1626 R. Liao et al.: A Comparative Study of Physicochemical, Dielectric and Thermal Properties of Pressboard Insulation
1070-9878/11/$25.00 © 2011 IEEE
A Comparative Study of Physicochemical, Dielectric
and Thermal Properties of Pressboard Insulation
Impregnated with Natural Ester and Mineral Oil
Ruijin Liao
1
, Jian Hao
1, 2
, George Chen
2
, Zhiqin Ma
1
and Lijun Yang
1
1
State Key Laboratory of Power Transmission Equipment & System Security and New Technology
Chongqing University, Shapingba District, Chongqing 400044, China
2
School of Electronics and Computer Science
University of Southampton, Southampton SO17 1BJ, UK
ABSTRACT
Natural ester is considered to be a substitute of mineral oil in the future. To apply natural
ester in large transformers safely, natural ester impregnated solid insulation should be
proved to have comparable dielectric strength and thermal stability to mineral oil
impregnated solid insulation. This paper mainly focuses on a comparative study of
physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP
natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/pressboard
insulation after long term thermal ageing. The physicochemical and dielectric parameters
including moisture, acids and the ac breakdown strength of these two oil/pressboard
insulation systems at different ageing status were compared. The permittivity and ac
breakdown strength of these two oil/pressboard insulation systems at different
temperatures were also investigated. And a comparative result of the thermal stability
behavior of these two oil/pressboard insulation systems with different ageing status was
provided at last. Results show that though natural ester has higher absolute humidity and
acidity during the long ageing period, the lower relative humidity of natural ester helps to
keep its ac breakdown strength higher than mineral oil. The pressboard aged in natural
ester also has higher ac breakdown strength than that aged in mineral oil. The lower
relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial
to its dielectric strength. Using natural ester in transformer, the resistance to thermal
decomposition of the oil/pressboard insulation system could be also effectively improved.
Index Terms — Natural ester, mineral oil, pressboard, physicochemical, dielectric
strength, thermal stability.
1 INTRODUCTION
THE transformer plays an important role in providing a reliable
and efficient electricity supply and is one of the most critical
equipments in electric power transmission and distribution systems.
The majority of high voltage transformers are filled with liquids
that work as an electrical insulation as well as a heat transfer
medium. The most commonly used liquid in power transformers is
mineral oil due to its low cost and good properties. However the
performance of mineral oil starts to be limited due to environmental
consideration [1-3]. Firstly, conventional transformer oils are
usually non-biodegradable; they can contaminate soil and water
when a serious spill takes place [4, 5]. This may disturb the
plantation and other organisms. Secondly, the mineral oils were
extracted from petroleum, which is going to run out in the future
since petroleum is nonrenewable [5, 6].
Natural ester insulating fluid offers fire safety, environment,
and insulation ageing advantages over mineral oil and are
found to be suitable for the use in transformer insulation
system [7, 8]. Previous sealed tube ageing studies show that
the thermal ageing rate of virgin paper insulation in natural
ester is significantly slower than that in mineral oil [9-11]. At
present, two typical commercial products of natural ester are
BIOTEMP made by ABB in 1999 and FR3 developed by
Cooper in 2000. These two natural esters have currently been
used in small power and distribution transformers across the
United States, and further improvements are ongoing in the
hope that they will be widely applied in large power
transformers [3]. Moore [12] presented the requirements and
expectations of natural ester fluids for application in power
transformers. It has become clear that more research is
required to ascertain the long term safe operation of
transformers where natural ester is used.
Manuscript received on 18 June 2010, in final form 18 April 2011.
IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18, No. 5; October 2011 1627
Oil impregnated pressboard is widely used between
transformer windings as oil barriers for breaking up large oil
gaps and acting as mechanical support. It normally takes the
electrical stress under ac operating voltage and undergoes
degradation under a combined stress of thermal (the most
important factor), electrical, mechanical and chemical stresses
during routine operations. Therefore, the dielectric and
thermal stability are the most important properties of dielectric
material used in the transformer. However, the long term ac
breakdown behavior of the natural ester/pressboard insulation
has not been properly studied and the thermal stability of the
natural ester/pressboard insulation in the long ageing process
also has not been discussed.
In this paper, the physicochemical and dielectric parameters
including moisture, acids and the ac breakdown strength of the
natural ester/pressboard insulation and mineral oil/pressboard
insulation system with different ageing status were compared.
The permittivity and ac breakdown strength of these two
oil/pressboard insulation systems at different testing
temperatures were also investigated. Finally, the thermal
stability behaviors of the two oil/pressboard insulation
systems at different ageing conditions were analyzed.
2 EXPERIMENTAL
Accelerated thermal ageing in sealed systems is
recommended in the IEEE loading guide to simulate the real
ageing in modern sealed transformers [13]. In this paper,
accelerated thermal ageing experiment of natural
ester/pressboard insulation and mineral oil/pressboard
insulation at 110 °C was conducted for 120 days. The
insulation pressboard used in the experiment was provided by
Hunan No.1 Insulation Pressboard Co. Ltd, China. The
technical performances of the pressboard satisfy the
international standard IEC 641-3-1. These pressboards consist
of about 90% cellulose, 6-7% hemicellulose, and 3-4% lignin.
The pressboard has a thickness of 0.3 mm for a single layer.
The pressboard was cut into circular samples with a diameter
of 42 mm. The insulation oil used in this experiment was
Karamay 25# naphthenic mineral oil, which was degassed and
provided by ChuanRun Lubricant Co. Ltd., China. It is an
inhibited insulation oil. The mass concentration of 2, 6-di-tert-
butyl-4-methylphenol (DBPC) in the oil is 0.3%. This oil has
good electrical properties and oxidation stability, which
satisfies the ASTM D3487-2000(II).
The natural ester used in
this research was BIOTEMP natural ester provided by ABB
Chongqing Transformer Co. Ltd.
2.1 ACCELERATED THERMAL AGEING
EXPERIMENT
The pretreatment of the samples were as follows: firstly, in
order to simulate the real ageing conditions in modern sealed
transformers, all pressboard samples were put into a vacuum
box and dried at 90 °C for 48 h. Then the temperature of the
vacuum box was adjusted to 40 °C. Secondly, the new mineral
oil or new natural ester was infused into the vacuum box. The
vacuum box was left for 24 h at 40 °C before cooled down to
room temperature. Thirdly, 63 g pressboard samples were
taken out of the vacuum box each time and put into a glass
bottle (1000 ml). Then new mineral oil (or new natural ester)
was poured into each bottle at a mass weight ratio of
liquid/pressboard equal to 10:1 (each bottle has 630 g oil and
63 g pressboard). In order to simulate the effect of copper ion
in real transformers, 175 cm
2
copper sheet was put into every
bottle (according to the adopted proportion by Chongqing
ABB Ltd.)[14]. Then every bottle was filled up with nitrogen
and sealed (0.075 MPa=0.75 atm). These bottles were finally
put into the ageing ovens and heated to 110 °C for the
accelerated thermal ageing experiment. The initial moisture
content of new mineral oil and new natural ester are 5 mg/Kg
and 36 mg/Kg, respectively. The initial moisture content of
new mineral oil impregnated pressboard is 0.37%, and the
initial moisture content of natural ester impregnated
pressboard is 0.41%. During the ageing process, the
physicochemical and dielectric parameters, as well as the
thermal stability behavior of oils and oil impregnated
pressboards at different sampling intervals were measured.
The abbreviation of the samples analyzed is shown in Table 1.
Table 1. Abbreviation of samples analyzed in the experiment.
Sample name Sample composition
NE natural este
r
MO mineral oil
PINE pressboard impregnated in natural ester
PIMO pressboard impregnated in mineral oil
2.2 ANALYTICAL TECHNIQUES
2.2.1 PHYSICOCHEMICAL AND DIELECTRIC
PROPERTIES OF OILS AND OIL IMPREGNATED
PRESSBOARDS
The moisture content for various oils was measured at room
temperature (27±0.1 °C) using Karl Fischer Titration method. 3
ml oil sample was injected into the automatic coulometric Karl
Fischer titration unit (METTLER TOLEDO DL32) containing
KFR-C04 Karl Fischer Reagent for Coulometric Method
Pyridine-free.
Absolute moisture content of oil impregnated pressboards was
measured using 0.1±0.02 g pressboard sample, carefully
handled to avoid moisture exchange with air. Absolute moisture
content in pressboard was extracted in METTLER TOLEDO
DO308 oven (140 ºC) and carried by a dry nitrogen gas flow
(120 ml/min) to the coulometric titration cell. Then the absolute
moisture content of pressboard sample was acquired.
2.2.1.1 THE ACIDS CONTENT OF OIL AND OIL
IMPREGNATED PRESSBOARD
The acid number of oil was measured according to ASTM
D974-2. At present, no standardized method exists for
measuring acidity in oil impregnated cellulose paper. L. E.
Lundgaard et al studied the method to test the acids content in
insulation paper using the extraction/titration method [15].
Firstly, water was used to extract the acids in paper. Secondly,
the aqueous phase was titrated according to the method for oil
acidity (IEC 60296). In this research, a similar method of
extraction/titration was developed. Oil impregnated
pressboards with different ageing status were cut into
rectangular shape with a dry weight of 2±0.5 g. In order to
1628 R. Liao et al.: A Comparative Study of Physicochemical, Dielectric and Thermal Properties of Pressboard Insulation
avoid exposure to atmosphere, the pressboard sample was
immediately placed into 100 ml distilled water (27±0.1 °C)
contained in a 250 ml bottle. These bottles including
pressboard samples were left for 12 days (stirring one time
every 3 days) at 27±0.1 °C for a complete acid extraction.
Thereafter, 100.3 ml neutralization fluids (50 ml Absolute
Alcohol+50 ml Ether+0.3 ml Phenolphthalein) were added
into 30 ml water contained in the bottle and the sample was
titrated using KOH in alcoholic solvent (0.1 mol/L). Prior to
pressboard weighing, the pressboard samples were dried in the
vacuum box at 90 °C for 24 h. All the pressboard samples
were weighted to the nearest milligram, and the acidity was
calculated in terms of milligram of KOH per gram dry
pressboard.
The ac breakdown voltage of oils was measured using
standard cell with electrode space of 2.5 mm according to IEC
60156. The voltage with a frequency of 50 Hz was applied at
a rise rate 2 kV/s until breakdown. Tests were performed at
room temperature (27±0.1 °C) and atmosphere pressure. To
obtain a homogenous particle distribution, the oil was mixed
with a stirrer during the measurement breaks. Five breakdown
voltages were obtained for each oil sample and the average
value was used for comparison.
The ac breakdown voltage of oil impregnated pressboards
was carried out according to the standard IEC 60243-1, which
defines the experiment procedures of solid insulation material
under ac power frequency voltage. The electrodes consist of
two brass cylinders with a diameter of 25 mm, as shown in
Figure 1. The electrode edges were rounded to a radius of
3mm, creating an environment similar to the oil wedge
explained previously in [3, 16], as shown in Figure 2. The
voltage with frequency of 50 Hz was applied at a rise rate of 2
kV/s until breakdown. Five breakdown voltages were
obtained for each oil impregnated pressboard sample and the
average value was used for comparison.
Figure 1. The structure of electrode to measure ac breakdown strength of
pressboard
Figure 2. Location of oil wedge in transformer design.
2.2.2 THE AC BREAKDOWN STRENGTH OF OILS
AND OIL IMPREGNATED PRESSBOARDS UNDER
DIFFERENT TEMPERATURES
The ac breakdown strength of oils and oil impregnated
pressboards under different temperatures were investigated in
an oven where the testing temperature was controlled. The
initial moisture contents of new mineral oil and new natural
ester are 15 mg/Kg and 46 mg/Kg, respectively. The initial
moisture content of pressboard immersed in natural ester is
0.50%, and the initial moisture content of natural ester
impregnated pressboard is 0.49%. The testing temperature
range is from 40 to 70 °C. The test method of ac breakdown
strength of oils and oil impregnated pressboards at different
temperatures are the same as described above (Section 2.2.1).
2.2.3 THERMAL STABILITY BEHAVIOR OF OILS
AND OIL IMPREGNATED PRESSBOARDS
Thermogravimetry (TG) is a technique in which the mass of
a substance is measured as a function of temperature whilst
the substance is subjected to a controlled temperature
programme [17-21]. It is a useful technique applied to
determine the thermal stability of oils and fibers (cellulose)
[18-21]. The thermal stability behaviors of natural
ester/pressboard insulation system and mineral oil/pressboard
insulation system aged for different times were compared
firstly in this paper.
Data was collected on Q50 TG Analyzer (TA, America).
Each mineral oil sample (15.0-15.4 mg) and natural ester
sample (15.0-15.4 mg) were tested from 33 to 250 °C and 450
°C, respectively. The temperature scanning rate was 3° C/min
under nitrogen flow (50 ml/min). Each pressboard sample
(5.0-5.2 mg) was tested from 33 to 500 °C at a temperature
scanning rate of 5 °C/min under nitrogen flow (50 ml/min). In
this research, the thermal characteristic parameters, including
the initial decomposition temperature (IDT), the maximum
speed of decomposition (MSD) and the temperature at
maximum decomposition speed (TMDS) were focused on.
3 EXPERIMENTAL RESULTS
3.1 MOISTURE CONTENT OF OILS AND OIL
IMPREGNATED PRESSBOARDS
3.1.1 MOISTURE CONTENT OF OILS
The absolute moisture content of natural ester and mineral oil
during the ageing process is shown in Figure 3. The absolute
moisture content of natural ester increases initially and then
decreases, lastly increases again slightly when the ageing time
increases. While the absolute moisture content of mineral oil has
a slight decrease at first, and then increases all the time. Figure 3
indicates that the natural ester has much higher absolute
moisture content than mineral oil after experiencing the same
ageing length. This is because natural ester has a greater affinity
for moisture than does mineral oil [1, 5, 22, 23].
Considering that the saturation moisture content of oil is a
function of pressure and especially the temperature, the relative
moisture reflects more than just the moisture content [5, 24]. In
this paper, the relative moisture content of natural ester and
mineral oil dependence on ageing time was analyzed. The
IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18, No. 5; October 2011 1629
relative moisture for oil is the dissolved moisture content of the
oil relative to the maximum capacity of moisture that the oil can
hold. The relative moisture W
rel
at a given temperature T is
defined in terms of the actual moisture content in a liquid W
abs
versus the saturation limit W
L
(T) [25], as following:
.
()
abs
rel
L
W
W
WT
(1)
The moisture maximum solubility W
L
(T) at an absolute T
can be expressed in the form [26, 27]:
() *
H
T
L
WT Ke
(2)
The constant K depends on the liquids themselves and is
determined experimentally. According to the dependence of
the moisture saturation limit of natural ester and mineral oil as
a function of absolute temperature previously published in [5],
at room temperature, the saturation limits of natural ester and
mineral oil are about 3000 and 60 mg/Kg, respectively.
According to equation (1), the relative moisture content of
natural ester and mineral oil during the ageing process was
calculated, as shown in Figure 4. It can be seen that the
mineral oil has a higher relative moisture content than natural
ester, except for ageing 30 days, where the relative moisture
content of mineral oil is almost the same as that of natural
ester. Since the moisture has a detrimental effect on the
electrical performance of oil [1, 5], the lower relative moisture
content of natural ester may potentially make the natural ester
have better dielectric breakdown strength than mineral oil
during the ageing process.
0 20 40 60 80 100 120 140
0
20
40
60
80
100
120
140
160
MO NE
Moisture Content of Oil (mg/Kg)
Aging Time (days)
Figure 3. Absolute moisture content of natural ester and mineral oil in the
ageing process
0 20 40 60 80 100 120 140
0
2
4
6
8
10
12
14
MO NE
Relative Moisture Content of Oil (%)
Aging Time (days)
Figure 4. Relative moisture content of natural ester and mineral oil in the
ageing process.
3.1.2 MOISTURE CONTENT OF OIL IMPREGNATED
PRESSBOARDS
The absolute moisture content of oil impregnated pressboards
is shown in Figure 5. Moisture moves between the pressboard
and dielectric fluid to reach equilibrium in terms of relative
saturation [23, 28, 29]. Due to the higher absolute moisture
content of new natural ester and the great moisture affinity of
pressboard, the pressboard which has very low moisture content
will absorb moisture from natural ester at the beginning of
ageing in order to keep moisture equilibrium between oil and
pressboard. Therefore, the moisture content of the pressboard in
natural ester increases in the first stage of ageing. In addition,
moisture would be generated because of oil/pressboard
insulation deterioration. When sampled at 58 days, the moisture
content of natural ester increased to 1.25wt%, which is the
maximum. On the other hand, moisture reacts with the natural
ester via hydrolysis. The reaction consumes dissolved moisture
in the fluid causing additional moisture to move from the
pressboard into the fluid in order to maintain the equilibrium.
Hence, the moisture content of pressboards aged in natural ester
reduces after ageing 58 days.
In mineral oil/pressboard insulation system, the moisture
content of pressboard also increases because of oil/pressboard
insulation deterioration generating moisture. However, when
the oil and the air inside the bottles being relatively drier than
the moisture condition of pressboard, there is always a
migration of moisture from the pressboard to the oil and then
to the air [28, 29]. Thus the moisture content of pressboard
has a decline. The longer the periods of ageing, the larger is
the amount of moisture migrating out from the pressboard
[28]. Therefore, the moisture content of the pressboard aged in
mineral oil also shows a decrease after ageing 58 days.
0 20406080100120140
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
PIMO PINE
Absolute Moisture Content
of Pressboard (%)
Aging Time (days)
Figure 5. Absolute moisture content of pressboard aged in natural ester and
mineral oil.
3.2 ACIDS CONTENT OF OILS AND OIL
IMPREGNATED PRESSBOARDS
3.2.1 ACIDS CONTENT OF OILS
Figure 6 shows the dependence of acidity of natural ester and
mineral oil on ageing time. Under normal condition the acidity
of natural ester is higher than mineral oil [1, 22]. This is
reflected in Figure 6 at the beginning of the ageing experiment
where the acidity of new natural ester is higher than the new
mineral oil. Degradation of both natural ester and
mineral oil creates acids, so it is not surprising to see that the
1630 R. Liao et al.: A Comparative Study of Physicochemical, Dielectric and Thermal Properties of Pressboard Insulation
0 20 40 60 80 100 120 140
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
MO NE
Acidity of Oil (mgKOH/g)
Aging Time (days)
Figure 6. Acidity of natural ester and mineral oil in the ageing process.
acidity of natural ester and mineral oil increases with the
ageing time. However, it is worthy to mention that the acidity
of natural ester is considerably higher than mineral oil over
the whole ageing period. This is because the natural ester
degrades in a different manner to mineral oil.
Recognized mechanism of mineral oil is chain reaction of
free radical oxidation: including chain off, chain continuity
and chain breaking out [22, 30]. Natural ester consists
primarily of triglycerides. Triglycerides are glycerol
molecules with three long chain fatty acids attached at the
hydroxy groups via ester linkages. Unsaturated double bonds
in the fatty acids are active sites for many reactions, including
oxidation, lowering the oxidation stability of natural ester. The
greater the level of un-saturation, the more double bonds, and
the more susceptible to oxidation the natural ester becomes
[31]. Natural ester oxidation is initiated by formation of free
radicals. Free radicals can easily be formed from the removal
of a hydrogen atom from the methylene group next to a
double bond. Free radicals rapidly react with oxygen to form a
peroxy radical. The peroxy radical can then attack another
lipid molecule to remove a hydrogen atom to form a
hydroperoxide and another free radical, propagating the
oxidation process [31]. In addition, natural ester can hydrolyze
in the presence of moisture, which releases fatty acids and
glycerol, as presented in equation (3). However, there is no
hydrolysis in mineral oil. Therefore, the different chemical
reaction in oils results in the natural ester having much higher
acidity than mineral oil. On the other hand, due to the
differences in the chemical structures of acids formed by
natural ester and mineral oil, the acids formed in mineral oil
are detrimental[15, 22, 35], while the acids produced by esters
are beneficial [22, 23, 32-34].
(3)
From the components of natural ester and mineral oil, it can
be known that the main type of acids in natural ester is high
molecular acids, such as oleic acids, while the main type of
acids in mineral oil is low molecular acids, such as formic,
acetic and levulinic acids [15, 22, 32-35]. The low molecular
acids accelerate the ageing of the paper, but the high
molecular acids do not influence the paper ageing
significantly [15, 22, 35]. The high molecular acids produced
by hydrolysis of natural ester can react with the cellulose via
transesterification [22, 23, 34, 36], as shown in equation (3).
Under accelerated ageing, the reactive OH (hydroxyl) groups
on the cellulose molecule become esterified with fatty acid in
natural ester, which restrains the paper ageing [22, 23, 34, 36].
3.2.2 ACIDS CONTENT OF OIL IMPREGNATED
PRESSBOARDS
Unfortunately, there is no standardized method for measuring
acidity in oil impregnated cellulose. In this paper, the acid content
in the pressboard aged in natural ester and mineral oil was
compared at the first time, as shown in Figure 7. The pressboard
aged in natural ester has a much higher acid content than the
pressboard aged in mineral oil before 93 days. L. E. Lundgaard et
al studied the acids in mineral oil-paper insulation system and
proposed that the hydrophilic acids are mostly concentrated in the
insulation paper [15, 35]. The lower the molecular weight of acids,
the more easily it is absorbed by insulation paper [15, 35]. It is
known, the natural ester has higher acidity than mineral oil and
there is hydrolysis reaction which can release acids in natural ester.
Compared with the pressboard in mineral oil, the pressboard in
natural ester may absorb more acids from the oil. This may be one
reason why the pressboard in natural ester has a much higher acid
content than the pressboard in mineral oil. Another reason is that
there is natural ester in the natural ester impregnated pressboard.
The acidity of natural ester is much higher than mineral oil, which
can also contribute to the acid content of the pressboard immersed
in natural ester. However, the higher acid content in pressboard
immersed in natural ester is not bad thanks to the transesterification
reaction [22, 23, 34, 36].
Figure 7 shows that the acid content of the pressboard aged
in mineral oil is very close to that aged in natural ester after
aged for 93 days. It is known that the ageing of pressboard
will produce moist soluble carboxylic acids. The severer the
deterioration of the pressboard, the larger the quantity of acids
it generates [15, 35, 37]. The natural ester can restrain the
ageing rate of pressboard effectively [2, 6, 9, 22, 23, 34]. This
means that less carboxylic acid will be generated by the
pressboard aged in natural ester. Therefore, compared with the
0 20406080100120140
1.0
1.5
2.0
2.5
3.0
3.5
4.0
PIMO PINE
Acids Content of
Pressboard (mgKOH/g)
Aging Time (days)
Figure 7. Acids content of pressboard aged in natural ester and mineral oil.
