Transformer Oil Regeneration

Transformer oil regeneration can extend the average service life of a transformer, often from 25 to 30 years. As insulating oil ages, it oxidizes and begins to degrade, forming by-products, sludge, acids, and other contaminants that accumulate in the cellulose insulation. In other words, insulating oil becomes corrosive to paper insulation and initiates a paper degradation process that can eventually lead to catastrophic transformer failures.

If preventive maintenance is started early enough during transformer operation, paper degradation can be significantly slowed down or even prevented. This requires proper monitoring and planning, which allows maintenance to be scheduled just in time and appropriate preventive measures to be implemented.

General information about transformer oils

Transformer oil is both a heat-dissipating medium and an integral part of the transformer insulation system.

High-quality transformer oil shall meet specific criteria:

• High electrical insulating properties, because it acts as one of the dielectric media in the equipment.

• Relatively high heat capacity and thermal conductivity, along with low viscosity, as it is intended to provide cooling of the equipment.

• Ability to consistently retain its properties for a sufficiently long time, since it is intended for use without the need to be frequently changed in the equipment.

Challenges of Oil Aging

During the operational period of the equipment, its insulation is exposed to various external influences (heating, moisture ingress, etc.), which lead to changes in the properties and structure of the material and a decrease in dielectric strength, contributing to its aging.

The main causes of aging include the following types of impact:

• Short-term voltage increases (overvoltages) during discharges and switching operations;

• ionization, which causes oxidative processes or promotes the development of surface discharges;

• mechanical stresses caused by own weight, shocks during passage of overcurrents, vibration, wind loads, etc.;

• volumetric contamination (for example, in oil by oxidation products or carbon) and surface contamination;

• heating, which causes intensive aging in the case of violated operating conditions and has the most destructive effect on organic materials;

• volumetric humidification, which leads to an increase in leakage currents and, consequently, an increase in temperature.

As transformer oil ages under high temperatures, it oxidizes, forming sludge, acids, and water. Sludge deposits impair heat dissipation, disrupt electrical insulation, and increase the risk of transformer failure. Acids corrode metal components and degrade solid insulation, while water reduces the dielectric strength of the oil.

The aging process is influenced by factors such as temperature, the presence of metals (e.g., copper, nickel, and iron), and contaminants. These elements act as catalysts, accelerating oil oxidation, particularly in the presence of moisture and oxygen. The primary indicator of oil aging is an increase in its acid number, which indicates the need for transformer oil regeneration.

Approaches to oil purification

While in use, dielectric fluids accumulate oxidation products, contaminants, and other impurities, which significantly degrade oil quality. Oils that contain such contaminants fail to meet the required specifications and should be either replaced with new oil or treated.

Methods for treating waste transformer oils are determined by the nature of their contamination and can be classified as physical, physicochemical, chemical, or combined.

Oil that does not comply with dielectric strength requirements due to moisture or contamination, but whose physical properties have not significantly changed, can be typically restored by settling, centrifugation, filtration through a porous medium, vacuum spray dehydration, or drying with hot air or inert gas.

In cases of severe degradation, regeneration is required, i.e. the restoration of oxidized oil by removing the aging products. In practice, regeneration is usually applied to in-service oil with an acid number not exceeding 0.3–0.4 mg KOH/g. Restoration of such oils is carried out using the methods based on various adsorbents. The oil is restored by filtration through a layer of adsorbent granules. For this purpose, the adsorbent is placed in a special unit — an adsorber — through which the oil is run by a pump.

Transformer oil regeneration plant — FLD 12R

The FLD-12R transformer oil regeneration plant is designed to restore oil by removing by-products of the aging process. The process involves circulating the oil through Fuller’s earth, which effectively captures contaminants on its porous surface, improving oil quality and extending transformer service life.

A unique feature of FLD-12R systems is their use of Fuller’s earth, which is highly effective in removing impurities. The sorbent is reusable, with a regeneration capacity of 300–500 cycles, corresponding to 1.5–2.5 years of continuous operation. After its service life, the sorbent can be safely disposed of as non-hazardous waste.

During the oil treatment process, the sorbent becomes saturated with impurities. The reactivation mode of the FLD-12R plant clears the sorbent pores, removing contaminants and extending the useful life of the material.

Transformer oil regeneration reduces dielectric loss tangent and acidity. After regeneration, the properties of waste oil align with those of new oil, significantly improving transformer performance.