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
[4]. There is also an extensive bibliography on this subject found in
IEEE Std. C57.104–1991 [5].
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.
