What are the factors affecting the electrical performance of oil immersed transformer
Writer： Hengfeng you electric Time：2023-04-19 views：times
There are many factors affecting the electrical performance of oil immersed transformer, among which the main factors affecting the withstand voltage strength of transformer include water content, gas content, impurities, temperature, flow rate and so on. By analyzing the influence trend of these influencing factors on the withstand voltage strength of transformer oil, it shows that the higher the voltage level of transformer, the stricter the control requirements for water content, gas content and impurities of oil. With advanced technology, the impurities in transformer oil can be removed by coarse and fine device to make the oil meet the requirements of each voltage level.
1. Analyze the factors affecting the electrical performance of transformer.
1.1 water content of oil.
There are three forms of moisture in transformer oil: deposition, dissolution and combination. The smaller the moisture content in the oil, the higher the power frequency breakdown voltage. When the moisture content is greater than 200 × At 10-6, the breakdown voltage remains unchanged, because the excess water sinks into the oil bottom at this time, which will not affect the breakdown voltage value during oil test.
When the moisture content in the oil is (300 ~ 400) × At 10-6, the water content exceeds the saturated dissolution, and the pressure resistance value of the oil in the water deposition part is the same as that at the saturated dissolution. Effect of water content in oil on dielectric loss index (TG) of oil δ） And the electrical properties of solid insulation are also greatly affected. With the increase of moisture content, TG δ The value rises rapidly. With the increase of moisture, the breaking voltage of oil impregnated paper decreases rapidly in a curve. When the moisture content is 3%, the electrical strength decreases by about 10%. When 500 kV transformer leaves the factory, the moisture content of insulating paper shall be controlled below 0.5%.
Generally, when the transformer is running, the oil temperature rises, the water content in the oil increases, and the water content in the paper decreases, that is, the water content in the paper diffuses into the oil; When the operating temperature decreases, the diffusion direction is opposite. Therefore, when the transformer with high oil temperature quits operation under low temperature environment, or the water content in the oil is too high, part of the water content of the oil diffuses into the paper. In addition, due to the decrease of oil temperature, the water content in the oil is greater than the saturated dissolution, and the excess water will be separated out from the oil and sink into the bottom of the oil tank or cooler. When the transformer is put into operation again, the water at the bottom of the cooler will be introduced into the transformer coil by the oil pump, and the water will move to the high-strength area of the transformer, causing potential danger. This situation must attract the attention of the transformer operation Department, and the water content of the oil must be controlled within the required value.
Reducing the water content of oil plays an important role in improving the operation safety of transformer and slowing down the aging of oil. In order to reduce the water content of the oil, the vacuum heating method can be used to treat the oil, heat the oil temperature to 60 ~ 70 ℃, improve the vacuum and reduce the water content of the oil.
1.2 impurities in oil.
The breakdown field strength of pure oil is very high. When there are impurities and moisture in the oil, the breakdown voltage of oil decreases obviously. There are a lot of insulating materials in the transformer, and the oil contains fiber impurities, in which the fiber containing water is more conductive. The dielectric coefficient is large, and it is easy to arrange into impurity bridges along the direction of electric field. The leakage current along the bridge is large and generates more heat, which is easy to cause water evaporation, so as to expand the bubbles, and the breakdown will occur in these bridges and bubbles. The more uniform the site is, the greater the influence of impurities on the breakdown voltage and the greater the dispersion of breakdown voltage. The less the influence of impurities on withstand voltage and impulse voltage in uneven electric field, because when partial discharge occurs at the highest intensity on the spot, the oil disturbance makes it difficult for impurities to form a bridge. At the same time, under the instantaneous action of impulse voltage, impurities have no time to form a bridge.
The insulation strength of suspended particles in oil under power frequency voltage is related to the number, size and performance of particles.
Two pressurization methods: (1) smooth pressurization at the speed of 10kV / S（ 2) Step by step pressurization, starting from 65% of the expected breakdown voltage within 1 minute and boosting with 3% of the expected breakdown voltage of each stage. The two voltage application methods show that the insulation strength decreases with the increase of particle content. Due to long-time withstand voltage, staged pressurization is more serious than smooth pressurization. The difference between the test results of the two pressurization methods is estimated to be about 15%.
At present, the oil purifier deals with impurities in the oil. 500 kV transformer adopts coarse filter and fine filter to remove impurities in oil and ensure that the withstand voltage level of oil reaches 60 kV / 2.5 mm.
1.3 oil and gas content.
Gas content is one of the main control indexes of transformer oil. Gas content directly affects the insulation performance of EHV Transformer. The gas content of transformer oil pressure should not exceed 4%, and the gas content of 500 kV transformer oil should be controlled within 0.5%. The normal amount of dissolved air in the oil is 10% - 11%. When the gas content of the oil exceeds the saturated dissolved amount, the gas will be released from the oil and suspended in the oil. When there are suspended bubbles in the oil, due to their different dielectric coefficients, the interfacial electric field will be deformed, the electrical strength of the gas is low, and bubble discharge will occur. For transformers above 60kV level, electrostatic treatment is required before vacuum oil injection and finished product test, in order to eliminate bubbles in the transformer and oil and prevent bubble discharge during product test. In addition, when the transformer is put into operation, too much dissolved gas in the oil will be gradually discharged and concentrated in the gas relay, resulting in misoperation.
1.4 uniformity of electric field.
Improving the uniformity of electric field can significantly improve the power frequency failure voltage of high-quality transformer oil. For the oil containing impurities, under the action of impulse voltage, the impurities have no time to form a bridge. Improving the uniformity of electric field can improve the withstand voltage of oil. The impurities in the oil gather and arrange under the action of power frequency withstand voltage, so that the electric field is deformed and the increase of damage voltage is not obvious. Manufacturing defects in production, such as metal foreign matters, bubbles, poor conductor shielding, burrs in conductors and grounding parts, etc., Affect the uniformity of transformer electric field, resulting in partial discharge and withstand voltage damage of products.
Take the following measures, such as adding iron core shielding, wire shielding, oil tank protective tube, coil electrostatic plate, voltage equalizing ball, etc., It can improve the uniformity of transformer electric field, not only reduce the insulation distance of insulation structure, but also improve the product quality. Check the hanging core of the product before leaving the factory, remove impurities and foreign matters in the transformer, and ensure the cleanliness of the product to the greatest extent.
1.5 oil flow rate.
When transformer oil flows, static electricity is generated by friction with insulating materials. The higher the flow rate, the higher the voltage. The electrification phenomenon caused by the flow of oil in the transformer is called oil flow electrification. Electrified oil flow will deform the transformer electric field. When the superimposed electric field strength exceeds the local discharge field strength or breakdown field strength of the insulating material, the safe operation of the transformer will be endangered. When the oil flow velocity is 0.5m/s, the partial discharge pulse generated by oil flow electrification begins to appear. In transformer manufacturing, the maximum flow rate is 0.33m/s. Oil flow electrification has a greater impact on EHV Transformer. Therefore, the transformer must control the oil flow speed, increase the section of the oil flow channel and reduce the flow rate. The insulating parts of the oil flow channel should be rounded. For large capacity and high voltage transformers, large flow forced oil circulation cooler oil pump is adopted to reduce the voltage of oil flow and prevent insulation partial discharge or insulation breakdown caused by oil flow electrification. In order to suppress the oil flow of transformer during operation, the insulator with voltage regulator can not only increase a certain proportion in 50K oil flow, but also be used in 50K oil flow.
2、 Conclusion.
The above discussion is the main factors affecting the electrical performance of transformer oil. In addition, there are other factors affecting the electrical performance of transformer oil in use, such as the compatibility between transformer oil and transformer insulating materials, the mixed use of transformer oil, and the electrical performance of transformer oil pressure, which are very helpful to the manufacture and use of transformer.
