Test laboratory for transformer materials

The color and appearance of transformer oil are useful for comparative evaluation: a quick color darkening or dark oil is a sign of oil ageing. The analyzed oil color is assigned a number ranging from 1 to 8, whereby the level of discoloration is indicated by means of the rising color number.

By analyzing the appearance, undesirable by-products can be detected. Cloudiness or sediment indicate free water, insoluble sludge or dirt particles. If these are present, then also the breakdown voltage and/or the loss factor and eventually also other oil values are out of the norm and measures must be taken.

The breakdown voltage (Ud) indicates how well insulating oil can withstand an electrical load and is therefore decisive for the operational efficiency of a transformer. The breakdown voltage is measured according to VDE 0370 part 5 (=IEC 60156). The initial measures to be taken in case of a drop below the insulation voltage limit values of the relative transformer type, depend on the values of the other characteristic oil values.

Water formation is caused by the ageing of cellulose- insulating material (paper, pressboard, laminated wood), ageing of oil and infiltration of humidity from the environment by means of badly maintained air dryers and/or defective sealing systems.

There is an equilibrium between the water content in oil and the moisture content in the solid insulation, it reduces the breakdown voltage and accelerates the ageing process. This equilibrium though, is temperature and time-dependent.

As there is never an even temperature in an operating transformer, -oil temperature in the tank is much higher at the top than at the bottom, and the same counts for the temperature distribution in the windings – the moisture distribution curves are only a rough estimate. In the moisture distribution curves the water content in the oil can theoretically be distinguished from the moisture content in the cellulose, based on the temperature. This is however valid only for insulation paper and not for pressboard and laminated wood.  A precondition is constant temperature and a set equilibrium.

Through progressive ageing (Oxidation) of the oil, polar decomposition products develop. These deteriorate the dielectric properties of the oil. The result of far advanced oil ageing is sludge formation. Sludge greatly compromises the windings because it leads to the formation of sediments, which prevent heat removal. This heat accumulation in turn causes the winding paper to age intensely. A timely diagnosis of the incipient acid formation is therefore important, so that timely counteractive measures can be initiated.

Literature: Cigre Brochure D1.30 – „Oxidation Stability of Insulating Fluids“

The dielectric dissipation factor of an insulating material is the tangent of the dielectric loss angle.

The dielectric loss angle, is the angle in which the phase difference between the applied voltage and consequential current diverges from π/2 rad, when the capacitor's dielectric comprises only of insulating material.

A rising dissipation factor is an indication of oil ageing or oil contamination. The dissipation factor is strongly influenced by polar components and is therefore a very sensitive parameter.

In addition to the neutralization value and the dissipation factor, interfacial tension is another indicator of sludge formation in the transformer. This test measures the concentration of polar molecules in oil, which form during the ageing process. The higher the concentration, the lower the interfacial tension and the likelier it is for the insulating oil to form sludge.

Inhibitors are age protecting agents which delay the decomposition of the insulating oil. The IEC 60296 accredited inhibitor DBPC (Di-tertiary-butyl-para-cresol), with a weight percentage of 0.25 ± 0.040 , is used. Inhibited oils are preferred for transformers > 200 MVA, for heavily loaded transformers such as traction transformers, for oven transformers or at special customer request. When the accelerated oil ageing process is caused by a transformer malfunction, the original inhibitor content (0,3), can be restored by adding a calculated amount of DBPC in powder form, once the failure has been fixed. Even in the case of normal DBPC- deterioration, the oil can be re-inhibited, although an oil change would be less expensive. This decision can be made only based on the results of the other oil value tests: if these are normal then a re-inhibition can be successful. The inhibitor content is determined using infrared spectrometry or gas chromatography with MS-detector.

The decomposition of paper is caused by the processes of hydrolysis, pyrolysis and oxidation. The degree of polymerization (DP-value) of paper was defined by IEC 60450 and it counts the number of polymerized glucose rings. During the paper decomposition, the DP-value is reduced and the tensile strength decreases. New cellulose has a DP-value of 1000-1100, aged cellulose on the other hand has a value of only 150-200, which means the end of the transformer's lifespan.

As it is not possible to take paper samples during running operation, the condition of the solid insulation is estimated based on the cellulose decomposition products (2- Furfural). For this purpose a regular trend analysis is necessary.

The furan content in oil depends on:

• Oil temperature
• Neutralization value
• Sludge content
• Oil/paper ratio
• Type of oil (inhibited, non-inhibited)
• Type of paper (thermostabilized, non thermostabilized)

An exact link between the furan content and DP-value cannot be deduced at present. However it is possible to make a trend analysis: based on the change of the furan content, information about thermal conduct of the solid insulation over the years can be obtained.

The testing and approval of new insulating liquids, or mineral oils, synthetic or natural ester or silicone liquids is also part of the laboratory's work.

Using our computer controlled and monitored oxidation equipment we are able to set and monitor the test conditions much more precisely than is required by the standard specifications. The result is a much better reproducibility and safe assessment.

The solid insulating materials in the windings include purified cellulose solid materials like pressboard and insulating paper as well as bonded laminates (e.g. laminated board, various fiber- reinforced synthetic materials).

In the material testing laboratory the physical properties of these materials are tested, hence verifying their applicability for transformer construction.

The following tests are carried out:

• Determination of thickness and density
• Chemical analysis to ensure that there is no metal contamination
• Shrinkage behavior
• Moisture content
• Bending strength, compressive strength and tensile strength
• Compatibility with insulating liquids
• Impregnation behavior
• Conductivity and ph value of the aqueous extract
• Ash content

Transformer cores are made up of grain-oriented electrical steel. There are three different qualities: conventional grain-oriented steel, highly permeable grain-oriented steel and domain refined electrical steel.

In the material testing laboratory the following tests are carried out on the grain-oriented electrical steel:

• Surface resistance using the Franklin Tester
• Magnetic loss measurement, using a Single Sheet Tester (SST)
• Magnetostriction

Copper materials are mainly used in the windings and in the leads as single flat conductors, multiple flat conductors and continuously transposed conductors. The condition of the enamel paint and of the paper wrapping is decisive for safe transformer operation.

The following tests are carried out on copper materials:

• Resistance of the enamel insulation after accelerated ageing in insulating liquids
• Thickness and hardness of the enamel insulation
• Adhesion and elasticity during bending and elongation
• Proof stress and elongation at break of the copper
• Breakdown voltage varnished copper conductors
• Determination of the solidification factor of continuously transposed conductors with epoxy bonding

The tank, tank cover, manhole cover and valves of a transformer are sealed using sealing materials that are resistant to insulating liquid, for example:

• Nitrile Butadiene Rubber (NBR)
• Fluoric rubber (FPM)
• Fluorosilicone rubber (FMQ)
• Asbestos-free fibrous materials (FA 200)

Outside coating materials are used as corrosion protection for tanks, radiators, coolers and other construction parts. Interior coatings on the other hand are used to protect the transformer from possible metal particle contamination.

The main coating materials used are environmentally friendly. These can be for example, water-based, “High Solid” or cathodic dip coating.

In the material testing laboratory we inspect the condition of the coating using the following procedures:

• Layer thickness measurement
• Cross-cut test
• Effect of the internal coating materials on the insulating liquid (DGA/oil values)

Adhesives, cast resins, desiccants, glide waves and detergents are only some of the numerous auxiliary materials used in transformer production or repair. The behavior of each of these auxiliary materials must be understood to the smallest detail, particularly in their interaction with other materials. We therefore test the adhesion of glues, the adsorption behavior of desiccants and the compatibility of different auxiliary materials with insulating liquids.