Transformer Oil Dehydration System for Long-Term Reliability of Power Transformers

Transformer oil dehydration system is essential for long-term operation of oil-filled power transformers. The quality of transformer oil directly determines the duration of failure-free service, since this type of process oil concurrently performs electrical insulation and heat removal functions. While in use, transformer oil is continually exposed to electrical stress, elevated temperatures, oxygen, and environmental factors, which leads to contamination and aging. As the properties of oil deteriorate, its dielectric strength decreases, dielectric losses escalate, and the risk of internal faults greatly increases.

Contamination, Aging, and Moisture

The most influential factors affecting the transformer oil performance include particulate matter, soluble oxidation products, and moisture. Particulate matter constitutes insoluble substances, which are present in oil as a suspension or as a sediment. These include carbon deposits, cellulose fibers, dust, and metal particles. Their ingress into oil during use is due to degradation of insulation materials, dissolution of paints and varnishes, and wear of metallic components. Particulate matter significantly reduces the dielectric strength and causes the oil color to turn dark. The carbon that developed in response to electrical arcing near the oil face is particularly dangerous due to its high electrical conductivity, which significantly reduces the breakdown voltage. Cellulose fibers in solid insulation also adversely affect the dielectric strength, while dust slows down the oil circulation inside the transformer and affects thermal performance. Metal particles, particularly those of copper and aluminum contained in the windings and busbars, intensively accelerate the oil oxidation.

As transformer oil ages, sludge accumulates and deposits on the transformer windings, leads, busbars, and oil channels. This sludge impairs the heat transfer and, when in contact with cellulose insulation, accelerates its aging, further compromising the transformer's operational reliability. The cricial factors beyond solids include the soluble impurities that develop during oxidative aging. Over long-term operation, the colloid particles initially present in new oil increase in size and concentration. These particles consist primarily of acidic oxidation products, resins, varnish constituents, and other polar compounds. Their accumulation leads to an increase in acid number and dielectric losses; even a small resin content of about 0.5% can increase the dielectric loss tangent up to twenty times.

 The acids released by varnish films accelerate the oil oxidation, while asphaltenes and resinous compounds that developed from naphthenic constituents deposit on the windings and significantly reduce the cooling efficiency. The soap-like sediments resulting from the interaction of metals and acids are particularly dangerous in the presence of water, as these can cause internal flashovers and adversely affect the dielectric loss tangent of transformer oil.

Moisture is the final and most critical degradation factor. Water enters transformer oil from the atmosphere or results from chemical reactions inside the transformer. Dielectric strength is extremely sensitive to moisture content. Free water usually settles at the transformer bottom, while emulsified water consists of microscopic droplets that are difficult to remove and can align under an electric field to form conductive bridges, significantly reducing the breakdown voltage and substantially increasing the dielectric losses. Bound water is chemically associated with oil molecules and present even in new oil; although bound water is difficult to remove, it has a less severe impact on operational performance. The combined effect of moisture with metal particles, cellulose fibers, and colloid impurities accelerates oil aging at a much faster rate than any single factor alone.

Transformer Oil Dehydration System by Fluidex and Regeneration Solutions

A modern transformer oil dehydration system manufactured by Fluidex is designed to remove dissolved gases, free water, dissolved moisture, and solids from transformer oil in a single run. These oil purifiers are developed with capacities of 5 gpm (1.36 m³/h) up to 77 gpm (17.5 m³/h) for degassing, dehydration, and filtration mode, and those of up to 79.25 gpm (18 m³/h) for heating and filtration mode. The typical system consists of a vacuum chamber, input and output pumps, an oil heater, coarse and fine filters, a control cabinet, a vacuum pump with a backing pump, and oil storage tanks. Oil is heated and distributed as a thin film under high vacuum, allowing for efficient moisture and gas removal.

The transformer oil dehydration system increases the dielectric strength of oil to at least 75 kV, ensures simultaneous drying of transformers during oil processing, and significantly improves the maintenance efficiency. It can be used for initial filling of electrical equipment with insulating oil, evacuation of transformers and other high-voltage equipment, as well as for removal of air and moisture to prevent dielectric breakdown. The system is designed for easy operation and maintenance, as well as for long-term and cost-effective use. In order to achieve deeper reclamation of aged oil, Fluidex also offers the regeneration solutions that expand beyond thermal vacuum processing and filtration. The FLD 12R regeneration system reclaims oxidized transformer oil to as good as new condition by removing the aging products that cannot be eliminated through dehydration alone. Regeneration is performed by running the oil through adsorption layers filled with Fuller’s earth, a purified clay that effectively removes polar contaminants and oxidation by-products. The system operates at a flow rate of 2-4.5 gpm (0.5–1 m³/h) with a sorbent reactivation time of approximately 19 hours. The Fluidex regeneration technology allows Fuller’s earth to be reused 500 times or more, decreasing the oil losses by over 90%, resolving the disposal problems, and substantially reducing the need for change of transformer oil.