Transformer drying process

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The transformer body is affected by moisture, which will reduce the electrical resistance of insulation and accelerate the aging speed, so it must be dried. But with the development of production, more and more advanced drying equipment and drying methods will be used to improve drying quality.

1. Significance of transformer drying treatment
The purpose of transformer drying is to remove moisture in transformer insulating material, increase insulation resistance and increase flashover voltage. Transformers with voltage above 3kV must be dried.
Transformer body is mainly composed of iron core and coil and insulation assembly, assembly, before joining transformer oil, must after drying process, in order to remove the moisture in the insulation material and gas, the water content within the limits of control in the product quality requirements, to ensure that the insulation of the transformer has enough strength and operating life. For high voltage transformers, the water content of insulation materials should be within 0.5%.

2.Transformer insulation drying standard and drying treatment
(1) Drying standard:
Transformer insulation oil does not contain moisture. The breakdown voltage of the oil shall not be lower than 75% of the delivery data;
Insulation resistance is not less than 70% of the factory data;
The tangent of medium loss Angle shall not be greater than 130% of the factory data.

(2) The transformer shall be dried under the following conditions:
Replace winding or insulation during maintenance;
(2) When repairing or installing the body, the body is exposed to air for longer than the corresponding prescribed time;
The insulation resistance and absorption ratio are measured to measure the moisture exposure of the transformer winding.

3.Common methods of transformer drying treatment
(1) Induction heating. The tank body is placed in the original tank, the outer winding coil of the tank passes the current and USES the eddy current heating of the tank skin to dry. At this time, the wall temperature shall not exceed 115℃ ~ 120℃, and the body temperature shall not exceed 90℃ ~ 95℃. For the convenience of winding coil, as far as possible to make the number of turns of the coil or a small current, the general current is 150A, the wire can be used 35 ~ 50mm2. The oil tank wall can be lined with asbestos strips and the wire is wound around the asbestos strip. The power required for induction heating depends on the type of transformer and the drying conditions.

(2) Hot air drying method. Put the transformer in the drying chamber and let hot air to dry it. The drying chamber may be laminated with wainboard according to the size of transformer body, and the wainboard shall be covered with asbestos board or other canvas or asbestos linen impregnated with fire-proof solution. The drying chamber shall be as small as possible, and the space between the siding and the transformer shall not be greater than 200mm. It can be heated by electric furnace or steam coil.
When drying, the inlet hot air temperature should gradually rise, the highest temperature should not exceed 95℃, in the hot air inlet should be installed filter or installed metal mesh to eliminate Mars, dust. The hot air should not blow directly to the body, but evenly from the bottom of the body to each part, so that the moisture is released through the vent hole in the box.

(3) Vacuum drying method. This drying method is based on air as the heat carrier, under atmospheric pressure, the transformer body or winding is gradually preheated to about 105℃, and then vacuum treatment is started. Due to slow heat transfer and uneven internal and external heating (internal cooling and external heating), transformers with high voltage and large capacity often need more than 100h for preheating due to thick insulation layer, long production cycle and incomplete drying, so it is difficult to meet the insulation requirements of transformers. But the equipment is simple and easy to operate.

(4) Gas-phase vacuum drying method. This drying method USES a special kerosene vapor as a heat carrier. The kerosene vapor introduced into the vacuum tank condenses on the transformer and releases a large amount of heat energy, thus heating the body of the dryer. Due to the large heat energy of kerosene steam, the transformer body drying and heating is more thorough, more uniform, high efficiency, and the damage degree to the insulation material is also very small. However, due to its complex structure and high cost, it is currently only applied in the drying treatment of large transformer body of 110kV and above.

4. Requirements that should be met after the transformer is dried
(1) No matter which way is used to heat and dry the transformer, the body temperature of the transformer shall not exceed when there is no oil
The transformer body is affected by moisture, which will reduce the electrical resistance of insulation and accelerate the aging speed, so it must be dried. But with the development of production, more and more advanced drying equipment and drying methods will be used to improve drying quality.

(2) The transformer shall be dried under the following conditions:
Replace winding or insulation during maintenance;
When repairing or installing the body, the body is exposed to air for longer than the corresponding prescribed time;
The insulation resistance and absorption ratio are measured to measure the moisture exposure of the transformer winding.

5. Problems that should be noticed in the drying process of transformer
(1) If there is no vacuum in the drying chamber, the air hole should be opened on the box cover or the throttle hole should be used to let out the moisture.

(2) When heating with oil, insulation layer shall be installed outside the oil tank. Insulating materials such as asbestos cloth and glass cloth shall be used in the insulation layer. Inflammable materials shall not be used and corresponding fire prevention measures shall be taken

(3) In order to improve the drying quality of winding, there are two major factors must be seriously considered: one is to control the drying temperature; The second is to improve the vacuum degree of equipment. For the first point, the general drying equipment can meet the requirements of the process, for the second point, is affected by many factors, must be considered as a whole, reasonable arrangement, in order to achieve a good drying effect.

(4) In the vacuum pumping process during drying, in the low temperature stage when drying begins, it is not suitable for vacuum pumping or drying under the low vacuum condition, otherwise it is not conducive to the increase of core temperature and the elimination of moisture. When the temperature rises to 70 ~ 80℃, the vacuum degree begins to increase. When drying for 1 to 2 hours, there is more water vapor in the oil tank, and the thermal radiation capacity increases. The internal temperature tends to be uniform, and the moisture also gradually decreases, and the thermal radiation capacity decreases again.

(5) The method of insulation appraisal after drying and the technical indexes of the transformer tested. After the transformer is dried, its insulation performance needs a comprehensive appraisal to check its drying effect. The identified items are the same as the test items during the hoisting core overhaul except casing.

6. The main factors affecting the drying effect of transformer
(1) There must be enough heat energy. A large transformer has hundreds of kilograms to several tons of insulation and contains a lot of water, so a considerable amount of heat energy is required during the drying process.

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

(3) The diffusion coefficient of the insulating material depends on the moisture content, temperature and ambient air pressure of the insulating material.

(4) The drying time should be short, and the drying time mainly depends on the final moisture content index.

7 the conclusion
To sum up, the transformer insulation dry processing is the moisture in the material transfer to the surrounding media process, in order to improve the drying speed of insulating materials, people from raising body temperature and vacuum degree within the tank dry, take a lot of dry method, to deal with many practical problems encountered in the drying process. Each drying method has its own application and advantages and disadvantages, which 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 drying methods to improve the drying quality of transformer.