Gassing Characteristics of Transformer Oil Under Thermal ... - Neta

The Official Publication of the InterNational Electrical Testing Association Fall 2005
Chemist’s Perspective
Gassing Characteristics of Transformer
Oil Under Thermal Stress
In June 2005, ASTM International adopted a new test method for test-
ing transformer oil. The aim of the test is to determine the gassing
pattern of an oil subjected to thermal stress under what is considered
to be low temperatures, i.e. 120°C. The method is entitled “ASTM D 7150,
Standard Test Method for the Determination of Gassing Characteristics
of Insulating Liquids Under Thermal Stress at Low Temperature.”
Interest in the development of this test resulted from research that
Doble performed in the early 1990s and more recently the research that
the CIGRE Task Force TF 15/12-01-11 (TF11) performed in trying to
discern the differences in gassing behavior of some transformer oils at
low temperatures. CIGRE has termed the unusual gassing behavior as
“stray gassing.”
The CIGRE research was mainly looking at the gassing behavior of new
oils in which transformers were exhibiting increasing hydrogen levels, yet
no apparent cause for the abnormal gassing could be determined. As a
result, the research centered on the oil and found that some oils produced
more gas (in this case hydrogen) than others when they were aged under
thermal conditions in both sealed transformers (gas blanketed, sealed or
bladder/diaphragm transformers) and free breathing transformers (open
conservators). Doble had also observed a similar pattern and as a result
conducted a massive research study into the phenomenon that has lasted
over three years and still goes on today. What was also determined was
the fact that not only could the refining process be partially responsible
for the abnormal production of dissolved gases, but different types of
contamination in the oil, incompatible materials in the transformer, and
the addition of additives such as metal passivators could also cause abnormal
gassing to occur.
by Lance R. Lewand
Doble Engineering
Company
NETA WORLD Fall 2005 1
and
Paul Griffin
Doble Engineering
Company
The reason that this information
is so important is that it can significantly
affect the results of the dissolved
gas-in-oil (DGA) test. DGA
is one of the most widely used
diagnostic tools for assessing the
condition of electrical transformers
and in more recent years load
tap-changers and bulk oil circuit

eakers. The test is very sensitive
to a wide range of problems and is
used to characterize incipient fault
conditions. Categories of abnormal
conditions such as overheating,
partial discharge, and arcing can
be further identified by the insulating
materials involved (paper or
oil) or by the energy (high, moderate,
or low temperatures). DGA is
ideally suited for condition-based
maintenance programs as it allows
early detection of problems and
can then be used to follow most
deterioration processes as they
evolve. Then, the decision can be
made as to when to perform more
definitive tests to identify the
specific problem or take remedial
action. Indeed, DGA is sensitive
enough that it can be used to detect
problems during factory testing of
new transformers helping manufacturers
avoid shipping defective
units to the field.
It is imperative that DGA results
are interpreted accurately to ensure
the correct action is taken concerning
any incipient faults within the
transformer. It is well known that
oils can age differently, particularly
where there is ample oxygen such
as may be found in older units, or
free-breathing transformers. A 1991
study revealed that oils exposed to
relatively low temperatures could
have different gassing behaviors
and oxygen consumption rates
under the same controlled conditions.
Gassing rates were different
for two oils tested if the oxygen
content was fairly low at 3000 ppm
or high at 30,000 ppm and this is
one of the reasons why the new
ASTM D 7150 test is performed
using air and nitrogen sparged
samples. It was also shown that
passing the dissolved gas-in-oil
test for a factory heat run could
depend on the oil chosen.
Early Studies on Gassing Behavior
In 1994, the Doble Oil Committee performed a study on the low
temperature gassing behavior of an oil manufactured in France that
was being used for the first time by a French transformer manufacturer.
The transformer did not pass the dissolved gas-in-oil limits used by the
purchaser. The manufacturer performed some tests which showed that
the oil used (Shell Diala F) gassed much more than the oils they used
historically. To evaluate the gassing behavior of the Shell Diala F oil in
comparison to other products, air-saturated samples and some samples
that had been degassed (all gases removed except for small amounts of
oxygen and nitrogen) were aged in ground-glass matched barrel and
plunger syringes at 95ºC for 168 hours. The following oils were tested
in the study:
• Shell Diala F, uninhibited, manufactured in France
• Esso Univolt 52, uninhibited, manufactured in France
• Exxon Univolt N61, inhibited, manufactured in US
• Shell Diala A, uninhibited, manufactured in US
The results are provided in Table 1 (air saturated) and Table 2 after the
oil had been vacuum processed.
TABLE 1
Gassing Characteristics of New Oils Aged at 95ºC, ppm vol./vol.
Oil Hydrogen Methane Ethane Ethylene Acetylene CO CO 2 TCG
Diala F 378 106 63 2 0 215 908 764
Univolt 52 122 101 58 10 0 113 1414 404
Diala A 81 34 20 1 0 65 608 201
Univolt N61 35 0 0 0 0 46 489 81
TABLE 2
Gassing Characteristics of Vacuum-Processed
New Oils Aged at 95ºC, ppm vol./vol.
Oil Hydrogen Methane Ethane Ethylene Acetylene CO CO 2 TCG
Diala F 165 133 99 2 0 116 370 515
Univolt 52 0 4 2 0 0 0 42 6
The results listed in Tables 1 and 2 show clearly that the Shell Diala F
product produced larger amounts of combustible gases than the other oils
tested. Even when oxygen is removed or reduced in the test samples, the
overall concentration of gases is reduced but the same oil still produces
the largest quantity of dissolved gases. Table 2 reveals that degassing the
two oils to reduce the oxygen content had a much greater effect on the
Univolt 52 oil than the Diala F product. This type of experiment shows
that the gassing characteristics of new oils under thermal stress can be
drastically different. In the case of a transformer heat run, one oil would
cause the transformer to fail the test while the other oil would not be of
concern.
Recent Studies on Gassing Behavior
Doble, with the inspiration of the CIGRE work and its own previous
work, undertook a study to evaluate the gassing characteristics of oils
commercially available in 2001 and 2002 and in many cases still sold
today. The study was to look at a wide range of oils to determine the
amount of variation.
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Testing was performed under the following conditions:
• Air purged, 16 hours at 120°C
• Air purged, 164 hours at 120°C
• Nitrogen Purged, 16 hours at 120°C
• Nitrogen Purged, 164 hours at 120°C
The aging was performed for differing times to assess
initial gassing rate and when a plateau or equilibrium
rate was reached. The samples were either air
or nitrogen purged to have a range of oxygen contents
representative of service conditions. The aging times
were as follows:
• 16 hours – provides indication of initial generation
of gases and would be similar to the time for a factory
heat run test.
• 164 hours – indicates if the gases reach a plateau or
a constant rate of generation
The oils were assigned a sequential number based
on the total amount of combustible gases formed during
the experiment with air saturated oils aged for 16
hours. For example, number 1 had the least amount
of total combustible gas (TCG) and number 30 the
greatest amount of TCG. In some cases there was more
than one product from a refiner. Only the results of the
164-hour testing are presented here as this is the time
that was decided upon for the ASTM method.
Air Saturated Samples Aged
for 164 Hours at 120°C
For the samples saturated with air and aged for
164 hours, the data shown in Figure 1 reveals that
there is a large difference in the total combustible gas
(TCG) content between some of the oils and that very
significant amounts of combustible gases are formed
in some cases. There were four gassing patterns that
developed:
• Mostly carbon monoxide
• Hydrogen and carbon monoxide
• Mostly hydrogen
• Mixture of hydrogen, carbon monoxide, methane
and ethane
The predominant gas formed was hydrogen. However,
other gases such as methane, ethane, ethylene,
carbon monoxide, and carbon dioxide were also
formed. In general, if the oxygen content remained
high, the first three gassing patterns occurred (as listed
above). If the oxygen was depleted, the methane and
ethane were generated in greater concentrations. Ethane
was not generated in significant amounts without
methane. The oxygen consumption rate was quite different
for the various products.
Figure 1 — Total Combustible Gas
of Air Saturated Samples Aged for 164 Hours
Nitrogen Purged Samples Aged 164 Hours
at 120°C
Samples purged with nitrogen and then aged 164
hours did not contain as much combustible gas as
those saturated with air before aging for the same
amount of time; however, there were five oils that
still exhibited high concentrations of hydrogen and
total combustible gases. In these cases the hydrogen
made up a large percentage of the composition of the
TCG.
What is interesting in nitrogen-sparged samples is
that the methane and ethane values are consistently
low but make up a greater percentage of the TCG for
more samples than was the case when there was ample
oxygen at the start of aging. This is due to the fact that
the hydrogen produced in nitrogen-sparged samples
is not as high in concentration. Similarly, the carbon
oxide gases are consistently lower for the nitrogenpurged
samples than when saturated with air.
Figure 2 — TCG and Hydrogen Content
of Nitrogen Saturated Samples Aged for 164 Hours
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Conclusions
• There is a new ASTM Method D 7150 that provides
the protocol for determining the gassing characteristics
of oils under air and nitrogen (sealed) conditions
and at low temperature (120°C).
• It is not meant to be a routine test method in the
sense that DGA currently is. However, it can be applied
in those situations where there is a question of
whether the oil is involved in producing the gassing
that is being observed.
• DGA is a very important test, but care needs to be
taken in the interpretation of results to understand
the low temperature gassing behavior of the oil
when certain types of problems are detected. As
hydrogen is often the predominant gas formed for
the oils with the highest combustible gas generation
rates, it is possible that low temperature thermal
problems could be confused with low energy
partial discharge activity. Stray gassing may also
be mistaken for excessive gassing at modest temperatures.
• Some of the “breaking in” characteristics that are observed
for the gassing behavior of new transformers
or newly processed transformers may be attributed
to the oil and will stabilize over time.
• Stray gassing can also be due to the presence of
contamination in the oil, the presence of incompatible
materials, and the presence of additives.
• It is clearly important to know the properties and
characteristics of the oil in individual transformers,
especially when performing DGA. Similarly, it is
important to make sure the gassing characteristics
of oils used to ‘top off’ transformers are also well
understood.
References
1. Griffin, P.J., Lewand, L. R., Heywood, R., and Lapworth,
J. “Gassing Characteristics of Transformer
Oils at Modest Temperatures, Part 1: Transformer
Experiences”, Proceedings of the Seventy-First Annual
Conference of Doble Clients, 2004.
2. Griffin, P.J., Lewand, L. R., Heywood, R., and Lapworth,
J. “Gassing Characteristics of Transformer
Oils at Modest Temperatures, Part 2: Laboratory
Experiments”, Proceedings of the Seventy-First
Annual Conference of Doble Clients, 2004.
3. Schmidt, J., Eitner, R., and Hartwig, R., “Gassing Behavior
of Transformer Oils at Temperatures Above
60ºC”, 7 th International Symposium on High Voltage
Engineering, Aug. 26-30, 1991, pg. 33-36.
4. Griffn, P. J., “Gassing Characteristics of New Oils
Used in Factory Heat Runs”, Doble Oil Committee
Minutes, 1994, pg. 18-21.
Lance Lewand received his Bachelor of Science degree from
St. Mary’s College of Maryland in 1980. He has been employed
by the Doble Engineering Company since 1992 and is currently
the Laboratory Manager for the Doble Materials Laboratory and
Product Manager for the DOMINO®. product line. Prior to his
present position at Doble, he was Manager of the Transformer
Fluid Test Laboratory and PCB and Oil Services at MET Electrical
Testing in Baltimore, MD. Mr. Lewand is a member of ASTM
Committee D 27.
Paul J. Griffin received his BS degree at the American International
College and his MS at the University of Rhode Island. He has
been employed by the Doble Engineering Company since 1978 and
is currently Vice President of Laboratory Services. He is secretary
of the Doble Oil Committee, a member of ASTM committee D 27,
US Technical Advisor to IEC TC10 for Fluids for Electrotechanical
Applications, member of the IEEE Insulating Fluid subcommittee
of the Transformer committee, and a member of the CIGRE Working
Group 15.01 Fluid Impregnated Insulating Systems.
Article available for reprint with permission from NETA. Please inquire at neta@netaworld.org.
4 NETA WORLD Fall 2005