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In oil-immersed transformers, the primary insulating material is transformer oil, which is a type of natural mineral oil extracted from petroleum. More than 95% of its main components are carbon-based compounds, predominantly alkanes, cycloalkanes, and aromatic hydrocarbons, with the remaining portion consisting of non-hydrocarbon compounds. It has excellent insulating and heat dissipation properties. This can be further analyzed as follows:
(1)Insulation: Pure transformer oil has a very high dielectric strength, typically ranging from 200 to 250 kV/cm, which is many times higher than that of air. When used in conjunction with solid insulation materials, the effect is even better. Therefore, using transformer oil for insulation can significantly reduce the size of the transformer. Additionally, due to the fluidity of the liquid medium, after a breakdown occurs, the insulating strength can be restored, and no permanent discharge channels are left. After filtration, it can continue to be used. Transformer oil also has the ability to penetrate the inner parts of the transformer, filling the entire space, replacing the air, and improving the withstand voltage strength. Furthermore, it protects the windings and other solid insulation components from moisture.
(2)Heat Dissipation: During operation, the transformer generates heat due to active power losses in the windings and core, which causes temperature rise. If the heat is not dissipated in time, the insulation could be damaged. Transformer oil has a high specific heat and good fluidity, relying on convection to carry away heat. Specifically, the oil near the core and windings gets heated, becomes lighter in density, and rises, with cooler oil replacing it. This forms a natural circulation, and the hot oil is then dissipated through the oil tank walls and coolers. To ensure smooth circulation, oil channels are left between the windings and the core, between the windings themselves, and inside the windings. In large-capacity transformers, forced oil circulation and directed cooling are used to enhance heat dissipation. To make better use of transformer oil and ensure its quality, a series of technical indicators are established as standards for evaluating its quality. These indicators include the following:
(3)Dielectric Strength: The dielectric strength of transformer oil refers to its withstand voltage under an AC field. This is usually determined through testing. The test method typically involves placing the oil into a standard test cup, with a pair of metal electrodes of 25mm in diameter placed inside, with a gap of 2.5mm between them. An AC voltage is applied across the electrodes, and the voltage is increased until the oil breaks down. The breakdown voltage value is taken as the dielectric strength. The standard value increases with the voltage rating of the transformer. New oil generally requires a higher dielectric strength than oil in service (see Table 2-16).
(4)Specific Gravity: The specific gravity of transformer oil is related to its chemical properties. Oils with lower specific gravity are preferred, as impurities and moisture in the oil are more likely to settle out.
(5)Viscosity: Viscosity is an important indicator of the oil's flowability. Oils with lower viscosity flow better and have better heat dissipation properties.
(6)Pour Point: The pour point refers to the temperature at which the transformer oil loses its flowability and solidifies. It is typically represented by its grade, such as Grade 25 oil indicating a pour point of -25°C, and Grade 40 oil indicating a pour point of -40°C. Oils with lower pour points can maintain good flowability in cold temperatures, so oils used in northern regions generally require a lower pour point than those used in southern regions.
(7)Flash Point: Like gasoline, transformer oil can evaporate, though at a slower rate. The evaporation can deteriorate the oil, increasing its viscosity, reducing its volume, and creating fire hazards and harmful vapors. Therefore, it is preferable to use oils with lower volatility. The flash point indicates the temperature at which the vapor emitted by the oil reaches a level that can be ignited. The higher the flash point, the better.
(8)Impurities: Transformer oil often contains impurities such as ash, acids, alkalis, and sulfur. These impurities can corrode the windings, insulation components, and other metal parts of the transformer. Therefore, the lower the content of impurities, the better. Ash content should not exceed 1/10,000. The presence of water, free carbon, active sulfur, soluble acids, alkalis, and mechanical mixtures should be completely absent.
(9)Acid Value: When transformer oil comes into contact with oxygen in the air, oxidation occurs, producing free acids that increase the acid value and degrade the oil quality. The acid value is used to determine the degree of oxidation, and it is expressed by the amount of potassium hydroxide (KOH) required to neutralize all the free acids in one gram of transformer oil.
(10)Stability: Stability, also known as resistance to aging, refers to the oil's ability to resist deterioration. The higher the stability, the better. Stability is typically measured by the acid value and the amount of precipitates after artificial oxidation. The lower the content of these, the better the stability.
To date, domestic and international research on transformer oil breakdown theory has been conducted extensively, but a unified conclusion has not yet been reached. The commonly accepted theory is the "small bridge" theory. According to this theory, the breakdown of oil occurs due to the distortion of the electric field caused by air, moisture, and fiber impurities in the oil. Specifically, due to the high dielectric constant of moisture and impurities, they easily polarize and orient along the electric field direction, forming conductive "bridges" between the electrodes. Initially, these form electrical channels, and eventually, they develop into breakdown channels.
In the oil-immersed transformer insulation structure, the insulating performance is greatly dependent on the oil's breakdown strength. Theoretically, the dielectric strength of oil is very high; experiments have shown that specially purified oil can achieve breakdown strengths over 4000 kV/cm. However, in practice, purified transformer oil has a dielectric strength of around 200-250 kV/cm, and the breakdown voltage in standard test cups is generally above 35 kV/2.5mm (see Table 2-15). The significant variation is mainly due to the presence of moisture and impurities in the oil. According to current research, the dielectric strength of transformer oil (breakdown strength) is influenced by the moisture content, impurities, temperature, pressure, uniformity of the electric field, duration of voltage application, and the oil's volume. These factors will be discussed in more detail below.
1.Moisture
Moisture is one of the most harmful impurities in transformer oil. Even if the oil is initially dry, it will gradually absorb moisture from the environment and reach a saturation point, which is influenced by ambient humidity. There are two forms of water in oil: suspended (forming droplets) and dissolved (dispersed in molecular form). Suspended water greatly impacts the dielectric strength, while dissolved water has minimal effect.
2.Impurities
According to the "small bridge" theory, impurities like fibers, metal particles, and carbon particles significantly affect the breakdown voltage of oil. The presence of fibers is especially detrimental.
3.Temperature
The effect of temperature on the oil's breakdown voltage is complex. In a uniform electric field, the dielectric strength of oil has a maximum value around 60-80°C, and a minimum value near 0°C. At temperatures above 80°C, the moisture in the oil vaporizes, which causes a decrease in dielectric strength. At low temperatures, moisture solidifies, further decreasing breakdown strength.
In addition to transformer oil, other insulating materials commonly used in oil-immersed transformers include insulation paper, insulation board, and various other materials. These are described as follows:
1.Insulation Paper and Insulation Board
The insulation paper and boards used in transformers are typically made from unbleached sulfite pulp fibers and are cellulose-based insulating materials. They have high porosity, which allows them to absorb oil and moisture effectively.
2.Other Insulating Materials
Other materials such as varnished cloth, glass fiber, resin composites, porcelain, and wood are also used for specific components like core supports, spacer rings, and external insulation in transformers.
