The influence of moisture, impurities and oil temperature on transformer oil's insulation strength are mutually infiltrated and restricted.
Moisture has the greatest impact, even if the transformer oil contains a small amount of moisture, the insulation strength of the transformer oil will be significantly reduced. (This is why we must vacuum drying the transformer oil to ensure that the water content of the transformer oil is within an acceptable range). The water content of the new transformer oil reaches saturation within the range of about 100 ppm ~ 200 ppm, but such a small amount of water can also reduce the insulating strength of the oil several times. When the moisture content continues to increase, the change in dielectric strength becomes slower. Because water is a polar molecule, under the action of electric field force, it is easy to form a small conductive bridge between the two electrodes, connecting the two electrodes, so that the insulation strength drops sharply (you can check the previous article-the "small bridge of insulating oil breakdown" "theory). However, the solubility of water in oil is not large, only a certain amount of water is dissolved or suspended in the oil, and most of the water is deposited at the bottom of the container, so when the water increases, the insulation strength will not be significantly reduced.
In addition to water, there are other pollutants in oil, so the relationship between water content and breakdown voltage cannot be expressed by a formula. Moreover, the breakdown voltage depends not only on the water content, but also on the state in which the water exists in the oil. For the same water content, the degree of reducing the breakdown voltage is different. Emulsified water has a greater impact on breakdown voltage, followed by dissolved water. This is because the dissolved water is highly dispersed in the oil, and it is not easy to form small conductive bridges under the action of the electric field, while the suspended emulsified water, especially when the water in the oil is all milky, the small water droplets are very easy to be under the action of the electric field. If it is elongated and forms a small conductive bridge, the breakdown voltage of the oil will drop significantly.
When there are impurities such as water, fiber, dust, etc. in the transformer oil, it can aggravate the drop of breakdown voltage. If the oil contains no impurities, water can only reduce the breakdown voltage by about 20%. When there is no water, pure carbon reduces the breakdown voltage by 15%, and when there is water, the breakdown voltage can be reduced by 30%. If the ash and moisture in the air are immersed in the oil at the same time, the breakdown voltage of the transformer oil can be greatly reduced by about 50%.
For transformer oil without impurities and moisture, oil temperature has little effect on the insulation strength. It is because under a certain electric field and temperature, oil molecules are not easy to crack and ionize, but when the temperature rises to a certain level, the oil molecules will crack and ionize, and as the temperature rises, the viscosity of the oil will significantly decrease, the movement speed of ions and electrons produced by oil ionization will increase, resulting in breakdown of the oil and a significant decrease in insulation strength.
At the same temperature, transformer oil with moisture and impurities has a lower breakdown voltage than transformer oil without moisture and impurities.
When the temperature is low, the water in the oil is in the form of a suspension emulsion, and its breakdown voltage is low. When the temperature rises, the emulsion water gradually turns into dissolved water, and the breakdown voltage of the oil also increases. When the temperature rises to a certain height, the moisture in the oil will evaporate, and the more bubbles will be caused in the oil. And because of the decrease in viscosity, the resistance will decrease, so the moisture, impurities and the bubbles will be more in the oil. It is easy to form a small conductive bridge, and as a result, the breakdown voltage of the oil drops quickly. Especially when impurities and water in the oil exist at the same time, this "bypass" effect is more prominent, and the breakdown voltage drops more obviously.(Thereby, vacuum transformer oil treatment plant is a necessary maintenance equipment during transformer installation, maintenance and repair work.)
When the oil contains free carbon and moisture, the breakdown voltage of the oil decreases with the increase of the amount of carbon particles.
PureTech is a professional manufacturer of transformer oil treatment plants and oil regeneration plants. Through the technology of thermal-vacuum drying and multi-stage precision filtration, the dissolved water and emulsified water, dust, fiber and other impurities in the transformer oil are removed, the insulation strength and breakdown voltage of the old transformer oil are improved to ensure the safe and normal operation of power transformers, meanwhile save operating costs.
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