Kerosene Vapor Phase Drying Process For 44MVA Transformer

In the production process of transformer, drying treatment is a very key step, the merits of the transformer drying treatment process on the service performance of the transformer and the length of life plays a decisive role.In the actual use of the transformer, because of its bad working environment, coupled with the sealing treatment of the transformer is not in place, the transformer is damp and can not work normally.Now commonly used main transformer drying process of circulating air vacuum drying, dry cycle hot oil, kerosene and oil and vapor phase pressure valve vacuum drying drying, compared with the former three kerosene drying technology of oil and vapor phase with uniform heating, heating speed, drying effect is good wait for an advantage, widely applied to the transformer dry processing.However, the kerosene vapor phase drying process also has its own limitations, leading to some problems affecting the drying effect in the drying process of the transformer. Therefore, it is necessary to improve the kerosene vapor phase drying process to meet the needs of the transformer drying. The model RQ-KP-4556 kerosene and vapor phase drying equipment independently developed and produced by Hangzhou Ruiqin Mechanical Equipment Co., Ltd. was experimentally studied. The size of the drying oven of this model is 4500 x 5000 x 6000 mm.

1. Kerosene and vapor phase drying process
1.1 Principle of kerosene vapor phase drying process
The kerosene vapor phase drying process refers to using special kerosene as heat carrying medium to heat kerosene in a vacuum environment. Because the specific heat capacity of kerosene is lower than that of water, the saturated vapor pressure is between water and transformer. Therefore, after heating kerosene is easy to volatilization, volatilization at the same time will be taken away with the residual moisture in the transformer, after the completion of drying, all the kerosene in the transformer after heating is easy to evaporate, there is almost no residual in the transformer, will not affect the normal work of the transformer.In addition, kerosene is also an excellent detergent, taking away moisture at the same time, but also the dust, particles, impurities in the transformer together with the transformer out, to clean the transformer to improve the overall performance of the transformer, so as to improve the efficiency of the transformer.

1.2 Advantages of kerosene vapor phase drying process
(1) Heating evenly.Because kerosene steam can enter most parts of the transformer, and then through the phase transition from vapor to liquid state, transfer heat to the transformer.vapor phase kerosene can to surround type of heating transformer as a whole, the lower the temperature of transformer is, the more part of the kerosene steam heat, which in the area of low temperature and pressure vapour phase behavior change drastically, the more you will release heat, transformer temperature rise of each part of the body, will be faster temperature difference to narrow, form a uniform heating.

(2) High heating temperature.Because the whole process of kerosene vapor phase drying process is in a vacuum without oxygen, the phenomenon of kerosene combustion caused by excessive temperature can be avoided when kerosene is heated.Therefore, the heating temperature can be raised to 130 ~ 135℃, although only 20℃ higher than the traditional transformer drying process temperature, but the partial pressure difference of water vapor in the transformer has been increased by more than 2 times, therefore, the drying effect of the transformer is very different, so as to minimize the moisture content of the transformer.

(3) The heating speed is fast.The kerosene vapor phase drying process uses kerosene steam as the heat carrier medium. When the temperature of the transformer is low, the kerosene steam releases a large amount of heat through phase transformation condensation, so that the transformer is heated quickly and the temperature rise of the transformer is faster.When the transformer rises to a certain temperature, the kerosene steam transfers the heat to the transformer through convection heat transfer. Because the contact area between the kerosene steam and the transformer is very large, the convection heat transfer efficiency is very high, so as to rapidly improve the temperature of the transformer.

2. Kerosene-vapor phase drying process and improvement measures

2.1 Preparation stage
The temperature of the evaporator will be raised to the specified temperature, and the vacuum tank will be vacuumed to 700Pa at the same time. The general duration of this stage is about 2 hours, which shall be subject to the actual conditions.

2.2 Heating stage
Start the heating switch of the evaporator and heat the kerosene to form steam. The kerosene steam enters the transformer and heats the transformer through heat exchange.As the heating process continues, the moisture in the transformer is gradually transferred to the vacuum tank, and the pressure in the vacuum tank continues to rise.When the pressure in the vacuum tank rises to a certain extent, the vacuum pump should be opened to vacuum and the pressure difference between the collection tank and the vacuum tank should be maintained.During the heating process, not only the pressure difference between the collection tank and the vacuum tank should be paid attention to at all times, but also the change of the moisture content in the vacuum tank should be observed. When the moisture content in the vacuum tank almost stops increasing, the heating stage will end.

In the heating stage, it is found that the length of the heating stage is affected by many factors. Under the premise of ensuring the drying quality of the transformer, in order to shorten the whole working time and improve the working efficiency of the whole heating stage, the improvement is as follows:

(1) According to the actual heating situation, appropriately extend the time of the heating stage, the original heating time only accounts for 1/3 of the whole drying time, now by extending the heating time, it accounts for about 60% of the total time.After many experiments, it is found that the efficiency of drying work can be improved by this improvement, and the whole drying time can be shortened by 10% ~ 15%.

(2) Improve the temperature of the evaporator, the temperature of the evaporator will be raised from the original 120℃ to 135℃ in the heating process, the temperature of the transformer increases faster, shortening the whole drying time, and the maximum temperature of the transformer is about 10℃ higher than before, and the drying effect of the transformer will also be better.

(3) Improve the temperature control accuracy.In the drying experiment, it is found that when the moisture in the transformer is more, the iron core may rust.The main reason is that the temperature control is not done well in the early heating, leading to a large temperature difference between the core and the transformer, through the water vapor brought out of the kerosene to meet the lower temperature of the core, the core condenses into small water droplets, leading to the corrosion of the core.Therefore, in the early heating stage, the temperature rise rate should be strictly limited, controlled at 15 ~ 20℃/h, to ensure that the temperature difference on the core is not too big, not more than 30℃ is appropriate, through this improvement measure, can avoid the rust of the core.

2.3 Hypotensive treatment stage
Turn off the heating switch of the evaporator, stop conveying kerosene steam to the vacuum tank, and then vacuum the vacuum tank.With the increase of the vacuum of the vacuum tank, the kerosene liquid stuck on the transformer is turned into kerosene vapor by the vacuum pump due to the decrease of the boiling point.Before vacuuming, the pressure in the vacuum tank is about 10000Pa. After depressurization treatment, it is reduced to 1/4 of the original pressure, which is less than 2500Pa. The process lasts about 3 ~ 6 hours.

2.4 High vacuum stage
The vacuum tank is further vacuumed to fully remove the residual moisture and kerosene in the transformer, so as to ensure the thoroughness of the transformer drying.At this stage, when the dew point reaches the preset value and the vacuum degree reaches below the set value, the vacuum pumping can be stopped.

3.Conclusion
To sum up, using kerosene to dry the transformer is a complicated and comprehensive process, which will be affected by a variety of interference factors, but the results of kerosene drying treatment are good, which can meet the actual work requirements of the transformer.In this paper, the whole process of kerosene vapor phase drying process of transformer has been analyzed in detail, and it has been improved to a certain extent, hoping to give enlightenment to the relevant staff engaged in transformer drying, and then do a good job of transformer drying work.

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