By Ivanka Hàƒ¶hlein-Atanasova and Thomas Hammer, Siemens Energy Sector, Nàƒ¼rnberg, Germany
Diagnostics of oil-filled electrical equipment rely on insulating fluid markers, and the insulating fluids market is experiencing intensive development. Mineral oil has been used most, but several groups of insulating oils now exist: uninhibited, without added synthetic inhibitor; partially inhibited, with up to 0.08 percent ditertiary-bytyl paracresol (DBPC); and fully inhibited, up to 0.40 percent DBPC.
Carbon Oxides as Markers for Thermal Stress
Degassed oil contains only negligible amounts of carbon oxides. When in contact with ambient air, the oil absorbs environmental gases–oxygen (O2), nitrogen (N2), carbon dioxide (CO2)–over time. Air-saturated oil with a total gas content of approximately 10 percent contains roughly 300 parts per million CO2. Partly saturated systems contain proportionally lower CO2 values. In transformers, the rate of air absorption depends on the breather construction and the unit’s oil temperature range. Hermetically sealed systems don’t saturate with environmental gases; however, low amounts will be absorbed by leaks anyway. In nitrogen-blanketed systems, the oil will saturate only with N2, while leaving O2 and CO2 on low levels. Because the carbon monoxide (CO) content of the atmosphere is low and its solubility in oil is low, the amount of dissolved CO in an air-saturated oil is typically fewer than 10 parts per million. This leads to a CO2-to-CO ratio of greater than 30. Thermal stress leads to the formation of carbon oxides in oil. The formation of CO and CO2 is governed by temperature. Finally, the type of oil plays an important role in the overall gassing behavior. In the case of mineral oils, inhibition as well as oil, influences the gassing.
Carbon Oxides Deriving From Natural, Synthetic Esters
To estimate the amount of carbon oxide gases, which derive from oil itself (with or without catalytic reactions with copper) and from interactions in the presence of paper, the following setup has been tested:
Insulation fluids have been stored at 150 C for 164h in headspace vials. Each has been investigated, and a mean value within the group has been calculated. Within mineral oils, CO2 content is a concern. Although mineral oils form a certain amount of CO2, the presence of paper increases this amount significantly. In comparison, CO levels don’t vary much in the presence of paper with pure oil. Natural esters show a lower tendency than mineral oils concerning the evolution of CO (see Figure 1). CO2 might evolve from the degradation of the fluid under the influence of copper, as well as from paper degradation and is comparable to the amounts evolving from mineral oil. Synthetic ester shows a much higher evolution of CO and CO2 from the liquid than mineral oils do (see Figure 2).
Ethane–another dissolved gas anaysis (DGA) Thermal Stress Marker
Natural esters develop lower amounts of methane compared with mineral oils but very high amounts of ethane, which are relatively independent to the presence of copper and paper (see Figure 3). A certain dependence of ethane production on oxygen susceptibility exists. Ethane is also the fault gas most encountered in transformers with natural fluid and is developed according to the lipid peroxidation mechanism (see Figure 4).
This mechanism is common for all Omega-3 unsaturated fatty acids. Ethane development also is typical for the stray gassing behavior of some uninhibited oils, and it seems to be strongly related to oxidation. Ethane is considered the key-gas for thermal-oxidative degradation in vegetable oils.
Carbon Oxide Distribution in Closed-type Transformers Filled With Mineral Oil
CO concentration is different in a free-breathing transformer (likely to escape through atmosphere exchange over the conservator) compared with a transformer that is hermetically sealed or is equipped with a rubber bag. Closed-type transformers are often higher loaded; nevertheless, the insulating fluid is often in better condition than in a free-breathing transformer because it is not as affected by oxidation. The degradation of the combined fluid-solid insulation is accompanied by the formation of carbon oxides. The guide for interpretation of DGA IEC 60599 points out that the quotient CO2-to-CO < 3 is significant for solid degradation because of an electrical fault. Usually in cases of tracking affecting paper, further fault gases such as hydrogen, methane or acetylene are significantly increased.
Generally CO formation shows a temperature dependence and, in case of absence of a severe fault (no acetylene), is mainly derived from the insulating fluid. The CO2 content indicates degradation of the solid insulation. As a consequence, a growing trend ratio of CO2-to-CO quotient seems to be a better indicator of solid insulation degradation than carbon oxides alone. The usual used criteria of the ratio CO2-to-CO >3 and <10 should be further specified and, in case of closed-type transformers, revised. For closed-type systems or transformers working on a steady high, load ratios of 0.6 to 5 often have been observed with no fault indication (see table above).
Carbon Oxide Distribution in Free-breathing Transformers Filled With Mineral Oil
For free-breathing systems where excess oxygen is present causing accelerated oxidation, a value of 10 indicates accelerated solid insulation degradation. The DGA history of an aged population of transformers and the correlation of these values to degree of polymerization (DP) profiles show that a ratio CO2-to-CO > 10 is significant. Even after 35 years of service, the lower loaded network transformers show quotients of CO2-to-CO < 10 corresponding to DP values greater than 500. In generator transformers, the CO2-to-CO ratios are much higher than 10, corresponding to DP values between 150 and 400 (see Figure 5). A similar correlation also is seen the absolute values of CO2 in correlation to the DP values. In the investigated cases, a significant difference exists between the CO2 values for the network transformers (below 6,000 parts per million) and the generator transformers (above 10,000 parts per million) (see Figure 6).
Carbon oxides, both in closed-type and free-breathing equipment, can indicate the fluid’s and solid insulation’s thermal stress. Far before the hydrocarbon heating gases reach reliably interpretable values, the carbon oxides are clear indicators for incipient thermal oil aging.
Ivanka Hàƒ¶hlein-Atanasova is laboratory manager and Thomas Hammer is manager of technology and innovation with Siemens Energy Sector in Nàƒ¼rnberg, Germany.
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