What is the dielectric loss?
Dielectric Loss, such as those caused by polarization of the insulation medium, discharge over the surface of the insulation medium, partial discharge inside the insulation medium, etc., are produced inside the insulation medium when it is subjected to alternating current voltage.
What are the possible reasons for the increase in dielectric loss of large transformer?
- 1).The oil is completely saturated with water-soluble contaminants.
Transformer residual oil or solid insulating material has sol-gel impurities; installation and operation might produce soluble impurities.
The conductivity of the oil determines the dielectric loss factor of the oil, which is proportional to the conductivity coefficient. After the presence of sol-gel particles in the oil, the conductivity coefficient caused by the electrophoretic phenomenon may exceed the normal conductivity of the medium several times or tens of times, increasing the tgδ value. The settling balance of colloidal particles, so that the dispersion system in each level of concentration is not equal, the closer the concentration of the bottom layer of the container, the greater, can be used to explain the transformer oil upper layer medium loss factor is small, lower layer medium loss factor is large phenomenon.
- 2). The low viscosity of the oil contributes to an increase in the dielectric loss factor by increasing electrophoretic conductivity.
Some oils are more likely to be contaminated in the same pollution situation while having properties such as viscosity, specific gravity, flash point, etc., that are within the authorized range. This is due to the fact that an increase in the dielectric loss factor is caused by an increase in the total conductivity coefficient, which in turn is caused by an increase in the electrophoretic conductivity, when the oil’s viscosity is low enough to allow it to make contact with solid material during dust migration, increasing the number of soluble particles per unit volume of oil.
- 3). The hot oil circulation process increases the oil charged tendency and thus the dielectric loss factor.
The hot oil circulation drying process should be performed after the installation of large transformers. In general, the manufacturer provides new oil with a very low charged tendency. However, when the oil is injected into the transformer, some retain the low charging tendency of new oil while others exhibit increased charging tendency. And after the hot oil cycle, heating will cause all of the oil’s charged tendency to increase to varying degrees. The charged tendency of oil and its dielectric loss factor have a close relationship, with the dielectric loss factor of oil increasing and increasing with its charged tendency. As a result, the increased charged tendency of the oil following hot oil circulation is one of the reasons for the increased dielectric loss factor of the oil.
- 4). Microbial bacterial infection.
The invasion of flies, mosquitoes, and bacteria-like organisms during installation and overhaul is primarily to blame. Some mosquitoes and insects were discovered attached to the surface of the windings at the transformer site for lifting cover inspection. Microscopic insects, bacteria, mold-like organisms, and other organisms live in the oil’s lower sediment layer. Pollution and the presence of water, air, carbide, organic matter, various minerals, and microscopic amounts of elements in oil contributed to the growth of bacteria-like organisms. Temperature on the growth of microorganisms in the oil and oil performance has a large impact on the test, which found that the winter medium loss factor value is more stable.
The growth of microorganisms in the oil is directly related to environmental conditions, and the number of microorganisms in the oil determines the electrical properties of the oil. Microorganisms are naturally colloidal due to their high protein content. As a result, oil pollution caused by microorganisms is a type of microbial colloid pollution, and microbial colloids are charged, which affects the increase of oil conductivity, causing conductivity loss.
- 5). High water content of oil causes the increase of dielectric loss.
When the volume fraction of water content in oil is low (such as 30-40ppm), the effect on the tgδ value of oil is negligible; only when the water content in oil is high is there a significant effect.
There are two ways to solve transformer oil dielectric loss that exceeds the standard: one is to replace unqualified transformer oil with qualified transformer oil based on electrical testing and chemical analysis of the indicators. Another option is to regenerate the transformer oil.
- Replace the unqualified transformer oil.
To reduce the length of time the system is down, unqualified oils should be replaced with qualified oils; this is done by draining the old oil in a clean transformer, flushing it out with qualified transformer oil, and then injecting oil into the transformer using a vacuum. A longer outage can be avoided with this method of therapy. Oil acid value is high, oil color is dark yellow or brown, free water or oil turbidity phenomena is present, and the situation has deteriorated generally due to the equipment running for an extended period of time.
Oil regeneration treatment is a physical chemical or chemical method for removing harmful substances from transformer oil and restoring or improving the oil’s physical and chemical index. The two most common regeneration treatment methods are adsorbent and fuller’s earth. Adsorbent methods are appropriate for transformer oil with light degradation; fuller’s earth methods are appropriate for transformer oil with heavy degradation.
To summarize.
When the dielectric loss factor of transformer oil is found to be increasing, the first step should be to analyze the degree of oil deterioration and the cause of the increase in dielectric loss, so that appropriate measures can be taken to address it. To ensure the safe and stable operation of the equipment, restore the transformer oil to normal performance.
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