2.2 Dielectric loss

2.2.1 Dielectric loss

Under the action of AC voltage, part of the electrical energy in the dielectric will be converted into heat energy. This part of the energy is called dielectric loss. It is mainly caused by conductivity and slow relaxation polarization. It is also the root cause of electric breakdown of the dielectric.The energy consumed per unit time is usually called dielectric loss power

2.2.2 The form of dielectric loss

Divided into: free loss, dipole loss, conductivity loss, uneven dielectric loss

2.3 Insulation aging

2.3.1 The concept of insulation aging

During the operation of electrical equipment, due to various factors for a long time, a series of irreversible chemical and physical changes occur in the insulating material, which leads to the deterioration of electrical and mechanical properties. This irreversible change is often called aging.

Insulation aging is divided into thermal aging, environmental aging and electrical aging.The factors that cause aging are heat, electricity, light, oxygen, radiation and microorganisms.

2.3.2 Thermal aging

Due to heat, the aging of insulating materials is called thermal aging.In the process of thermal aging, polymer insulating materials often undergo thermal degradation, low molecular weight production or escape.The insulation properties and mechanical properties of the insulating material are reduced.

2.3.3 Electrical aging

In electrical equipment, the performance of insulating materials under the action of an electric field changes irreversible until they fail. This process is called electrical aging.The main cause of electrical aging of insulating materials is partial discharge.Due to the partial discharge of ozone, ozone is a strong oxidizing agent, which causes the addition reaction of macromolecules containing double bonds, and ozone cracking of the material occurs. Zhan Jun

2.3.4 Environmental aging

Environmental aging refers to the polluting chemical reactions caused by insulating materials under the action of light, oxygen, radiation, acid-base and other factors, of which ultraviolet rays in sunlight are the main factors.3 Solid insulation material

The solid insulating material used in transformers refers to the insulating material that is solid in its own form or has undergone chemical reactions and physical changes to become solid.There are many kinds of solid insulation materials for transformers, such as insulating paper, insulating cardboard, Nomex tape, adhesive paper, electrician laminated wood, epoxy glass cloth board, low dielectric loss laminate, insulating paint, insulating glue, cotton tape, tightening tape, mesh weft-free polyester tape and so on.This training will introduce the main solid insulation materials used in transformers to everyone.

3.1 Insulating paper

The insulating paper described here refers to pure sulfate wood pulp paper, and other insulating paper includes synthetic fiber paper (such as NOMEX paper), thickened insulating paper (such as Denison paper) and so on.

3.1.1 Insulating paper is divided into two categories: plant fiber paper and synthetic fiber paper.

3.1.2 The relationship between the molecular structure of cellulose and the characteristics of insulating paper

3.1.2.1 Impregnation

Since the molecular structure of cellulose consists of hydroxyl groups and primary alcohols, cellulose has good hygroscopicity.If cellulose is placed in humid air, water molecules will penetrate between the macromolecules of cellulose, moistening the capillaries in the cellulose structure.Therefore, the insulating paper has a certain impregnability. The impregnation of insulating paper is closely related to the processes of pulping, beating and paper making.From the perspective of pulping, the purer the pulp is made, the better the quality and the greater the impregnation.

3.1.2.2 Mechanical properties

The mechanical strength of the paper first depends on the molecular weight of the cellulose.The greater the molecular weight, the greater the strength.Starting from this basic concept, if you require higher-strength insulating paper, you can use wood as a raw material instead of hemp fiber or cotton fiber, because the molecular weight of cotton and hemp fiber is much larger than that of wood fiber. Molecular weight.

3.1.2.3 Electrical performance

3.1.3 The main insulating paper for oil-immersed transformers

In oil-immersed transformers, the insulating papers often used are power cable paper, high-voltage cable paper and transformer inter-turn insulating paper. Their performance indicators are introduced below.

3.1.3.1 Power cable paper

Power cable paper is used for the insulation of power cables and transformers or other electrical products of 35KV and below.The products are divided into three levels: DLZ-V, DLZ-A and DLZB, and the thickness is divided into three types: 80um, 130um and 170um.This cable paper is web paper.

3.1.3.2 High voltage cable paper

High-voltage cable paper is suitable for 110~330KV transformers, which is web paper

3.1.3.3 Transformer turn insulation paper

Transformer turn insulation paper is also a kind of high-voltage cable paper, but the performance of the paper is better. It can be used for 550KV transformers, transformers and reactors. There are three forms of water content in insulation paper and insulating cardboard; 1. Chemically bound water.2. Physical and chemical binding to water (adsorption of water).3. Physical and mechanical combination of water.

Among them, the chemical binding water has the strongest binding force, and the general content is below 0.1%. If the transformer is dried, if the binding water is treated, the degree of polymerization of the insulating cardboard will decrease and the material will age.For ultra-high voltage transformers, the water content of the dried material is between 0.2% and 0.3%.

Physical and chemical binding water refers to the physical adsorption of binding water, which is based on the molecular interaction between the material and the water.Due to the internal and external activity of cellulose fibers, the liquid is adsorbed by physical and chemical gravity.The moisture content absorbed by the physical and chemical forms is between 8% and 9%.

Physically and mechanically bound water refers to free water. Through the surface tension of the liquid in the capillary of the material, it is present in the large capillary system, that is, it is present in the cell cavity and cell gap, and the material is physically and mechanically bound. In the manufacture of insulating paper and cardboard, the free water is first removed during the drying process of paper and cardboard, and then the water is partially adsorbed, and its water content is controlled at about 6%.

3.1.3.4 Telephone paper

Telephone paper is made of unbleached sulfate wood pulp and is mainly used in the production of telephone cables. Some transformer factories also use it in the production of transformers. Telephone paper is also used as the base paper for insulating crepe paper.

Telephone paper is divided into three categories: A, B, and C according to quality, of which A and B are used to manufacture communication cables.Telephone paper is divided into two specifications: DH-50 and DH-75，

Telephone paper is web paper.

3.1.3.5 Capacitor paper

Capacitor paper is divided into category A and Category B. Category A is used for metallized paper dielectric capacitors in the electronics industry, and Category B is used for standard capacitor paper for power capacitors, which is characterized by high tightness and thin thickness.

3.1.3.6 Roll-wrapped insulating paper

Winding insulating paper is used as the base plate of the adhesive paper (The adhesive paper has single-sided adhesive paper, double-sided adhesive paper and grid adhesive paper, and the adhesive paper is used to wrap the insulating tube and the capacitive sleeve.The characteristic of rolled insulating paper is that the water absorption is higher than that of cable paper and lower than that of impregnated paper.Its thickness is 0.07mm, 0.09mm.

3.1.3.7 Impregnated insulating paper

Impregnated insulating paper is an insulating paper used to suppress laminated products after impregnating resin. According to the purpose of laminated products, impregnated insulating paper is divided into: sulfate wood pulp paper, sulfite wood pulp paper.The biggest feature of impregnated insulating paper is that it has good impregnation properties, its water absorption height is the highest among insulating papers, and the impregnated insulating paper is neutral.

3.2 Insulating crepe paper

Insulating crepe paper is made of insulating paper for electricians by wrinkling.There are wrinkles along its transverse direction, and the wrinkles are pulled apart when stretched.Due to different wrinkling processing procedures, crepe paper with different elongation can be made.The thickness of the insulating paper is generally 0.05mm~0.12mm, and the elongation range is 5%~200%.Insulating crepe paper is commonly used in the winding insulation of oil-immersed transformers, such as the insulating bandage of winding heads, leads and insulating screens.

3.3 Denison paper

The modified insulating paper is turned into nitrile paper, and in the manufacturing process, bis-nitrile amine, ethylene cyanide, etc. are added to the pulp.Produced by Denison in the United States, it is characterized by the fact that after the paper is wrinkled and calendered, it has good mechanical strength and appropriate elongation. It is an ideal material for the insulation of the windings of large transformers. It can be used to bandage the transposition wire to ensure tightness without swelling. Phenomenon.