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Oil is a vital part of the transformer body and (similarly to blood in a human being body) keeps responsibility for the condition of the entire organism. Oil is particularly responsible for functional serviceability of the entire insulation system. The insulating oil must be kept in pristine condition, since its condition can be a decisive factor, which determines the life span of the transformer. Fields and laboratory experiences have shown that transformer oil contains a vast amount of information. Oil analyses can be extremely useful in monitoring the condition of power transformers. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. To ensure long-term reliability of oil filled power transformers, it is important to identify early sign of degradation of the insulating oil. In this paper, oil degradation was monitored with various ASTM test methods. Investigations were performed on service-aged oil samples as well as on oil samples aged in laboratory conditions. Many key parameters actually used to monitor the condition of transformer oil relative to oxidation/degradation were investigated. From the obtained results, correlations were found between some of them. The results indicate that Dissolved Decay Products (DDP) and turbidity, which change with a higher rate than interfacial tension (IFT) and Acid Number (AN) values, can be possibly used as an effective index for insulating oil degradation assessment. Limits are suggested which provide a “picture” of the fluid condition.

It is a well-known fact that the service reliability of power transformers largely depends upon the condition of the dielectric fluid. The steady deterioration (under the influence of the multiple stresses) of the insulation has an important impact on the condition of the transformers themselves. This contribution reports some investigations on some commercially available ester, silicone and mineral oil fluids. Some of their physicochemical properties are studied by submitting them to various stresses: electrical, thermal and oxidation. Many comparisons are made according to the quality test results of mineral oil, as it is very familiar to the transformer industry. A new technique using a reactive free radical reagent, 2,2-diphenyl-1- picrylhydrazyl (DPPH), added to both new oil for reference purposes and different aged oil to assess free radical concentration is presented. The gassing tendency under either thermal or electrical stress, along with the physicochemical properties of the fluids, is affected; it is assessed with the production of oxidative aging by-products. The gassing performance characteristics of natural ester fluids are far superior to those of conventional mineral oil. A significant reduction in insulation aging rate was observed with synthetic ester fluids.

Oil/paper insulation degradation in transformers involves chemical and physical changes in the materials. Some of the chemical reactions involve very reactive intermediates called free radicals. Free radicals play a major role in a wide variety of ageing processes. The detection of these reactive species in oil may, in principle, provide useful information for monitoring oil degradation. This manuscript details a laboratory technique, which determines the relative content of free radicals in insulating oils of petroleum origin by a spectrophotometric method. Free radicals may be formed in oils under operating or test conditions. The procedure enables the determination of the relative concentration of free radicals, which can act as the precursors of decay products such as charge carriers, oxidized molecules, as well as polymerization products. The technique involves using a reactive free radical reagent, 2,2-diphenyl-1-picrylhydrazyl (DPPH), added to oil to assess free radical concentration. This method is applicable to new, reclaimed, or used oils as well as naturally or artificially oxidized oil (the cause of aging can be chemical, physical, or electrical). In this contribution, free radicals were assessed following electrical discharge application in oil.

This article presents the results of an industrial experiment, which demonstrate the possibility of arresting or reducing the oxidative decay of oil-paper insulation in freely breathing power transformers by using a new type of nitrogen blanketing system.

International audience; Lifetime extension of power transformers is a subject of high importance for electric power systems utilities. The decision of replacing, refurbishing or repairing a service aged power transformer requires considering several factors, especially the cost and time to execute the work. The lifetime of the power transformer being related to the condition of the insulation system; one way of improving the situation is to reclaim insulating oil by Fuller??s Earth treatment. This procedure is economically attractive because of increasing prices for both mineral and synthetic transformer coolants, effective cost and environmentally sounds. Reclamation rejuvenates the transformer oil by eliminating contaminants. In this paper, a series of experiments has been performed with service aged oils reclaimed in laboratory conditions. Fast, inexpensive and reliable laboratory testing procedures developed by ASTM (D 6802 and D6181) were also used to monitor decay products as traces impurities. The results obtained in laboratory conditions, indicate that a large number of reclamation passes (around 15 passes) are required to regenerate inservice aged oil to a grade close to new oil. It is also shown that not only the reclamation improves the gassing tendency of oil, but also the type of Fuller??s Earth is very important for reclamation process. By upgrading the analytical chemistry of oil, the predictive maintenance of this non-renewable resource is modernized, its cost diminished and the service reliability of transformers enhanced. Currently, Fuller??s Earth is only used once. After depletion it must be disposed of in a land fill. By using two organic solvents that are recoverable by atmospheric distillation, it is shown, in laboratory conditions, that this mineral absorbent can be reactivated and successfully reused many times.

International audience; Since gas evolvement deteriorates the dielectrical properties of insulating fluid, its ability to resist decomposition under electrical discharge is of paramount importance for the safety of power transformers under operating conditions. In this paper, a series of experiments have been performed under electrical discharge according to ASTM D6180. Various insulating fluids were considered. The samples include minerals oils, synthetic and natural esters, and silicone fluid. The results indicate that the gassing tendency of natural esters is much lower than their counterparts. No significant differences between synthetic esters and silicone fluid were observed. Severely hydrotreated and hydrocracked oils indicate a higher gassing tendency. It is also shown that service-aged oil gassing tendency decreases with reclamation passes. The reclamation of service aged oil by three commercially available Fuller's Earth (FE) showed a declined gassing tendency when FE helped reducing the DDF and turbidity. In addition, the theoretical premises that, oil born decay products contribute to the oil gassing, is experimentally confirmed under laboratory conditions. Extended stability test (3 times longer than the standard specification) also provide convincing evidences that each brand of mineral insulating oil might contain a limited amount of unstable hydrocarbons and volatile compounds. This finding indicates that when in service conditions these are gradually decomposed, the gassing tendency should go down.

ISSN : 2153-3725 E-ISBN : 978-1-4244-7354-0 Print ISBN: 978-1-4244-7352-6 INSPEC Accession Number: 12221039; International audience; It is known that the oxidation decay of oil in power transformer deteriorates the paper insulation of windings. To avoid the formation of incipient electrical failures, service providers recommend timely reclamation of liquid insulation. Even though the purity of oil is improved, the decay products previously adsorbed by the paper cannot be removed. Another setback is the unknown gassing tendency of reclaimed oil. This contribution shows that not only the reclamation improves the gassing tendency of oil, but also the type of Fuller's Earth affects this important property. By upgrading the analytical chemistry of oil the predictive maintenance of this non-renewable resource is modernized, its cost diminished and the service reliability of transformers enhanced.

There is a general agreement that in service conditions the quality of mineral insulating oils gradually deteriorates under the impact of electrical, thermal and chemical stress. It is also widely accepted that only the incipient electrical failures such as hot spots and partial discharges are responsible for the gassing of oil. Nevertheless, the relationship between the cause and the symptoms of paper insulation deterioration is not clearly understood. This article intends to show that the gassing of oil is a more complex phenomenon. It provides experimental evidence that the chemical composition of hydrocarbon blend and the oil born decay products are also contributing factors to oil gassing.

There is a general agreement that in service conditions the quality of insulating fluids gradually deteriorates under the impact of electrical, thermal and chemical stress. Two ASTM methods are used to monitor the deterioration of liquid insulation step by step. A comparison is made between the performances of a commercial based mineral oil and ester fluids. The results obtained using a laboratory grade spectrophotometer and a ratio-turbidimeter, indicate that the absorbance increases by a significant and easily observable margin with ageing rate. Under the same aging condition, synthetic ester ages considerably slower than mineral oil; natural ester showing an intermediate behaviour.

Gassing of oil is defined scientifically as the chemical decomposition of certain hydrocarbons, under the impact of electrical and thermal stress. Indeed, the homolytic breakdown of vulnerable covalent bonds generates both small and large free radicals. The small ones become gases dissolved in the oil, while the large fractions generate insoluble colloidal suspensions. Currently, the electrical power industry believes that the gassing of oil is caused solely by hidden incipient electrical failures. Consequently, these are detected and diagnosed by periodic dissolved gas analysis (DGA). This paper provides experimental evidence that the formation in service conditions of undetected oil-born decay products is a contributing factor to the gassing of oil.

There is a general agreement that in service conditions the quality of insulating fluids gradually deteriorates under the impact of electrical, thermal and chemical stresses. An important side effect of oil gassing related to the invisible colloidal suspensions is also investigated. Two ASTM methods are used to monitor the deterioration of liquid insulation step by step. The results obtained using a laboratory-grade spectrophotometer and a ratio-turbidimeter indicate that the absorbance increases by a significant and easily observable margin with ageing rate. A comparison is made between the performances of a naphtenic commercial-based mineral oil and a synthetic and natural ester fluid. Under the same aging condition, the obtained results indicate that esters have a better capability to dissolve sludge. Currently, the gassing of oil is solely related to hidden incipient electrical failures. Consequently, these are detected and diagnosed by periodic dissolved gas analysis. The results performed in laboratory conditions reported in this contribution on oil stability under electrical stress indicate that, undetected oil-born decay products contribute to the gassing of oil.