Various research organizations, such as Westinghouse Electric Corporation, Analytical Associates, Inc., that did extensive research in the 1970s quickly led to the widespread use of dissolved gas-in-oil analysis as a predictive maintenance tool . There is also an extensive bibliography on this subject found in IEEE Std. C57.104–1991 .
The basic theory is straightforward: Transformer dielectric fluids are refined from petroleum and are very complex mixtures containing aromatic, naphthenic, and paraffinic hydrocarbons. At high temperatures, some of these molecules break down into hydrogen plus small hydrocarbon molecules such as, methane, ethane, ethylene, acetylene, propane, and propylene. This process is known as cracking.
The kraft paper materials that are used to insulate transformer windings are made up of cellulose. At high temperatures, cellulose oxidizes to form carbon dioxide (CO2), carbon monoxide (CO) and water (H2O). High concentrations of CO2 and or CO are indications of overheated windings.
All of the breakdown products are gases that dissolve readily in transformer oil in different concentrations, depending on the specific gas and the temperatures that produce them. By taking samples of transformer insulating oil, extracting the dissolved gases and doing a quantitative analysis of the various gases in the samples through gas chromatography, it is possible to infer the temperatures at the sites where these gases were produced.
At temperatures below 150°C, transformer oil starts breaking down into methane (CH4) and ethane (C2H6). At temperatures above 150°C, ethylene (C2H4) begins to be produced in large quantities while the concentration of ethane decreases.
At around 600°C, the ethylene production peaks while the concentration of methane continues to increase. Acetylene (C2H2) production starts at around 600°C and methane concentration peaks at 1000°C. Hydrogen (H2) production is not significant below 700°C and continues to increase along with acetylene at temperatures above 1400°C.
Therefore, the relative concentrations of the key gases change over a wide range of temperature. This is basis for the application of dissolved gas in-oil analysis for predictive and diagnostic use. An approximate formula uses the ratio of C2H4/C2H6 to derive the temperature of oil decomposition between 300°C and 800°C:
T(°C) = 100 C2H4/C2H6 + 150
The so-called Rogers ratio method takes the ratios of several key gases into account to develop a code that is supposed to give an indication of what is causing the evolution of gas. The codes for the four ratio method are given in Table 8.2. A fairly detailed diagnosis of transformer trouble can be derived from various combinations of codes, shown in Table 8.3.
The diagnoses shown above were derived from empirical observation. The problem with the four-ratio Rogers code is that a code generated from the gas concentrations will often not match any of the ‘‘known’’ diagnoses.
So like a rare disease with strange symptoms, many cases of transformer trouble cannot be diagnosed at all using this method. Another method, called the three-ratio method, sometimes works when the four-ratio method does not.
In the three-ratio method, the values of A, B, and C are given in Table 8.4 with the corresponding diagnoses for the various combinations given in Table 8.5. Not only are the ratios of the key gases important, but the total quantity of dissolved gas and the rate of increase are also important factors in making a diagnosis. One of the criteria for making a judgment call is the total combustible gas concentration. The combustible gases include H2, CH4,
C2H4, C2H6, C2H2, which are produced by oil decomposition, and CO, which is produced by cellulose decomposition. Each utility has a different philosophy and a different threshold for concern.
Table 8.6 gives one set of guidelines based on good utility practice that is useful for determining the overall health of a power transformer based on the total concentration of combustible gases.
It is generally accepted that if the rate of combustible gas generation exceeds 100 ppm per day on a continuing basis, or if the presence of C2H2 exceeds 20 ppm, then consideration should be given to taking the transformer out of service to perform additional tests and inspection.
IEEE Std. C57.104-1991 Table 3 also provides a set of actions based on the total dissolved combustible gas (TDCG) concentrations as well as the daily rate of TDCG production.
According to the IEEE Guide, a rate of 30 ppm per day is the threshold for considering removing the transformer from service. Oil samples are taken from the bottom drain valve in a sealed syringe to prevent the dissolved gases from escaping.
The samples are sent to a chemical laboratory where the dissolved gases are extracted from the sample under vacuum and analyzed using a gas chromatograph. The results are reported as ppm dissolved in oil.