IEC 60599 Guide to the interpretation of dissolved and free gases analysis
IEC 60599 Mineral oil-impregnated electrical equipment in service - Guide to the interpretation of dissolved and free gases analysis
4 Mechanisms of gas formation
4.1 Decomposition of oil
Mineral insulating oils are made of a blend of different hydrocarbon molecules containing CH3, CH2 and CH chemical groups linked together by carbon-carbon molecular bonds. Scission of some of the C-H and C-C bonds may occur as a result of electrical and thermal faults, with the formation of small unstable fragments, in radical or ionic form, such as H, CH3, CH2, CH or C (among many other more complex forms), which recombine rapidly, through complex reactions, into gas molecules such as hydrogen (H-H), methane (CH3-H), ethane (CH3-CH3), ethylene (CH2=CH2) or acetylene (CH≡CH). C3 and C4 hydrocarbon gases, as well as solid particles of carbon and hydrocarbon polymers (X-wax), are other possible recombination products. The gases formed dissolve in oil, or accumulate as free gases if produced rapidly in large quantities, and may be analyzed by DGA according to IEC 60567.
Low-energy faults, such as partial discharges of the cold plasma type (corona discharges), favour the scission of the weakest C-H bonds (338 kJ/mole) through ionization reactions and the accumulation of hydrogen as the main recombination gas. More and more energy and/or higher temperatures are needed for the scission of the C-C bonds and their recombination into gases with a C-C single bond (607 kJ/mole), C=C double bond (720 kJ/mole) or C≡C triple bond (960 kJ/mole), following processes bearing some similarities with those observed in the petroleum oil-cracking industry.
Ethylene is thus favoured over ethane and methane above temperatures of approximately 500 °C (although still present in lower quantities below). Acetylene requires temperatures of at least 800 °C to 1200 °C and a rapid quenching to lower temperatures, in order to accumulate as a stable recombination product. Acetylene is thus formed in significant quantities mainly in arcs, where the conductive ionized channel is at several thousands of degrees Celsius, and the interface with the surrounding liquid oil necessarily below 400 °C (above which oil vaporizes completely), with a layer of oil vapour/decomposition gases in between. Acetylene may still be formed at lower temperatures (< 800 °C), but in very minor quantities. Carbon particles form at 500 °C to 800 °C and are indeed observed after arcing in oil or around very hot spots.
Oil may oxidize with the formation of small quantities of CO and CO2, which can accumulate over long periods of time into more substantial amounts.
4.2 Decomposition of cellulosic insulation
The polymeric chains of solid cellulosic insulation (paper, pressboard, wood blocks) contain a large number of anhydroglucose rings, and weak C-O molecular bonds and glycosidic bonds which are thermally less stable than the hydrocarbon bonds in oil, and which decompose at lower temperatures. Significant rates of polymer chain scission occur at temperatures higher than 105 °C, with complete decomposition and carbonization above 300 °C. Mostly carbon monoxide and dioxide, as well as water, are formed, in much larger quantities than by oxidation of oil at the same temperature, together with minor amounts of hydrocarbon gases and furanic compounds. The latter can be analyzed according to IEC 61198, and used to complement DGA interpretation and confirm whether or not cellulosic insulation is involved in a fault. CO and CO2 formation increases not only with temperature but also with the oxygen content of oil and the moisture content of paper.
4.3 Other sources of gas
Gases may be generated in some cases not as a result of faults in the equipment but through rusting or other chemical reactions involving steel, uncoated surfaces or protective paints.
Hydrogen may be produced by reaction of steel with water, as long as oxygen is available from the oil nearby. Large quantities of hydrogen have thus been reported in some transformers that had never been energized. Hydrogen may also be formed by reaction of free water with special coatings on metal surfaces, or by catalytic reaction of some types of stainless steel with oil, in particular oil containing dissolved oxygen at elevated temperatures. Hydrogen may also be formed in new stainless steel, absorbed during its manufacturing process, or produced by welding, and released slowly into the oil.
Hydrogen may also be formed by the decomposition of the thin oil film between overheated core laminates at temperatures of 140 °C and above (see [1] * of annex C).
Gases may also be produced by exposure of oil to sunlight or may be formed during repair of the equipment.
Internal transformer paints, such as alkyd resins and modified polyurethanes containing fatty acids in their formulation, may also form gases.
These occurrences, however, are very unusual, and can be detected by performing DGA analyses on new equipment which has never been energized, and by material compatibility tests. The presence of hydrogen with the total absence of other hydrocarbon gases, for example, may be an indication of such a problem.
NOTE The case of gases formed at a previous fault and remnant in the transformer is dealt with in 5.3.
