Transformer Vacuum Drying Equipment

Oil-immersed power transformer insulation drying operation is a very important process in transformer manufacturing process, which has a great impact on the final quality of transformer. Some of them are not thoroughly dry, and the residual water in the insulating materials will form electrodes under high electric field and continuously produce free discharge, which gradually destroys the insulating properties of the insulating materials. At the same time, in the normal operation of transformer, the aging process of insulation material itself will be accelerated, and the life of transformer will be shortened. If excessive drying leads to crystalline water escaping from insulating materials, it will lead to premature aging of insulating materials and decrease of mechanical strength. These problems are reflected in the vacuum drying equipment and technology applied in our country at the present stage, such as backward drying mechanism, large energy consumption, long processing product cycle, low level of detection and control, and unable to judge the end point of drying, which are difficult to meet the requirements of product quality control and international standards. Comparing the kerosene vapor coherent drying and low frequency thermal drying commonly used at home and abroad, the vacuum drying equipment with variable pressure method has the advantages of low cost, short drying cycle, accurate judgment of drying end point, no danger and pollution caused by kerosene, no special requirements for geographical environment and factory conditions, and wide application range.

Principle of Vacuum Drying by Variable Pressure Method
In the process of transformer drying, besides the direct evaporation of water on the surface of insulating material, the moisture inside the material also has a process of diffusion evaporation. The greater the difference between the partial pressure of water vapor in insulating material and that in surrounding medium, the faster the diffusion and evaporation of water in insulating material. Therefore, it is necessary to increase the temperature of insulating materials in order to increase the water pressure inside the materials. At the same time, to reduce the vapor partial pressure of the surrounding medium, vacuum is needed. Thereupon, heating and vacuum pumping become the main contradiction in operation. Correct handling of the relationship between them is the key problem of drying technology. The new vacuum drying technology of pressure-swing method, which adopts the method of “pressure-swing” to actively control the change of pressure in the drying process, solves the above problems and achieves satisfactory results. The application of pressure-swing vacuum drying technology is mainly embodied in equipment and process, which is embodied in the following two aspects.

Application of Variable Pressure Vacuum Drying Technology
The drying process is divided into four stages: preheating, variable pressure dehydration, final drying and final judgment. Firstly, heating is the main step in the preheating stage. Gas ballast effect is used to make the drying tank in a low vacuum state. At the same time, air and evaporated water vapor in the tank are fully utilized to transfer heat from the heater to the transformer body through convection and conduction, so that the temperature of winding and core can rise quickly. The evaporated water vapor can be pumped out and the temperature distribution in the drying tank can be uniform by means of the stirring action of air filling through the cyclic air extraction and charging process in the preheating process. When the air temperature, winding temperature and core temperature in the tank reach the set value, it can enter the stage of variable pressure dehydration. Secondly, in the stage of variable pressure dehydration, we should make full use of the evaporated water vapor and convective heat from insulating materials to supplement the heat lost by the transformer due to dehydration and vaporization, and at the same time continuously extract the dehydrated water vapor in time. By pumping vacuum step by step, the moisture stored in insulating materials will gradually decrease. At this time, only radiation heat transfer is needed to meet the basic requirements of vaporization heat. When the pressure and temperature in the tank reach the set value, it will enter the final drying stage. Thirdly, in the final drying stage, the pressure in the pot decreases continuously after continuous vacuum pumping, which further enlarges the difference between the partial pressure of water vapor inside and outside the insulating material, and continuously extracts the residual moisture from the insulating layer until the final dry pressure is reached in the pot. Finally, in the judgment stage of drying end point, when the vacuum degree in the drying tank reaches the final dry pressure and the temperature and leakage rate of each monitoring point meet the judgment conditions, the vacuum drying end point can be judged by voltage conversion method to determine whether the drying of transformer has met the requirements.

Application of Variable Pressure Vacuum Drying Equipment
The drying equipment consists of four parts: vacuum drying tank, heating system, air pumping system and automatic control system. The automatic control system takes programmable logic controller (PLC) as the core, and has various control and protection functions required by pressure-changing method. It can automatically control the pressure-changing drying process and complete the whole process and final judgment. More than 20 different programs can be stored to suit transformers with different capacities.

Characteristics of Vacuum Drying Technology by Variable Pressure Method
The main characteristics of pressure-swing vacuum drying are as follows:
(1) It is a new gas-phase drying technology, in which water vapor removed by drying is used as heat carrier. It makes full use of vaporization heat to improve heating efficiency, and uses the method of controlling pressure change in drying process to correctly handle the relationship between heating and vacuum pumping, breaking through the traditional concept, so it can use simpler equipment with the advantages of high heat transfer efficiency, energy saving, short drying cycle, uniform heating and energy saving.
(2) The on-line automatic judgment of vacuum drying endpoint was realized by using pressure-swing method. By using the temperature and vacuum measurement system and programmable logic controller needed for process control, continuous on-line measurement, calculation and judgment can be carried out without adding special measuring equipment. It is not only accurate and reliable, but also completely automatic.
(3) High degree of automation, high-performance programmable controller as the control core, with complete control and protection functions, drying process is completely realized by the system itself. At the same time, the equipment control mode can be fully automatic operation and manual operation.
(4) The equipment has reasonable structure design, good safety performance, energy saving and pollution-free. Moreover, the equipment investment and operation cost are low, which fully meets the requirements of modern industry.

Application of Vacuum Drying Technology with Variable Voltage Method for Oil-immersed Power Transformer
Throughout the application in recent years, the vacuum drying process of oil-immersed power transformer with variable voltage method has the following effects compared with the original process:
(1) The drying cycle is generally shortened by more than 50%.
(2) Energy consumption is reduced by 30%-50%. Each equipment can save 250 tons of coal and 160,000 kWh of electricity per year.
(3) The quality of products has been greatly improved. Because the temperature rise can be controlled and the temperature distribution in the tank is uniform, the phenomenon of coil surface scorching in the past has been avoided and the problem of iron core rusting has been solved. The drying quality of transformer products is superior to the national standard of excellent products (JB / DQ2247 – 87), the insulation resistance is doubled, and the qualified rate of one-time delivery reaches 100%.
(4) The comprehensive production capacity is increased by 20%~30%, and the production capacity of vacuum drying process is more than double that of the original process equipment. Only by ensuring that the medium and small transformers are turned on every night, the actual production needs can be met, the passive situation that the previous process has to wait for the drying process has been changed, and the production cycle of the product has been shortened.
(5) No pollution, the labor intensity of workers has been greatly reduced. Variable-voltage vacuum drying technology is an ideal drying technology with less investment and quick effect, which is very suitable for medium and small transformer plants.