Figure 1 - High-power transformers at the Siemens transformer factory (Nuremberg, Germany).
High-power transformers are some of the most important elements in the supply of energy (see Figure 1). All transformers comprise essentially the following main materials: copper; magnetic sheet steel; insulating materials (i.e., paper, pressboard, and wood); and transformer oils, which are used for both insulation and cooling purposes. Organic substances in particular (all solid and fluid insulating materials) are subject to an aging process that is dependent on the operating conditions. Under normal operating conditions, high-power and distribution transformers frequently reach an age of over 30 years. However, some events are considered totally abnormal during the course of the transformer service life, e.g., transient voltages, overheating during normal operation, dynamic stresses, and disruption to cooling. These processes result in accelerated aging of the aforementioned materials (see Figure 2).
Figure 2 - Effect of oil-aging products on the aging rate.
Fast, simple, and convenient off-line tests can replace on-site, regularly used, expensive analyses for the successful operation and early detection and correction of accelerated aging conditions. Special monitoring methods have been developed by the Siemens transformer factories throughout many years of successful management, in conjunction with operators and manufacturers. Of central importance are the interfacial tension and viscosity measurements. Commercial measuring instruments have been used successfully for this purpose for years, with regular model changes ensuring increased efficiency.
This article describes the way these tests function and the significant role they play in transformer life management and in the design and testing of new materials.
Interfacial tension measurements
Figure 3 - Left: water/air interface. Right: amphiphilic molecules on the oil/water surface.
The interfacial tension (IT) in transformer oils provides information regarding the presence of polar substances that form due to the decomposition of oil and paper. The molecules that are directly on the surface of an oil making contact with air are observed. As can be seen in Figure 3, these mutually attracted molecules (the greater the distance, the less the force) are drawn increasingly down, since the molecules in a gas are, on average, much farther away. A similar imbalance also results on the interface of two nonmiscible liquids, e.g., between water and oil. This manifests itself in an elastic “skin” that presses the liquid contained therein into as small a surface as possible, i.e., the water forms in the oil spherical drops, which, depending on the density difference, either sink or rise.
The molecules of surface-active molecules, such as surfactants in cleaning agents, generally have an oil-loving hydrocarbon chain and a water-loving polar “head.” For these molecules, the energetically most favorable position is directly on the oil/water interface. They will thus preferably settle there with a certain temporal delay, reducing the interfacial energy and, hence, the IT.
Since very small concentrations are sufficient to coat the surface, IT measurement can even detect the smallest provable traces of this group of substances, which could only be done previously with a great deal of analytical effort.
Ring method according to du Noüy
Figure 4 - Ring method according to du Noüy.
Figure 5 - Fully automatic, mobile ring tensiometer according to du Noüy during the measurement of the IT between water and insulating oil already discolored by oil decomposition products.
Using the well-established du Noüy method according to ASTM D971-99a and ASTM D1331 (Figure 4), the following measurement was made (Figure 5). With an extremely sensitive load cell by means of a horizontal platinum ring, the maximum force (Fmax) exerted on the surface of the lamella as is it removed from the oil just before it breaks is measured. The unit of measurement for the IT is generally given in milliNewton/m (mN/M).
Drop volume method
Figure 6 - Drop volume method.
The drop weight/drop volume method according to ASTM D2285 is an alternative way to determine the IT (Figure 6). With this method, using a vertically suspended syringe on a levelly ground capillary, drops of water are created in the insulating oil to be tested. The volume of the drop created on the end of the capillary is increased by means of pressing the syringe plunger down. The drop will then sink through the oil to the bottom of the collecting cuvette. During this process, the sample droplet is detected by a light barrier and, thus, from the measured distance of the syringe plunger and the known syringe cross-section, the volume and, hence, the IT, of the drop is determined (Figure 7).
Figure 7 - The PC-controlled drop volume tensiometer supplies information regarding the adsorption behavior of amphiphilic impurities on the surface.
The accuracy of both methods described above is dependent on external influences, such as the cleanliness of the apparatus that comes into contact with the sample, the homogeneity of the samples (e.g., to be produced by stirring), and the occasional introduction of impurities via the transfer into or from other laboratory sample vessels.
IT as the index of insulating oils
IEC 60422 forms the foundation for the monitoring and maintenance of insulating liquids. In addition to the quantitatively difficult-to-measure parameters of color and appearance, the analytically determined water content, the neutralization number, and the loss factor, the IT is also an indicator of the quality of the transformer oil. This method measures the concentration of the polar molecules in the oil that develop during the aging process. The higher this concentration, the lower the interfacial tension, and the higher the tendency of the insulating oil to form sludge. The cause of this is the advanced aging (oxidation) of the oil through, for example, penetrating water or the aging of the cellulose, and reduces the formation of sludge. This process minimizes the dielectric properties of the insulating system, especially solid insulation. Sludge severely impairs the winding cooling, thus preventing heat removal. This heat accumulation, in turn, makes the winding paper age very quickly.
Interfacial tensions of less than 15 mN/m indicate the possibility of sludge formation. Therefore, maintenance measures are recommended for values under 22 mN/n, irrespective of the values of the neutralization number.
The viscosity of the insulating oil is critical to the design of the cooling system of a transformer. The lower the viscosity, the better is the cooling. IEC 60296 defines a maximum viscosity of 12 mm2/sec at 40 °C for transformer oils.
The viscosity that demonstrates the flowability of an oil sample is another important characteristic of oils in general and transformer oils in particular. Indirectly, viscometry also tests the quality of the insulating papers. One distinguishes between the density-dependent viscosity, i.e., dynamic (unit: 1 cp = 1 mPas) from the kinematic viscosity (unit: 1 cSt = 1 mm2/sec). The latter can be determined very accurately with Ubbelohde glass viscometers (Figure 8) by measuring the time it takes for a specific amount of sample to pass through a well-defined capillary under the influence of gravity. Light barriers (Figure 9) replaced the eye and stopwatch long ago.
Figure 8 - The Ubbelohde method is used to determine the kinematic viscosity.
Figure 9 - Modern PC-controlled measuring system using the Ubbelohde method for the accurate determination of the viscosity of insulating oils (ochre colored) and the DP value of insulating papers dissolved in cupriethylendiamine (CED) (blue).
The Ubbelohde viscometry method has long since been tried and tested, and is now available in a highly automated version. Not only can time measurements be carried out automatically, but dosage and cleaning can be completely automated as well. Modular systems are especially suitable for this, since they are designed to precisely meet the user’s requirements.
Determination of quality of insulating papers
Paper is a polymer, i.e., it is made up of long strands of polymerized glucose rings. The differential pressure (DP) value of the paper is a measurement of the number of glucose rings in the cellulose chain. The DP value is directly linked to its mechanical strength, i.e., the tensile strength. The mechanical properties of paper are no longer guaranteed when the tensile strength of the transformer papers reaches 25% of the original value. New paper typically has a DP value of between approximately 1000 and 1100, which can diminish to about 200 at the end of its serviceable life.
The solvent viscosity is a thoroughly tested, easy-to-use method for assessing the strand length changes in polymers. The intrinsic viscosity (IV) determined from the ratio between the viscosity value of the polymer solution and the solvent and the concentration (the IV value) correlates with the degree of polymerization (i.e., the strand length). As with other polymers, standards determine which solvents are suitable, which weights of the contents and measuring temperatures are to be selected, and how the data are to be measured and evaluated. The solvent CED is frequently used for paper and cellulose, since it ensures the gentle (i.e., nondestructive) development of the paper molecules in the solvent.
Figure 10 - Dependency of paper aging on the moisture content of the paper (parameter: DP value).
The degree of polymerization of an insulating paper is determined by means of solvent viscosity in accordance with IEC 60450. New cellulose comprises approximately 1000–1100 sugar molecules, i.e., the DP value is 1000–1000. Aging causes a reduction in the DP value and the associated mechanical strength. The mechanical strength is important for the mechanical load of the transformer, e.g., in the case of transportation costs. However, mechanical strength is most important for short-circuit loads. A reduction in the DP value to around 200–150 DP units means the end of the service life of the transformer. The DP value is also an important quantity for evaluating the condition of the solid insulation, taking into consideration the type of paper (thermally stabilized/not thermally stabilized) and the moisture content of the oil and paper.
The rate of aging depends greatly on the temperature and the water content in the insulation (Figure 10). Transformers with damp insulation and high operating temperatures can age up to 20 times faster.
Resistance test of insulating oils in extreme climates
Transformers are used frequently in areas with very severe climatic conditions, e.g., in Arctic regions. It is well-known that the viscosity of the (additive-free) oils, like other liquids, is dependent on the temperature. In Arctic regions, the temperature in the switched-off transformer can plummet to below –40 °C, resulting in a significant increase in the viscosity and, hence, a change in the insulating properties; under certain circumstances, this can result in damage to the transformer. It is therefore necessary to recreate the potential extreme temperatures and while testing the oils under such conditions. Again, the Ubbelohde viscosity measurements, which can be carried out at temperatures lower than –40 °C using cryostats and appropriate laboratory apparatus, are also suitable for the measurement of the temperature dependency well above the operating temperature.
Because they are modular, the same viscosity measuring systems used for measuring the DP can also be used for resistance testing. For example, a measuring stand is placed in a suitable cooling thermostat and, due to the increased viscosity, the Ubbelohde viscometer is replaced by one with a greater capillary radius. The measurements can be made simultaneously to the DP tests using the same device.
Fully automated tensiometer and viscosity measuring systems, in conjunction with precise temperature technology, simplify the monitoring of important characteristics of transformer oils and insulating papers. They are essential testing methods in the management of transformer life and in the development of new materials.
Kachler, A.J. Diagnostic and Monitoring Technology for Large Power Transformers (Fingerprints, Trend Analysis from Factory to On-site Testing), CIGRE SC12, Sydney Colloquium, Oct 6–10, 1997.
Kachler, A.J. On-Site Diagnostics of Power and Special Transformers, ISEI, Anaheim, Apr 2–5, 2000, CA.
Siemens—Transformer Life Management: Customer Service for Aging Analytics and Laboratory Diagnostics. Guidelines for Transformers in Operation. Order no.: E50001-U410-A34.
Siemens—Efficient Recording and Diagnosis of the Condition of High-Performance Transformers.
Siemens—All Services for Transformers.
Siemens brochures can be ordered through www.transformer-service.com.
Dr. Hofmann is Sales Manager, Measuring Instruments, Lauda Dr. R. Wobser GmbH & Co. KG, Postfach 12 51, 97912 Lauda-Königshofen, Deutschland, Germany; tel.: +49 9343 503 186; fax: +49 9343 503 188; e-mail: firstname.lastname@example.org.