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Dissolved Gas Analysis (DGA)
Test Method: ASTM D 3612 – 02
Dissolved Gas Analysis is the single most important test performed on oils from transformers. As the insulating materials in a transformer break down due to thermal and electrical stresses, gaseous by-products are formed. These gaseous decomposition products of varying composition dissolve in the oil. The nature and amount of the individual component gases are indicative of the type and degree of the abnormality responsible for the gas generation.
Method A— Dissolved gases are extracted from a sample of oil by introduction of the oil sample into a pre-evacuated known volume. The evolved gases are compressed to atmospheric pressure and the total volume measured. The gases are introduced into a gas chromatograph
Method C— Method C consists of bringing an oil sample in contact with a gas phase (headspace) in a closed vessel purged with argon. The dissolved gases contained in the oil are then equilibrated in the two phases in contact under controlled conditions. At equilibrium, the headspace is over pressurized with argon and then the content of a loop is filled by the depressurization of the headspace against the ambient atmospheric pressure. The gases contained in the loop are then introduced into a gas chromatograph
(Click link to see DGA presentation) (Click link to see Paper)
Practical Experience Gained from Dissolved Gas Analysis at an Aluminium Smelter--Eurotehcon 2011. Click here to download.
Practical Experience Gained from Dissolved Gas Analysis at an Aluminium Smelter--Eurotehcon 2011-presentation. Click here to download.
Furanic Compounds In insulating Oil
Test Method: ASTM D 1533 –00
Furanic compounds are generated by the degradation of cellulosic materials used in the solid insulation systems of electrical equipment. Furanic compounds, which are oil soluble to an appreciable degree, will migrate into the insulating liquid. High concentrations or unusual increases in the concentrations of furanic compounds in oil may indicate cellulose degradation from aging or incipient fault conditions. Testing for furanic compounds may be used to complement dissolved gas in oil analysis. Experience is required in evaluating the furanic compound data, as there are factors such as type of insulation preservation/oil expansion system, type of conductor wrapped insulation, family of transformer, and treatment of the oil or the transformer, which can influence the interpretation. Tests for furanic compounds should be performed initially for all power transformers to have a baseline, for important or older transformers, when high carbon oxides are generated, for highly loaded transformers, and when other tests indicate accelerated aging.
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WATER CONTENT
Test Method D 1275 – 03
Water, in minute quantities, is harmful in power equipment because it is attracted to the places of greatest electrical stress and this is where it is the most dangerous. Water accelerates the deterioration of both the insulating oil and the paper insulation, liberating more water in the process. The dielectric breakdown voltage of insulating material is a function of the water content. The water migrates between the solid and liquid insulation in a transformer with changes in temperature.
DIELECTRIC STRENGTH
Test Method: IEC 156/ASTM (D 877, D 1816)
The dielectric strength is the measure of the insulating oils ability to withstand electrical stress without failure. The test involves applying an ac voltage at a controlled rate to two electrodes immersed in the insulating fluid. The gap is a specified distance. When the current arcs across this gap the voltage recorded at that instant is the dielectric strength breakdown strength of the insulating liquid. Contaminants such as water, sediment and conducting particles reduce the dielectric strength of insulating oil. Combination of these tends to reduce the dielectric strength to a greater degree.
ACIDITY OR NEUTRALISATION NUMBER (NN)
Test Method: ASTM D974/ D 664 – 01
Insulating oil may contain acidic constituents that are present as additives or as degradation products formed during service, such as oxidation products. Acidic products of oxidation within the oil will increase rapidly once present. These contaminants have a very aggressive effect on the materials of the transformer. The acidity value of the oil sample will indicate the oil life remaining before it contributes to the deterioration within the transformer. If the acidity increases above a certain level the oil requires replacement /regeneration to prevent deterioration of both current carrying and insulating components within the transformer. The life of a transformer can be no greater than the life of the insulating paper and pressboard within the transformer. Although limits are set for the acidity value, it is good practice to process oils well before these limits are reached if the transformer is to give long and reliable service.
INTERFACIAL TENSION (IFT)
Test Method: ASTM D971
The Interfacial Tension (IFT) measures the tension at the interface between two liquids (oil and water) which do not mix. The test is sensitive to the presence of oil decay products and soluble polar contaminants from solid insulating materials. Interfacial tension measurements on electrical insulating oils provide a sensitive means of detecting small amounts of soluble polar contaminants and products of oxidation. A high value for new mineral insulating oil indicates the absence of undesirable polar contaminants. The test is frequently applied to service-aged oils as an indication of the degree of deterioration.
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Dissipation Factor (Tan D) or Power Factor, Resistivity and Relative Permittivity (Dielectric Constant)
Test method IEC 247
Either separately or together are important indicators of the intrinsic quality and degree of contamination of an insulating fluid. These parameters may be used to interpret the deviation from desired dielectric characteristics and the potential influence on performance of equipment in which the fluid is used.
Dissipation Factor (or Power Factor)— This is a measure of the dielectric losses in an electrical insulating liquid when used in an alternating electric field and of the energy dissipated as heat. A low dissipation factor or power factor indicates low ac dielectric losses. Dissipation factor or power factor may be useful as a means of quality control, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling
Resistivity-of a liquid is a measure of its electrical insulating properties under conditions comparable to those of the test. High resistivity reflects low content of free ions and ion-forming particles, and normally indicates a low concentration of conductive contaminants. Low resistivity values being a sign of oil, which contains particulate contaminants and oxidative products.
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Polychlorinated Biphenyls (PCBs)
Test Method D 4059 – 00
Polychlorinated biphenyls PCBs, (or askarels as they are often known), are highly regulated therefore insulating liquids that may contain PCBs should be tested to ensure proper handling and disposal.
Transformer owners have a responsibility for identifying PCB contaminated installations and for actions required to dispose of contaminated oil and equipment. Using modern techniques it is possible to establish the presence of PCB’s down to levels of one part per million. Under proposed South African legislation, an inventory and plant labeling is required for plant containing PCB. Also, contaminated equipment must be disposed of through a licensed company.
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Oxidation Inhibitor Content
Test Method D 4768 – 03 This test provides a method for the quantitative determination of the amount of oxidation inhibitor (2,6-ditertiary butyl-paracresol DBPC or 2,6 ditertiary phenol) present in an inhibited oil. Control of the inhibitor content is an important factor in maintaining long service life of inhibited insulating oils.
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Oxidation Stability (acid/sludge)
Test Method ASTM D 2440 is a method of assessing the oxidation resistance of oil by determining the amount of acid/sludge products formed when tested under certain prescribed conditions.
Good oxidation stability is necessary in order to maximize the service life of the oil by minimizing the formation of sludge and acid. Oils that meet the requirements specified for this test tend to minimize electrical conduction, ensure acceptable heat transfer, and preserve system life. There is no proven correlation between performance in this test and performance in service, since the test does not model the whole insulation system (oil, paper, enamel, wire). However, the test can be used as a control test for evaluating oxidation inhibitors and to check the consistency of oxidation stability of production oils.
Corrosive Sulfur (ASTM D1275 Method A or B) test detects the presence of objectionable quantities of elemental and thermally unstable sulfur-bearing compounds in oil. The presence of corrosive sulfur compounds will result in deterioration of these metals. The extent of deterioration is dependent upon the quantity and type of corrosive agent and time and temperature factors.
Detection of these undesirable impurities, even though not in terms of quantitative values, is a means for recognizing the hazard involved.
Method B is more rigorous and the preferred method.
Density or Specific Gravity (D 1298) of oil is the ratio of the weights of equal volumes of oil and water determined under specified conditions. In extremely cold climates, density has been used to determine whether freezing of water in oil-filled apparatus, will resulting in ice formation and possibly cause flashover of conductors extending above the oil level. The specific gravity of mineral oil influences the heat transfer rates. Oils of different specific gravity may not readily mix when added to each other and precautions should be taken to ensure mixing.
Viscosity (D 445) is the resistance of oil to flow under specified conditions. The viscosity of oil used as a coolant influences heat transfer rates and consequently the temperature rise of an apparatus. The viscosity of an oil also influences the speed of moving parts in tap changers and circuit breakers. High viscosity oils are less desirable, especially in cold climates. Standard viscosity curves can be generated using Method D 341 by measuring two or three data points and plotting the data on special chart paper. The resulting curve can be used to interpolate or extrapolate values at temperatures where the viscosity is not measured directly.
Color (D 1500) of a new oil is generally accepted as an index of the degree of refinement. For oils in service, an increasing or high color number is an indication of contamination, deterioration, or both. However, color is not always a reliable guide to product quality and should not be used indiscriminately in product specifications
Metal and Particle analysis (ASTM D5185 & D7151-05)
This test method can be used to monitor equipment condition and help to define when corrective action is needed. It can also be used to detect contamination such as from silicone fluids (via Silicon) or from dirt (via Silicon and Aluminum). This test method can be used to indicate the efficiency of reclaiming used insulating oil.
Dissolved Metals:
The high temperatures associated with some incipient-fault conditions will cause the amount of dissolved metals associated with the problem to increase along with the dissolved gases in oil. Comparison of the metal-in-oil content with baseline values before the occurrence of the incipient-fault condition can help locate the source of the gassing and the problem
Wear Metals:
Pumped cooling systems are susceptible to bearing wear, which when excessive can create metal particles which are deleterious to the insulation system.
Degree of Polymerisation of New and Aged Electrical Papers and Boards (ASTM D 4243 – 99)This test method is used for determining the average degree of polymerization (abbreviated DP) of new or aged electrical papers.
The degree of polymerisation (DP) test is another means for assessing insulation aging. This test is performed on paper samples. The DP test provides an estimate of the average polymer size of the cellulose molecules in materials such as paper and pressboard. Generally, paper in new transformers has a DP of about 1000. Aged paper with a DP of 150-200 has little remaining mechanical strength, and therefore makes windings more susceptible to mechanical damage during movement, particularly during extreme events such as through-faults. As insulation aging in transformers can be uneven due to thermal, moisture, oxygen, and by-product concentration gradients samples from various locations are needed to provide the best diagnosis of the overall insulation condition.
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