Analysis of Insulation Failures in Power Transformers and Corresponding Remedial Measures

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Currently, the most widely used power transformers are oil-immersed transformers and dry-type resin transformers. The insulation of a power transformer refers to the insulating system composed of insulating materials; it is a fundamental requirement for the normal operation and functioning of the transformer. The service life of a transformer is determined by the lifespan of its insulating materials (such as oil-paper or resin). Practice has shown that most transformer damages and failures are caused by damage to the insulation system. According to statistics, accidents caused by various types of insulation failures account for over 85% of all transformer accidents. For transformers that are operating properly and for which maintenance is carried out carefully, their insulating materials have a very long service life. Therefore, protecting the normal operation of transformers and ensuring proper maintenance of their insulation systems can largely guarantee that transformers have a relatively long service life. Preventive and predictive maintenance is the key to extending the service life of transformers and enhancing the reliability of power supply. In oil-immersed transformers, the main insulating materials are insulating oil, as well as solid insulating materials such as insulating paper, cardboard, and wood. The aging of transformer insulation refers to the decomposition of these materials due to environmental factors, which leads to a reduction or loss of their insulating strength. 1. Failures of solid paper insulation
Solid paper insulation is one of the main components of the insulation system in oil-immersed transformers. It includes insulating paper, insulating boards, insulating gaskets, insulating rolls, insulating binding tapes, etc. Its main component is cellulose, with the chemical formula (C6H10O6)n, where n represents the degree of polymerization. Generally, the degree of polymerization of new paper is around 1300; when it drops to around 250, its mechanical strength has decreased by more than half. The degree of polymerization at which extreme aging leads to the end of its useful life is between 150 and 200. After insulation paper ages, its degree of polymerization and tensile strength gradually decrease, and water, CO, CO2 are produced, as well as furfural (furanal). Most of these aging products are harmful to electrical equipment; they can reduce the breakdown voltage and volume resistivity of insulating paper, increase dielectric loss, decrease tensile strength, and even corrode the metal materials in the equipment. Solid insulation exhibits irreversible aging characteristics; the degradation of its mechanical and electrical strength is irreversible. The lifespan of a transformer primarily depends on that of its insulating materials. Therefore, the solid insulating materials used in oil-immersed transformers should possess good electrical insulation properties and mechanical characteristics; moreover, after years of operation, their performance should degrade slowly—that is, they should have favorable aging characteristics. 1 Properties of paper fiber materials: Insulating paper fiber materials are the primary insulating components in oil-immersed transformers. Paper fibers are part of the basic solid structural components of plants; the molecules that make up these fibers contain positively charged atomic nuclei and negatively charged electrons that orbit around them. Unlike metal conductors, insulating materials contain very few free electrons, and the minimal electrical current that can flow through insulators comes from ionic conduction. Cellulose is composed of carbon, hydrogen, and oxygen; thus, due to the presence of hydroxyl groups in its molecular structure, there is a potential for water to be formed, which gives paper fibers their water-containing properties. Furthermore, these hydroxide groups can be regarded as centers surrounded by various polar molecules (such as acids and water); they are bonded together through hydrogen bonds, which makes the fibers susceptible to damage. At the same time, the fibers often contain a certain proportion (about 7%) of impurities, including a certain amount of moisture. Due to the colloidal nature of the fibers, this moisture cannot be completely removed. This also affects the properties of the paper fibers. Polar fibers are not only prone to absorbing moisture (as water makes a medium highly polar), but when paper fibers absorb water, the intermolecular forces between hydroxide groups weaken. This leads to a sharp decline in mechanical strength due to the instability of the fiber structure. Therefore, paper-insulated components generally need to be dried or treated with vacuum drying, and then coated with oil or insulating paint before they can be used. The purpose of applying this coating is to keep the fibers moist, thereby ensuring higher insulation and chemical stability as well as greater mechanical strength. At the same time, after the paper is sealed with paint, its absorption of moisture is reduced, oxidation of the material is prevented, and gaps can be filled to eliminate bubbles that may affect insulation performance and cause partial discharges and electrical breakdown. However, some believe that after dipping in paint and then in oil, some of the paint may gradually dissolve into the oil, affecting its properties; due care should be taken when using such paints. Of course, the properties of fiber materials with different compositions, as well as the varying qualities of fiber materials with the same composition, result in different degrees of influence and performance differences. For example, cotton has the highest fiber content; its fibers are the strongest. Some imported insulating boards, thanks to their superior processing, exhibit significantly better performance compared to certain domestic boards. Most insulating materials used in transformers are paper of various types (such as paper tape, paperboard, and pressure-molded paper products) for insulation. Therefore, it is very important to select appropriate insulating paper materials as fiber raw materials in transformer manufacturing and maintenance. The special advantages of fiber paper are its high practicality, low cost, and ease of use and processing. It is simple and flexible to form and process at moderate temperatures. Additionally, it is lightweight, has moderate strength, and readily absorbs impregnating materials (such as insulating varnish, transformer oil, etc.). 2 Mechanical strength of paper insulating materials. In the selection of paper insulating materials for oil-immersed transformers, in addition to factors such as the fiber composition, density, permeability, and uniformity of the paper, the requirements regarding mechanical strength are also important. These include tensile strength, impact strength, tear strength, and toughness: ① Tensile strength: It is required that the paper fibers be able to withstand a maximum stress without breaking when subjected to tensile forces ； ② Stamping strength: A measure of the ability of paper fibers to withstand pressure without breaking ； ③ Tear strength: The force required to tear the paper fibers must meet the corresponding standards ； ④ Toughness: It refers to the strength of paper or cardboard when folded or bent, which must meet the relevant requirements. To assess the solid insulation performance, it is possible to measure the degree of polymerization of the paper or cardboard by taking samples, or to determine the furfural content in the oil using high-performance liquid chromatography. This helps to determine whether there are issues with the solid insulation when a fault occurs inside the transformer, whether low-temperature overheating is causing localized aging of the coil insulation, and to assess the degree of aging of the solid insulation. When using paper fiber insulation materials in operation and maintenance, it is necessary to control the transformer’s rated load, ensure good air circulation and effective heat dissipation in the operating environment, and prevent the transformer’s temperature from rising too high as well as oil depletion in the tank. It is also necessary to prevent accelerated aging of the fibers caused by oil contamination and deterioration, which could impair the insulation properties, service life, and safe operation of the transformer. 3 Degradation of paper fiber materials mainly involves three aspects: (1) Fiber embrittlement. When excessive heat causes moisture to be released from fibrous materials, it further accelerates the embrittlement of these materials. Due to the embrittlement and flaking of paper, insulation failures may occur under the influence of impact forces such as mechanical vibrations, electrical stress, and operating waves, leading to electrical accidents. (2) The mechanical strength of fiber materials decreases. The mechanical strength of fiber materials decreases as the heating time increases. When the heat generated by the transformer causes moisture in the insulating materials to be expelled once again, the value of the insulation resistance may increase, but its mechanical strength will **decrease; the insulating paper will no longer be able to withstand mechanical forces such as short-circuit currents or shock loads. (3) Shrinkage of the fiber material itself. Fiber materials contract after embrittlement, reducing the clamping force; this may lead to contraction movement, causing the transformer windings to shift and rub against each other under electromagnetic vibrations or surge voltages, thereby damaging the insulation. II. Faults in Liquid Oil Insulation Oil-immersed transformers with liquid insulation were invented in 1887 by the American scientist Thomson, and in 1892 they were adopted for use in power transformers by companies such as General Electric in the United States. The liquid insulation referred to here is transformer oil insulation. 1 Characteristics of oil-immersed transformers: (1) **It improves the electrical insulation strength, reduces the required insulation distance, and decreases the size of the equipment.** (2) **It enhances the effective heat transfer and cooling capabilities of the transformer, allows for a higher current density in the wires, and reduces the weight of the equipment. Heat generated within the transformer is transferred to the transformer’s casing and radiators through the thermal circulation of the transformer oil, thereby improving the efficiency of cooling.** (3) The oil-sealing mechanism reduces the degree of oxidation of certain internal components of the transformer, thus extending its service life. 2 Properties of transformer oil: During operation, transformer oil must possess stable and excellent insulating and thermal conductivity properties. Among them, insulation strength tg8, viscosity, freezing point, and acid value are the main property indicators of insulating oil. Insulating oil, derived from petroleum, is a mixture of various hydrocarbons, resins, acids, and other impurities; its properties are not always stable, as it undergoes continuous oxidation under the influence of factors such as temperature, electric fields, and light. Under normal conditions, the oxidation process of insulating oil proceeds very slowly; with proper maintenance, it can maintain its quality and avoid aging even after 20 years of use. However, metals, impurities, gases, and other substances that mix into the oil accelerate this oxidation process, causing the quality of the oil to deteriorate – its color to darken, its transparency to become cloudy, and the levels of moisture, acid value, and ash content to increase, all of which lead to a decline in the oil’s properties. 3 Reasons for Transformer Oil Degradation: The deterioration of transformer oil can be divided into two stages, depending on the severity: contamination and degradation. Contamination refers to the presence of water and impurities in oil, substances that are not products of oil oxidation. Contaminated oil has reduced insulating properties, a lower breakdown electric field strength, and an increased dielectric loss angle. Degradation is the result of oil oxidation. Of course, this oxidation doesn’t refer solely to the oxidation of hydrocarbons in pure oil; rather, impurities present in the oil accelerate the oxidation process, especially metal particles such as copper, iron, and aluminum. Oxygen originates from the air inside the transformer; even within a completely sealed transformer, there is still approximately 0.25% oxygen present. Given its high solubility, oxygen makes up a relatively large proportion of the gases dissolved in the oil. When transformer oil oxidizes, moisture acting as a catalyst and the heat from accelerators cause the oil to form sludge. The effects of this are mainly as follows: under the influence of an electric field, the precipitate particles become larger; impurities precipitate primarily in areas where the electric field is strongest, forming conductive “bridges” that compromise the insulation of the transformer; the precipitate does not form uniformly but instead takes the shape of separate, elongated strands, which may also align along the direction of the electric field lines. This undoubtedly hinders heat dissipation, accelerates the aging of insulating materials, and leads to a decrease in insulation resistance and overall insulation performance. 4. Process of transformer oil degradation. The main products formed during the various stages of oil degradation are peroxides, acids, alcohols, homologs, and sludge. Early stage of deterioration. The peroxides formed in the oil react with the insulating fiber material to produce cellulose oxide, which reduces the mechanical strength of the insulating fibers, leading to embrittlement and insulation shrinkage. The acid produced is a mucilaginous fatty acid; although its corrosivity is not as strong as that of mineral acids, its growth rate and impact on organic insulating materials are significant. Late degradation stage. It is sludge that is formed when acids corrode materials such as copper, iron, and insulating paint; this reaction results in the creation of a viscous, asphalt-like polymeric conductive substance. It can dissolve to some extent in oil, and its formation occurs rapidly under the influence of an electric field. It adheres to insulating materials or the edges of transformer casings, and deposits in oil pipes and cooler fins, thereby raising the operating temperature of the transformer and reducing its electrical strength. The oxidation process of oil is determined by two main reactive conditions: one is an excessively high acid value in the transformer, which makes the oil acidic. Secondly, the oxides dissolved in oil transform into compounds that are insoluble in oil, thereby gradually degrading the quality of the transformer oil. 5 Analysis, assessment, and maintenance of transformer oil quality (1) Deterioration of insulating oil. Its physical and chemical properties change, which in turn deteriorates its electrical properties. By testing parameters such as the acid value, interfacial tension, sludge formation, and water-soluble acid value of insulating oil, it is possible to determine whether such defects are present. Regenerating the insulating oil may eliminate the products resulting from its deterioration, but this process may also remove the natural antioxidants contained in it. (2) The insulating oil becomes damp due to water infiltration, as water is a highly polar substance. Under the influence of an electric field, it tends to ionize and decompose, thereby increasing the conductive current of the insulating oil. As a result, even trace amounts of moisture can cause a significant increase in the dielectric loss of the insulating oil. By testing the microwater content of the insulating oil, it is possible to determine whether such defects are present. Pressure vacuum oil filtration of insulating oil generally eliminates moisture. (3) Insulating oil is contaminated with microbial bacteria. For example, when installing the main transformer or removing its core, insects attached to the surface of insulating components, as well as residues such as bolts left by the installation crew, can all carry bacteria that may contaminate the insulating oil; or the insulating oil itself may already be infected with microorganisms. The main transformer generally operates in an environment of 40–80°C, which is highly favorable for the growth and reproduction of these microorganisms. The insulating properties of minerals and proteins in microorganisms and their excretions are much lower than those of insulating oil, which leads to an increase in the dielectric loss of the insulating oil. It is difficult to address this defect using on-site recycling methods, as no matter what approach is taken, some microorganisms always remain on the insulating solid. After treatment, the insulation of the main transformer improves to some extent in the short term. However, since the operating environment of the main transformer is highly favorable for the growth and reproduction of microorganisms, these remaining microorganisms continue to grow and multiply over the years, resulting in a gradual decline in the insulation of certain main transformers. (4) The alkyd resin insulating paint containing polar substances dissolves in oil. Under the influence of an electric field, polar substances undergo dipolar relaxation polarization; energy is consumed during this alternating polarization process, which leads to an increase in the dielectric loss of the oil. Although the insulating paint is cured before leaving the factory, it may still be incomplete in its curing process. After the main transformer has been in operation for some time, the inadequately treated insulating paint gradually dissolves in the oil, causing its insulation properties to decline over time. The timing of the occurrence of such defects is related to the thoroughness of the insulating paint treatment, and certain results can be achieved through one or two adsorption treatments. (5) The oil contains only moisture and impurities. This level of contamination does not change the basic properties of the oil. Moisture can be removed by drying; impurities can be eliminated through filtration; air in the oil can be removed by vacuuming. (6) Mixing and using two or more types of insulating oil from different sources. The properties of the oils should comply with relevant regulations; they should have the same specific gravity, freezing point, viscosity, and similar flash points; moreover, the stability of the oils after mixing must also meet the required standards. For oil that has deteriorated as a result of mixing, its quality has changed, leading to the formation of acidic substances and sludge; therefore, chemical methods of oil regeneration are required to separate these degraded products in order to restore its original properties. III. Insulation and Characteristics of Dry-Resin Transformers Dry transformers (referring here to those with epoxy resin insulation) are primarily used in locations with high fire safety requirements. Such as high-rise buildings, airports, oil depots, etc. 1 Types of resin insulation: Transformers with epoxy resin insulation can be classified into three types based on their manufacturing processes: those that are cast under vacuum using an epoxy quartz sand mixture, those that are cast under vacuum with reinforcement using epoxy and alkali-free glass fibers, and those that are impregnated using alkali-free glass fiber winding. (1) Vacuum casting insulation using epoxy quartz sand mixture. In this type of transformer, quartz sand is used as a filler for the epoxy resin. The coils, which have been wound and impregnated with insulating paint, are placed into a coil casting mold; under vacuum conditions, a mixture of epoxy resin and quartz sand is then poured over them. Since the casting process fails to meet quality requirements, such as residual bubbles, local unevenness in the mixture, and potential localized thermal stress cracking, transformers with such insulation are not suitable for use in humid and hot environments or areas with large load variations. (2) Epoxy alkali-free glass fiber reinforced vacuum differential pressure casting insulation. Epoxy alkali-free glass fiber reinforcement uses alkali-free glass short fiber glass mats as the outer insulation for the winding layer insulation. The thickness of the outermost insulating coating is generally 1 to 3 mm of thin insulation; it is mixed using an epoxy resin casting compound, and bubbles are removed under high vacuum during pouring. Due to the thinness of this insulating coating, local discharge points can easily form if the impregnation is inadequate. Therefore, it is necessary to ensure thorough mixing of the casting compound, complete removal of bubbles under vacuum, as well as proper control over the low viscosity of the casting compound and the pouring speed, in order to guarantee high-quality impregnation of the wire coil during the pouring process. (3) Alkali-free glass fiber winding impregnation insulation. In the case of alkali-free glass fiber-wound and impregnated transformers, the inter-layer insulation treatment and impregnation of the coils are carried out simultaneously during the winding process. This method does not require the winding molds used in the aforementioned two impregnation methods; however, it demands that the resin have low viscosity, with no tiny bubbles remaining in the resin during both the winding and impregnation processes. Insulation characteristics and maintenance of resin transformers: The insulation level of resin transformers is not significantly different from that of oil-immersed transformers; the key factors are the temperature rise and partial discharge of these transformers. (1) The average temperature rise of resin transformers is higher than that of oil-immersed transformers; therefore, higher heat resistance levels are required from the insulating materials. However, since the average temperature rise does not reflect the temperature at the hottest points in the windings, choosing the heat resistance level of the insulating materials based solely on this average value can lead to inappropriate selection, or prolonged overload operation of the resin transformer, which in turn affects its service life. Since the temperature rise measured in transformers often does not reflect the temperature at their hottest points, it is advisable, whenever possible, to use an infrared thermometer to check those hottest areas of resin-filled transformers while they are under maximum load. By adjusting the direction and angle of the fan cooling systems accordingly, it is possible to control the local temperature rise and ensure the safe operation of the transformers. (2) The level of partial discharge in resin transformers is related to factors such as the electric field distribution within the transformer, the uniformity of the resin mixture, and the presence of residual bubbles or cracks in the resin. The degree of partial discharge affects the performance, quality, and service life of resin transformers. Therefore, measuring and testing the level of partial discharge in resin transformers is a comprehensive assessment of their manufacturing process and quality. Partial discharge measurements should be carried out during the handover inspection of resin transformers as well as after major repairs, and the stability of their quality and performance is evaluated based on any changes in partial discharge levels. As dry-type transformers are used more and more widely, when selecting such transformers it is necessary to have a thorough understanding of their structural design, insulation design, and insulation configuration. It is important to choose products that come from manufacturers with sound production processes and quality assurance systems, as well as strict production management and reliable technical performance, in order to ensure the product quality and heat resistance of the transformers. This will in turn enhance the safe operation of the transformers and the reliability of power supply. IV. Main factors affecting transformer insulation failures The main factors that influence the insulation performance of transformers include temperature, humidity, oil protection methods, and overvoltage effects. 1. Effect of temperature: Power transformers have oil and paper insulation, and the moisture content in these materials follows different equilibrium curves at various temperatures. Under normal circumstances, as the temperature rises, the moisture in the paper will migrate out into the resin; conversely, the paper will absorb the moisture from the oil. Therefore, when the temperature is high, the moisture content in the insulating oil inside the transformer is higher; conversely, the moisture content is lower. At different temperatures, the degree to which cellulose is depolymerized and its chains are broken, along with the amount of gas generated, varies. At a certain temperature, the generation rate of CO and CO₂ remains constant; that is, the concentration of CO and CO₂ gases in the oil varies linearly over time. As the temperature continues to rise, the production rates of CO and CO₂ tend to increase exponentially. Therefore, the levels of CO and CO₂ in the oil are directly related to the thermal aging of the insulating paper, and changes in these levels can be used as one criterion to determine whether there are any abnormalities in the paper layers of sealed transformers. The lifespan of a transformer depends on the degree of insulation aging, which in turn is determined by the operating temperature. For an oil-immersed transformer, under rated load the average temperature rise of the windings is 65°C, while the temperature rise at the hottest point is 78°C. If the average ambient temperature is 20°C, then the temperature at the hottest point will be 98°C. At this temperature, the transformer can operate for 20–30 years. However, if the transformer operates under overload conditions, the increased temperatures shorten its lifespan. The International Electrotechnical Commission (IEC) considers that for transformers with Class A insulation, within the temperature range of 80–140°C, every increase of 6°C in temperature doubles the rate at which the effective insulation life of the transformer decreases. This is known as the 6°C rule, indicating that the thermal restrictions are stricter than the previously accepted 8°C rule. 2 Effect of humidity: The presence of moisture will accelerate the degradation of paper cellulose. Therefore, the generation of CO and CO₂ is also related to the moisture content of the cellulose material. At a constant humidity, the higher the water content, the more CO₂ is released. Conversely, the lower the water content, the more CO is produced during decomposition. Trace moisture in insulating oil is one of the important factors affecting its insulation properties. The presence of trace amounts of water in insulating oil poses a serious threat to both the electrical and physical-chemical properties of the insulating material. Water can cause the spark discharge voltage of the insulating oil to decrease, increase the dielectric loss factor tgδ, accelerate the aging of the insulating oil, and lead to a deterioration of its insulating properties. When equipment gets wet, it not only reduces the operational reliability and lifespan of electrical equipment, but can also lead to equipment damage and even pose a threat to human safety. 3. Influence of oil protection methods: The presence of oxygen in transformer oil accelerates the insulation degradation process, and the oxygen content is related to the oil protection method used. Furthermore, different pool protection methods result in varying dissolution and diffusion of CO and CO₂ in the oil. Since the dissolution of CO is low, CO in open-type transformers tends to diffuse into the oil surface space; therefore, the volume fraction of CO in such transformers is generally not greater than 300x10-6. In sealed transformers, since the oil level is insulated from air, CO and CO2 do not evaporate easily, resulting in higher concentrations of these gases. 4 Effects of overvoltage ① Effects of transient overvoltage. Under normal operation, the phase-to-ground voltages generated by a three-phase transformer are 58% of the phase-to-phase voltages. However, in the event of a single-phase fault, the voltage across the main insulation increases by 30% in systems with a grounded neutral point, and by 73% in systems without a grounded neutral point; this can lead to damage to the insulation. ② Impact of lightning overvoltage. Due to the steep wavefront of lightning overvoltage, the voltage distribution across the longitudinal insulations (between turns, between phases, and the insulation itself) becomes highly uneven, which may leave discharge traces on the insulations and thus damage the solid insulation. ③ Effect of switching overvoltage. Since the front of the switching overvoltage is quite gentle, the voltage distribution is approximately linear. When the switching overvoltage wave moves from one winding to another, it is roughly proportional to the number of turns between these two windings, which can easily lead to degradation and damage of the main insulation or the insulation between phases. 5 Effect of short-circuit electromotive force: The electromotive force during an outlet short circuit can cause the transformer windings to deform and the leads to shift, thereby altering the original insulation distances. This leads to heating of the insulation, accelerated aging, or damage that results in discharge, arcing, and short-circuit faults. In summary, understanding the insulation properties of power transformers and ensuring proper operation and maintenance are directly related to the safe operation, service life, and reliability of power supply. Power transformers are important and critical components in power systems. Those responsible for their operation and maintenance, as well as managers, must be familiar with the insulation structure, material properties, manufacturing quality, maintenance methods, and scientific diagnostic techniques associated with these transformers. Only by implementing optimized and proper operation management can the efficiency, service life, and reliability of power transformers be ensured. http://bbs.bjx.com.cn/data/attachment/forum/201611/03/13240151ecchb158w2ehe1.jpg