Dielectric loss refers to the sum of polarization loss and conductance loss caused by transformer oil under the action of an alternating electric field. The dielectric loss factor can reflect the insulation characteristics of the transformer, reflect the aging degree of the transformer oil under the action of electric field, oxidation and high temperature, and reflect the degree of pollution such as polar impurities and charged colloids in the oil. During the long-term use of the transformer, the dielectric loss factor test can reflect the operating conditions of the transformer oil.
1. Analysis of the causes of excessive dielectric loss
1. The influence of impurities.
During the installation of the transformer, there are impurities such as dust in the oil or solid insulating materials. After a period of operation, the colloidal impurities gradually precipitate out. The colloidal particles have a small diameter and slow diffusion, but they have a certain amount of activity energy. Particles can automatically coalesce, from small to large, in a coarsely dispersed system, in a non-equilibrium and unstable state, and when they exceed the range of the colloid, they accumulate due to gravity. 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 or tens of times, resulting in an increase in the dielectric loss.
2. The influence of transformer structure.
From the analysis of the transformer manufacturing structure, some transformer manufacturers currently consider the elimination of the oil purifier (thermosiphon) from the perspective of reducing the leakage of the transformer, which has a certain impact on the increase of the transformer oil dielectric loss factor. If the transformer is equipped with an oil purifier to help stabilize the quality of the insulating oil, it can "suck out" the internal moisture of the insulation during the operation of the transformer, improve the electrical performance of the insulation, and slow down the increase of moisture in the insulation.
3. The influence of microbial contamination.
Microbial infections are mainly caused by bacterial immersion during installation and overhaul. Due to pollution, the oil contains water, air, carbon compounds, organic matter, various minerals and trace elements, which constitute the basic conditions for the growth, metabolism and reproduction of fungi organisms. Since microorganisms are rich in protein, they have colloidal properties. Therefore, the contamination of oil by microorganisms is actually a kind of microbial colloid pollution, and the microbial colloids are charged, which increases the conductance of the oil, so the conductance loss also increases. . Transformer oil is in a fully sealed, oxygen-deficient and non-glossy body, and the microorganisms in the oil are anaerobic and anaerobic. Conduct a dielectric loss test after being placed for a long time, especially when placed in a colorless transparent glass bottle, the dielectric loss value will become smaller. Transformers carry different loads and operating oil temperatures in different periods, and the reproduction speed of microorganisms at different temperatures is also different. The oil temperature runs in the range of 50~C～70~C, and the reproduction speed is the fastest, so the dielectric loss increases relatively. quick. Therefore, temperature has a great influence on the growth of microorganisms in the oil and the performance of the oil. Generally, the dielectric loss factor in winter is relatively stable.
4. The influence of metal ions.
Wear or corrosion of the copper metal components of the transformer body (such as oil pump shaft or impeller wear, exposed copper lead corrosion), severe overheating or burning of the winding copper wire, etc. will cause the copper ions to dissolve into the oil ff1, and make the copper ion concentration in the transformer oil Increased, resulting in an increase in dielectric loss.
5. The influence of water content.
Although the insulating material is dried during the manufacturing process of transformers and other electrical equipment, the deep layer still retains moisture. If the protection measures are improper during transportation and installation, the insulating material will become damp again, and the respiratory system will enter moisture during operation and pass through the oil. The surface penetrates into the oil. In addition, during the operation of solid insulating materials and transformers, water is generated due to the oxidative thermal cracking of transformer oil. When the insulating oil is at operating temperature and there is dissolved oxygen, its oxidation will accelerate, and organic acids and water will be produced. Lead to excessive moisture in the oil. For pure oil, when the water content in the oil is low (such as 30mg/L-40mg/L), it has little effect on the medium loss of the oil, but when the water content in the oil is greater than 60mg/L, the medium The loss factor increases sharply.
2. Preventive measures
1. Strengthen the supervision of the transformer manufacturing, factory test, transportation and installation process, especially to prevent impurities mixed in the oil from being brought into the main transformer, resulting in a rapid increase in transformer oil dielectric loss within a short period of time after the transformer is put into operation . Through the oil supervision during the installation of large transformers, it is possible to reduce transformer failures due to poor quality of new oil to a certain extent.
2. Since the temperature of transformer oil can reach 60℃～80℃ during operation, when transformer oil is in contact with air or moisture, it will accelerate oxidation under the action of transformer core and copper wire as a metal catalyst. When oil is in use, antioxidants will continue to be consumed, supplementing with T501 antioxidant can delay oil aging.
3. During the operation of the transformer, the desiccant in the respirator should be replaced in time to avoid oil deterioration. At the same time, the transformer oil should be tested strictly in accordance with the pre-test regulations, and treatment measures should be taken in time when a certain index is found to be unqualified.
3. Treatment methods for increased dielectric loss of transformer oil There are two methods to solve the problem of excessive dielectric loss of transformer oil:
1. Replace the unqualified oil.
Replacing the unqualified oil can shorten the power failure time of the system. You only need to drain the old oil in the transformer, flush the transformer with qualified oil, and then vacuum oil the transformer. This kind of treatment is more suitable for the unit that does not allow long-term power outage; the unit has been running for a long time, the oil has a high acid value, the oil is dark yellow or brown, free water or oil is turbid, and it is completely degraded. However, a simple oil change is not as thorough as the "washing" of the transformer by the oil filter, and the oil change is expensive. It is not conducive to energy saving and environmental protection, and the oil change treatment should not be the first choice for the over-standard oil.
2. Regeneration treatment.
Regeneration treatment refers to physical-chemical or chemical methods to remove harmful substances in oil and restore or improve the physical and chemical indicators of oil. The common methods of regeneration treatment are: adsorbent method and sulfuric acid-white clay method. The adsorbent method is suitable for the treatment of oil with a lighter degree of deterioration; the sulfuric acid-white clay method is suitable for the treatment of oil with a heavier degree of deterioration. The adsorbent method can be divided into contact method and percolation method. The contact method uses powdery adsorbent (such as clay, 801 adsorbent, etc.) and oil to regenerate under stirring contact mode; while percolation method is to force oil through Purifiers for granular adsorbents (such as silica gel, granular clay, activated alumina, etc.) are subjected to percolation regeneration treatment. For severely degraded transformer oil, the sulfuric acid-white clay method can be used for regeneration. Sulfuric acid treatment can remove a variety of aging products in the oil, and clay treatment can eliminate the undesirable substances remaining in the oil after acid treatment. In actual production and operation, it is often encountered that after the oil is vacuum, filtered and purified, the water content of the oil is very small, and the medium loss factor of the oil is relatively high. This is because the medium loss factor of the oil is not only related to the content of the oil. The amount of water is related to many factors. From the above analysis, it can be found that the increase in the dielectric loss factor of most transformer oils is caused by the increase of soluble polar substances (such as sols, etc.) in the oil. For sol particles, the diameter is between 10-gm and 10-Tm, which can pass through the filter paper. Therefore, the dielectric loss factor of the two-stage vacuum oil filter can not achieve the purpose, so the oil dielectric loss factor caused by this reason can be dealt with. To increase the problem, usually the percolation method can be used to regenerate the treatment to get good results.