220KV Oil-immersed Power Transformer-VP Drying

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The basic principle of VP drying(kerosene vapor phase drying) is to use kerosene to heat, evaporate under vacuum to become high-temperature kerosene vapor, which condenses in the vacuum tank when it encounters a relatively low-temperature workpiece and releases the latent heat of condensation (about 306.6kJ/kg) to the transformer body. The condensed kerosene is collected, re-transported to the evaporator through the kerosene delivery system and heated again to evaporate. After so many cycles, the temperature of the work piece continues to rise and the water in it continuously evaporates into water vapor, which is pumped away by the vacuum system and finally reaches the purpose of thoroughly drying.

220KV oil-immersed power transformer insulation drying operation is a very important process in the transformer manufacturing process, which has a great influence on the final quality of the transformer. If the drying is not thorough, the residual moisture in the insulating material will form an electrode under a high electric field and continuously generate free discharge, thereby gradually destroying the insulating properties of the insulating material. At the same time, in the normal operation of the transformer, the aging process of the insulation material itself will be accelerated, and the life of the transformer will be shortened. If the drying is excessive, the crystal water in the insulating material will escape, which will lead to the aging of the insulating material in advance and the mechanical strength will decrease.

Process stage of kerosene vapor phase drying equipment: divided into preparation stage, heating stage, depressurization stage and high vacuum stage.

1)Preparation stage
Place the product in a vacuum tank and evacuate to reduce the pressure to below 700Pa. At the same time, the vacuum tank body is heated and insulated,the evaporator preheats the kerosene, so that the evaporator kerosene steam outlet temperature is gradually increased from 98 to 130 ℃. The kerosene changes from a liquid state to a vapor state and diffuses in the vacuum tank to fully heat the product.

2)Heating stage
At this time, only one set of vacuum holding pump is working,the vacuum degree of the vacuum tank and the condensation collection tank is maintained at the vacuum level specified in the process. Kerosene steam exchanges heat with the transformer, condenses and flows to the buffer tank, and then returns to the evaporator to form a cycle. As the temperature continues to rise, the vapor in the kerosene steam and the insulating material gradually increases the pressure in the vacuum tank. At this time, the above mixed gas is pumped to the condensation recovery system by a maintenance pump for condensation and separation. The condensed and separated kerosene liquid is then input into the evaporator, and the above process continues until the temperature of the body reaches the process requirements before entering the depressurization stage.

3)Depressurization stage(Low vacuum stage)
Stop the delivery of kerosene to the evaporator and continue to draw the mixed gas from the vacuum tank out of the vacuum tank to re-evaporate the kerosene remaining in the insulating material for condensation recovery. According to the different product capacity and voltage level, the intermediate voltage reduction stage and heating stage can be alternated several times to achieve the best drying effect.

4) High vacuum stage
Start the main vacuum system to vacuum the vacuum tank. The final pressure reaches below 10Pa. The temperature of the body reaches the specified temperature, and the water output rate of the insulation material is less than per ton of insulation material per hour less than 10g, the drying process ends.

Edited By: Jessica WU;
Copyright: SIMUWU Vacuum Furnace

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Kerosene Vapor Phase Drying Plant