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Vacuum Drying Process Of Oil – Immersed Transformer

1. Current status of oil-immersed transformer drying process
Transformer drying process is mainly to remove excess water in the insulating material, and excess water mainly in the form of an adsorption adhesion in insulating materials, theoretical analysis, the water in the process of to get enough ability can turn into water vapor molecules, and then through the clearance of materials to spread around, and from the site.
Different drying methods, such as hot oil circulation, vacuum drying, zero sequence drying, short circuit, hot air drying and heat loss, are mainly adopted for the insulation drying of oil-immersed transformers according to the degree of moisture exposure and field conditions.Hot oil cycle is a cycle mode in which heat is brought to the heating equipment by the use of heat-conducting oil.However, it is found in practical application that there are many problems with zero sequence, short circuit, hot air, heat loss and other methods, which have little value in practical application.At present, the methods of hot oil circulation and vacuum drying are mainly used.Therefore, the vacuum drying technology of variable pressure method is discussed in the following paragraphs.

2. Vacuum drying technology by variable pressure method.
The moisture in oil-immersed transformer is mainly attached to the interior and surface of insulating material, so the drying process can be divided into three links: surface drying, internal drying and deep drying.Generally speaking, the rate of water evaporation in oil-immersed transformers is directly related to the pressure of water vapor in the surrounding medium.The traditional vacuum drying method is carried out in an environment close to the surface atmospheric pressure. Air is used as the medium. Firstly, the transformer needs to be heated to more than 100C and then vacuum treatment. Most studies have confirmed that the thermal conductivity of traditional vacuum drying technology is not high, and it takes a long time due to slow heat transfer, and the drying effect is not ideal.Due to uneven heating, the internal temperature cannot reach the ideal degree, and the deep drying cannot be realized. Besides, the insulation layer of oil-immersed transformer with large capacity and high voltage is usually thicker. If traditional vacuum drying technology is adopted, it will consume 100 hours in the preheating process, and the treatment efficiency is low.However, this method also has some advantages, mainly focusing on the advantages of simple operation and economic benefits.But generally speaking, the application value of traditional vacuum drying technology is not high and the efficiency is low.Therefore, at present, many enterprises reform and optimize the traditional vacuum drying technology.

3. The transformer vacuum drying oven of SIMUWU Shanghai Ge Hang Technology Co., Ltd. is mainly vacuum drying through the pressure change method. The vacuum drying process of the pressure change method is mainly divided into four parts: preheating, pressure change dehydration, final drying and end point judgment.
(1) Preheating: Heating is the main task of preheating. This method is mainly to transfer the heat from the heater to the transformer through convection and conduction through the air in the tank and the evaporated water vapor, so as to rapidly heat up the internal environment.During the preheating process, the evaporated water vapor can be discharged by circulating aeration, and the stirring effect ensures the uniform heating inside the tank.When the temperature of air, winding and core in the tank reaches predetermined parameters, the next step can be carried out.
(2) Variable pressure dehydration: in this process, it is necessary to make full use of the evaporation of water vapor and convective effect of the insulation material to supplement the heat loss caused by vaporization, and at the same time timely discharge the evaporation of water vapor in the material.After the step by step vacuum treatment, the residual moisture in the insulating material is relatively less, and the vaporization heat radiation can complete the internal drying and deep drying requirements of the material.The final drying process can be carried out when the pressure, temperature and other parameters in the tank reach the standard value of the vacuum drying technology of the variable pressure method.
(3) Final drying: Continuous vacuum pumping can make the pressure inside the tank decrease continuously, so as to further increase the difference between the partial pressure of water vapor in the inner and outer space of the insulation material, so as to continuously discharge the remaining moisture inside the insulation until the pressure inside the tank reaches the final drying.
(4) End point judgment: After the vacuum in the tank reaches the end dry, the temperature and leakage rate of each monitoring point are checked. If the standard parameters are reached, the end point judgment can be made to determine whether the drying treatment of oil-immersed transformer is qualified.

4. The advantages of vacuum drying under variable pressure are as follows.
(1) The method of variable pressure vacuum drying technology using heat of vaporization effect to improve the efficiency of drying process, and use the pressure change, improve the effectiveness of preheating and vacuum treatment between, breakthrough the limitation of the traditional vacuum drying technology can be implemented and can be used in a simple device to promote the efficiency of heat transfer, so as to shorten the time of the drying process, and can reduce energy consumption, improve the economic benefits of enterprises.
(2) Terminal judgment adopts online automatic judgment technology, mainly using PLC equipment and online monitoring device, which can realize continuous online measurement, calculation and judgment, with high reliability and reduced manual workload.
(3) The application of automation technology improves the operating efficiency of the system. Combined with the control and protection functions of PLC equipment, automatic control can be realized, thus improving the modernization level of the system.
(4) This technology has high safety and environmental protection, low operating cost, and can meet the modern society’s requirements for oil-immersed transformer insulation drying process.This technology is suitable for small and medium-sized transformer plants, and has the advantages of small investment and high efficiency, which can shorten the production cycle of products and improve the economic benefits of enterprises.

5. Conclusion
To sum up, oil-immersed transformer is an important equipment in modern power network, and the safety of equipment operation has a direct impact on the power supply network.Insulation drying treatment is related to the overall performance of oil-immersed transformer, so it is necessary to pay attention to the improvement of oil-immersed transformer insulation drying treatment process.Found in the actual work, vacuum drying process in oil-immersed transformer insulation dry processing has a good application effect, especially the method of variable pressure vacuum drying technology, this technology has the drying process of high efficiency, low energy consumption and drying effect good advantages, to avoid rusting problem, coil sear, core can improve the production efficiency of transformer factory, is worth popularizing in oil-immersed transformer in practical production.