At present, the most widely used power transformers are oil-immersed transformers and dry-type resin transformers. The insulation of power transformers is the insulation system composed of transformer insulation materials. It is the basic condition for the normal operation and operation of the transformer. The service life of the transformer is determined by It is determined by the life of the insulating material (that is, oil paper or resin, etc.).
Practice has proved that most of the damage and failure of transformers are caused by the damage of the insulation system. According to statistics, accidents caused by various types of insulation faults account for more than 85% of all transformer accidents. For transformers that operate normally and pay attention to maintenance and management, their insulating materials have a long service life.
According to theoretical calculations and experimental studies abroad, when the actual temperature of a small oil-immersed distribution transformer continues at 95°C, the theoretical life span will reach 400 years. The experience of design and field operation shows that the actual life of a well-maintained transformer can reach 50 to 70 years. According to the design requirements and technical indicators of the manufacturer, the expected life of the transformer is generally set at 20-40 years. Therefore, protecting the normal operation of the transformer and strengthening the reasonable maintenance of the insulation system can to a large extent ensure that the transformer has a relatively long service life, and preventive and predictive maintenance is the key to improving the service life of the transformer and improving the reliability of power supply .
In oil-immersed transformers, the main insulating materials are insulating oil and solid insulating materials, insulating paper, cardboard, and wooden blocks. The so-called aging of transformer insulation means that these materials decompose due to environmental factors, reducing or losing their insulation strength.
1. Solid paper insulation failure
Solid paper insulation is one of the main parts of oil-immersed transformer insulation, including: insulation paper, insulation board, insulation pad, insulation roll, insulation binding tape, etc. Its main component is cellulose, and its chemical expression is (C6H10O6)n, Where n is the degree of polymerization. Generally, the degree of polymerization of new paper is about 1300. When it drops to about 250, its mechanical strength has dropped by more than half. The degree of polymerization at the end of life due to extreme aging is 150-200. After the insulation paper ages, its degree of polymerization and tensile strength will gradually decrease, and water, CO, CO2 will be generated, followed by furfural (furaldehyde). Most of these aging products are harmful to electrical equipment, and will reduce the breakdown voltage and volume resistivity of the insulating paper, increase the dielectric loss, decrease the tensile strength, and even corrode the metal materials in the equipment. Solid insulation has irreversible aging characteristics, and the aging reduction of its mechanical and electrical strength cannot be restored. The life of the transformer mainly depends on the life of the insulating material. Therefore, the solid insulating material of the oil-immersed transformer should have good electrical insulation and mechanical properties, and after many years of operation, its performance will decline slowly, that is, the aging characteristics are good.
(1) The properties of paper fiber materials. Insulating paper fiber material is the most important insulating component material in oil-immersed transformers. Paper fiber is the basic solid tissue component of plants. Among the atoms that make up the material molecule, there are positively charged atomic nuclei and negatively charged electrons orbiting the atomic nucleus. The difference between conductors is that there are almost no free electrons in the insulating material, and the very small electric current in the insulator mainly comes from ion conduction. Cellulose is composed of carbon, hydrogen, and oxygen. Therefore, due to the presence of hydroxide radicals in the molecular structure of cellulose, there is a potential for the formation of water, which makes paper fibers have water-containing properties. In addition, these hydroxide radicals can be considered as the center surrounded by various polar molecules (such as acid and water). They are combined by hydrogen bonds, making the fiber vulnerable to damage: at the same time, the fiber often contains a certain proportion (about 7%). About) impurities, these impurities include a certain amount of water, because the fiber has a colloidal nature, the water cannot be completely removed. This also affects the performance of paper fibers.
Polar fibers are not only easy to absorb moisture (moisture makes a strong polar medium), and when paper fibers absorb water, the interaction force between hydroxide radicals becomes weak, and the mechanical strength deteriorates sharply under the condition of unstable fiber structure Therefore, paper insulating parts generally need to be dried or vacuum-dried and immersed in oil or insulating varnish before they can be used. The purpose of varnishing is to keep the fiber wet. To ensure that it has higher insulation and chemical stability and has relatively high insulation and chemical stability. High mechanical strength. At the same time, after the paper is sealed with lacquer, it can reduce the absorption of water by the paper, prevent the material from oxidizing, and also fill the gaps to reduce the air bubbles that may affect the insulation performance, cause partial discharge and electrical breakdown. However, some people think that after dipping the paint and then immersing in the oil, some paints may slowly dissolve into the oil and affect the performance of the oil. The application of this type of paint should be fully paid attention to.
Of course, the properties of fiber materials with different components and the different qualities of fiber materials with the same composition have different effects and performances. For example, cotton has the highest fiber content and hemp fiber is the strongest. Some imported insulating paperboards are processed well due to their good processing. The performance is obviously better than some domestic cardboard materials. Most of the insulating materials of transformers are insulated with various types of paper (such as paper tape, cardboard, paper pressure forming parts, etc.). Therefore, it is very important to choose insulating paper materials with good fiber raw materials in the manufacture and maintenance of transformers. The special advantages of fiber paper are practicality, low price, convenient use and processing, simple and flexible molding and handling when the temperature is not high, light weight, moderate strength, and easy absorption of impregnating materials (such as insulating paint, transformer oil, etc.).
(2) Mechanical strength of paper insulation material. In addition to the fiber composition, density, permeability and uniformity of the paper, the most important factors for selecting paper insulation materials for oil-immersed transformers also include the requirements for mechanical strength, including tensile strength, punching strength, tear strength and toughness:
1) Tensile strength: When the paper fiber is subjected to tensile load, it has the maximum stress that it can withstand without being broken
2) Punching strength: a measure of the ability of paper fibers to withstand pressure without being broken.
3) Tear strength: The force required for the paper fiber to tear is required to meet the corresponding standards.
4) Toughness: the strength of paper folded or cardboard can meet the corresponding requirements.
To determine the performance of solid insulation, you can try to sample and measure the degree of polymerization of paper or cardboard, or use high-performance liquid chromatography to measure the furfural content in oil, so as to analyze whether there is a fault in the transformer, whether it involves solid insulation or whether there is a local aging of the coil insulation Low temperature and overheating, or judge the aging degree of solid insulation. In the operation and maintenance of paper fiber insulation materials, attention should be paid to controlling the rated load of the transformer, and the operation environment is required to have good air circulation and heat dissipation conditions to prevent the transformer from exceeding the standard temperature rise and the tank body lacking oil. It is also necessary to prevent the accelerated aging of the fiber caused by oil pollution and deterioration, which will damage the insulation performance, service life and safe operation of the transformer.
(3) Deterioration of paper fiber material. It mainly includes three aspects:
1) The fiber is brittle. When excessive heating causes moisture to escape from the fiber material, it will accelerate the embrittlement of the fiber material. Due to the embrittlement and peeling of paper, insulation failure may occur under the influence of mechanical vibration, electric stress, operation wave and other impact forces, which may cause electrical accidents.
2) The mechanical strength of the fiber material decreases. The mechanical strength of the fiber material decreases with the extension of the heating time. When the insulation material is discharged again due to the heat of the transformer, the value of the insulation resistance may become higher, but its mechanical strength will be greatly reduced, and the insulating paper will not be able to withstand short-circuit current Or impact load and other mechanical forces.
3) Shrinkage of the fiber material itself. The fiber material shrinks after embrittlement, which reduces the clamping force, which may cause shrinkage and movement, causing the transformer windings to shift and rub under electromagnetic vibration or impulse voltage to damage the insulation.