Aging factors of insulation structure of the hotte

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Insulation structure, aging factors and corresponding measures of voltage transformer

in the design of transformer, the insulation part generally adopts the traditional design method, that is, the inspection and statistics method is used to determine the insulation life of transformer. In recent years, modern design methods have appeared in the insulation design methods of transformer, such as finite element analysis method, reliability design technology and optimization design technology. With the development of computer technology, large-scale professional analysis and optimization software is used for analysis and design. The scheme obtained by using modern design methods is more economical and reasonable than that designed by using traditional design methods. Because modern design methods are still in development, the specific content of modern design methods is difficult to determine, but at present, there are many more mature methods, which can be summarized as follows: optimization design, finite element analysis, reliability analysis, modeling design, value engineering, design expert system; Computer aided design, etc. Moreover, these methods can be cross applied to form a variety of methods with unique application objects, such as reliability optimization design

I. insulation structure of voltage transformers

voltage transformers are divided into dry-type, cast-in-place, oil immersed and gas type according to the insulation medium. The insulation structure of voltage transformers is based on different voltage levels According to the different use environment, different methods are adopted. After the size of the data block is specified by the computer (the formula can be changed as needed. Usually, the low-voltage transformer specially used for measurement is dry-type, and the high-voltage or ultra-high-voltage sealed gas insulated (such as sulfur hexafluoride) transformer is also dry-type. The pouring type is applicable to voltage transformers of 35kV and below, and products of 35kV and above are oil immersed. Compared with the traditional oil immersed and gas filled transformer, epoxy resin cast transformer has the advantages of no oil, no gas, no shell, lifelong maintenance free, good insulation performance and so on. It is a kind of transformer widely used in the world at present

1. Dry insulation structure

dry insulation structure is simple to manufacture and low in cost, but the insulation strength is not high, so the voltage of this structure is generally not more than 380V. The wire adopts QZ enameled round copper wire, and the insulation materials used in the insulation structure mainly include: insulating paper, glass fiber cloth belt, phenolic plastic, etc. Bakelite or plastic framework insulation is used between the coil and the iron core, interlayer insulation and the main insulation between the primary and secondary coils, generally using yellow wax silk or polyester film. The insulation between the annular core and coil of instrument voltage transformer is generally made of insulating cardboard, and then wrapped with oneortwo layers of glass ribbon, yellow wax silk or polyester film tape. The dry-type structure is simple, but the volume is large. It is only suitable for low-voltage indoor devices, or instrument voltage transformers of 10kV and below

2. Cast insulation structure

the so-called cast insulation refers to the solid insulation formed by curing and molding after mixing resin, filler, pigment and curing agent in a certain proportion and pouring them into the mold containing the primary and secondary windings and other parts of the transformer. The cured mixed glue not only fixes the relevant parts, but also is the main insulation of the transformer. Casting insulation has the advantages of good insulation performance, high mechanical strength, moisture resistance, fire prevention and so on. At room temperature or high temperature, the resin mixed glue has good fluidity, can fill small gaps, and is easy to cast into complex shapes. The resin mixed glue also has a strong bonding effect, and can firmly bond metals and many insulating materials together. It is an ideal insulation molding material for instrument transformers. At present, it is widely used in indoor instrument transformers with voltage levels of 35kV and below

at present, epoxy resin and unsaturated resin are widely used in instrument transformers in China. Unsaturated resin is cheap, can be cured at room temperature, and the pouring process is simple, but its electrical and mechanical strength is low, and its heat resistance is poor; In addition, the saturated vapor pressure of unsaturated resin is high, so the mixed glue should not be degassed in vacuum, nor should it be vacuumized during pouring, and there are bubbles in the pouring body; The curing shrinkage of unsaturated resin is large, and the mixed adhesive is easy to crack when curing. Therefore, unsaturated resin is only suitable for low voltage products. The curing shrinkage of epoxy resin is small, and the saturated steam pressure is low. It is suitable for pouring under high temperature and high vacuum, so that the fluidity of the mixed adhesive is better, and the degassing can be maximized, so as to obtain a castable with ideal performance

there are two kinds of cast insulation: half cast insulation and full cast insulation: half cast insulation is that the coil is cast separately, and then the iron core is installed; Full pouring type is to pour the coil and iron core together after installation. Epoxy resin full cast voltage transformer has the following characteristics: (1) it realizes oil-free and protects the environment; (2) It realizes maintenance free, saves a lot of human and material resources and power outage time; (3) Its manufacturing materials are non combustible or flame-retardant and self extinguishing substances, which are flame-retardant and explosion-proof

the insulation of the coil to the iron core and ground is borne by the resin, and bubbles or conductive impurities are required not to be sandwiched during pouring. Cable paper or composite insulating paper is used for insulation between coil layers. The main insulation between primary and secondary coils adopts epoxy resin cylinder, phenolic paper cylinder or cable paper cylinder impregnated with vacuum pressure paint. The plastic pouring type has compact structure and convenient maintenance, and is suitable for 3-10kV indoor devices

epoxy resin is a chemical raw material that has been widely used for a long time. It is not only a fire-retardant and flame-retardant material, but also has superior electrical properties. Later, it was gradually adopted by the electrical manufacturing industry. The cast-in-place voltage transformer has compact structure and simple maintenance. With the development of outdoor resin, it will gradually be used in outdoor products above 35kV. Cast voltage transformers are divided into semi cast and full cast according to the casting form. Among them, the primary winding and each low-voltage winding, as well as the two bushings at the outlet end of the primary winding are cast into a whole, and then the iron core is assembled. It is a commonly used semi cast structure. The advantages are that the casting body is relatively simple and easy to manufacture, but the disadvantages are that the structure is not compact enough, and the exposed iron core will produce corrosion, which requires regular maintenance; The winding and core are cast into one, which is called full cast type. It is characterized by compact structure and almost no maintenance, but the casting body is more complex and the setting of core buffer layer is more troublesome

the external insulation of cast transformer is different according to the indoor and outdoor structural forms. The primary winding lead of indoor transformer is cast into a sleeve with mixed glue to ensure the insulation distance between the lead terminal and the base or exposed core. The cast sleeve is generally a cone, cylinder or square cylinder, and sometimes an umbrella skirt is set as required. There are many umbrella skirts for the external insulation of outdoor transformer to increase the creepage distance along the surface. Sometimes it is also designed as a large and small umbrella skirt to improve its anti fouling and anti condensation flashover ability. The rubber of outdoor pouring transformer is different from that of indoor pouring transformer. The epoxy resin, filler and curing agent used should meet the outdoor operation conditions

3. Oil immersed insulation structure

in China, oil immersed voltage transformers account for a large proportion at present, and their structural forms are widely used in 35kV and above voltage transformers. Indoor products with lower voltage levels also adopt this structure. Oil immersed voltage transformers can be divided into single-stage type and cascade type. The single-stage type is used for 220kV and below voltage levels, and the cascade type is used for 66kV and above voltage levels

the insulation of oil immersed voltage transformers can be divided into: internal insulation in oil and external insulation in air. The main insulation is the insulation of the primary winding and high-voltage lead to the core or grounding part and to other windings. The insulation between cores and between cores of cascade voltage transformers and between cores and ground is also regarded as the main insulation. Longitudinal insulation refers to the insulation between turns, layers and segments of windings

4. Gas and make it subject to the impact of economic cycle. Smaller insulation structure

sf6 gas is colorless and tasteless. It has high electrical strength, excellent arc extinguishing performance, good cooling characteristics, nonflammability and strong arc extinguishing ability. It is an excellent insulating material. Using it in electrical equipment can avoid the threat of fire, reduce the size of equipment and improve the reliability of system operation. The appearance of SF6 metal enclosed switchgear (GIS) has reduced the floor area of outdoor substations and improved the safety and reliability of operation. Generally, SF6 gas insulated transformer is used in the supporting equipment of GIS equipment. After the transformer is installed on GIS, it is filled with SF6 gas and has good insulation performance

sf6 voltage transformer adopts single-phase double column iron core, and its body structure is similar to oil immersed single-stage voltage transformer, including winding end insulation, high-voltage lead insulation, and insulation between primary winding and iron yoke, shell and other grounding metal parts. If three single-phase voltage transformers are installed in one shell, phase to phase insulation is also included. The interlayer insulation adopts weft giant adhesive tape and juku film, and the primary winding section adopts rectangular or graded pagoda shape. The lead insulation varies according to whether the transformer is matched or

independent. At present, domestic manufacturers use the SF6 gap between the high-voltage lead and other accessories to ensure its insulation strength

the error of the transformer using SF6 gas is stable. At present, only the grounding type is produced, and the monomial type is used for the split phase fully enclosed combined electrical apparatus. The three-phase consists of three single-phase transformers, which are used for three-phase totally enclosed switchgear. In addition, there are independent single-phase SF6 gas insulated transformers for general open substations

II. Factors causing insulation aging

during long-term operation, the insulation of electrical equipment will undergo a series of physical changes such as softening or melting of solid media, volatilization of low molecular compounds and plasticizers, and chemical changes such as oxidation, electrolysis, ionization, generation of new substances, resulting in gradual deterioration of its electrical, mechanical and other properties, such as increased conductivity and dielectric loss, embrittlement, cracking, etc, These phenomena are collectively referred to as insulation aging. Insulation aging eventually leads to insulation failure, and power equipment cannot continue to operate. See Figure 1 for the relationship between the service life of the insulating material and aging time

Fig. 1 Relationship between the service life of insulating materials and aging time

there are many reasons for insulation aging, mainly due to thermal, electrical and mechanical forces, as well as moisture (moisture), oxidation, various rays, microorganisms and other factors. The speed of insulation aging is closely related to insulation structure, materials, manufacturing process, operating environment, voltage, load, etc

1. Thermal aging

the insulation temperature of electrical equipment rises due to the high ambient temperature or the heating of electrical equipment itself during operation. In 1930, ntsinger first proposed the empirical relationship between insulation life and temperature, that is, the rule of 10 ℃. He believed that the insulation life would be reduced by about half for every 10 ℃ increase in temperature. But in fact, the aging rate of different insulation should be different, so the 10 ℃ rule cannot be simply applied to all insulation systems. In 1948, Dakin put forward a new view that thermal aging is actually an oxidation effect with polymerization chain splitting and other effects, which is essentially a chemical reaction process. Therefore, the chemical reaction rate equation should be followed:

lnl=ln α+ B/T

Where, α、 B is the constant determined by the specific aging reaction, l is the insulation life, t is the absolute temperature. This equation provides a theoretical basis for the high temperature accelerated aging test and the extrapolation of the test results, and makes up for the shortcoming that the montsinger 10 ℃ rule is difficult to distinguish the differences of aging under different conditions

under the action of high temperature, the mechanical strength of insulation decreases, and the structure is deformed due to oxygen

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