Transformer drying treatment methods

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1. Vacuum drying method for iron loss in oil tanks: Vacuum drying is carried out using the transformer oil tank itself, so a power supply with low capacity and voltage is sufficient； Its disadvantages are heavy workload, long drying time, and the need for large amounts of materials, equipment, and necessary calculations. 　　2. Winding copper loss drying method: This method utilizes the heat generated by passing electricity through the transformer windings to directly heat the insulation of those windings; as a result, the temperature rises rapidly, the workload is low, and the drying time is short ； Its drawback is that it requires a large power supply capacity. 　　3. Zero-sequence current drying method: Electricity is passed through the transformer windings to heat the oil tank, core, and windings ； Its disadvantage is that oil must be drained, the core must be lifted, and the winding connection pattern must be changed before and after drying the transformer; this involves a lot of work, temperature control is difficult, and local overheating may occur. 　　4. Zero-sequence short-circuit drying method: This is a composite method that lies between the zero-sequence current drying method and the winding copper loss drying method. The former relies on iron loss for heating, while the latter depends on copper loss for heating; the zero-sequence short-circuit drying method is therefore a composite approach that utilizes both iron loss and copper loss for heating. 　　5. Induction heating method: The device is placed in its original oil tank, and coils are wound around the outside of the tank; an electric current is passed through these coils, and the eddy currents generated cause the tank material to heat up, thereby enabling drying. At this time, the wall temperature of the tank should not exceed 115°C to 120°C, while the temperature of the tank body should not exceed 90°C to 95°C. For the convenience of winding the coil, it is advisable to keep the number of turns or the current level as low as possible; generally, a current of 150A is used, with wire of 35–50 mm2 being suitable. Several asbestos strips can be placed on the wall of the oil tank, and the wire is wound around these asbestos strips. The power required for induction heating is determined by the type of transformer and the drying conditions. 　　6. Hot air drying method: Place the transformer in a drying chamber and pass hot air through it for drying. The drying chamber can be constructed using wall panels adapted to the size of the transformer core, with asbestos sheets or other canvas or asbestos cloth impregnated with fire-resistant solutions filled inside the panels. The drying chamber should be as small as possible, and the distance between the wall panels and the transformer should not exceed 200 mm. It can be heated using an electric furnace or steam coils. 　　7. Vacuum hot oil atomization spray drying method: After the transformer oil is reduced to a large extent, it is heated to 100 degrees; then, using a vacuum oil filter and special atomizing nozzles, the oil is continuously circulated back into the tank, thereby raising the temperature of the tank and the equipment itself and allowing water vapor to escape. At the same time, some of the water vapor present in the insulation is removed by the vacuum pump. It is an efficient and high-quality Guangzhou method; its disadvantages are the large amount of work required, as well as the variety of specialized equipment needed. 　　8. Vacuum drying method: In this drying method, air is used as the heat transfer medium; under atmospheric pressure, the transformer core or windings are gradually preheated to around 105°C before vacuum extraction is carried out for processing. Due to slow heat transfer, heating is uneven inside and outside (cold inside and hot outside). High-voltage, high-capacity transformers, with their thicker insulation layers, often require more than 100 hours for preheating; this results in a long production cycle, as well as incomplete drying, making it difficult to meet the insulation requirements of such transformers. But the equipment is simple and easy to operate. 　　9. Vapor 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 surface of the transformer, releasing a large amount of heat energy that is used to heat the part to be dried. Due to the high thermal energy of kerosene vapor (the heat of vaporization of kerosene is 306×103 J/kg), it enables more thorough and uniform drying and heating of the transformer body, resulting in high efficiency as well as minimal damage to the insulating materials. However, due to its complex structure and high cost, it is currently only used for the drying treatment of large transformers of 110 kV and above. 　　Note: First of all, regardless of the method used to heat and dry the transformer, when there is no oil present, the temperature of the transformer’s core must not exceed 95°C; when drying with oil, the oil temperature must not rise above 80°C, in order to prevent the oil from deteriorating. If oil drying does not increase the insulation resistance, all the oil should be drained and drying should be carried out without oil. Secondly, when using the oil-dried method, the insulation resistance and the breakdown voltage of the oil should be measured every 4 hours. When the oil breakdown voltage remains stable and the insulation resistance value stays constant for 6 consecutive hours, drying can be stopped.