Modern electrical transformers demand more stringent performance from transformer oils to ensure high reliability and efficiency. Historically, transformer oils have been based on conventional paraffinic or naphthenic oils, often with performance enhancing additives. Although many of these oils have a long and proven track record, they can have limitations, including lack of global availability and widely variable consistency, composition and therefore performance.
A new generation of transformer oils has been developed that overcomes these limitations. Formulated from gas-to-liquid base oils, these products have improved additive response, essentially zero sulfur content and consistent composition and performance. The use of such oils contributes to increased transformer reliability and efficiency.
Fighting Oxidation & Corrosion
The key properties of transformer oils are insulation, cooling and corrosion resistance. These properties depend on a number of factors, including the base oil, how the oil has been processed, and the type and level of additives.
Crude oils typically used to make insulating oils can contain a wide variety of aromatic, naphthenic and paraffinic hydrocarbons, as well as sulfur, nitrogen and other heterocyclic compounds. If left in the oil, unsaturated and aromatic hydrocarbons and heterocyclic compounds can be prone to rapid oxidation.
Oxidation produces oil-insoluble products that generate deposits such as sludge and oil-soluble organic acids and polymeric compounds that generate thickened, potentially corrosive oil. These conditions can shorten oil life and service interval, causing costly and time-consuming downtime and maintenance.
While all insulating oils degrade with time, the rate of degradation can be controlled by carefully selecting base oil and additive chemistry. Depending on their type and quantity, sulfur compounds in crude and finished insulating oils can act either as natural antioxidants or as sources of potentially corrosive sulfur. Total sulfur in crudes can be 2 percent by weight or more, but refining by distillation, hydrotreatment or solvent extraction can convert or remove the most unstable components.
Knowing the type and level of sulfur can indicate whether an oil could potentially become corrosive. However, given the complex range of sulfur compounds potentially present and how they may interact in a transformer, a detailed chemical understanding and explanation is still far from clear and is the subject of active study.
Currently, amongst the most widely used tests for predicting the field performance of insulating oils are the International Electrotechnical Commission’s IEC 61125C oxidative stability test and the IEC 62535 test for potentially corrosive sulfur content.
Table 1 lists oxidative stability testing results for a conventional inhibited (stabilized against oxidation) naphthenic and a GTL inhibited hydrocarbon insulating oil. The results show that the GTL-based oil, like the naphthenic oil, exceeded not only the conventional level of oxidative stability, as defined by the IEC 60296 specification, but also the higher levels laid out in Section 7.1 of IEC 60296.
Typically, the lower the sulfur content in the fresh oil, the less potential risk of corrosive sulfur developing in insulating oil during service. More highly refined inhibited oils have the lowest sulfur content, often less than 40 parts per million. GTL-based transformer oils have no detectable sulfur.
However, two-dimensional gas chromatography shows that the number of sulfur containing components in an oil is relatively large even for the lower total sulfur oils. Therefore, before commercialization, transformer oils are thoroughly tested to ensure they meet the latest and most stringent tests for corrosive sulfur, such as IEC 62535.
The IEC 62535 test for potentially corrosive sulfur immerses a copper strip wrapped in a Kraft paper in oil and heats the oil to detect its corrosive behavior towards copper. After the test, samples are compared visually and rated. These empirical methods can be a source of misinterpretation and doubt more than a guarantee of an accurate response for an oil over its full service life.
As a result, given the extremely long service life and variable operating conditions that oils typically see in service, it is difficult to predict with certainty what sulfur compounds could and will ultimately form. Therefore, modern transformer oils are formulated to have significantly lower levels of total sulfur than earlier products. GTL transformer oils offer the possibility of zero detectable sulfur to significantly minimize long-term corrosion risks.
Thermal Properties
The thermal properties of transformer oils such as specific heat capacity and thermal conductivity are proportional to their density. The graphs below plot the calculated values for two Shell products – one a high-performance conventional oil, the other made with GTL base stock.
As can be seen, both specific heat capacity and thermal capacity are higher for GTL oils, indicating enhanced thermal properties. This may result in cooling benefits for transformers in operation, which can extend oil and transformer life, and allow either higher loading or reduce the need for forced cooling. In addition, GTL oils typically have a significantly higher flash point and lower density than conventional hydrocarbon inhibited oils, providing safer transformer operation and opportunities for transformer design optimization.
Can Oils Mix?
An important property of transformer oils is their compatibility with oils from other suppliers or with different formulations. Only compatible oils should be mixed in service to avoid loss of performance.
To test this property, mixed inhibition (uninhibited and inhibited) and unmixed inhibition oils (conventional and GTL based), both aged and unaged, were tested in different ratios and combinations. The only additive in the inhibited oils was a butylated hydroxytoluene antioxidant. The oils were tested according to IEC 61125C to simulate aging in service and to identify potential miscibility and compatibility issues. Results showed no deposit formation or evidence of incompatibility when mixing these used and fresh oils.
Testing confirmed that the GTL inhibited transformer oil produced comparable results when mixed with fresh and used uninhibited naphthenic base oils. The data indicates that a mixed inhibition oil system typically displays performance properties that are an average of the type and quantity of its constituent parts. This is irrespective of whether the oil is unused or used, or whether the oil types are naphthenic, paraffinic or a mixture of the two.
Interestingly, mixtures containing the GTL based oil show significantly improved oxidative stability and can, therefore, provide optimal resistance to degradation in service. The conclusion from this is that GTL transformer oil has superior antioxidant response compared to state-of-the-art naphthenic based products.
Compatibility of insulating oils is evaluated within IEC 60422, which states in Section 6.12 that unused oils containing the same or no additives are considered to be compatible and can be mixed in any proportion. It recommends oils of the same type be used for topping up or refilling. This section states that normally no problems develop when less than 5 percent unused oil is added to lightly used oil, but higher addition levels to heavily aged oils may lead to precipitation. If in doubt, it is advisable to run a compatibility test or consult the oil supplier.
Boosting Oxidation Stability
Further tests were run to evaluate the oxidative stability (or aging characteristics), compatibility and miscibility of mixed uninhibited (used and fresh naphthenic based) and inhibited (naphthenic and GTL based) oils. Testing consisted of filling two glass vials containing copper strips wrapped with Kraft paper with oil, and heating to 100 degrees C (in the dark). One vial was sealed to prevent air from entering; the other had a tube to allow air in, simulating transformer breathing. A sealed third vial contained only oil and was used to compare color changes. The test was run for up to 14 days with regular visual checks for signs of oil degradation (color change, turbidity, and deposit formation).
After only five days, the conventional uninhibited oil began to undergo oxidative degradation. After 10 days, this oil showed significant color change and deposit formation. Degradation was most pronounced in the vial that was open to the atmosphere. In contrast, the inhibited oil showed no significant visible signs of oxidative or other degradation after 10 days and even after 14 days.
When this test was run on selected mixed inhibition oil mixtures, results showed that the major component of the mixture controlled the rate and degree of overall degradation of the sample. Degradation dropped significantly with no visible deposit formation when inhibited GTL oil was the main component at 85 percent with the remainder an aged commercial uninhibited transformer oil. While the oil darkened slightly, neither mixture produced visible deposits when the oils were mixed at the start of the test.
The absence of visible deposit formation in mixtures of aged uninhibited and unused inhibited GTL oil indicates that the inhibited GTL oil is just as compatible and miscible with the aged oil as an inhibited naphthenic oil. It also indicates that differences in solvency between the naphthenic and GTL oils are not significant.
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