2008 Annual Report Conference on Electrical Insulation Dielectric Phenomena
Particle Effect on Breakdown Voltage of Mineral
and Ester Based Transformer Oils
X. Wang and Z.D. Wang
School of Electrical and Electronic Engineering, the University of Manchester,
Manchester M60 1QD, UK
zhongdong.wang@manchester.ac.uk
Abstract- ac breakdown voltage tests are usually used for quality
check of mineral oils during transformer operation. When
studying ester-based oils, previous experimental research
including the ones carried out at University of Manchester
laboratories, were based on ‘as-received’ oil samples and particle
effects were not quantified. Similarly, water effects on oils
determined previously were also ambiguous since water can be
combined with particles to reduce breakdown voltages. This paper
used mineral and ester-based transformer oils, i.e. 10 Gemini X
as mineral oil, Midel7131 as synthetic ester-based oil and FR3 as
nature ester-based oil, in the forms of “as-received” and
“processed”, and their AC breakdown strengths were
comparatively studied. Weibull distributions of ac breakdown
voltages of three oils were discussed. It can be seen from the
results that particle effects on ac oil breakdown voltages are
dominant for “as–received” mineral oil and the ‘processed’
mineral oil has a distinctly different breakdown characteristic
from ‘as-received’ mineral oil in terms of the effect of water
contents.
Introduction
Transformer oil is one of the most essential components for
conventional oil immersed transformers as it acts both as
electrical insulation and thermal coolant. The dielectric
strength of the oil and more specifically the level of oil
contamination determine the dielectric safety margin of the
transformer insulation system. In addition, transformer oil
may be used in power transformers for decades without
changing, and the transformer oil, together with the insulation
paper, can deteriorate due to ageing. This long term ageing
produces water, acid and many other by-products, soluble or
insoluble. Maintenance guides consider the oil to be a
component that could be treated and managed separately from
the oil-paper insulation system. Ac breakdown voltage tests,
together with many other test techniques on oil samples, were
used for quality check and control.
Ester-based transformer oils have gained the success in
distribution transformers at certain countries and regions, and
they are currently under research and development for
application in large power transformers. Ester-based oils have
been shown to extend the remaining life of paper as it is more
hygroscopic than mineral oil; more water is therefore
absorbed and reacted by ester-based oils so the paper is kept
dry.
Ester oils are also advantageous compared to traditional
mineral oil as these oils are less flammable, non-toxic and
more biodegradable. However, using ester-based oils also has
978-1-4244-2549-5/$25.00 © 2008 IEEE
certain disadvantages, i.e. that vegetable oils can more easily
oxidize and hydrolyze. Especially in United Kingdom, large
power transformers are typically of free breathing design so
measures must be taken to control oxidation and hydrolysis.
Research activities have been increasingly active on ester
based transformer oils for recent years and many test results
on ac breakdown voltages were published, including some
statistics analysis results [1]. However, the oil samples used in
those tests were often not filtered or, in another word, not
particle controlled, leaving the ac breakdown test results more
relevant to the particles or impurities rather the oil itself.
In this paper, the withstand voltages of ‘as-received’ and
‘processed’ mineral and ester-based transformer oil were
investigated. As the withstand voltage of a dielectric fluid is a
statistically distributed quantity corresponding to a probability
of failure, a large sample of breakdown voltages was analyzed
and their statistical distributions compared. The mean
withstand voltages, as well as the highest and lowest
breakdown voltages were discussed. In addition, the water
effect on withstand voltages for both ‘as-received’ and
‘processed’ mineral oil were studied. It can be found that the
water needs to be combined with particles to have a
significant effect on mineral oil ac breakdown voltages.
Experimental procedures
2.1 Experiment samples
A type of mineral oil, 10 Gemini X provided by National
Grid, was used as traditional mineral oil. Midel7131 provided
by M&I materials, was used as the synthetic ester-based oil,
and FR3, provided by Cooper Power Systems, was used as the
nature ester-based oil. For each type of oil, both the
‘as-received’ and ‘processed’ oil samples were tested. The
‘as-received’ oil here refers to oil samples processed through
degassing and dehydrating only. The ‘process’ oil here refers to
oil samples processed through degassing, dehydrating and
filtering. The processing procedure will be described below.
2.2 Preprocessing procedures
The oil samples were filtered through a 0.2µm membrane
filter unit for 3 cycles. It was found that after 3 cycles, the oil is
clean enough and the particle number in oil samples could not
decrease further even more filter cycles were given. The
particle number in oil samples was decided by HIAC 8011
liquid particle counting system, which can detect particles with
598
diameter from 1µm to 200µm. The particle counting results are
shown in Figure 1. Figure 1.a, 1.b and 1.c show the results for
mineral oil, FR3 and midel7131 separately.
1000000
'As-received' mineral
Particle number (in 100ml oil)
100000
'Processed' mineral
10000
1000
100
10
1
0
10
20
30
40
Particle diameter (µm)
50
60
1000000
'As-received' FR3
Particle number (in 100ml oil)
100000
'Processed' FR3
10000
2.4 Water uptake procedures
1000
In order to decide the water effect of ‘as-received’ and
‘processed’ mineral oil, a desiccator and glycerol solution were
used to control the relative humidity in the transformer oil.
According to the Raoult’s law, the relative humidity of the air
in the desiccator can be controlled by the proportion of glycerol
solutions. The Raoult’s Law is expressed as P=xaPa, where P is
the resulting vapor pressure, x is the mole fraction of
component a in solution and Pa is the vapor pressure of pure a.
Oil sample container was deposited in the desiccator with a
certain relative humidity for one week. After conditioning, the
container was sealed and stored for another week before testing
to allow further diffusion of moisture to create a high degree of
consistency in the sample. Care was taken during the water
uptake procedure for ‘processed’ oil sample in order not to
contaminate the sample.
The moisture content of each oil sample was determined by
Karl Fischer titration method. A metrohm 648 Coulometer with
oven were used to provide accurate moisture reading. The dry
oil samples with water content less than 5% RH were achieved
by the drying method as specified in preprocess procedures.
100
10
1
0
10
20
30
40
50
60
Particle diameter (µm)
100000
'As-received' midel7131
Particle number (in 100ml oil)
measurements with partial sphere electrodes and 1mm gap
distance, following the ASTM D1816 test standard. 2 series of
5 breakdowns are required by the standard, but in order to
compare the statistical distributions of different oils, 8 series of
5 breakdowns were taken to give 40 breakdown voltages in
total. It is believed that the size of 40 breakdowns were big
enough to give a reasonably accurate distribution to estimate
the failure voltage at a low risk rate.
It needs to be noted that ester-based transformer oils have
higher viscosities and lower interfacial tensions than mineral
oil, leading to slow ejection of gas bubbles created during the
breakdown. Therefore, it seems sensible for ester-based
transformer oils to have a longer standing time after each
breakdown. However Cooper Power System published a test
guide which only requests extra standing time after pouring the
oil sample into the test cell and before the start of the tests,
therefore 1 minute standing time between two successive
breakdowns specified by ASTM 1816 was used for all the oil
samples.
10000
'Processed' midel7131
1000
100
10
1
0
10
20
30
40
Particle diameter (µm)
50
60
Figure 1. Compare of particle counting results
The ‘clean’ oil is defined by CIGRE working group 12.17 as
the particle content with a diameter larger than 5µm is down to
300 per 100ml in a oil sample [2]. As can been seen from the
filtering results, the particle numbers in ‘processed’ oil samples
were almost reaching the number given in the definition of
‘clean’ oil.
Subsequently, the oil samples were individually degassed
and dried at less than 1kPa for 2 days at 80°C, then were given
a further day to cool down to ambient temperature under
vacuum conditions. It was found that the relative water
contents in the oil samples were all less than 3% RH after
drying, which could be considered as very dry oil. This applies
to all three oils, i.e. mineral oil, Midel7131 and FR3.
2.3 AC withstand voltage tests
A Baur DPA75 was used for AC withstand voltage
Experimental Results and Discussions
Previous research has drawn the conclusion that ester-based
oil has comparable dielectric strength as mineral oil, with
similar withstand voltage which is defined as the breakdown
voltage at a specified low risk of failure [3]. In order to gain a
better understanding on how the oil properties were influenced
by particles, a test was carried out to compare the AC withstand
strengths of ‘as-received’ and ‘processed’ mineral and
ester-based transformer oils.
Researches have also been conducted concerning the water
content effect on the breakdown voltage of mineral oil. It is
generally considered that the breakdown voltages are decreased
with the increase of relative water content, a significant
reduction of breakdown voltage was noticed by many
researchers for the relative water content between 5 to 10
599
percents [4]. However, little research has investigated the water
effect on very clean oil. Therefore tests were carried out to
investigate the difference of water effect on both ‘as-received’
and ‘processed’ mineral oil.
99.5
Probability (%)
95
3.1 AC withstand strength comparison
70
40
10
1
20
25
30
35
40
45
50
55
60
Breakdown Voltage (kV)
Figure 2. Distribution for breakdown voltages of mineral oil
'Processed' FR3 oil
'As-received' FR3 oil
Probability (%)
95
70
40
10
1
25
30
35
40
45
50
55
Breakdown Voltage (kV)
Figure 3. Distribution for breakdown voltages of nature ester oil
95
Probability (%)
The AC withstand strength results for ‘as-received’
ester-based oil were reported, previously produced at the same
laboratory at the University of Manchester [4]. In order to
verify the result, a successive of 40 breakdown tests was
carried out using the same configuration for ‘as-received’ oils.
The breakdown tests were also carried out on ‘processed’ oils
to give a distribution of breakdown voltages.
Figure 2 through Figure 4 show the breakdown voltage
distributions for ‘as-received’ and ‘processed’ mineral oil,
synthetic ester and nature ester oils. The mean breakdown
voltage is given in the figures by the dot lines.
According to the weakest link theory, their breakdown
voltage distributions are fitted by 2 parameter Weibull
distribution given by equation (1)
(1)
Where
x: electric field
λ: scale parameter
k: shape parameter
For all the 3 types of transformer oils, the breakdown
voltage distributions follow the Weilbull distribution well. The
Weibull fitting distributions are given in the figure by the solid
lines. From the results, the dielectric strengths of ‘processed’
oils were increased significantly comparing to those of
‘as-received’ oil, in terms of mean breakdown voltage, lowest
and highest breakdown voltages. The increase of lowest
breakdown voltages, however, was larger than that of highest
breakdown voltage, i.e. the lowest breakdown voltage was
increased by 9.4 kV and the highest breakdown voltage was
increased by 3.2kV for FR3 oil. The higher dielectric strength
for ‘processed’ oils is caused by fewer particles suspended in
the oil. The fitting parameters of different oil are given in Table
1.
From the results, it can be easily seen that both the scale and
shape parameter are increased after oil processing and the
‘processed’ oil follows the Weilbull distribution much better
than ‘as-received’ oil. Especially at low probabilities, the
breakdown voltages for ‘processed’ oil almost spot on the
fitting curve, while the ‘as-received’ oil breakdown voltage was
far from the fitting curve, usually lower than the fitting
breakdown voltage. This deviation may be explained by the
theory that breakdown is caused by weak links. It is easy to
understand that the breakdown characteristic of ‘processed’ oil
is steady because there are fewer particles in the oil, which
means less weak links. For ‘as-received’ oil, the more the
particles with different sizes, the more possibilities for different
sized particles to be in the path between the two electrodes to
initiate streamers, the less predictable the breakdown voltage
may be. This results in the deviation between the measured
breakdown voltage and the fitting value at low probabilities.
'Processed' mineral oil
'as-received' mineral oil
'Processed' Midel7131
'As-received' Midel7131
70
40
10
1
0.1
30
35
40
45
50
55
Breakdown Voltage (kV)
Figure 4. Distribution for breakdown voltages of synthetic ester oil
Table 1. Weibull distribution parameters for three oils – particle effect
Λ
k
Mineral
6.3
38.8
‘As-received’ oil
FR3
Midel7131
8.2
12.7
43.0
43.4
Mineral
13.4
49.6
‘Processed’ oil
FR3
Midel7131
11.9
12.9
46.5
46.9
The distribution parameters of oil dielectric strength are
given in Table 2. Compared with the previous results reported
in [4], all the mean breakdown results for mineral, synthetic
and nature esters are a little smaller. However, this difference is
within the toleration level, when considering the results were
based on a successive 100 breakdowns. It is suggested by many
researchers that successive breakdown could increase the test
results, attribute to the removal of surface irregularities or
heating up the local oil [5].
Comparing the distribution parameters of ‘as-received’ and
‘processed’ oil samples, conclusion can be drawn that the
‘processed’ oils have higher dielectric strengths and less
600
standard deviations than ‘as-received’ oils. It is well known
that breakdown is initiated by the ‘weak link’ in the oil, such as
gas bubbles, particles or surface defects. Since gas contents and
electrode surfaces have been pre-controlled well, less particles
in the ‘processed’ oil produce less ‘weak links’, therefore it is
more difficult to initiated breakdown in the ‘processed’ oils and
consequently, the breakdown voltages are higher and more
consistent.
Breakdown Voltage (kV)
60
Table 2. Breakdown voltages & distribution parameter for different oil samples
‘As-received’ oil
Mean breakdown
voltage (kV)
Lowest breakdown
Voltage (kV)
Highest breakdown
Voltage (kV)
Standard
deviation (kV)
Coefficient of
variation
‘Processed’ oil
Mineral
FR3
Midel
7131
Mineral
FR3
Midel
7131
36.0
40.5
41.6
47.7
44.5
45.1
23.8
25.6
27.9
35.8
35
36.2
48.9
49.9
48
58.2
53.1
53.3
6.3
5.8
4.5
4.1
4.56
4.3
17.5%
14.3%
10.5%
8.6%
10.3%
9.7%
50
'As-received' mineral
40
30
20
10
0
0
20
40
60
80
Relative Water content (%)
100
Figure 5. Breakdown voltages as a function of relative water content for
'as-received' mineral oil
Table 3. Distribution characteristics for 'as-received' mineral oil
The particle contents are quite different for different types of
‘as-received’ oils, therefore it is more meaningful to compare
the dielectric strengths of ‘processed’ oils with similar particle
contents. For ‘processed’ oil samples, it can be noticed that the
ester-based oils have comparable dielectric strengths with
mineral oil. The coefficient of variation for ester-based oil is a
little larger than that of mineral oil, indicating the breakdown
for ester-based oil is more unpredictable and there are more
chances for ester-based oil to breakdown at lower voltages for
a low risk.
Water content
(%RH)
Mean breakdown
voltage (kV)
Standard
deviation (kV)
Coefficient of
variation
3%
10%
25%
30%
35%
55%
35
29
20
15
16
7
4.0
4.5
4.1
3.8
4.9
2.9
11%
16%
21%
25%
31%
41%
It can be seen from the result that the standard deviation, for
different relative water content, was kept around 4 kV.
However, as the mean breakdown voltage decreased the
coefficient of variation increased from 10% at water content of
3% RH to nearly 40% at water content of 55% RH.
b) Water effect on ‘processed’ mineral oil
The same test was carried out on the ‘processed’ mineral oil.
3.2 Water effect on withstand strength
The ‘processed’ mineral oil is clean oil with less than 500
Water affects the breakdown voltages of oil by influencing particles with diameter larger than 5µm in 100ml oil sample.
the conductivity of microscopic particles suspended in the oil. The particle counting result could be seen in Figure 1. Instead
These particles could create larger discharges in approaching of 20 breakdowns, 40 breakdown voltages were measured for a
an electrode when they are more conductive, therefore increase more precise evaluation of distribution performance. The
the likelihood to initiate gas bubbles and lead to breakdown. It average value of breakdown voltages, with the highest and
has been proved that the type of particles suspended in the oil lowest voltages, as a function of relative water content is given
also affect the water influence on the oil as the water and in Figure.6.
particles have combined effect, which could amplified the
The mean breakdown voltage decreasing tendency, as can
reduction of dielectric strength of oil [6].
be seen from the results, can be divided into 3 parts to discuss.
First, the breakdown voltage does not change much when the
a) Water effect on ‘as-received’ mineral oil
For each oil sample of different water content, 20 water content is less than 40% RH. Then, it gradually drops as
breakdowns were measured to give a distribution. The average the water content is increased between 40% and 60% RH, and
value of breakdown voltages, with highest and lowest finally it reduces to a plateau when the water content is larger
breakdown voltage, as a function of water content expressed in than 70% RH. This trend is quite different from that of the
‘as-received’ mineral oil.
relative humidity is given in Figure 5.
The lowest and highest breakdown voltage could also be
The mean breakdown voltage decreased significantly with
divided
into two parts. When the water content in oil is less
water content between 3% and 10% RH. The reduction in the
than
40%
RH, the highest breakdown voltage is kept at about
dielectric strength of oil is exponential for water content less
50
kV
and
the lowest breakdown voltage is reduced as water is
than 40% RH. When water content in oil is larger than 40% RH,
increased.
This
means although the mean breakdown voltage is
the breakdown voltage gradually reduced to a plateau. Both the
kept
nearly
the
same at this stage, there is more chance for the
lowest and highest breakdown voltage decreased with the
oil
with
larger
water content to breakdown at lower voltage.
increase of water content in oil. This result is in consistence
When
the
water
content is larger than 40% RH, both the
with previous research, bearing in mind that the particle
highest
and
lowest
breakdown voltage are reduced as the water
number and size is first time quantified here. The distribution
content
is
increased.
Consequently, it emphasizes the necessity
characteristics of oil breakdown voltages are given in Table 3.
to use the highest and lowest breakdown voltage, not just the
601
mean breakdown voltage, to evaluate the dielectric strength of
oil.
created, resulting in the significant drop of dielectric strength.
Conclusions
Breakdown Voltage (kV)
60
a) The ‘as-received’ oil samples were processed to become
clean oil. The dielectric withstand strengths of both the
‘as-received’ and ‘processed’ mineral, synthetic ester and
nature ester oil samples were investigated.
The oil dielectric strength is increased after processing, in
terms of mean breakdown voltages, highest and lowest
breakdown voltages. The standard deviation is also decreased.
The ester-based oils have comparable dielectric strength as
mineral oil, but has larger coefficient of variations, indicating
its unpredictability and probably a higher chance for
ester-based oil to breakdown at lower voltages.
'Processed' mineral
50
40
30
20
10
0
0
20
40
60
Water content (%)
80
100
b) The water effect on ‘as-received’ and ‘processed’ mineral oil
were also investigated to show the combined effect of particle
and moisture.
For ‘as-received’ mineral oil, the breakdown voltage drop
sharply with the increase of water content expressed in relative
humidity. For ‘processed’ mineral oil, the breakdown voltage is
kept steady when water content is less than 40% RH, and
decreased gradually when relative water content is increased
afterwards. The coefficient of variation is increased with water
content for both ‘as-received’ and ‘processed’ oil samples.
Figure.6. Breakdown voltages as a function of water content in % RH for
'processed' mineral oil
Table 4. Distribution characteristics for 'processed' mineral oil
water content
(%RH)
Mean breakdown
voltage (kV)
Standard
deviation (kV)
Coefficient of
variation
3%
22%
41%
67%
83%
98%
44
40
39
24
16
14
3.0
3..2
4.4
3.5
3.6
3.7
7%
8%
11%
15%
23%
27%
The distribution characteristics of oil breakdown are shown
in Table 4. The standard deviations for oil samples with
different water content, is kept around 3.5 kV, and the
coefficient of variation increased from 7% at 3% water content
RH to 27% at water content of 98% RH. However, both the
standard deviations and the coefficient of variations of
‘processed’ mineral oil are much less than those of
‘as-received’ mineral oil, indicating that the breakdown voltage
is much consistent for clean oil, and the water effect on the
dielectric strength of ‘as-received’ mineral oil is more
unpredictable. Therefore, not only the mean breakdown voltage
of ‘as-received’ mineral oil is lower, also is the withstand
voltage in the presence of water.
These results verified Chadband’s ‘weakest link’ theory that
the breakdown is caused by the weak link in the oil [7], for this
case the weak link is the conductive particles which is formed
by particles absorbing water.
When the oil is clean, there are fewer particles suspended in
the oil. As the oil is moistening by water, there is little chance
for the water molecule to combine with particles to form
conductive ‘weak link’ in the oil. At low quantities of water,
the water dissolved in the oil evenly by forming hydrogen
bonds between itself and the polar part of the oil. Although the
oil is generally considered to be non-polar, there will always be
a small quantity of polar structures within itself. The dielectric
withstands strength, therefore, will not be influenced much by
the water content. Once there is more water in the oil, both the
chance for the water to combine with particles and the chance
for the water molecules to form water cluster are increased,
leading to more ‘weak links’ suspended in the oil. Therefore
the dielectric withstand strength starts to reduce.
For ‘as-received’ oil, there are many particles and even at
low water content large quantity of ‘weak links’ could be
Acknowledgement
The first author would like to thank EPSRC DHPA award to
provide scholarship for his PhD study. AREVA T&D, EdF
Energy, M&I Materials, Cooper Power Systems, National Grid,
Scottish Power, TJ|H2b Analytical Services and Electricity
North West for their financial and technical support to form the
research consortium on ‘Alternative fluids for large power
transformers’ at The University of Manchester. Grateful thanks
are given to Dr Hongzhi Ding for his helpful suggestions.
References
[1] D. Martin, "Evaluation of the Dielectric Capability of
Ester-based Oils for Power Transformers," in School of Electrical
and Electronic Engineering. vol. PhD Manchester: Unviersity of
Manchester, 2007, p. 223.
[2] CIGRE, "Effect of Particles on Transformer Dielectric
Strength," in Working Group 17 of Study Committe 12, June 2000.
[3] D.Martin, "Evaluation of the Dielectric Capability of
Ester-based Oils for Power Transformers," in School of Electrical
and Electronic Engineering. vol. PhD Manchester: Unviersity of
Manchester, 2007, p. 223.
[4] D. Martin and Z. D. Wang, "A Comparative Study of the Impact
of Moisture on the Dielectric Capability of Esters for Large Power
Transformers," in Electrical Insulation and Dielectric Phenomena,
2006 IEEE Conference on, 2006, pp. 409-412.
[5] M. G. Danikas, "Technical Report of Particles in Transformer
Oil," IEEE Electrical Insulation Magazine, vol. 7, pp. 39-40,
March/April, 1991.
[6] K. Miners, "Particles and Moisture Effect on Dielectric Strength
of Transformer Oil Using VDE Electrodes," IEEE Transactions on
Power Apparatus and System, vol. PAS-101, pp. 751-756, March,
1982.
[7] W. G. Chadband, "The Electrical Breakdown of Insulating Oil,"
Power Engineering Journal, vol. 6, pp. 61 - 67, Mar. 1992.
602