2021 22nd International Middle East Power Systems Conference (MEPCON), Assiut University, Egypt
Thermal Aging Influence on Relaxation Time of
Transformer Insulation Paper Impregnated in
Natural and Synthetic Ester Oils
2021 22nd International Middle East Power Systems Conference (MEPCON) | 978-1-6654-1998-7/21/$31.00 ©2021 IEEE | DOI: 10.1109/MEPCON50283.2021.9686229
Walaa A. Madkour
Department of Electrical Power and
Machines Engineering
Faculty of Engineering, Tanta
University, Egypt
walaa.madkour.3112@gmail.com
Diaa-Eldin A. Mansour
Department of Electrical Power and
Machines Engineering
Faculty of Engineering, Tanta
University, Egypt
mansour@f-eng.tanta.edu.eg
H. F. Abosheiasha
Department of Engineering Physics and
Mathematics
Faculty of Engineering, Tanta
University, Egypt
hatem_fouad@f-eng.tanta.edu.eg
ester oil takes approximately triple aging hours at 130 °C and
ten times aging hours at 150 °C to attain similar aging of that
impregnated in mineral oil.
Abstract—For many centuries, mineral oil was used as an
insulating fluid with power transformers. However, mineral oil
is produced from limited petroleum products and is not
biodegradable. These reasons motivated researchers to
investigate the usage of alternative insulating liquids such as
natural and synthetic esters. But, for practical application of
ester oils, there is a need to study the influence of the thermal
aging on the paper insulation impregnated with such oils, which
is the main aim in this paper. The aging of kraft paper in
natural, synthetic esters and mineral oil was compared. The oilpaper insulation was aged at 120 °C for 72 hrs, 144 hrs and 240
hrs. The dielectric behavior was investigated via using
frequency domain spectroscopy analysis over a frequency range
from 10 HZ to 1 MHZ. The dielectric relaxation times were
calculated for unaged and aged oil impregnated paper samples.
The results indicated the better behavior of synthetic ester over
natural ester and mineral oil.
In spite of numerous studies discussed the aging
suppression of paper insulation impregnated in ester oils, there
are limited studies that investigated the relaxation time in such
oils which is crucial in clarifying the corresponding physical
mechanisms behind aging suppression. In [8], the relaxation
time was investigated for thermally aged pressboard samples
impregnated in only natural ester oil using Cole–Cole
expressions. It was found that thermal aging decreases the
relaxation time of pressboard samples, and this decrease is
more pronounced with increasing the aging temperature.
In this research, the aging of paper insulation impregnated
in natural and synthetic ester oils was investigated using
dielectric spectroscopy, and then the obtained results were
compared to that impregnated in mineral oil. Various
dielectric parameters were extracted, and then the relaxation
times were calculated for different samples.
Keywords— Oil-paper insulation, natural ester, synthetic
ester, dielectric spectroscopy, relaxation time.
I. INTRODUCTION
The demand for the reliable operation of power
transformers is increasing due to their direct impact on the
reliable operation of electrical grids. Most of power
transformers are currently insulated by mineral oil due to its
advantages as a cooling and electrical insulating medium.
However, there are some drawbacks associated with the usage
of mineral oils. These drawbacks can be summarized in low
biodegradability, dependence on depleted resources, and low
moisture tolerance. Accordingly, ester oils were proposed as
an alternative for mineral oils [1-3]. Ester oils can be natural
ester or synthetic ester. Ester oils could improve safety of
power transformers due to their high flash and fire points [4].
II. EXPERIMENTAL SETUP
A. Preparation of test samples
The kraft paper samples, which used as a transformer
insulation papers, had a 0.18 mm thickness and 40 mm
diameter. The mineral oil used in our experiments was Shell
DIALA S2 with a density of 875 Kg/m3, flash point of 140 ºC
and a kinematic viscosity of 9.4 mm2/s. The used natural ester
oil and synthetic ester oil are MIDEL eN 1204 and MIDEL
7131, respectively. Their specifications are summarized in
Table I.
B. Oil-paper samples with different aging periods.
Twelve samples of transformer insulation kraft paper were
weighted and put into the vacuum oven for 48 hours at 80 ºC
On the other hand, the lifetime of power transformers in
service depends to a large extent on the condition of their solid
insulation, mainly paper and pressboard insulation. So, it is
aimed to elongate the service life of such insulating materials
when operating into transformers. With using ester oils, the
aging rate of paper insulation was lower than that occurred
when using mineral oils [5-7]. In [5], the degree of
polymerization for paper impregnated in mineral oil decreased
to about 200 after 47 days of thermal aging, while it was kept
higher than 400 for paper impregnated in mineral oils. In [7],
it was found that Kraft paper impregnated in rapeseed-based
TABLE I. SPECIFICATIONS OF USED NATURAL AND SYNTHETIC ESTER
OILS.
Natural ester
3
Synthetic ester
970 kg/m3
Density
920 kg/m
Flash point
315 °C
260 °C
Fire point
350 °C
>300 °C
Kinematic viscosity at 40 °C
37 mm2/s
28 mm2/s
407
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A. Analysis of relative permittivity
Relative permittivity of impregnated paper samples
describes the dielectric polarizability, which is a function of
frequency and temperature. It is also affected by any
contaminants existing into the paper, such as moisture content
and acids. The dielectric spectroscopy of impregnated paper
samples was evaluated during the aging process and was
compared to unaged samples. Fig. 2 shows the variation of
relative permittivity (ε’) against frequency for various samples
under various aging conditions.
5.0
M0
4.5
M72
M144
ε'
4.0
M240
3.5
3.0
2.5
2.0
1
10
Fig. 1. Prepared oil-paper samples with different aging periods.
TABLE II. PREPARED SAMPLES OF TRANSFORMER INSULATION KRAFT
PAPER AND THEIR SYMBOLS.
72
144
240
Mineral oil
M0
M72
M144
M240
Natural ester oil
N0
N72
N144
N240
Synthetic ester oil
S0
S72
S144
S240
2
10
3
10
4
10
5
10
10
7
(a) Mineral oil-impregnated paper.
4.6
N0
4.4
N72
N144
N240
4.2
ε'
to remove the moisture content. The percentage loss in the
samples of kraft paper due to drying in vacuum oven ranged
between 5% and 6%. Then, twelve samples of mineral oil,
natural ester and synthetic ester (4 samples of each type) were
put into the vacuum oven for 24 hours at 80°C. After that, nine
kraft paper samples were impregnated in the insulation oil
samples and put into the aging oven for 72, 144, 240 hours.
The aging temperature was set at 120 °C, which is widely
adopted in literature [9, 10]. The remaining three samples
were impregnated in in the insulation oil samples without any
aging for the sake of comparison. These samples are shown in
Fig. 1. The symbol ‘M’ denotes to mineral oil, while the
symbols ‘N’ and ‘S’ denotes to natural and synthetic ester oils,
respectively. Table II summarizes all the prepared samples
and their symbols.
4.0
3.8
3.6
1
10
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz)
(b) Natural ester oil-impregnated paper.
5
4
FREQUENCY DOMAIN SPECTROSCOPY ANALYSIS OF
OIL-PAPER SAMPLES
ε'
III.
6
Frequency (Hz)
Aging duration (hours)
0
10
3
The dielectric characteristics were obtained using the LCR
meter Agilent E4980A device. The device has a cell with two
parallel electrodes. In a parallel-plate capacitor system,
measurements were made using the frequency domain
spectroscopy method. Between the capacitor metal plates the
impregnated paper sample was inserted. The dielectric
properties were analyzed within a frequency range from 10 to
1 MHZ.
S0
S72
2
S144
S240
1
1
10
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz)
(c) Synthetic ester oil-impregnated paper.
Fig. 2. Relative permittivity of oil-paper insulation with different aging
periods.
408
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For all aged kraft paper samples, either impregnated in
mineral oil, natural ester oil or synthetic oil, the ε’ values
increased with increasing the aging period. The increase in ε’
values is more pronounced at low frequency region. This can
be attributed to the increasing contribution of the conductivity
at low frequency, where aging increased the number of bound
charges by paper insulation [11]. It is worth mentioning that
the synthetic ester-impregnated paper has much higher ε’
value at low frequency region than both natural esterimpregnated paper and mineral oil-impregnated paper for the
same aging period. While, at high frequency region natural
ester-impregnated paper has the highest ε’ value.
Oil-impregnated paper has several types of polarization
when exposed to an alternating electric field. In the lowfrequency region, the Maxwell–Wagner effect determines the
change of ε’ value with aging time. This effect occurs along
inner layers of paper insulation or at their interfaces with
electrode. On the other hand, with increasing frequency to the
high-frequency region, dipole orientation polarization
becomes dominant and other polarization processes
diminishes. This causes all ε’ values to reduce. As the
frequency of the electric field further increases, the dipoles
become unable to keep up with the fast-changing polarity. As
a result, ε’ values reach a somewhat constant value [12].
Relative permittivity alone cannot be used to judge the
superior performance of certain impregnated paper over
another one.
0.07
M0
0.06
M144
ε''
0.05
+
1+
0.04
0.02
0.01
1
10
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
(a) Mineral oil-impregnated paper.
N0
0.10
N72
N144
ε''
0.08
N240
0.06
0.04
0.02
10
1
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
(b) Natural ester oil-impregnated paper.
0.0007
S0
0.0006
S72
S144
0.0005
S240
ε''
0.0004
0.0003
0.0002
0.0001
C. Analysis of relaxation time
It takes a finite amount of time for polarization to develop
when a direct voltage is applied before it reaches its maximum
value. Similarly, when a direct voltage is applied to a dielectric
for a long enough time, the decay of polarization to zero value
is not instantaneous, but requires a certain amount of time. The
term “dielectric relaxation” is used to describe the
phenomenon and the relaxation time refers to the required
time for dipoles to recover their random alignment after
removing the applied electric field. To analyze the behavior of
a dielectric in alternating fields, the equivalent circuit shown
in Fig. 4 is utilized, which visualize the lossy dielectric as an
equivalent to an ideal dielectric in series or parallel with a
resistance [14]. The complex impedance of this equivalent
circuit can be represented by the following equation:
Ƶ =
M240
0.03
B. Analysis of dielectric losses
Figs. 3 shows the dielectric losses as a function of
frequency. There is a significant increase in the dielectric loss
factor within the frequency range from 103 to 106 Hz. As
shown in the equation ε″(ω)=σ0/(ε0.ω), the dielectric loss
factor ε″(ω) is significantly affected by dc conductivity σ0
[13]. This can explain the high dielectric losses for natural
ester-impregnated paper compared to mineral oil-impregnated
paper, where natural ester has high dc conductivity due to its
high affinity for absorbing moisture and acids. On the other
hand, the synthetic ester has the lowest dielectric loss values.
Because of the accelerated thermal aging, the dielectric loss
factor significantly increases for the natural ester compared to
that for the synthetic ester.
∗
M72
0.0000
1
10
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
(c) Synthetic ester oil-impregnated paper.
Fig. 3. Dielectric losses of oil-paper insulation with different aging periods.
Fig. 4. The equivalent circuit of a lossy dielectric.
where Ƶ*= Ƶ′ - jƵ″, Rs is the series resistance, Rp is the parallel
resistance and ω is the angular frequency which equal 2π .
By separating the real and imaginary components of the
complex impedance:
(1)
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Ƶ′ =
Ƶ″ =
+
(2)
1+
1.2x10
+
M72
M144
0.8
Z'' (Ω)
Considering the the relaxation time distribution parameter α,
which is less than 1 for paper insulating material [15], the
expression of the complex impedance can be given as follows:
Ƶ∗ =
M0
1.0
(3)
1+
6
M240
0.6
0.4
1+(
(4)
)
0.2
Thus, the expression for Ƶ" will be given as follows:
Ƶ″ =
[cos
1 + 2 sin
2
(
2
(
)
)
+(
0.0
0
10
]
)
(1 − )
[sin "
$(
2
Ƶ″ =
(1 − )
1 + 2 cos "
$(
)
2
(
)
)
1.4x10
)
(
)
[1 + 2 sin
2
(1 − )'( (
( '( )
2
− ( '( )
(
)
]
' ()
+ ( '( )
]
(
)]
10
3
10
4
10
5
10
6
10
7
6
N0
N72
1.0
[1
N144
N240
0.8
0.6
0.4
0.2
0.0
0
10
(7)
-For )*+, , ωmaxτ0 = 1, where ωmax is the angular frequency
at which Ƶ″ is maximum. So, τ0 can be obtained for various
samples. Fig. 5 illustartes the plots of Ƶ″ against frequency
for various impregnated paper samples. For all samples, the
relaxation time decreases against aging period. This can be
attributed to the formation of multiple oil-paper interfaces,
where thermal stresses changes the microstructure of
cellulose kraft paper in the amorphous region and weakens
intermolecular interactions. The decrement in relaxation time
against aging is more pronounced in mineral oil-impregnated
paper indicating more impact for the thermal aging process.
IV.
2
1.2
Z'' (Ω)
[cos
10
Frequency (Hz)
(6)
By differentiating Ƶ" with respect to the angular frequency:
%Ƶ"
=
%
1
(a) Mineral oil-impregnated paper.
]
+(
10
(5)
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz)
(b) Natural ester oil-impregnated paper.
1.6x10
6
S0
1.4
S72
1.2
S144
S240
Z'' (Ω)
1.0
0.8
0.6
CONCLUSIONS
0.4
In this paper, the aging of paper insulation impregnated
in natural and synthetic ester oils was investigated using
dielectric spectroscopy in comparison to that impregnated in
mineral oil. For relative permittivity, the synthetic esterimpregnated paper had much higher values at low frequency
region than both natural ester-impregnated paper and mineral
oil-impregnated paper for the same aging period. While, at
high frequency region natural ester-impregnated paper had
the highest relative permittivity values. On the other hand, the
synthetic ester has the lowest dielectric loss values. By
analyzing the relaxation time, it was found that the relaxation
time in mineral oil-impregnated paper decrement against
aging with a higher rate than that in both natural esterimpregnated paper and synthetic ester-impregnated paper.
So, as a general remark, the aging process of paper insulation
is suppressed when using ester oils.
0.2
0.0
0
10
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz)
(c) Synthetic ester oil-impregnated paper.
Fig. 5. Dielectric losses of oil-paper insulation with different aging periods.
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