid local reinforcement".
The insulation system (commonly known as the insulation structure) covers a wider field than the insulation material system. It refers to the insulation of the electrical equipment (or its independent components), including not only the insulation materials and their combinations, but also the insulation and conductors. Or the relationship between magnets, the relationship with electric fields, the relationship between insulation and the surrounding environment (gas or liquid and its conditions, surface contamination, heat dissipation conditions, mechanical force or radiation effects, etc.), etc., and its adaptability to the operating parameters of the power system It's the insulation fit. The air flow and heat dissipation conditions, insulation stress conditions, etc. in dry-type transformers are all within the scope of the insulation system that must be considered.
Insulating paper
Plant fiber paper is divided into wood fiber, cotton fiber and hemp fiber. Among them, the most commonly used is pure sulfate wood pulp fiber paper. Its raw material is wood. Commonly used softwood is Pinaceae wood, such as yellow pine, white pine, and red pine. The main component of wood such as fir and red pine is cellulose, which is a natural polymer compound. The manufacturing method of insulating paper adopts chemical methods, such as the sulfate method. In this method, the main component of the cooking liquid is sodium sulfide (Na2S). The sodium sulfide is hydrolyzed to generate sodium hydrogen sulfide and sodium hydroxide. Sodium hydrogen sulfide can react with wood other than cellulose. The cellulose reacts and dissolves it in the alkali solution. The cooking liquid is relatively mild, so the molecular weight of the cellulose decreases very little. Commonly used plant cellulose insulation papers in transformers include: power cable paper, high-voltage cable paper and transformer inter-turn insulation paper, etc.
1) Cable paper: Cable paper is made from sulfate pulp. The grades are DL08, DL12 and DL17. The thicknesses are 0.08mm, 0.12mm and 0.17mm respectively. It is 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 power cable paper in the air is 6~9×103kV/m, and after drying and impregnating transformer oil, its electrical strength reaches 70~90×103kV/m. 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; insulating crepe paper is made from electrical insulation paper Made of wrinkle processing, there are wrinkles along its transverse direction, and it is pulled apart when stretched. It is often used for wrapping insulation of oil-immersed transformers, such as insulation wrapping of coil heads, leads and electrostatic shielding devices; high-density cable paper is also an insulating material. A kind of crepe paper, its electrical strength is 100% to 150% higher than ordinary crepe paper, and its mechanical strength is 50% higher. It has high electrical strength, good oil resistance, good elasticity, and is easy to stretch. It can be used as a lead instead of varnished cloth tape. and insulation of wire connections and bends.
2) Telephone paper: Telephone paper is also made of sulfate pulp and is commonly used in telephone cables. It has poor mechanical strength and is generally used as turn insulation, layer insulation or covering insulation of conductors.
3) Capacitor paper: Capacitor paper is divided into Class A and Class B according to usage requirements. Class A capacitor paper is used for metallized paper dielectric capacitors in the electronics industry. Class B is mainly used as the interelectrode dielectric of power capacitors. Capacitor paper is characterized by high tightness and thin thickness. Generally, capacitor paper is often used for current transformers and less frequently for transformers.
4) Wound insulating paper: Wound insulating paper is used as the base paper of the adhesive paper. The adhesive paper is used to wind the insulating cylinder (tube) and capacitive bushing. Its characteristic is that the water absorption height is higher than that of the cable paper and lower than that of the impregnated paper. Paper. Gum paper is divided into single-sided or double-sided glue (phenolic or epoxy resin), which is cured at low temperature. When rolling paper tubes or pressing laminated products, the glue is finally cured when heated and pressurized. , the roll generally uses single-sided tape, and the pressed tape board uses double-sided tape. In addition, there is diamond-dot adhesive paper (grid-dot adhesive paper), which is used for interlayer insulation of oil-immersed foil-wound coils. After curing, it ensures the adhesion between insulation and between insulation and foil, enhancing the It has good strength and good oil permeability.
Commonly used transformer insulating papers are cable paper, crepe paper and rhombus dotted paper, which are used in transformers as inter-turn insulation, inter-layer insulation, lead binding, etc. Generally, the prices of various types of insulating paper are about the same. It will be too big, about 20 yuan/kg.