1.Breakdown VoltagePN-EN 60156kV
2.KF water contentPN-EN 60814
PN-EN 60422
3.Acid NumberPN-EN 62021 mgKOH/g
4.Dielectric loss factor tgΔPN-EN 60247 -
5.ResistivityPN-EN 60247 GΩm
6.Flash-pointPN-EN 2719 °C
7.DensityPN-EN ISO 3838 g/cm3
8.ViscosityPN-EN ISO 3104 mm2/s
9.Surface tensionISO 6295
10.Solid particlesPN-ISO 4407; PN-ISO 4406 -
11.Sulfur corrosionPN-ISO 62535-
12.PCBPN-IEC 997 ppm
13.DGAPN-EN 60567 ppm
14.Furan compounds contentPN-EN 61198 mg/kg
15.Polarity indexPN-81/C-04952
PN-EN 60247
16.Refractive index of lightPN-81/C-04952-

Breakdown Voltage

The breakdown voltage value gives information on the possibility of insulating oil, shows the presence of contaminants as well as the need for centrifugation. This depends on the humidity of the sample and the presence of impurities such as solid particles. Measurement conditions influence the size to a large extent and therefore standardized measurement procedures and appropriate equipment are used.

Laboratory analysis is performed in accordance with DIN EN 60156 “Insulating Liquids. Determination of the breakdown voltage at power frequency. “We perform measurements using the Baur oil tester which is used for automated measurements of electrical breakdown strength of insulating liquids. Designation results are presented as the mean of six tests due to the low recurrence of a single measurement.

Water content

The presence of water in oil and cellulose insulation affects the deterioration of dielectric parameters and, above all, electric strength. Water in the transformer may be derived, among other, from the diffusion of atmospheric moisture at the time of inspection of internal thermal aging of oil and cellulose insulation and leakage of the water cooling system. As a result of moisture, partial discharges or increase in pressure in the transformer tank can occur which in turn may lead to equipment breakdown.

Examination of water content in oil is carried out in accordance with DIN EN 60814 “insulating liquids – paper and pressboard are saturated with oil. Determination of water by Karl-Fisher automatic coulometric titration”. Analysis is performed using a Metrohm 899 COULOMETRU. In order to obtain accurate results, the equipment is compatible with the highest class of Sartorius Coubis analytical weights. Reagents used are of high quality, which ensures high accuracy of measurement.

Acid Number

This parameter is an indirect indicator indicating the possibility of precipitates and is used for the quality control of transformer oil. If the value exceeds 0.1 acidic decomposition of acidity products of oil causes the acceleration of the aging process of cellulose insulation. Oils of high acidity can react with metal equipment elements.

Determination of the acid number is performed in accordance with DIN EN 62021-1 “electrical insulating liquids. Part 1. Measurement is performed using a semi-automatic Metrohm TI-TOUCH OIL 916 device. In order to confirm questionable results, we use the colorimetric titration method in accordance with DIN EN 62021-2: 2008. This method is used to identify possible changes that may occur during use under oxidizing conditions.

Dielectric loss factor and resistivity

These figures provide information about oil deterioration or by deterioration of its properties. These parameters are affected by pollution, temperature, electric field strength, the time voltage is applied, water reacting with oxidation products resulting in a noticeable increase in value. These figures are important indicators of quality and degree of contamination of the sample. They are used to interpret deviations from the desired properties and the possible impact on the operation of the transformer.

Determination of dielectric loss factor and resistivity are made in accordance with DIN EN 60247: 2008 “Insulating Liquids.” Measurement of relative permittivity, dielectric loss factor and resistivity at constant current. “The measurement is done using an automated measuring bridge for measuring the resistivity, dielectric constant and the coefficient oil loss of Baur DTL type oil.


Flashpoint gives information on fire safety, as characterized by the creation of flammable mixtures and is the value at which the ignition is in contact with flame.

The measurement is performed in accordance with DIN EN ISO 2719, “Determination of flash point. Pensky-Martens closed cup method. “The measurement is done using a semi-automatic INKOM device to mark the flash point dedicated to determine this parameter of mineral oil and other flammable liquids.


Knowledge of oil density allows the calculation of the total weight of the transformer as well as to avoid the occurrence of ice fragments at low temperatures (such a situation can occur when water will be weaning from the motion transformer) and, consequently, would lead to device failure. In addition, the oil-containing aromatic compound has a higher density than the oil, naphthenic compounds or paraffin.

Designating the density is performed in accordance with PN-EN ISO 3838: 2008 “Petroleum and liquid or solid petroleum products. Designating the density or relative density. Methods of using the pycnometer with stopper and capillary pycnometer and dual capillary partitions.


Viscosity is a measure of the internal resistance placed by oil, subjected to shearing, forcing it to flow. This property of the oil affects the cooling effect of the transformer, therefore – in this case – the desired oil is of low viscosity. Its value is influenced, among other, by the temperature, i.e. .: the higher the temperature, the greater the decrease in viscosity. In case of increase of this parameter, cooling properties deteriorate consequently raising the operating temperature and the deterioration of the oil and cellulose.

Our laboratory performs designation of viscosity in accordance with DIN EN ISO 3104 “Petroleum products. Transparent and opaque liquids. Determination of kinematic viscosity and calculation of dynamic viscosity. “Measurement is performed using a Lauda heat thermostat, and high-precision measuring capillary.

Surface tension

Surface tension strongly depends on the oil content of polar groups. Surface tension of the oil is a good indicator of oil purity during storage and aging in operation. On the surface, tension affects the formation of slime in the oil operated (sludge) – so it is important when making decisions about replacing or regenerating oils. Fresh oils should have a surface tension of at least 40mn / m, and it is usually a value between 45 and 50. Very exploited oils may have surface tension values less than 20 mN / m.

In our laboratory, we measure the surface tension using the tensiometric measurement method using the German produced STA1. The device is compatible with the highest quality Sartorius Cubis weight.

Solid particles

Transformer oil should undergo examination in order to control pollution emissions. Storage method, transportation, and oil carbonization of cellulose degradation affect the increase in their content. One of the consequences of oil aging is the formation of precipitate which is deposited on the surface of the winding resulting in cooling conditions deteriorating. The exact power consumption of the sample and determining the type and particle size allows for qualitative and quantitative determination of fluid impurities. On the basis of marking, we can observe, among other, whether the pollution is formed by metal which is the product of wear, rust or paper fibers from the system.

The measurement is performed according to the ISO 4407 standard “hydraulic control actuators. Contamination of operation fluid. Designating the impurities of solid particles counting method using a microscope “and ISO 4406” and control of hydraulic actuators. Operating fluids. The method of encoding levels of contaminants in solid particle form. “The measurement is done using a Conbest stereoscopic microscope and a Moticam camera.

Sulfur corrosion

Sulfur Corrosion is a parameter that does not apply to all types of oil. Between 1995-2005 oils containing DBDS (benzyl disulfide) appeared on the market, which at the time of transformer operation proved to be corrosive. This compound under specific conditions of temperature leads to the formation of deposits of copper sulfide (Cu2S) and silver sulfide (Ag2S). Sulfides may appear on the elements of tap changer, as well as paper insulation. The presence of sulfides reduces the dielectric strength of the insulation system and can also lead to coil shorting. Early detection of the appearance of sulfur compounds in the oil may prevent the occurrence of breakdown, and also allows for early intervention and thus preventing corrosion, for example after the recovery of oil.

Our laboratory offers analysis of transformer oil for the potential presence of corrosive sulfur. The method is performed according to PN-EN 62535 “Insulating Liquids – The method for potential corrosive sulfur detection in fresh and used oils electro-insulator”. Our laboratory has approved ASTM corrosive strips that allow accurate assessment of corrosion classes of analyzed oil.


In accordance with EU and national requirements, by the end of June 2010 all equipment containing PCBs at a concentration greater than 50 ppm (mg / l) must be withdrawn from service. (Regulation of the Minister of Economy of 24 June 2002 pertains). PCS’s were used as a component of transformer oil due to its excellent electrical insulating properties, but it turned out that at high temperatures degraded dioxin compounds were considered highly toxic. Hazardous properties have led to the absolute disposal of transformer oil which exceeds the limits of PCB’s. PCBs do not exhibit adhesion properties (adjacent) to metal surfaces. Thus, if the amount of PCB’s in the oil does not exceed 25 ppm, the oil can be regenerated.

The PN-C 96050 “Used Oils” norm presents two ways of proceeding with used oils depending on their content of PCBs. If the PCB content does not exceed 25 ppm (mg / L) such oils should be subjected to regeneration. Content exceeding 50 ppm should be withdrawn from service. Determination of polychlorinated biphenyls derivatives has been made using the gas chromatography (GC-MS) technique with respect to the IEC 997 standard.

Analysis of gases dissolved in DGA oil

Analysis of dissolved gas in oil (DGA) by gas chromatography determines the basic method of noninvasive diagnostic, allowing identification of the type and rate of change slowly developing defects of the insulating transformer. The advantages of DGA include: high sensitivity measurement, substantially higher than the electrical measurement technique that enables detection and tracking defects in the early stages of their development, a small volume of sample required for analysis.

The basic document on the interpretation of the test results and the classification of DGA defects in the transformer insulation systems is the international standard IEC 60599: 1999 “Guide to the interpretation of dissolved and free gases analysis”. We perform DGA analysis in the laboratory using a Japanese Schimadzu gas chromatograph according to PN-EN 60567 “Electrical oil equipment- Sampling and analysis of greenhouse gas free and dissolved- Guidelines”. To make sure that the chromatography vials are properly prepared we use a so-called revolving table for flushing argon vials. It is also a guarantee of obtaining good reproducibility measurements. In order to provide our customers with the highest quality of services and the most accurate measurement, three-point calibration curves are made using two types of patterns: standard gas mixtures and Morgan Schaffer liquid oil patterns.

Furan compounds content

It is advised to perform furan contents measurements in order to rule out or confirm local overheating in the area of winding insulation in addition to the DGA. 2FAL is treated as a quantitative indicator of the degradation process. The 2FAL concentration value of oil above 1.5 ppm indicates the location of the thermal defect in cellulose insulation on the warning level. In the presence of local overheating other furan contents appear apart from 2FAL mainly 5- hydroxymethyl-furfural (5HMF) and 5-methyl-furfural (5MEF). 5HMF occurs in the initial phase of local overheating, often simultaneously with 2FAL. When the process is already advanced, 5MEF appears.

Determining the content of furan compounds is made by using liquid chromatograph from the Japanese Schimadzu company using packed columns distinguished by high efficiency and selectivity, and a UV detector. Designation performed in accordance with DIN EN 61198 “insulation mineral oils – 2 Methods for determining furfural and related compounds.” Great attention is given to the method of sample preparation, and therefore we use special SPE sample treatment equipped with a vacuum pump and columns filled with sorbent. In order to ensure measurement accuracy, we have created two four-point calibration curves based on patterns of liquid.

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