Oil regeneration is considerably more advanced procedure of its properties improvement than oil treatment. In addition oil is purified from chemical aging products such as organic acids, aldehydes, ketones and unsaturated hydrocarbons, which accelerate the degradation of cellulose insulation. In the process of the oil regeneration there are removed contaminations and aging products in order to obtain oil properties comparable to the properties of a new one.

ParameterMethodOld oilNew oilThe regenerated oil
Breakdown voltage [kV]IEC 60156~ 20> 70> 70
Acid number [mg KOH / g]IEC 60296 / 82~ 0,2< 0,03< 0,01
Water content [mg / kg]IEC 60814> 10< 10< 10
Loss factor tgδ at 90°CIEC 60247~ 0,09< 0,003< 0,004
Surface tension [mN / m]ISO 606295~ 25> 40> 50
Resistivity at 90°C [GΩm]IEC 60247~ 1> 150> 150

The progress of the regeneration process can be controlled even by direct observation of the color change of regenerated oil.

Oil regeneration or replacement?

The regeneration in many aspects is more profitable than an oil replacement. The first and obvious reason for choosing regeneration is the economic factor. In most cases, the cost of regeneration is between 50 to 60% of an oil replacement.

Regeneration gives also other technical benefits:

  • The regeneration process is carried out by a long term oil circulation. As a result, apart from the improvement of oil properties, there are gradually removed contaminants from the cellulose insulation. During the oil replacement, even while maintaining the procedure of cleaning and the tank and windings washing, usually it is possible to remove only a small amount of impurities from the insulation.
  • The regeneration can be performed during operation of the energized transformer. Under such conditions, the additional benefit is speeding up the process of insulation purification from contaminations.
  • The procedure of oil inhibition increases its resistance to aging.

When to perform the oil regeneration?

The recommendation for the regeneration of oil can be increased values of the acid number or surface tension, dielectric losses coefficient and oil resistivity. Accelerated sludge deposition and cellulose degradation occur when the acid number of the oil is higher than 0.1 mg KOH/g. Another factor indicating for the need of oil regeneration is the presence of corrosive sulfur in oil.

Acid number

It is the most important property describing the aging process of the oil. Acids created in the process of oxidation lead to metals corrosion and accelerate the degradation of cellulose insulation. The products of oxidation are insoluble sludges and other deposits, which deteriorate the heat transfer from the active part of the device.

Oil recycling technology

ENERGO-COMPLEX uses for the regeneration of oil the mobile container-station REGEN produced by ARRAS MAXEI, which can be connected to the transformer tank. The regeneration process involves oil circulation in the device MAS 6000, where oil is heated, and a built-in filter traps solid particles. Then, oil enters the sorption columns in REGEN device, where in a result of physical and chemical processes products of aging are removed from the oil. In the column oil is pumped back into the MAS 6000 device, where it is the dehydrated and degassed.

Oil regeneration device produced by ARRAS MAXEI

Fuller’s earth clay

Regeneration system performs two alternate regenerative processes and activation of used absorbent material (Fuller’s earth clay). With this technology, the sorbent can be used up to 300 times without deterioration of the filtering properties.

Regenerative cycle repeated several times steadily improves physico-chemical properties of the oil. This process can be terminated when the oil properties reach the expected values


The regeneration process of the oil is very beneficial for the environment as it allows to reduce the amount of waste oils. During regeneration, there appears a very small amount (only about 2%) of waste oil, which is no longer suitable for use.

See more: Regeneration of transformer oil

See more: Protection against corrosion of copper

The influence of regeneration cycles on acid number

PropertyUnitValue before the regenerationValue after the regeneration
Acid Number[mgKOH/g]0,3610,004
Breakdown voltage[kV]27,998,8
Resistivity at 90°C[Ωm]5,6 x 10⁹1,5 x 10ᶦᶦ
Dielectric losses coefficient tgδ0,0354350,00125
Water content by KF[ppm]20,43,2

Oil Treatment

The treatment of electroinsulating oil is a common and very important procedure performed during operation of the high-voltage devices with paper-oil insulation. This procedure is applied to both new and used oils, and its aim is to give the oil properties that are required by the standards and instructions and allow HV devices and power transformers to be energized.

Transformer oils during operation are subject to aging, which results i.a. in a higher tendency to dissolve water and the absorption of solid contaminants, which lowers their electrical properties. In addition, solid particles and corpuscular water tend to accumulate in the areas of the most intense electric field and this phenomenon is extremely dangerous for the paper-oil insulation.

The use of a vacuum unit (centrifuge) allows for oil purification from the solids, drying of moisture excess as well as its degassing. Due to this treatment the most important electrical properties are improved, the risk of insulation breakdown is lowered, and the generation and deposition of sludge on elements of devices is slowed down.

In the aggregate the oil is heated to a proper temperature and atomized in a vacuum chamber. In the conditions of reduced pressure a quick evaporation of the water and gases takes place. Essential in this process is the highest possible degree of dispersion, which is achieved by means of a rotor atomizing the oil into tiny particles in the form of a mist. The centrifuge removes from the oil the suspended solids, pollutants and corpuscular water, which are retained in the rotor. There is also removed dissolved moisture and air, which are vaporized. Centrifuged and partially dried and deaerated oil is directed to a vacuum container of a size allowing for efficient removing of the rest of moisture and air.

The use of suitable centrifuges provides very efficient oil treatment. ENERGO-COMPLEX uses for oil treatment under operating conditions ighly efficient mobile unit MAS 6000, which parameters are given in the table below. The service is performed at the customer’s on-site installation of devices.

Parameters relating to the MAS 6000 aggregate

Flow rate6000 l/h
Heating power78 kW
Total power91 kW
Filtration on cartridge1 μm
Flow to atmosphere240 m³/h
Ultimate vacuuum against closed suction0.08 mbar
Vacuum0,5 do 5,0 mbar