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Condition Monitoring of Vegetable
Oil Insulation in In-Service Power
Transformers: Some Data Spanning
10 Years
Key words: transformer, insulation, oil insulation, condition monitoring
Introduction
Power transformers are expected to operate for several decades [1]. The insulating oil of a power transformer can be replaced if it becomes too degraded. However, it is obviously financially preferable to a utility that the insulating oil last as long
as practically possible.
Vegetable oils were among some of the earliest types of dielectric liquid, e.g., a team led by George Westinghouse used
caster and linseed oils from the late 1880s onward [2]. However,
a disadvantage was that the vegetable oils readily oxidized, and
so mineral oils were adopted. In the mid to late 1990s there was
renewed interest in vegetable oil–based dielectrics, to which
antioxidants had been added [3]. Subsequently, vegetable oil–
based transformer oils became commercially available. Initially,
they were used only in smaller distribution transformers. However, as the electrical industry became more confident, they began to be used in ever larger power transformers from the early
2000s.
A topic of high interest to utilities is the behavior of vegetable oil–based dielectrics in the field. Although laboratory-based
investigations have been performed over several years, there is
comparatively little data for operating transformers. Transformers that have used vegetable oil from the time of entering service
are still only around 10 years old, and the utilities are not always
willing to share their experiences.
In Australia, the first power transformer using vegetable oil
insulation (Envirotemp FR3), reported in this study, began operation in 2005. The substation was located in an environmentally sensitive area of New South Wales, and so a readily biodegradable and nontoxic transformer dielectric was chosen to
minimize the impact of a possible spill. Since then the utility has
purchased a further 15 power transformers using Envirotemp
FR3, and in 2006 a power transformer using Envirotemp FR3
was manufactured for a UK utility.
44
Daniel Martin and Tapan Saha
Power and Energy Systems, University of
Queensland, St. Lucia, QLD 4072, Australia
Lindsay McPherson
Primary Systems, Essential Energy, Port
Macquarie, NSW 2444, Australia
Data for 17 fault-free vegetable oil–
filled power transformers, in service
for up to 10 years, are presented and
analyzed. No significant changes in
the properties of the oil during service were found.
In this article we present condition monitoring data for these
17 transformers, aiming to add to current knowledge of the longterm use of vegetable oil dielectrics.
Long-Term Use of Vegetable Oil–Based
Dielectrics
A vegetable oil is composed of triglyceride molecules [4].
Over time, if oxygen reacts with the triglycerides, acids and water will be generated, which then degrade the cellulosic paper
insulation of the transformer [5], causing it to become brittle.
Once the tensile strength of the paper falls to a quarter of its initial value, the paper is considered to be at the end of its function-
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al life [6]. In addition, the viscosity of a vegetable oil exposed to
oxygen gradually increases, until a gel forms [7].
To limit the extent of oxidation, the manufacturers of vegetable oil–based dielectrics add antioxidants to their products,
e.g., 2,6-ditertiary-butyl-para-cresol, and specify that they must
be used only in sealed transformers.
Accelerated aging studies of vegetable oil–based dielectrics
have been performed in the laboratory environment to verify
their long-term stability. Their performance has been compared
to that of mineral oil [8]–[11]. In general, vegetable oils are not
as chemically stable as mineral oil. However, it has been recognized that accelerated aging tests are not reliable indicators
of in-service performance, because the conditions differ [11].
Importantly, even if a vegetable oil is not as chemically stable as
mineral oil, it may remain serviceable for as long as industry requires. Currently, very little data on in-service power transformers are available to compare with those from accelerated aging
studies. This article presents a limited amount of such data.
The Power Transformers
Data on the condition of the vegetable oil in the 17 transformers mentioned in the Introduction were obtained from two
utilities:
a) a utility located in New South Wales, Australia (16 transformers, numbered 1–16 in Table 1, filled with Envirotemp FR3 manufactured by Cargill) and
b) a utility located near London, UK (1 transformer, numbered 17 in Table 1, manufactured in the UK, also filled
with Envirotemp FR3).
Table 1. Specifications of the 17 transformers filled with Envirotemp FR3
Transformer
Rated
voltage
(kV)
Rated
power
(MVA)
Year of
manufacture
1
33/11
5/8
2008
KNAF
2
33/11
5/8
2007
KNAF
3
33/11
5/8
2007
KNAF
4
33/11
5/8
2009
KNAF
5
66/11
20
2014
KNAN
6
66/11
20
2014
KNAN
7
33/11
5/8
2007
KNAF
8
33/11
5/8
2007
KNAF
9
33/11
5/8
2006
KNAF
10
33/11
5/8
2010
KNAF
11
33/11
5/8
2010
KNAF
12
132/11
30
2013
KNAN
13
33/11
10/16
2005
KNAF
14
33/11
10/16
2007
KNAF
15
33/11
5/8
2008
KNAF
16
33/11
5/8
2011
KNAF
17
132/33
45/90
2006
KNAF
Cooling
type1
K indicates that the oil had a high fire point (>300°C); N indicates that it
flowed naturally around the transformer tank, i.e., was not pumped; and AN
and AF indicate, respectively, that air flowed naturally over the radiator bank
or was forced with fans.
1
These transformers had KNAF or KNAN cooling, where K
indicates that the oil had a high fire point (>300°C); N indicates
that it flowed naturally around the transformer tank, i.e., was not
pumped; and AN and AF indicate, respectively, that air flowed
naturally over the radiator bank or was forced with fans. KNAF
class transformers have two power ratings, the higher/lower rating for active/inactive fans.
The data presented in this article are concerned with the condition of the vegetable oil, up to 10 years into its useful life. At
such times the dielectric strength of the oil may fall to an unsafe
level, or the oil may become acidic and shorten the remaining
service lifetime of the transformer’s paper insulation.
The IEEE [12] and IEC [13] have published guides describing test methods for fresh vegetable oils to be used in electrical equipment. The present study involved measurements of dielectric dissipation factor (DDF), interfacial tension (IFT), acid
number, water content, and AC breakdown voltage. Dissolved
gas analysis was also carried out.
Measurements
DDF
Over time the oil in a transformer ages. Atmospheric oxygen enters the transformer tank and reacts chemically with the
oil. As the oil oxidizes its DDF rises, and water is created by
chemical reactions. The IEEE recommends limits for the DDF
March/April — Vol. 33, No. 2
of mineral oil in a standard [14], but limits have not yet been
recommended for in-service vegetable oils because of lack of
data [12]. Instead, it is recommended that a DDF value exceeding 0.03 (or 3%), at room temperature, should trigger “prompt
investigation” [12].
Figure 1 shows room temperature DDF values for oil samples
taken from 10 of the transformers filled with Envirotemp FR3,
and from two normally operating 33/11-kV, 10-MVA transformers M1 and M2 filled with mineral oil (not listed in Table 1). M1
and M2 were installed in the early 1980s. As can be seen, the
DDF values for the vegetable oil samples are much higher than
those for the two mineral-oil samples. This is to be expected
because of differences in molecular structure [15]. Specifically,
the oxygen atoms and carbon-carbon double bonds present in
vegetable oil increase its polarity. It should be noted that the
DDF values for the vegetable oil samples do not exceed the 3%
limit suggested in [12].
IFT
The IFT data are shown in Figure 2. The IFT of new mineral
oil in new equipment should be higher than 38 mN/m [16]. As
can be seen, the IFT values for the 20 vegetable oil samples are
45
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Figure 1. Room temperature dielectric dissipation factors
(DDF) for 15 oil samples taken from 10 Envirotemp FR3–filled
transformers, and for 4 mineral oil samples, 2 from each of
transformers M1 and M2 (indicated by red arrows). The numbers on the vegetable oil symbols correspond to the transformer
numbers in Table 1.
considerably lower than 38 mN/m; this difference is caused by
the different molecular structure of vegetable oil compared with
mineral oil. The IFT value for sample 17 is noticeably lower
than those for the other samples. This difference is probably due
to a measurement error, since the IFT value for sample 17 recorded in later measurements was much larger (the IFT values
for transformer oil samples are not expected to increase without
intervention).
Acid Number
The acid number of an oil is the mass of the alkali potassium
hydroxide (KOH) required to completely neutralize the acids in
1 g of oil. Some of the compounds in vegetable oil react with
water or oxygen to form acids, which may be benign or aggressive toward the paper insulation of a transformer, depending on
their molecular structure and the chemical reactions involved.
Oxidation reactions generate short-chain acids, which then degrade the paper insulation, whereas hydrolysis reactions produce
long-chain acids, which do not degrade the paper insulation [5].
Unfortunately, the acid number test used in industry [8] does not
Figure 2. Room temperature interfacial tension (IFT) values for
20 oil samples taken from 14 Envirotemp FR3–filled transformers.
46
differentiate between long-chain and short-chain acids present
in the oil. Vegetable oils contain higher concentrations of acid
than mineral oil because of the hydrolysis reactions mentioned
above; these reactions do not occur readily in mineral oil. Thus,
the acid number limits recommended for vegetable oils should
be higher than those recommended for mineral oil. The IEEE
Std C57.152-2013 [14] recommends an in-service maximum
acid number of 0.015 mg of KOH/g of oil for mineral oil and
0.06 mg of KOH/g of oil for natural ester (vegetable) oil. The
maximum acid number recommended in standard IEC 60422
[16] for mineral oil is 0.03 mg of KOH/g of oil; at present the
IEC does not recommend a maximum acid number for vegetable
oil.
The acid numbers for 22 vegetable oil samples taken from
16 Envirotemp FR3–filled transformers are given in Figure 3. It
will be seen that there is no consistent increase in acid number
with time, suggesting that any degradation of the oil was insignificant over a period of 10 years.
Degradation is caused by reactions with oxygen, which generate unwanted acids. Thus, a rise in acid number over time
could indicate that the sealing of the transformer has failed, allowing ingress of air.
Water Content of the Oil (WCO)
The mass of water within the tank of a transformer can increase due to ingress of moisture, or to chemical reactions between the insulation and oxygen. The WCO is defined as the
mass of water dissolved in the oil per unit mass of oil, and is
usually expressed in ppm. Excessive levels of water can cause
two problems:
1) The breakdown voltage of the oil falls. Limits for WCO
in mineral oil are set to ensure that the breakdown voltage
remains sufficiently high for continued reliable operation
of the transformer. The breakdown voltage is influenced
by the water concentration in the oil, and the number (and
size distribution) of particles floating in the oil [10]. However, the relationship between water concentration in the
oil and breakdown voltage is a function of the percentage
Figure 3. Acid numbers for 22 oil samples taken from 16 Envirotemp FR3–filled transformers. Some data points overlapped
sufficiently to be indistinguishable.
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water saturation of the oil, not the absolute WCO value
[10], [17], [18]. The percentage saturation at temperature
T [PS(T)] is defined as the WCO (expressed in ppm) divided by the corresponding WCO when the oil is fully
saturated with water, multiplied by 100. Thus,
PS(T ) =
WCO
10A−B /T
⋅ 100,
(1)
where A = 6, B = 881.39 K [19], and T is the absolute
temperature. Vegetable oil can dissolve much more water
than mineral oil at the same temperature, e.g., 1,000 ppm
for vegetable oil and 50 ppm for mineral oil at ambient
temperature [11].
2) The paper insulation may also become wet, defined in an
IEEE guide [20] as the condition when the water content
of the paper exceeds 2% by weight. Under such conditions
the transformer could fail during emergency loading, if the
temperature of the insulation became sufficiently high that
water bubbles were ejected from the insulation and moved
to points between the turns of a winding. Flashover might
then occur.
It has been reported [21] that the temperature at which bubbles form in Envirotemp FR3 is several °C higher than the corresponding temperature in mineral oil. However, the higher viscosity of Envirotemp FR3 would probably cause a transformer
to operate, at rated load, at a temperature slightly higher than
that at which it would operate if filled with mineral oil [22].
Utilities should bear this in mind in connection with emergency
loading.
WCO will change with the loading of a transformer because
the paper insulation adsorbs/desorbs water from the oil when
the oil cools/heats. Although the temperature of the oil is usually measured on sampling, the immediate loading history of
the transformer is usually not known. Since the time constant
for the migration of water in and out of the paper insulation is of
the order of days [23], the WCO of an oil sample may not have
equilibrated at the time of measurement.
Measured WCO values for 87 oil samples taken from 17
transformers are shown in Figure 4, and the corresponding oil
temperatures in Figure 5. Since vegetable oil can dissolve more
water than mineral oil at the same temperature, it is probably
more informative to express water content in terms of percentage saturation, calculated using (1). The relevant plot appears in
Figure 6, in which data for the two 10-MVA mineral oil–filled
transformers M1 and M2, analyzed in [24], are also presented.
The percentage saturation values for the two M2 samples are
very similar to those for the vegetable oil samples, but the two
M1 samples significantly exceed the two M2 samples.
Figure 4. Water content of oil values for 87 oil samples taken
from 17 Envirotemp FR3–filled transformers.
requirement of the transformer oil, it must be met regardless of
the type of oil. The recommended minimum breakdown voltage
for normal operation is dependent on the rated voltage of the
transformer, either 40 kV for a transformer rated below 72 kV, or
50 kV for a transformer rated between 72 and 170 kV.
The breakdown voltages of 21 Envirotemp FR3 samples,
taken from 16 transformers, and four mineral oil samples, two
from each of the transformers M1 and M2, are shown in Figure
7. Even after seven years in service there is little difference between the two oils.
Dissolved Gas Analysis and Fault Diagnosis
The concentrations of certain gases dissolved in the transformer oil are used to detect and diagnose thermal or electrical
faults within the transformer tank. Overheating and electrical
activity break down the oil, generating hydrogen (H2), ethane
(C2H6), ethylene (C2H4), methane (CH4), acetylene (C2H2), carbon monoxide (CO), and carbon dioxide (CO2). These concentrations and the concentration ratios between certain pairs of
gases are used to diagnose the type and severity of faults.
Diagnosis standards for faults in mineral oil–filled transformers using gas ratios have been published by IEEE [26] and
IEC [27]. Duval et al. [28]–[30] and Khan [31] independently
AC Breakdown Voltage
The breakdown voltages of the vegetable oil samples were
compared with the limits for mineral oil recommended in IEC
standard 60422 [16], which is based on the method given in
standard IEC 60156 [25]. Since a high breakdown voltage is a
March/April — Vol. 33, No. 2
Figure 5. Temperatures of the same oil samples as in Figure 4.
47
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Figure 6. Percentage water saturation values for 87 oil samples
taken from 17 Envirotemp FR3–filled transformers, and for four
mineral oil samples, two from each of the transformers M1 and
M2 (indicated by red arrows).
investigated the gas concentration ratios associated with thermal
and electrical faults in transformers filled with Envirotemp FR3.
Their key findings were that Envirotemp FR3
• generates the same gases as mineral oil in the presence of
faults, but with different concentration ratios;
• generates these gases (except ethane) in lower concentrations than mineral oil, so that more precise measurement
equipment is required for Envirotemp FR3; and
• generates ethane in larger concentrations than mineral oil.
Further work on the dissolved gas content of Envirotemp FR3
in a fault-free operating power transformer has been reported by
Martin et al. [32], [33]. The motivation of that work was that,
while Duval and Khan [28]–[31] had investigated the effect of
faults on gas generation, Martin et al. believed that in 2010 to
2011 there were insufficient data showing beyond reasonable
doubt that a power transformer was in a fault-free condition.
Low concentrations of gases can accumulate in transformer in-
Figure 7. Breakdown voltage values for 21 vegetable oil samples, taken from 16 Envirotemp FR3–filled transformers, and
four mineral oil samples, two from each of the transformers M1
and M2 (indicated by red arrows).
48
Figure 8. Measured concentrations (in ppm) of ethylene in 92
samples of vegetable oil taken from 17 Envirotemp FR3–filled
transformers.
sulating oils during normal operation, complicating diagnosis
of the transformer condition. Thus, concentrations of several
tens of ppm of ethylene and methane were found in mineral oil
undergoing laboratory investigations at temperatures typical of
normal transformer operation [34]. Gases can also be generated
by chemical reactions that are not due to faults, e.g., chemical
reactions between core steel and oil have been known to generate hydrogen [27].
Since the Duval triangle method uses methane, ethylene, and
acetylene for diagnosis of faults in Envirotemp FR3, the concentrations of these three gases (and also of ethane, oxygen, and
hydrogen) in the oil of the transformers involved in the present
study were measured. In every oil sample the acetylene concentration was below the detection limit of the laboratory. This
result is significant because accumulation of acetylene can indicate serious arcing within the transformer tank [27]. A large majority of the measured concentrations of ethylene and methane,
shown in Figures 8 and 9 respectively, are of order several ppm,
much lower than would be expected if a thermal or electrical
fault were present. According to [27], typical gas concentration
values for operating mineral oil–filled transformers are 60 to 280
ppm for ethylene and 30 to 130 ppm for methane. Thus, it is very
unlikely that faults existed in any of the 17 transformers.
Figure 9. Measured concentrations (in ppm) of methane in 92
samples of vegetable oil taken from 17 Envirotemp FR3–filled
transformers.
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Figure 10. Measured concentrations (in ppm) of ethane in 92
samples of vegetable oil taken from 17 Envirotemp FR3–filled
transformers.
Figure 12. Measured concentrations (in ppm) of hydrogen in 92
samples of vegetable oil taken from 17 Envirotemp FR3–filled
transformers.
Some of the chemical constituents of vegetable oil, in particular linolenic acid, are known to react with oxygen, thereby
generating ethane [35]. It follows that an unexpected increase in
the concentration of ethane in the oil could be an indication that
oxygen is leaking into the transformer tank.
The measured concentrations of ethane, shown in Figure 10,
are similar to those measured in a 132/11/11-kV 50-MVA power
transformer, also filled with Envirotemp FR3 [32], [33]. The ethane levels for a given transformer do not increase consistently
with time in service, and therefore no leaks are suspected.
A large fraction of the 92 oxygen concentrations shown in
Figure 11 lie within the range 0 to 6,000 ppm by volume; only
eight exceed 10,000 ppm by volume. The latter are probably due
to the samples being contaminated by air, because subsequent
measurements yielded much lower concentrations.
The hydrogen concentrations in the Envirotemp FR3 oil samples are shown in Figure 12; the corresponding concentrations
in mineral oil [27] are of order 50 to 150 ppm by volume. The
concentrations in the oil samples from transformers 3 and 16 are
very high in comparison with those reported in previous studies
[32], [33]. It is stated in IEC guide 60599 [27] that
a) hydrogen can be generated by chemical reactions involving steel, uncoated surfaces or protective paints, or by the
decomposition of thin oil films between overheated core
laminates at temperatures of 140°C and above and
b) hydrogen has been detected in transformers that have never been energized.
In connection with (a), if core laminates are heated to 140°C and
above, it would be expected that gases indicative of a thermal
fault, e.g., methane, would also be found.
Duval developed two extra triangles to distinguish between
stray gassing and a fault [29]. These triangles compare the ratio of the concentrations of hydrogen and ethane, generated by
stray gassing, with the ratio of the concentrations of methane
and ethylene, generated by a thermal fault. For transformers 3
and 16 the levels of hydrogen and ethane are much higher than
those of methane and ethylene. Consequently, the diagnosis is
that the high hydrogen concentrations shown in Figure 12 do not
indicate a fault, and should therefore be disregarded.
Conclusions
Figure 11. Measured concentrations (in ppm) of oxygen in 92
samples of vegetable oil taken from 17 Envirotemp FR3–filled
transformers.
March/April — Vol. 33, No. 2
The test methods used to confirm normal operation of a mineral oil–filled transformer can also be used with vegetable oils.
However, the recommended gas concentration limits will be different. Although the performance of vegetable oils has been investigated under laboratory conditions, many more in-the-field
data are needed to establish gas concentration limits that can be
considered as representative of normal transformer operation. In
this article we have provided some such data.
Although vegetable oils are known to have an inherently lower oxidation stability than mineral oil, we believe that our data
strongly suggest that, as long as the tank is sealed, Envirotemp
FR3 oil is effective over the long term, say at least 10 years, as
transformer insulation. The three measures that indicate degradation of oil, i.e., acidity, DDF, and IFT, do not show a consis-
49
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tent change with time in service. Furthermore, the breakdown
voltage of the oil remains higher than the 40- or 50-kV limit
recommended in [16], indicating that the oil is still behaving
satisfactorily as insulation.
[22]
[23]
Acknowledgments
The authors wish to thank Paul Dyer, UKPN, for providing
measurement data for analysis, and for discussion of the experiences of a UK utility when using Envirotemp FR3–filled transformers.
[24]
[25]
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D. Martin, N. Lelekakis, J. Wijaya, M. Duval, and T. Saha, “Investigations into the stray gassing of oils in the fault diagnosis of transformers,”
IEEE Trans. Power Del., vol. 29, no. 5, pp. 2369–2374, 2014.
K. M. Schaich, “Lipid oxidation: Theoretical aspects,” in Bailey’s Industrial Oil and Fat Products, 6th ed. New York, NY: John Wiley and Sons
Inc., 2005.
Daniel Martin received the BEng degree
in electrical and electronic engineering
(with study abroad in Germany) from the
University of Brighton, UK, in 2000. He
joined Racal Electronics, which became
the international electronics company
Thales, working on communication and
aircraft systems. He left Thales in 2004 to
pursue his PhD degree in electrical insulation at the University of Manchester, UK. He investigated the
suitability of using vegetable oils and synthetic esters as substitutes for mineral oil within large power transformers, and graduated in 2008. He joined Monash University in Australia working on transformer condition monitoring, and quickly assumed
the directorship of the Centre for Power Transformer Monitoring. At the beginning of 2013 he moved to the University of
Queensland, Australia, continuing his work on developing innovative tools for transformer condition monitoring, funded by
the local utilities.
IEEE Electrical Insulation Magazine
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Tapan K. Saha (SM ’97) was born in
Bangladesh and immigrated to Australia
in 1989. Currently, he is a professor of
electrical engineering in the School of
Information Technology and Electrical
Engineering, University of Queensland,
Brisbane, Australia. Before joining the
University of Queensland, he taught at the
Bangladesh University of Engineering and
Technology, Dhaka, for three and a half years and then at
James Cook University, Townsville, Australia, for two and a
half years. His research interests include power systems, power
quality, and condition monitoring of electrical plants. Saha is a
Fellow of the Institution of Engineers, Australia.
Lindsay McPherson commenced work as
an electrical apprentice in 1973. He gained
experience and skills with six electrical
utilities in regional NSW, covering a broad
range of activities including electrical
design, construction, operation, maintenance, and management of distribution
and sub-transmission networks. During
March/April — Vol. 33, No. 2
the past 25 years his particular interest has been in the field of
primary high voltage substation assets. Currently, as part of a
diverse engineering team, he is responsible for the management
of Essential Energy’s primary Sub-Transmission and Zone
Substation Assets, which range up to 132 kV and are situated in
400 locations across regional NSW. As engineering manager of
Zone Substations, Essential Energy, he is part of the substation
Asset Management Team located in Port Macquarie and Wagga
Wagga, NSW. In this role he is responsible for new power
transformer procurement. In certain Essential Energy installations, transformers operating with vegetable-based oil have
been used to reduce fire and environmental risks. Since the use
of vegetable oils in Essential Energy’s network began in 2006,
he has developed a keen interest in understanding their behavior and aging characteristics, and associated transformer performance over time.
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