Drying process and principle of the hottest transf

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Transformer drying treatment process and principle

Abstract: the transformer body is damped due to immersion in water, which will reduce the electrical strength of insulation and accelerate the aging speed, so it must be dried. This paper expounds a series of problems related to transformer drying treatment in detail. However, with the development of production, more and more advanced drying equipment and methods will be used to improve the drying quality

1 significance of transformer drying treatment

the purpose of transformer drying is to remove moisture from transformer insulation materials, increase its insulation resistance and improve its flashover voltage. Transformers with voltage above 3KV must be dried

the transformer body is mainly composed of iron core, coil and insulation material assembly. After assembly, before adding transformer oil, it must undergo drying treatment process to remove moisture and gas in the insulation material, so that its moisture content is controlled within the limits of product quality requirements, so as to ensure that the transformer has sufficient insulation strength and operating life. For high-voltage transformers, the moisture content of insulation materials is required to be within 0.5%

there is no need for users to panic. 2. Transformer insulation drying C. standards and drying treatment during the experiment process

(1) drying standard:

① transformer insulation oil does not contain water. The breakdown voltage of oil shall not be lower than 75% of the factory data

② the insulation resistance shall not be lower than 70% of the factory data

③ the tangent of the medium loss angle shall not be greater than 130% of the factory data

(2) the transformer should be dried in the following cases:

① replace the winding or insulation during maintenance

② when repairing or installing the body, the exposure time of the body in the air exceeds the corresponding specified time

③ the transformer winding is damp by measuring the insulation resistance and absorption ratio

3 common methods for transformer drying

(1) induction heating method. Huang Qunhui, director of the Institute of industrial economics of the Chinese Academy of Social Sciences, believes that it is dry by placing the body in the original oil tank, winding the coil outside the oil tank with current and using the eddy current heat of the tank skin. At this time, the temperature of the tank wall does not exceed 115 ℃ ~ 120 ℃, The temperature of the body should not exceed 90 ℃ ~ 95 ℃ "today's Lilian thinking. For the convenience of winding the coil, try to make the number of turns of the coil less or the current smaller, the general current is 150A, and the conductor can be 35 ~ 50mm2. Multiple asbestos strips can be placed on the oil tank wall, and the conductor can be wound on the asbestos strip. The power required for induction heating depends on the type of transformer and drying conditions.

(2) Hot air drying method. Put the transformer in the drying room and dry it with hot air. The drying room can be built with wall panels according to the size of the transformer, and the wall panels are fully covered with asbestos boards or other canvas or asbestos linen impregnated with fire-proof solution. The drying chamber should be as small as possible, and the distance between the wall plate and the transformer should not be greater than 200mm. Electric furnace and steam coil can be used for heating

the power consumed when using the electric furnace is calculated according to the following formula: the amount of hot air passing through the drying chamber Q per minute is selected according to the volume of the drying chamber Q, which is generally calculated with q=15qm3

P≈0.07 γ Q (t2-t1)

where p- required electric furnace power, kw γ- The specific heat of the air at constant pressure (both 0.31) T2, t1- the temperature of the inlet hot air and the surrounding air temperature, ℃

the temperature of the inlet hot air should gradually rise during drying, and the maximum temperature should not exceed 95 ℃. Filters or metal grids should be installed at the inlet of the hot air to eliminate sparks and dust. The hot air should not be blown directly to the body, but evenly to all parts from under the body, so that the moisture can be discharged through the ventilation hole in the box

(3) vacuum drying method. This drying method uses air as the heat carrier, and gradually preheat the transformer body or winding to about 105 ℃ under atmospheric pressure before vacuum pumping for treatment. Due to the slow heat transfer and uneven internal and external heating (internal cooling and external heating), the transformer with high voltage and large capacity often needs to be preheated for more than 100h due to its thick insulation layer. The production cycle is very long and the drying is not complete, so it is difficult to meet the insulation requirements of the transformer. But the equipment is simple and easy to operate

(4) gas phase vacuum drying method. This drying method uses a special kerosene vapor as the heat carrier, and the kerosene vapor introduced into the vacuum tank condenses on the transformer body and releases a large amount of heat energy, so as to heat the dryer body. Because kerosene steam has a large heat energy (the heat of kerosene gasification is 306 × 103j/kg), so the transformer body can be dried and heated more thoroughly and evenly, with high efficiency and little damage to insulation materials. However, due to its complex structure and high cost, it is currently only used in the drying treatment of large transformers of 110kV and above

4 requirements that the transformer should meet after drying

(1) no matter which method is used to heat and dry the transformer, the body temperature of the transformer should not exceed 95 ℃ when there is no oil, and the oil temperature should not exceed 80 ℃ when it is dried with oil, so as to avoid oil aging. If the insulation resistance cannot be improved by drying with oil, all the oil shall be discharged and dried without oil

(2) using the drying method with oil, the insulation resistance and oil breakdown voltage should be measured every 4H. When the oil breakdown voltage is stable and the insulation resistance value remains stable for 6h, the drying can be stopped

5 problems that should be paid attention to in the drying process of transformer

(1) if the drying chamber does not vacuum, the air hole should be opened on the box cover or the throttle hole should be used to release the moisture

(2) when heating with oil, an insulating layer should be installed outside the oil tank. The insulating layer can be made of asbestos, glass cloth and other insulating materials. Flammable materials should not be used, and corresponding fire prevention measures should be taken

(3) in order to improve the drying quality of windings, two factors must be carefully considered: first, control the drying temperature; The second is to improve the vacuum of the equipment. For the first point, the general drying equipment can meet the process requirements. For the second point, due to the influence of many factors, it must be considered as a whole and reasonably arranged to achieve good drying effect

(4) in the process of vacuumizing during drying, it is not suitable to vacuumize or dry under low vacuum at the low temperature stage of drying, otherwise it is not conducive to the rise of core temperature and the elimination of moisture. When the temperature rises to 70 ~ 80 ℃, the vacuum degree begins to increase. When the drying is carried out for 1 ~ 2H, there is more water vapor in the oil tank, the heat radiation capacity is improved, the internal temperature tends to be uniform, the water content is also gradually reduced, and the heat radiation capacity is reduced

(5) the method of insulation identification after drying and the technical indicators of the transformer tested. After the transformer is dried, its insulation performance needs to be comprehensively evaluated to check its drying effect. The identified items, except for the casing, are the same as the test items during the overhaul of the lifting core

6 main factors affecting the drying effect of transformer

(1) there should be sufficient heat energy. A large transformer has hundreds of kilograms to tons of insulators, which contain a lot of moisture, so it needs a lot of heat energy in the drying process

(2) the greater the difference between the water vapor partial pressure inside the insulating material and the surrounding medium, the faster the evaporation, diffusion and migration of water in the insulating material. The difference can be increased by reducing the partial pressure of water vapor around the insulation and increasing the temperature of the insulation material

(3) the diffusion coefficient of insulating materials depends on the water content, temperature and surrounding air pressure of insulating materials

(4) the time of drying process should be short, and the drying time mainly depends on the final required water content index

7 conclusion

to sum up, the drying treatment of transformer insulating materials is the process of transferring the moisture in the materials to the surrounding media. In order to improve the drying speed of insulating materials, people start with improving the drying temperature of the body and the vacuum degree in the oil tank, and adopt many drying methods to deal with many practical problems encountered in the drying process. Various drying methods have their own applications, advantages and disadvantages, and should be carried out according to local conditions. With the development of transformer production, it is believed that there will be more and more advanced drying equipment and methods to improve the drying quality of transformers. (end)

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