Transfomer Vapor Phase Drying Plant

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Vacuum drying of transformer is an extremely important process in the process of transformer installation. The technical level of vacuum drying will directly affect the health of transformer. If the drying treatment is not complete, a small amount of water will remain in the internal insulation material, which will generate a large number of charged electrodes under the action of high electric field of the transformer, which is easy to cause free discharge and serious damage to the internal insulation level. This paper introduces the basic principle, advantages and disadvantages, and application process of vacuum drying, mainly analyzes the current use of more gas-phase vacuum drying method, and discusses the problems and precautions in the use of this method.

The internal insulation medium of oil immersed power transformer is generally isolated by cardboard, which contains a small amount of moisture. Before the transformer is put into operation, the internal insulation material must be vacuum treated to avoid the discharge of these water molecules under the action of high electric field strength, which will cause internal insulation damage. If the vacuum drying is not enough, the aging process of the insulating materials will be faster in the normal operation of the transformer, which will cause potential safety hazards and greatly shorten the service life of the transformer. Therefore, for the vacuum drying of large capacity transformer, the gas phase drying method has been widely used.

1. Overview of vacuum drying technology
The moisture in the internal insulation material of transformer generally absorbs the internal part of the insulation material. In the process of drying, the water is generally changed from liquid state to gas state by heating first, and then the water is discharged by the internal and external pressure difference generated by high vacuum. The higher the temperature is applied under the same vacuum condition, the faster the drying speed of the transformer will be. The following introduces the conventional vacuum drying technology:

(1) Traditional vacuum drying method
a. Process: use air as the heat carrier, raise the temperature inside the transformer to 105 ° C by heating under the standard atmospheric pressure, and then use the vacuum extractor to extract the air.
b. Advantages: the equipment used is simple and easy to operate.
c. Disadvantages: the heat transfer speed of air is relatively slow, and the internal and external heating of the transformer is uneven, which is prone to the situation of incomplete drying.

(2) Spray drying
a. Process: vacuum the transformer first. When the transformer is in high vacuum state, spray hot oil inside the transformer to heat the body.
b. Advantages: the heating of the transformer is relatively uniform.
c. Disadvantages: drying takes a long time.

(3) Cyclic pressure method
a. Process flow: in the process of vacuum drying, use a vacuum pumping machine to vacuumize. In order to achieve a certain degree of vacuum, use an air injection valve to inject a certain amount of dry air into the transformer, so as to improve the internal thermal conductivity of the transformer, so that the residual moisture on the insulating material can obtain enough energy to escape.
b. Advantages: the method of vacuum drying takes a short time, and the vacuum drying is complete.
c. Disadvantages: there are many equipments involved in drying.

2. Application of transfomer vapor phase drying plant method
Due to the large internal space and the large thickness of the insulating board of the large capacity transformer, if the above-mentioned conventional vacuum drying treatment is adopted, it will lead to the problems of long drying treatment time, many dead angles of drying, insufficient thoroughness and large investment, etc., while the gas-phase drying method can meet the requirements of high vacuum and high temperature for the large capacity transformer, and the drying treatment is relative Fast. The medium processed by transfomer vapor phase drying plant is mainly kerosene.
Process stage of kerosene gas phase drying equipment

Preparation stage:
The transformer is vacuumized with a vacuum extractor to reduce the internal pressure to below 700pa. At the same time, the vacuum tank body is heated and insulated, the kerosene evaporator is preheated to make the temperature of kerosene steam outlet rise from 98 to 130, the kerosene used for drying changes from liquid state to steam state, and diffuses in the vacuum metal tank.

Heating stage:
At this time, only one vacuum maintenance pump is needed for the transformer to keep working. Kerosene steam is injected into the transformer through high pressure for heat exchange. In case of condensation of insulating material or wall, it flows to kerosene collection tank and then returns to evaporator, forming a circulation system.
Step down stage (also called low vacuum treatment stage):
Stop injecting kerosene into the evaporator, and continuously extract the gas in the vacuum tank. Through the high temperature effect in the transformer, the kerosene remaining on the insulating board is evaporated again, and finally condensed for recovery. In view of the difference between the capacity and voltage level of the transformer, some treatment processes are carried out alternately in step-down stage and heating stage, so that the internal drying treatment effect can meet the requirements.

High vacuum stage:
Enable the main vacuum system to vacuum the tank. Make the final pressure in the vacuum tank drop below 10Pa. When the temperature of the transformer body reaches the value specified in the process, and the water yield per ton of internal insulating material per hour drops below 10g / T.H, the drying process can be completed.

3. Advantages of transfomer vapor phase drying plant equipment
Dryer body temperature uniformity
As the heat released by kerosene vapor condensation is used for heating, the lower the temperature is, the more intense the condensation is, that is, the higher the heat release is, so the temperature of the body is uniform.

High heating temperature
In the process of kerosene gas-phase drying, the insulation material aging process is very slow. Compared with the hot air circulation vacuum drying, the heating temperature can be increased by about 30 benefits, reaching about 130 benefits.

Fast drying
The body of the device is always in a vacuum state, so the water outflow process runs through the whole drying cycle. For example, it used to use hot air circulation vacuum drying method to dry, a sfp90000-150000kva / 220kV transformer needs 95 hours, now it only takes 43 hours to dry the same capacity transformer with vacuum transformation drying method, and the time is shortened by 1.21 times. It can be seen that the use of kerosene gas phase drying method greatly shortens the production cycle, and its economy and efficiency are reflected.

It can clean the body
After kerosene vapor condenses into liquid, it can clean the inside and outside of the transformer body, which has unique advantages in the treatment of repair transformer. After the treatment of the transformer body, the surface is clean and free of oil dirt and stains.

4. Problems to be noticed in transfomer vapor phase drying plant
(1) If the old transformer is dried in the process of maintenance, generally there is a lot of oil on this kind of transformer. In order to avoid impurities entering the vacuum pump in the heating process, liquid circulating vacuum pump can be used according to the actual situation on site.

(2) Prolonging the heating time in the drying process has a positive effect on shortening the drying time and improving the drying level. In practical operation, if the heating time is extended to about 60% of the whole drying process (usually only 1 / 3), the drying time can be effectively shortened by about 10% ~ 15%, and the residual water in the transformer will reach the operation standard.

(3) The water content in the transformer is high at the beginning. During the heating process, the humidity in the box will become larger. At this time, the iron core is prone to oxidation and rust often occurs. Solution: control the heating speed of the transformer body, generally keep the temperature rise speed within 15-20 ° f / h, and the temperature difference of the inner core within 30 ° F.

5. summary
Transfomer vapor phase drying plant method is usually used for the drying treatment of large-scale transformer, which has significant effect, low energy consumption and good economy. Based on the continuous summary of practical experience, this paper conducts in-depth study on the gas phase drying method, which plays a positive role in the safe and stable operation of transformer.