1 Introduction
Dielectric loss refers to the total of polarization loss and conduction loss caused in transformer oil under the action of an alternating electric field. The dielectric loss factor can reflect the quality of transformer insulation, the degree of aging of transformer oil under the effects of electric field, oxidation, and high temperature, and the degree of contamination by polar impurities and charged colloids in the oil. During long-term use of transformers, the dielectric loss factor test can reflect the operating condition of transformer oil.
2 Analysis of Causes for Excessive Dielectric Loss
(1) Influence of impurities. During transformer installation, impurities such as dust may exist in the oil or solid insulating materials. After a period of operation, colloidal impurities gradually precipitate. Colloidal particles have very small diameters (generally 10-gin to 10 m), diffuse slowly, but have certain activity energy. Particles can automatically coalesce, growing from small to large, forming a coarse dispersion system in a non-equilibrium unstable state. When they exceed the colloidal range, they deposit due to gravity. When sol exists in the oil and the precipitate exceeds 0.02%, it may cause the conductivity to exceed the normal dielectric conductivity by several times or even dozens of times, thereby increasing the dielectric loss value.
(2) Influence of transformer structure. From the analysis of transformer manufacturing structure, some transformer manufacturers have canceled the oil purifier (thermosiphon) to reduce oil leakage, which has a certain impact on the increase of the dielectric loss factor of transformer oil. If the transformer is equipped with an oil purifier, it is beneficial to stabilize the quality of insulating oil, as it can "absorb" moisture from the insulation during transformer operation, improve the electrical performance of insulation, and thus slow down the increase of moisture in the insulation.
(3) Influence of microbial contamination. Microbial bacterial infection is mainly caused by the invasion of bacterial organisms during installation and major overhauls. Due to contamination, the oil contains water, air, carbonaceous substances, organic matter, various minerals, and trace elements, thus constituting the basic conditions for the growth, metabolism, and reproduction of fungal organisms. Since microorganisms are rich in protein and themselves have colloidal properties, microbial contamination of oil is actually a kind of microbial colloid contamination, and microbial colloids carry charges, increasing the conductivity of the oil, so conduction loss also increases. Transformer oil is in a fully sealed, oxygen-free, and light-free body. Microorganisms in the oil are anaerobic and photophobic. After being placed for a long time, the dielectric loss test, especially in colorless transparent glass bottles, shows a smaller dielectric loss value. Transformers have different loads at different times and different operating oil temperatures. Microorganisms reproduce at different speeds at different temperatures. When the oil temperature is in the range of 50°C to 70°C, reproduction is fast, so the dielectric loss increases relatively quickly. 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 is relatively stable in winter.
(4) Influence of metal ions. Wear or corrosion of copper metal components in the transformer body (such as wear of oil pump shafts or impellers, corrosion of exposed copper leads), severe overheating or burning of winding copper conductors, etc., can cause copper ions to dissolve into the oil, increasing the concentration of copper ions in transformer oil, leading to an increase in dielectric loss.
(5) Influence of moisture content. Although insulating materials are dried during the manufacturing of transformers and other electrical equipment, residual moisture remains in their deep layers. If protective measures are improper during transportation and installation, insulating materials may become damp again. During operation, the breathing system may introduce moisture, which penetrates into the oil through the oil surface. In addition, solid insulating materials and transformers generate moisture due to oxidation and thermal cracking of transformer oil during operation. When insulating oil is at operating temperature and dissolved oxygen is present, oxidation accelerates, producing organic acids and water, which will lead to excessive moisture in the oil. For pure oil, when the moisture content is low (e.g., 30 mg/L to 40 mg/L), the impact on the dielectric loss of the oil is not significant, but when the moisture content exceeds 60 mg/L, the dielectric loss factor increases sharply.
3 Preventive Measures
(1) Strengthen supervision of transformers during manufacturing, factory testing, transportation, and installation, especially to prevent impurities from mixing into the oil and entering the main transformer, leading to a rapid increase in transformer oil dielectric loss shortly after commissioning. Through oil supervision during the installation of large transformers, to a certain extent, transformer failures caused by poor quality of new oil can be reduced.
(2) Since the temperature of transformer oil can reach 60°C to 80°C during operation, when transformer oil contacts air or moisture, oxidation accelerates under the action of the transformer core and copper wires as metal catalysts. During use, antioxidants are continuously consumed, and adding T501 antioxidant can delay oil aging.
(3) During transformer operation, the desiccant in the breather that has failed should be replaced in time to avoid oil oxidation and deterioration. At the same time, transformer oil should be tested strictly according to preventive test procedures. When any indicator is found unqualified, treatment measures should be taken promptly.
4 Treatment Methods for Increased Dielectric Loss in Transformer Oil
There are two methods to solve the problem of excessive dielectric loss in transformer oil: one is to replace the unqualified oil and refill with oil that has passed electrical tests and chemical analysis for all indicators; the other is to regenerate the excessive oil.
(1) Replacing unqualified oil. Replacing unqualified oil can shorten system outage time. It only requires draining the old oil from the transformer, flushing the transformer with qualified oil, and then vacuum oil filling. This treatment is suitable for situations where the unit cannot tolerate long outages; the unit has been running for a long time, the oil acid value is high, the oil is dark yellow or brown, free water or turbidity appears, and the oil is fully degraded. However, simple oil replacement is not as thorough as oil filtration in "flushing" the transformer, and oil replacement is costly, not conducive to energy saving and environmental protection. For excessive oil, oil replacement should not be the first choice.
(2) Regeneration treatment. Regeneration treatment refers to the use of physical-chemical or chemical methods to remove harmful substances from the oil and restore or improve the physical and chemical indicators of the oil. Common methods for regeneration treatment include: adsorbent method and sulfuric acid-clay method. The adsorbent method is suitable for treating oils with mild deterioration; the sulfuric acid-clay method is suitable for treating oils with severe deterioration. The adsorbent method can be further divided into contact method and percolation method. The contact method uses powdered adsorbents (such as clay, 801 adsorbent, etc.) and oil in a stirred contact manner for regeneration; the percolation method forces oil through a purifier filled with granular adsorbents (such as silica gel, granular clay, and activated alumina) for percolation regeneration. For severely deteriorated transformer oil, the sulfuric acid-clay method can be used for regeneration. Sulfuric acid treatment can remove various aging products in the oil, and clay treatment can eliminate undesirable substances remaining in the oil after acid treatment. In actual production and operation, it is often encountered that after vacuum, filtration, and purification treatment, the oil has very low moisture content, but the dielectric loss factor value is high. This is because the dielectric loss factor of oil is not only related to moisture content but also 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) in the oil. For sol particles, their diameter is between 10-gm and 10-Tm, and they can pass through filter paper, so treatment with a two-stage vacuum oil filter cannot achieve the goal. Therefore, to solve the problem of increased dielectric loss factor caused by this reason, percolation regeneration treatment usually achieves good results.
5 Conclusion
When an increase in the dielectric loss factor of transformer oil is found, the degree of oil deterioration and the causes of the increase in dielectric loss should be specifically analyzed so that corresponding measures can be taken to restore the transformer oil to normal performance and ensure the safe and stable operation of the equipment.
