Shell has invested in research programs with leading universities, technical institutes and private companies to accelerate the innovation process for maintaining transformer life. For instance, in 2012, the company joined a major European research consortium that is investigating transformer design and operation and the influence of the oil on aging and reliability.

Research Goals

The research, which is one of the biggest academic exercises in this field in Europe, is being led by the University of Manchesters School of Electrical and Electronic Engineering, a recognized technical center of expertise and excellence for electrical research on transformer oils and transformers in the U.K. The consortium also includes transformer manufacturers, utility companies, testing laboratories and insulation material manufacturers. Shell provides oil for the research.

Zhongdong Wang, professor of high voltage engineering at the University of Manchester, said, When considering safety, environment and economic impacts, the implications of a transformer failure can be extremely severe for a power utility, especially one that owns, operates and maintains a population of aging transformers. Therefore, it is of vital importance to understand transformer reliability and the influence of the oil on transformer aging and reliability.

Shell believes this program could help revolutionize the lifespan of future transformers because it is enhancing the body of scientific knowledge in this specialist area. For instance, it has provided better understanding of how oils and transformers age, and has helped identify the key attributes that need to be improved to increase the reliability and longevity of both oils and transformers in service. Tests at the university have helped to validate Shells inhibited transformer oil based on gas-to-liquid (GTL) technology – Shell Diala S4 ZX-I.

One test simulated the effects of high-voltage transients due to lightning strikes and switching operations in power systems and assessed the resilience of different oils to the effects of such transients. In the test, an inhibited Shell GTL oil and Shells conventional inhibited oil, both with water content of less than 10 parts per million, were evaluated for their lightning impulse breakdown in needle-sphere and needle plane geometries.

An eight-stage impulse generator with a maximum output of 800,000 Volts and 4,000 Joules delivered a standard lightning impulse that reached peak voltage in approximately 1.2 microseconds and decayed to 50 percent of peak voltage in approximately 50 microseconds after the beginning of the wave. The test showed that the GTL-based oil had a significantly higher lightning impulse breakdown voltage than the conventional oil.

As a result of this and other testing, Shell recently introduced the first transformer oil based on GTL technology. We believe that this oil, coupled with scientific findings from a major research program, could help to revolutionize the reliability and lifespan of transformers now and in the future.

What Transformers Need

With modern transformers getting smaller and operating at higher voltages, the stresses placed on the oil are higher than ever before. Also, corrosive sulfur species in conventional transformer oils can cause failures due to copper corrosion. But GTL-based transformer oils are essentially sulfur free, removing the risk of corrosion in the transformer and improving reliability.

In addition, GTL-based oils have an excellent response to antioxidant additives, which means that they resist degradation in demanding applications. And they show lower acidity and sludge formation on aging, and their oxidative stability is well above standard requirements.

Effective cooling in a transformer is determined chiefly by two oil parameters: thermal properties – specific heat capacity and thermal conductivity – and fluidity or viscosity. An oils thermal properties are proportional to its density. Calculations and measurements show that specific heat capacity and thermal capacity values are typically higher for GTL-based oils than for conventional transformer oils, which indicates better thermal properties. This may provide cooling benefits for transformers in operation and enable either higher loading or reduce the need for forced cooling or some other design change such as reducing transformer size.

Another important parameter that influences oils ability to provide cooling in a transformer is its fluidity or viscosity across the usual transformer operating temperature range. Usual temperature ranges are defined in specifications such as IEC 60076 Part 1, which defines the normal ambient lower temperature limit for power transformers as minus 25 degrees C.

At higher temperatures, most oils have comparably good low viscosity, which facilitates good cooling. At lower temperatures, most oils thicken considerably, which reduces their flow rate and cooling ability. Inhibited GTL oil thickens significantly less at lower temperatures; therefore, it maintains good fluidity and flow properties, even under extreme conditions.

Other important issues with transformer oils are compatibility and miscibility, which are not the same. Two fluids are miscible if they form a clear fluid when mixed. Compatibility goes much further than this. Obviously, compatible fluids must be miscible, but to be compatible one fluids performance must not be diminished by the other.

When testing for compatibility, pairs of fluids are mixed and observed under different conditions to see if they are miscible. Then, using performance tests, they are assessed to see whether any pair behaves differently. Incompatibility can show itself through the formation of deposits, worsening of oxidation stability or differences in any electrical properties of the fluid pairs.

Shell commissioned a series of tests on miscibility, compatibility and solvency, and concluded that GTL-based transformer oils can be used in combination with conventional hydrocarbon oils. Moreover, we see positive effects when fresh oil is added to aged oil.

To evaluate the effect of mixing different transformer oils in service, the properties of several mixed and unmixed inhibited oils, both aged and unaged, and in different ratios and combinations, were tested. The table on Page 44 shows the results for 15 percent aged uninhibited naphthenic oil mixed with 85 percent GTL-based inhibited oil. The mixture still shows the highest oxidation stability. The GTL-based oil compensates for the decreased performance of the aged oil: more than if the test had been conducted using a conventional inhibited oil.

Inhibited vs. Uninhibited

In chemical terms, an inhibitor is a synthetic antioxidant added to a transformer oil to slow the process of oxidative degradation. If allowed to proceed unchecked, this process shortens fluid life and decreases its performance. Depending on the level of refining, the mineral oils produced from crude oils contain sulfur-based chemical species that can act as mild inhibitors for oxidation. These species are exploited in so-called uninhibited oils. Strictly speaking, they are not uninhibited oils, but they do not have any added synthetic inhibitor.

Very highly refined mineral oils and synthetic oils contain no natural inhibitor, so they are prone to oxidation if chemical inhibitor is not added. This may appear to be a disadvantage, but it is actually a benefit. The complete system is more consistent and behaves in a more predictable and measurable way when aging occurs than uninhibited (naturally inhibited) mineral oils.

Furthermore, inhibited transformer oils, with their greater purity and predictability, are far more oxidation resistant than uninhibited oils. This is becoming essential, as the combination of longer oil life expectancy and transformer design and operation (for example, smaller transformers and higher voltages and loads) dramatically increases the stress on the fluid.

Ivanka Hoehlein, manager of Siemens Material Testing Laboratory, concurred, Our laboratory and field testing of transformer oils shows that inhibited oils offer enhanced performance compared with uninhibited oils.

Peter Smith is Technology Manager for Shell Global Solutions (UK). Email him at peter.w.r.smith@shell.com.