Analysis and Treatment of Insulation Faults of Power Transformers

2019-06-27 15:02:20 JuKe Technology (DongGuan) Co., Ltd. Read

Analysis and Treatment of Insulation Faults of Power Transformers

 Most transformer damage and failure are caused by damage to the insulation system. Therefore, protecting the normal operation of the transformer and enhancing the reasonable maintenance of the insulation system can largely ensure that the transformer has a longer service life. Next, the oil-immersed transformer manufacturer - Juke Industrial Xiaobian introduced the insulation of the Power Transformer.

Analysis and Treatment of Insulation Faults of Power Transformers.jpg

First, solid paper insulation failure
Solid paper insulation is one of the main parts of oil-immersed transformer insulation, including: insulating paper, insulating board, insulating mat, insulating coil, insulating banding, etc. The main component is cellulose. Generally, the degree of polymerization of the new paper is about 1300. When it is lowered to about 250, the mechanical strength has been reduced by more than half, and the degree of polymerization caused by extreme aging to terminate the life is 150 to 200. After the insulation paper is aged, its polymerization degree and tensile strength will gradually decrease, and water, CO, CO2 will be formed, followed by furfural. Most of these aging products are detrimental to electrical equipment, which can reduce the breakdown voltage and volume resistivity of insulating paper, increase dielectric loss, decrease tensile strength, and even cause corrosion of metal materials in equipment. Solid insulation has irreversible aging characteristics, and its mechanical and electrical strength aging reduction is unrecoverable. The life of a 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 properties and mechanical properties, and its performance declines slowly after years of operation, that is, the aging characteristics are good.
1. Properties of paper fiber materials.
Insulating paper fiber material is the most important insulating component material in oil-immersed transformer. Paper fiber is the basic solid tissue component of plants. The atoms that make up the material molecule have positively charged nuclei and negatively charged electrons running around the nucleus. The difference in conductors is that there is almost no free electrons in the insulating material, and the extremely small electrically conductive current in the insulator mainly comes from the ion conductance. Cellulose consists of carbon, hydrogen and oxygen, so that due to the presence of hydroxide in the molecular structure of the cellulose, there is a potential for water formation, giving the paper fibers watery character. In addition, these hydroxides can be considered as centers surrounded by various polar molecules (such as acid and water), which are combined by hydrogen bonding, making the fibers susceptible to damage: at the same time, the fibers often contain a certain proportion (about 7%). Impurities, including a certain amount of water, which are colloidal in nature, so that these moistures are not completely removed. This also affects the performance of the paper fiber.
Polar fibers are not only easy to absorb moisture (moisture makes a strong polar medium), but also weaken the interaction force between hydroxides when the paper fibers absorb water, and the mechanical strength deteriorates drastically under the condition of unstable fiber structure. Therefore, paper insulation parts are generally used after drying or vacuum drying treatment and oil immersion or insulating varnish. The purpose of immersion paint is to keep the fibers moist. Ensure high insulation and chemical stability and high Mechanical strength. At the same time, after the paper is sealed by paint, it can reduce the absorption of moisture by the paper, prevent the material from oxidizing, and also fill the gap to reduce bubbles that may affect the insulation performance, cause partial discharge and electric breakdown. However, some people think that after immersing the paint, they may be immersed in oil. Some paints may slowly dissolve in the oil and affect the performance of the oil. The application of such paints should be fully taken care of.
Of course, the properties of the fiber materials of different compositions and the different qualities of the fiber materials of the same composition have different effects on the size and performance. Most of the transformer insulation materials are insulated with various types of paper (such as paper, cardboard, paper pressure molded parts, etc.). Therefore, it is very important to select the insulating paper material of the fiber raw material in the manufacture and maintenance of the transformer. The special advantages of fiber paper are high practicability, low price, convenient use and processing, simple and flexible molding and treatment when the temperature is not high, light weight, moderate strength, easy to absorb impregnating materials, such as insulating paint, transformer oil and so on.
2. Mechanical strength of paper insulation.
Oil-immersed transformers The most important factor in selecting paper insulation is the mechanical strength requirements, including tensile strength, stamping strength, tear strength and toughness, in addition to the fiber composition, density, permeability and uniformity of the paper:
1 Tensile strength: When the paper fiber is subjected to tensile load, it has the maximum stress that can be tolerated without being pulled off.
2 Stamping strength: A measure of the ability of a paper fiber to withstand pressure without being broken.
3 Tear strength: The force required to tear the paper fibers meets the corresponding standards.
4 Toughness: It is the strength of paper folding or paperboard bending to meet the corresponding requirements.
To judge the solid insulation performance, you can try to measure the degree of polymerization of paper or paperboard, or use high performance liquid chromatography to measure the content of furfural in the oil, so as to analyze whether there is solid insulation inside the transformer or whether there is local aging of the coil insulation. Low temperature overheating, or to judge the degree of aging 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. It is required to have good air circulation and heat dissipation conditions in the operating environment to prevent the temperature rise of the transformer from exceeding the standard and the cabinet from being deficient in oil. It is also necessary to prevent accelerated deterioration of the fibers caused by oil pollution and deterioration, and damage the insulation performance, service life and safe operation of the transformer.
3. Deterioration of paper fiber materials.
It mainly includes three aspects:
1 Fiber is brittle.
When excessively heated to detach moisture from the fibrous material, it accelerates the embrittlement of the fibrous material. Due to the embrittlement and peeling of the paper, an electrical breakdown may occur due to an impact of mechanical vibration, electric stress, and operating waves.
2 The mechanical strength of the fiber material is reduced.
The mechanical strength of the fiber material decreases with the extension of the heating time. When the heat of the transformer causes the moisture of the insulating material to be discharged again, the value of the insulation resistance may become higher, but the mechanical strength will be greatly reduced, and the insulating paper will not be able to withstand the short-circuit current. Or the impact of mechanical forces such as shock loads.
3 shrinkage of the fiber material itself.
The fiber material shrinks after embrittlement, which reduces the clamping force, may cause shrinkage movement, and causes the transformer winding to displace friction under electromagnetic vibration or shock voltage to damage the insulation.

Second, liquid oil insulation failure
The liquid-insulated oil-immersed transformer was invented by American scientist Thomson in 1887. It was extended to power transformers by General Electric Company of the United States in 1892. The liquid insulation referred to here is transformer oil insulation.
Features of oil-immersed transformers:
1 greatly improved the electrical insulation strength, shortened the insulation distance, and reduced the size of the equipment;
2 greatly improve the effective heat transfer and heat dissipation effect of the transformer, increase the allowable current density in the wire, and reduce the weight of the device. It transfers the heat of the transformer body through the thermal circulation of the transformer oil to the transformer case and the radiator. Perform heat dissipation to increase the effective cooling and cooling level;
3 Due to the oil immersion seal, the oxidation degree of some components and components inside the transformer is reduced, and the service life is prolonged.
1. Performance of transformer oil.
In addition to the transformer oil in operation, it must have stable and excellent insulation and thermal conductivity.
Among them, the dielectric strength tg8, viscosity, freezing point and acid value are the main properties of insulating oil.
The insulating oil extracted from petroleum is a mixture of various hydrocarbons, resins, acids and other impurities, and its properties are not all stable, and will be continuously oxidized under the influence of temperature, electric field and photosynthesis. Under normal circumstances, the oxidation process of insulating oil proceeds very slowly. If it is properly maintained or even used for 20 years, it can maintain its proper quality without aging, but the metal, impurities, gas, etc. mixed into the oil will accelerate the development of oxidation, so that the oil The quality deteriorates, the color becomes darker, the transparency is turbid, and the moisture content, acid value, ash content, etc., deteriorate the properties of the oil.
2. The cause of transformer oil degradation.
Transformer oil quality deteriorates, and can be divided into two stages of pollution and deterioration according to the degree of severity.
Contamination is the mixing of moisture and impurities into the oil. These are not products of oil oxidation. The insulation performance of the contaminated oil will deteriorate, the breakdown electric field strength will decrease, and the dielectric loss angle will increase.
Deterioration is the result of oxidation of the oil. Of course, this oxidation does not only refer to the oxidation of hydrocarbons in the pure oil, but the presence of impurities in the oil will accelerate the oxidation process, especially copper, iron, aluminum metal dust and the like.
Oxygen is derived from the air in the transformer. Even if there is a volume of about 0.25% of oxygen inside the fully sealed transformer, the solubility of oxygen is high, so the gas dissolved in the oil occupies a high ratio.
When the transformer oil is oxidized, the moisture of the catalyst and the heat of the accelerator cause the transformer oil to form sludge. The effect is mainly as follows: the precipitate particles are large under the action of the electric field; the impurity precipitation is concentrated in the region with the strongest electric field, and the transformer is Insulation forms a conductive "bridge"; the deposits are not uniform but form separate slender strips, and may be arranged in the direction of the power line, which undoubtedly hinders heat dissipation, accelerates the aging of the insulating material, and leads to a decrease in insulation resistance and a decrease in insulation level. .
3. The process of transformer oil degradation.
The main products of the oil during the deterioration process are peroxides, acids, alcohols, ketones and sludge.
Early stage of deterioration. The peroxide formed in the oil reacts with the insulating fiber material to form oxidized cellulose, which deteriorates the mechanical strength of the insulating fiber, causing embrittlement and insulation shrinkage. The acid produced is a viscous fatty acid which, although corrosive is not as strong as mineral acid, has a large growth rate and a large influence on the organic insulating material.
Late deterioration stage. It is a sludge produced. When acid erodes copper, iron, insulating varnish and other materials, it reacts to form sludge. It is a viscous and asphalt-like polymeric conductive material that dissolves moderately in oil and generates velocity under the action of electric field. Soon, it adheres to the edge of the insulating material or the transformer case, and is deposited on the oil pipe and the cooler fins, etc., so that the working temperature of the transformer rises and the electric strength decreases.
The oxidation process of oil is composed of two main reaction conditions. One is that the acid value in the transformer is too high and the oil is acidic. The second is that the oxide dissolved in the oil is converted into an oil-insoluble compound, thereby gradually degrading the transformer oil quality.
4. Transformer oil quality analysis, judgment and maintenance processing.
1 The insulating oil has deteriorated.
Including its physical and chemical properties change, which deteriorates its electrical properties. By testing the acid value of the insulating oil, interfacial tension, sludge precipitation, water-soluble acid value and other items, it can be judged whether it is a defect of this type, and the regeneration of the insulating oil may eliminate the product of oil deterioration, but also during the treatment. Natural antioxidants may have been removed.
2 Insulating oil is infiltrated by water, because water is a strong polar substance.
Under the action of the electric field, it is easy to ionize and decompose, and the electrical conductivity of the insulating oil is increased. Therefore, a trace amount of water can significantly increase the dielectric loss of the insulating oil. By testing the micro-water of the insulating oil, it is judged whether it is such a defect. Pressure-type vacuum oil filtration of insulating oil generally eliminates moisture.
3 Insulating oil infects microbial bacteria.
For example, when the main transformer is installed or suspended, the insects attached to the surface of the insulator and the latches left by the installer: stains, etc., may carry bacteria and thus infect the insulating oil: or the insulating oil itself has infected the microorganisms. The main transformer - the general operation in the environment of 40-80 ° C, is very conducive to the growth and reproduction of these microorganisms. Since the insulation properties of minerals and proteins in microorganisms and their excretions are much lower than that of insulating oils, the dielectric loss of insulating oils is increased. This type of defect is difficult to handle by on-site recycling, because no matter how it is treated, some microorganisms remain on the insulating solid. After the treatment, the main transformer insulation will recover in a short period of time, but since the operating environment of the main transformer is very conducive to the growth and reproduction of microorganisms, these residual microorganisms will also grow and reproduce year by year, so that the insulation of some main transformers will decrease year by year;
4 Alkyd resin insulating paint containing polar substances is dissolved in oil.
Under the action of the electric field, the polar material will undergo dipole relaxation polarization, and energy is consumed during the AC polarization process, so the dielectric loss of the oil is increased. Although the varnish is cured before it leaves the factory, it may still be incomplete. After the main transformer has been running for a period of time, the incompletely treated insulating varnish is gradually dissolved in the oil, so that the insulation performance is gradually reduced. The time of occurrence of such defects is related to the thoroughness of the treatment of the insulating varnish, and a certain effect can be obtained by one or two adsorption treatments.
5 Only oil and impurities are mixed in the oil.
This contamination does not change the basic properties of the oil. The water can be removed by drying; the impurities can be removed by filtration; the air in the oil can be removed by vacuuming.
6 Two or more types of insulation 不同 from different sources are used in combination.
The nature of the oil should comply with the relevant regulations; the oil has the same specific gravity, the same solidification temperature, the same viscosity and similar flash point; and the stability of the oil after mixing also meets the requirements. For the oil which deteriorates after the oil mixing, since the oil quality has changed, acidic substances and sludge are generated, and the deteriorated product is separated by the chemical method of oil regeneration to restore its properties.
Third, the insulation and characteristics of dry resin transformer
Dry-type transformers (here referred to as epoxy-insulated transformers) are mainly used in places with high fire protection requirements. Such as high-rise buildings, airports, oil depots, etc.
1. Type of resin insulation.
Epoxy tree refers to the insulation transformer which can be divided into epoxy quartz sand mixture vacuum casting type, epoxy alkali-free glass fiber reinforced vacuum pressure difference casting type and alkali-free glass fiber wrapping impregnation type according to the manufacturing process characteristics.
1 Epoxy quartz sand mixture vacuum casting insulation.
This type of transformer is made of quartz sand as epoxy resin. The coil is wrapped by insulating varnish and wrapped in a coil casting mold. Under vacuum conditions, the mixture of epoxy resin and quartz sand is used for drip irrigation. pouring. Because the casting process is difficult to meet the quality requirements, such as residual bubbles, local unevenness of the mixture and possible local thermal stress cracking, such insulated transformers should not be used in areas with hot and humid environments and large load changes.
2 Epoxy alkali-free glass fiber reinforced vacuum differential pressure casting insulation.
The epoxy alkali-free glass fiber reinforcement is an outer layer of the outer layer of the winding insulation insulated by the alkali-free glass short fiber glass mat. The outermost layer of the insulation wrap is generally thin and has a thickness of 1 to 3 m. It is mixed by the proportion of the epoxy resin castable material, and the bubble casting is removed under high vacuum. Since the thickness of the wrap insulation is thin, when the dipping is poor It is easy to form a partial discharge point, so the mixing of the castable is required to be complete, the vacuum removal of the bubble should be thorough, and the low viscosity and casting speed of the castable should be mastered to ensure the high quality of the wire package impregnation during the casting process.
3 alkali-free glass fiber wrapped impregnation insulation.
The alkali-free glass fiber wrapped impregnated transformer is used to complete the coil interlayer insulation treatment and the coil impregnation while winding the transformer coil. It does not require the winding forming mold in the above two methods of impregnation, but requires the resin to have a small viscosity. The resin should not leave tiny bubbles during coil winding and impregnation.
2. Insulation characteristics and maintenance of resin transformers.
The insulation level of the resin transformer is not significantly different from that of the oil-immersed transformer. The key lies in the two indicators of temperature rise and partial discharge of the resin transformer.

1 The average temperature rise level of the resin transformer is higher than that of the oil-immersed transformer. Therefore, the corresponding heat resistance level of the insulating material is required to be higher, but the average temperature rise of the transformer does not reflect the temperature of the hottest part of the winding, when the insulating material is resistant. The heat rating is only selected according to the average temperature rise, or improperly selected, or the resin transformer is overloaded for a long time, which will affect the service life of the transformer.
Since the temperature rise of the transformer measurement often does not reflect the temperature of the hottest part of the transformer, it is better to check the hottest part of the resin transformer with an infrared thermometer under the maximum load operation of the transformer under certain conditions, and adjust it in a targeted manner. The direction and angle of the fan cooling device controls the local temperature rise of the transformer to ensure the safe operation of the transformer.
2 The partial discharge amount of the resin transformer is related to the electric field distribution of the transformer, the uniformity of resin mixing, and whether there are residual bubbles or cracking of the resin. The amount of partial discharge affects the performance, quality and service life of the resin transformer. Therefore, the measurement and acceptance of the partial discharge of the resin transformer is a comprehensive assessment of its process and quality. The partial discharge measurement test should be carried out after the acceptance and overhaul of the resin transformer, and it is evaluated according to whether the partial discharge changes. Its quality and performance stability.
With the increasing application of dry-type transformers, in the selection of transformers, the process structure, insulation design and insulation configuration should be clearly understood. Products with perfect production technology and quality assurance system, strict production management and reliable technical performance should be selected. To ensure the product quality and heat-resistant life of the transformer, the safe operation and reliability of the transformer can be improved.
Fourth, the main factors affecting transformer insulation faults
The main factors affecting the insulation performance of transformers are: temperature, humidity, oil protection and overvoltage effects.
1. The effect of temperature.
The power transformer is oil and paper insulation, and the water content in oil and paper has different equilibrium curves at different temperatures. Under normal circumstances, the temperature rises, the moisture in the paper should be precipitated into the poise; otherwise, the paper should absorb the moisture in the oil. Therefore, when the temperature is high, the micro-water content of the insulating oil in the transformer is large; on the contrary, the micro-water content is small.
When the temperature is different, the extent to which the cellulose is entangled, broken, and accompanied by gas generation is different. At a certain temperature, the production rate of CO and CO2 is constant, that is, the CO and CO2 gas contents in the oil are linear with time. The rate of CO and CO2 production tends to increase exponentially as the temperature increases. Therefore, the content of CO and CO2 in oil has a direct relationship with the thermal aging of insulating paper, and the content change can be used as one of the criteria for the presence or absence of abnormality in the paper layer in the sealed transformer.
The life of a transformer depends on the ageing of the insulation, which in turn depends on the operating temperature. For example, if the oil-immersed transformer is under rated load, the average temperature rise of the winding is 65 °C, and the hottest temperature rise is 78 °C. If the average ambient temperature is 20C, the hottest temperature is 98 °C; at this temperature, the transformer can run 20- In 30 years, if the transformer is overloaded, the temperature rises, which shortens the life.
The International Electrotechnical Commission (1EC) believes that Class A insulated transformers have a temperature range of 80 to 140 C. For every 6 °C increase in temperature, the effective life of the transformer insulation is doubled. This is the 6 °C rule, indicating the heat limit. It is more stringent than the 8°C rule approved in the past.
2. The effect of humidity.
The presence of moisture will accelerate the degradation of paper cellulose. Therefore, the production of CO and CO2 is also related to the water content of the cellulosic material. When the humidity is constant, the higher the water content, the more CO2 is decomposed. Conversely, the lower the water content, the more CO is decomposed.
Trace moisture in insulating oil is one of the important factors affecting insulation properties. The presence of trace moisture in the insulating oil has great harm to the electrical properties and physical and chemical properties of the insulating medium. The water can cause the spark discharge voltage of the insulating oil to decrease, the dielectric loss factor tg8 to increase, promote the aging of the insulating oil, and deteriorate the insulation performance. . The dampness of the equipment not only leads to reduced operational reliability and longevity of the electrical equipment, but also may result in equipment damage or even personal safety.
3. The impact of oil protection methods.
The action of oxygen in the transformer oil accelerates the insulation decomposition reaction, and the oxygen content is related to the oil protection mode. In addition, the pool protection is different, so that CO and CO2 are different in oil solution and diffusion conditions. If the dissolution of CO is small, the open transformer CO is easily diffused to the oil surface space. Therefore, the open transformer generally has a volume fraction of CO of not more than 300x10-6. Sealed transformers, because the oil surface is insulated from the air, make CO and CO2 less volatile, so the content is higher.
4. The effect of overvoltage.
1 The effect of transient overvoltage.
The phase and ground voltage generated by the normal operation of the three-phase transformer is 58% of the phase-to-phase voltage, but the voltage of the main insulation will increase by 30% for the neutral point grounding system and the neutral point ungrounded system will increase by 73 when the single-phase fault occurs. %, which may damage the insulation.
2 The effect of lightning overvoltage.
Due to the steep wave head, the lightning overvoltage causes the voltage distribution on the longitudinal insulation (interrogation, interphase, insulation) to be uneven, which may leave traces of discharge on the insulation, thereby destroying the solid insulation.
3 The effect of operating overvoltage.
Since the wave head for operating the overvoltage is fairly gradual, the voltage distribution is approximately linear. When the operating overvoltage wave is transferred from one winding to the other, it is proportional to the number of turns between the two windings, which easily causes main insulation or Deterioration and damage of the phase insulation.
5. The effect of short-circuit electric power.
The electric power when the outlet is short-circuited may cause the transformer winding to be deformed and the lead to be displaced, thereby changing the original insulation distance, causing the insulation to heat up, accelerating aging or being damaged, causing discharge, arcing and short-circuit failure.