In lowest temperature fault conditions the hydrogen generation is possible, of course it is with other hydrocarbon radicals; because the hydrocarbon molecule breakdown without new produced material containing carbon atom in related temperature is impossible.
The identification and significance of gases in electrical equipment was first used to distinguish between combustible and non-combustible gases produced in transformers as long ago as the 1920s. This was carried out by applying a light to the gas collected from the sample or vent tap of the Buchholz relay.
Transformer Oils consisting of high molecular weight hydrocarbon molecules can suffer degradation due to decomposition of these molecules into lighter more volatile fractions. This process is also accelerated by temperature. It is desirable that it should not occur at all within the normal operating temperatures reached by the plant, but it cannot be prevented at the higher temperatures generated by fault conditions.
Initially the procedure aimed to detect the presence of hydrogen, which meant that there was a ‘real’ fault within the transformer. Over the next 30 years the procedure was refined to enable hydrogen, acetylene and carbon monoxide to be detected, which enabled some indication of the nature of the fault to be deduced. In particular, the presence of acetylene meant that very high temperatures existed, and carbon monoxide was taken as an indication that solid insulation was involved.
The immediate effect of the breakdown of the hydrocarbon molecules as a result of the energy of the fault is to create free radicals. These subsequently recombine to produce the low molecular weight hydrocarbon gases. It is this recombination process which is largely determined by the temperature, but also influenced by other conditions. For the lowest temperature faults both methane and hydrogen will be generated, with the methane being predominant.
As the temperature of the fault increases ethane starts to be evolved, methane is reduced, so that the ethane/methane ratio becomes predominant. At still higher temperatures the rate of ethane evolution is reduced and ethylene production commences and soon outweighs the proportion of ethane. Finally, at very high temperatures acetylene puts in an appearance and as the temperature increases still further it becomes the most predominant gas.
The area indicated as including normal operating temperatures goes up to about 140°C, hot spots extend to around 250°C, and high-temperature thermal faults to about 1000°C. Peak ethylene evolution occurs at about 700°C. Also the organic polymer or aromatic polyamide material can be made into a range of papers and boards in a similar way to cellulose fibers but which remain stable at operating temperatures of up to 220°C.
However there are some hydrogen combination material which may be origin of free H2 in transformer (KOH).BS 148:1923 included an oxidation test with a limit to the amount of sludge produced. However, new oil was allowed an acidity equivalent to 2.0 mgKOH/g, a figure which is four times higher than the level at which oil would now be discarded.
In the UK the organisation EA Technology’s Dr M.K. Domun has studied and collated oil analysis data from around 500 transformers, mainly of 132 kV, for many years and as a result of this work has published figures in a paper presented to an IEE Conference on Dielectric Materials, Measurements and Applications in September 1992 as ‘optimal values’ for transformers which have been on load for a lengthy period and which are considered to be in a ‘healthy’ condition. These are listed in Table below.
Author : Hamid - From: Iran- Firouzabad in Fars
