If necessary

Further investigations on liquid and solid insulation

Testing of transformer oils is not only an indication for the status quo of the oil filling but also for the conduction of further inner transformer components. Nevertheless, it can be necessary to test them directly. Of course for these testings, too, PCL is at your disposal. Due to the equipment and experience PCL is able to realise rapidly all common testings with insulating oil according to standards, no matter if it is new or operation oil. The following routine examinations guarantee PCL’s high quality standard as a basis for a sound assessment of the results.


The acidity, formerly neutralization number, allows the determination of the content of acidic components in insulating oils. These carboxylic acids - oxidation products of hydrocarbons - are formed in the course of operation in the presence of atmospheric oxygen and can lead to unwanted corrosion of materials.

Breakdown voltage

The breakdown voltage is the defining characteristic of the insulating capacity of the oil. It describes the value at which it between two 2.5 mm distant spherical dome electrodes for flashover occurs. Crucial to the minimum requirement the breakdown voltage is the upper voltage of the equipment. Water and particle content are key parameters for the value of the breakdown voltage.

Buchholzgas analysis

The Buchholz gas analysis are in combination with the dissolved gas analysis valuable clues to the nature of the event that the response to Buchholz relay has performed. It indicates whether the incident solid insulation was involved, if not directly from the dissolved gas analysis can be derived.


New insulating oils are colorless and clear. During the aging, the color will get darker due to the formation of oxidation products. Nevertheless, the color index is only a minor criterion for assessing the aging state of an oil. But it completes the overall picture of an oil filling.

Corrosive sulphur

The test is used for the determination of potentially corrosive sulfur. Sulfur compounds are contained in the mineral oil and can usually lead to undesirable corrosion of materials. Since not all the available sulfur containing compounds are corrosive, is the determination of total sulfur content of the insulating oil is not always a meaningful criterion.

Degree of polymerisation

Cellulose is a polycondensation product of glucose units (grape sugar), which are chain-like linked. The average Number of sugar molecules per cellulose unit is represented by the average degree of polymerization (DP). Its determination is made viscometry and is a statement about the condition of the insulating paper.


Density is the quotient of mass and volume of a liquid. It is not a direct criterion of quality, however, the density of a insulating liquid may give an indication of their chemical nature (mineral oil, esters, silicone oil, ...).

Dielectric dissipation factor

The dielectric loss factor tan delta 90°C is expressed as the ratio of the active current and reactive current with a load of the oil with sinusoidal alternating voltage defined. The tan delta 90°C provides information on the level of dielectric losses occurring during operation the oil filling and is influenced by moisture, particles, or polar oxidation products.

Dissolved gas analysis

The analysis of gases dissolved in oil (DGA) is today one of the most commonly used method for the diagnosis and evaluation of oil-filled electrical operating esources. DGA allows an assessment of the transformer in respect of its thermal and electrical state, and (in conjunction with a Buchholz gas analysis) the characterization of acute and insidious faults.

Furfural content

Cellulose is thermally degraded, creating a stable, readily soluble in the insulating and stable decomposition product furfural (2-FAL). Its determination allows a conclusion on the state of the solid dielectric (paper/pressboard). The operator is thus a means to hand given to be able to plan the future of its energy supply much better.

Interfacial tension

The interfacial tension is indicative of the presence of polar compounds in the oil. Is the determination of this parameter during an operating oil an indication on the oil aging. During the operation time of a transformer oil, the interfacial tension increases due to the formation of oxidation products constantly from. New oils at the interfacial tension is indicative of the quality of the refining.

Inhibitor content

With the inclusion of an infrared (IR)-spectrum of the inhibitor content of an oil sample is determined. Inhibitors, usually 2,6-di-tertiary-butyl-para-cresol (DBPC), increase the aging resistance of mineral oils against thermal, oxidative influences. The inhibitor content of a new oil-filling is in usually 0.3% (mass) and this is broken down during operation.

Particle content

The number of solid particles can have a considerable influence on the dielectric strength. Loading due to aging of the solid Isolation emitted in the oil cellulose fibers with water will slow down the breakdown voltage. Oil sludge particles in insulating oil can in unfavorable cases, lead to a significant drop in cooling performance, or in the cooling channels of the windings can cause heat build-up or heat shock.

PCB content

The determination of polychlorinated biphenyls (PCB's) by gas chromatography. Limits and measures are regulated in the Altölverordnung datet 16.04.2002 (BGBI I S. 1368). The often quoted DIN 51527-1 is technically identical to DIN EN 12766-1, -2.


The purity gives an indication of whether the insulating liquid is free from suspended solids and contaminants that may have an adverse impact on electrical resistance. In addition to the visual inspection of the sample, an aliquot is filtered and assayed for possible residues.

Refractive index

The refractive index is the ratio of the velocities of light of a defined wavelength (sodium D line) in a vacuum and the material to be tested at 20° C. It is an independent measure of the aging state and may be helpful in identifying an oil sample.

Saponification value

The oxidation of insulating extends over alcohols, aldehydes (cetones), to the carboxylic acids. Alcohols and acids can condense to form esters and thus these acids are no longer recorded in the neutralization number. The esters are, in the course of testing split (saponified) and then then the content of "free" (formerly bound) acids determined. For routine condition assessment of plant oils is often sufficient to determine acidity.


The viscosity describes the flow behavior of insulating oils. All liquids, the viscosity is highly dependent on the temperature. Transformer insultating oils must still guaranteed a sufficient flow properties even at very low temperatures, so that during the operation resulting loss of heat can be dissipated sufficiently by convection.

Water content

The water content is determined by Karl Fischer coulometry. The water content is a very important criterion for the assessment of the use of state of the oil filling, since the dielectric strength decreases significantly with increasing water content. About the determination of water content in oil can roughly estimate the moisture content of the solid insulation.

Test procedures in the flexible accreditation category III