Analysis of Cause of Transformer Oil Dielectric Loss Exceeding Standard

Dielectric loss is the sum of polarization loss and conductance loss caused by transformer oil under the action of alternating electric field. Dielectric loss factor can reflect the transformer insulation characteristics, reflect the transformer oil in the electric field, oxidation and high temperature under the action of aging degree, reflect the degree of polarity impurities and charged colloid pollution in the oil. In the process of long-term use of transformer, the operation condition of transformer oil can be reflected by medium loss factor test.

1. Analysis of causes for excessive dielectric loss

1, the influence of impurities.

Dust and other impurities exist in the oil or solid insulation materials of the transformer during installation. After running for a period of time, colloidal impurities gradually precipitate out. Colloidal particles are small in diameter and diffuse slowly, but have a certain amount of active energy. Particles can automatically coalesce, from small to large, coarse dispersion system, in an unstable state of non-equilibrium, when beyond the colloid range, due to gravity and sedimentation. After the presence of sol in the oil, when the sediment exceeds 0.02%, it may cause the conductivity to exceed the normal conductivity of the medium several times or dozens of times, resulting in the increase of the dielectric loss value.

2. Influence of transformer structure.

From the transformer manufacturing structure analysis, some transformer manufacturers from the transformer to reduce oil leakage Angle to consider the cancellation of oil purifier (thermal siphon), the increase of transformer oil medium loss factor has a certain impact. If the transformer is equipped with net oil is conducive to the stability of the quality of insulation oil, it can "suck out" the internal moisture of insulation during the operation of the transformer, improve the electrical performance of insulation, thus slowing down the increase of moisture in insulation.

3. Effects of microbial contamination.

Microobject bacterial infection is mainly caused by bacterial immersion during installation and overhaul. Due to pollution, oil contains water, air, carbonized matter, organic matter, various minerals and trace elements, which constitute the basic conditions for the growth, metabolism and reproduction of fungi. Because microorganisms are rich in protein, they have colloidal properties, so microbial oil pollution is actually a kind of microbial colloid pollution, and microbial colloid are charged, so that the conductivity of the oil increases, so the conductivity loss also increases. Transformer oil in the sealed, anoxic and matt body, there are microorganisms in the oil anaerobic and anaerobic. For a long time after placing dielectric loss test, especially in the colorless transparent glass bottles placed, its dielectric loss value will become smaller. Transformer in different periods with different loads, operating oil temperature is different, microbial propagation speed is different at different temperatures, oil temperature in the range of 50~C ~ 70~C operation, propagation speed is the fastest, so the dielectric loss is relatively increased quickly. Therefore, temperature has a great influence on the growth of microorganisms in oil and the performance of oil, and the dielectric loss factor is relatively stable in winter.

4. The influence of metal ions.

Wear or corrosion of copper metal components of transformer body (such as oil pump shaft or impeller wear, exposed copper lead corrosion), serious overheating or burning of winding copper wire will make copper ions dissolve into oil FF1, so that the concentration of copper ions in transformer oil increases, leading to the increase of dielectric loss.

5. Influence of water content.

Transformer and other electrical equipment in the manufacturing process of insulating materials, although by drying treatment, but its deep residual moisture, if improper protection measures in the process of transportation and installation, will make insulating materials again damp, respiratory system into the moisture, and through the oil surface into the oil. In addition, solid insulating materials and transformers in the process of operation, due to the transformer oil oxidation thermal cracking and water, insulation oil at the operating temperature and the existence of dissolved oxygen, the oxygenation will accelerate, the production of organic acids and water, which will lead to excessive water in the oil. For pure oil, when the water content in the oil is low (such as 30mg/L-40mg/L), the medium loss of oil has little influence, but when the water content in the oil is greater than 60mg/L, the number of media loss factors increases sharply.

Second, preventive measures

1, strengthen the supervision of the transformer in the manufacturing, factory test, transportation and installation process, especially to prevent impurities mixed into the oil and brought into the main transformer, resulting in transformer oil medium loss in a short time after operation of the phenomenon of rapid rise. By supervising the oil service during the installation of large transformers, transformer faults caused by poor quality of new oil can be reduced to a certain extent.

2, because the transformer oil in operation can make the temperature of 60℃ ~ 80℃, transformer oil in contact with air or moisture, as a metal catalyst transformer core and copper wire, will accelerate the oxidation. In the use of oil, antioxidants will continue to be consumed, adding T501 antioxidant can delay the aging of oil.

3, transformer in the process of operation, the failure of the desiccant in the respirator should be replaced in time to avoid oil oxidation metamorphism. At the same time, the transformer oil should be tested strictly in accordance with the pre-test rules, and treatment measures should be taken in time when a certain index is found to be unqualified.

There are two ways to solve the problem of excessive dielectric loss of transformer oil:

1, replace the unqualified oil.

Replacing the unqualified oil can shorten the system outage time. You only need to drain the old oil in the transformer, wash the transformer with qualified oil, and then fill the transformer with vacuum oil. This treatment is more suitable for the unit does not allow a long time of power outage; The unit runs for a long time, the oleic acid value is high, the oil is dark yellow or brown, there is turbidity of free water or oil, and the situation of comprehensive degradation. But the simple oil change is not as good as the "washing" of the transformer by the oil filter, and the oil change costs a lot. It is not conducive to energy saving and environmental protection, and it is not the first choice for oil change treatment to exceed the standard.

2. Regeneration treatment.

Regeneration treatment refers to the physical - chemical or chemical process of removing harmful substances from oil to restore or improve the physical and chemical properties of oil. The common methods of regeneration treatment are: adsorbent method and sulfuric acid - clay method. The adsorbent method is suitable for treating the oil with light deterioration degree; Sulfuric acid - white soil method is suitable for the treatment of heavy degraded oil. Adsorbent method can be divided into contact method and filtration method, contact method using powder adsorbent (such as clay, 801 adsorbent, etc.) and oil in the stirred contact regeneration; The filtration method is to force the oil through the purifier equipped with granular adsorbent (such as silica gel, granular clay and activated alumina, etc.), filtration regeneration treatment. For transformer oil with serious deterioration, sulfuric acid - white earth method can be used for regeneration treatment. Sulfuric acid treatment can remove a variety of aging products in the oil, and clay treatment can eliminate the residues in the oil after acid treatment. In actual production and operation, often encountered oil after vacuum, filtration and purification treatment, the water content of oil is very small, and the oil medium loss factor value is higher, this is because the oil medium loss factor is not only related to water content, but also related to many factors. From the above analysis, it can be found that the increase of dielectric loss factor of most transformer oil is caused by the increase of soluble polar substances (such as sols) in the oil. For sol particles, its diameter between 10-GM ~ 10-TM, can pass through the filter paper, so the secondary vacuum oil filter processing its medium loss factor can not achieve the purpose, so to deal with the oil medium loss factor caused by this reason to increase the problem, usually using infiltration method regeneration treatment can get good results.

Four, conclusion

When it is found that the dielectric loss factor of transformer oil increases, it is necessary to analyze the degree of deterioration of oil and the reason for the increase of dielectric loss, so as to take corresponding measures to deal with it, so that the transformer oil can achieve normal performance and ensure the safe and stable operation of equipment.