Download Quantitative Assessment of moisture content in Transformer Oil

2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Quantitative Assessment of moisture content in
Transformer Oil-paper Insulation Based on Extended
Debye model and PDC
XIA Guoqiang. WU Guangning
College of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, Sichuan Province, China
Abstract—In order to assess the moisture content of
insulating pressboard in oil-immersed transformer, and
thus providing reliable guidance for insulation
condition and life estimation of oil-immersed
transformer, in this paper, we made four groups of
oil-paper insulation samples with different moisture
content under laboratory conditions and studied the
relationship between pressboard and moisture content
by using polarization and depolarization current (PDC)
technique. The experiment result shows that PDC
curve is very sensitive to the change of moisture
content, and the change of moisture content mainly
affects the tail of depolarization current curve; the
extended Debye model parameters can be obtained
from depolarization current curve, and as the moisture
content increases the number of branches in Debye
model increases, the largest time constant becomes
large; the peak of time domain spectroscopy turn up at
1000s., the time constant branch which is longer than
1000s presents the pressboard branches of Debye
model; the feature quantity Qpaper can show the internal
polarization of insulating paper and has a linear fitting
relationship with moisture content, so Qpaper can be
used to assess the moisture content of insulating
pressboard.
Index Terms—Debye model, insulating paper,
moisture content, oil-paper insulation, polarization and
depolarization current.
I. INTRODUCTION
Power transformer is one of the most expensive and
important equipment in power system. The condition
of the transformer is very important on ensuring the
safe and stable operation of power system [1]. The
main insulation system consist of fiber paper,
cardboard and insulating oil, its performance can
degrade under the thermal, electrical, mechanical,
chemical compound factors during its operation [2],
[3]. Moisture is one of the most important factors. If
the moisture content increase to twice as the original,
the lifetime of insulation system will be halved [4], [5].
The increase of moisture content in oil will rise
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dielectric loss and reduce breakdown voltage of
oil-paper insulation which will damage the insulation
structure and affect normal operation of equipment;
water is not only the degradation product of insulating
paper but also reaction catalyst, it will accelerate the
degradation process of insulating materials. However,
water mainly exists in insulating paper, and it has a
very low proportion in oil which also can be removed
through the oil filter. Therefore, the study on the
assessment of the insulation pressboard moisture
content is hot spot in current research.
In recent years, the demand of non-destructive test
methods and technical means on transformer
insulation system promotes the development of
diagnostic techniques based on the dielectric response.
Where, Polarization/Depolarization Current (PDC),
Return Voltage Method (RVM) are based on the
time-domain [6]–[9], Frequency Domain Spectroscopy
(FDS) is based on frequency domain[10], [11]. The
traditional way to measure the moisture content of
insulating paper is that the moisture of transformer oil
can be measured by Karl Fischer titration, then though
T.V.Oommen equilibrium curve we can obtain the
moisture content of insulating paper [12], [13]. But in
reality, there are many restrictions [13].
T.K.Saha presented the Extended Debye model of
oil-paper insulation based on PDC and connected the
relaxation time constant with branch of Debye model
[14]; JIANG Xiubo analyzed the moisture of oil-paper
insulation by RVM, studied the relationship between
moisture content and polarization spectroscopy [10],
but they did not come up with an effective feature
quantity to present the moisture content; XU Yuan
proposed a method to assess moisture content of
oil-paper insulation based on FDS [15], but this
method relative to PDC is more time-consuming. So,
in this paper, in order to obtain the feature quantity of
insulating paper of transformer, oil-paper insulation
samples were prepared with different moisture
contents under laboratory conditions. Then we
measured the PDC of samples, plotted the PDC curves
and time domain Spectroscopy; the parameters of
Extended Debye model can be calculate though the
PDC curves; finally, combining the relationship
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2016 China International Conference on Electricity Distribution (CICED 2016)
between oil-paper with the branch of Debye model, we
proposed a feature quantity of moisture content of
insulating paper.
II. THEORY
A. PDC principle [16]
According to Dielectric Physics Theory, when a step
voltage is applied on the dielectric material, dielectric
will produce charging current which is called
polarization current; when the voltage is removed, the
short circuit current of dielectric is called
depolarization current. Typical Circuit of PDC is
shown in Fig.1.
The relationship between polarization current and
time:
i p  t   C0U 0   0  f  t  
The geometric capacitance C0 can be measured by
power frequency capacitance method; insulation
resistance R0 is decided by the ratio of measuring
voltage and the sum of depolarization and polarization
current in the largest measuring time. The PDC in
oil-paper insulation can be seen as composed by
different time constant relaxation process, so it can be
modeled as the sum of a series of exponential
functions. The Extended Debye model branch
parameters can be obtained by formula (3) to (6).
n
id   [ Ai exp(t /  i )]
id  t   C0U 0  f  t   f  t  tc  
(2)
Wherein, U0 is test voltage source, C0 is geometric
capacitance of sample, f(t) is dielectric response
function, σ is dc conductivity, ε is permittivity of
vacuum, tc is measuring time of polarization.
(3)
i 1
Ai  U 0
(1)
The relationship between depolarization current and
time:
Xi’an, 10-13 Aug, 2016
ip 
1  exp(Tp /  i )
Ri
(4)
 i  Ri Ci
(5)
U0 n U0
  [ exp(t /  i )]
Rg i 1 Ri
(6)
Wherein, τi is the time constant of branch I, tp is the
largest measuring time, Rg is the conductivity of
samples.
III. DESIGN OF EXPERIMENT
ip
id
object
U0
Precision
ammeter
A
Fig.1 Typical Circuit of PDC measurement
B. Extended Debye model
Oil-paper insulation of transform is non-uniform
structure which is composed of two kinds of dielectric.
For a single dielectric, its polarization process can be
simulated as capacitors, non-uniform structure
dielectric’s polarization can be equivalent to series and
parallel of resistors and capacitors. In the research
content of T. K. Saha, the composite branches of
oil-paper Equivalent Model had been simplified and
finally the research obtained extended Debye model as
show in fig.2 [14].
R0
R1
R2
Ri
Rn
C1
C2
Ci
Cn
C0
In this paper, 1mm thick cardboard and 25#
naphthenic mineral insulating oil were selected as
experimental material. Four group oil-paper insulation
samples were prepared with different moisture
contents under laboratory conditions, and the specific
steps are as follows: first of all, insulation pressboard
was dried at 90 ℃/ 50 Pa vacuum box for 48 hours,
and transformer oil which has been degassed was
heated to 50 ℃ ; then, the transformer oil and
insulation pressboard were impregnated 2 days in
40 ℃ / 50 Pa vacuum box; after that the oil-paper
insulation samples naturally absorbed moisture in the
air, the moisture content of oil-paper sample was
decided by the weight of the moisture absorption
percentage in the air; finally we obtained four group
oil-paper insulation samples with moisture contents
1.15％、2.04％、3.02％、4.29％.
Four group samples’ PDC tests were carried out in the
three-electrode measuring device, as shown in figure 3.
PDC test equipment is PDC-Analyser-1MOD PDC
current tester, DC voltage is 500V, measuring time is
10000s. It's worth noting that before the start of the
test in each group, we need to place the entire
measuring device into 60℃ incubator for 5 hours, so
as to make sure that the internal and external
temperature of the entire system achieve a relatively
balanced.
Fig.2 Extended Debye model of oil-paper insulation
CICED2016 Session x
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2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Incubator
1E-8
Fixed Knob
Sample
Electrical
connections
Electrode
Polarization Current/A
Temperature
Probe
mc
mc
mc
mc
1E-9
1E-10
1E-11
1
.
10
b)
mc
mc
mc
mc
Depolarization Current/A
4.29%
3.02%
2.10%
1.15%
1E-10
1E-11
1E-12
1
10
100
1000
10000
t/s
a)
Depolarization current
CICED2016 Session x
1000
10000
Polarization current
Fig.4 Polarization and depolarization current with
A. Polarization and depolarization of samples
It can be seen from Figure 4(a) that when the moisture
content of oil-paper samples increases, the initial
phase of the depolarization current curve did not
change significantly, but still rise overall while at the
end of the curve has obvious increasing trend.
Reference [10] pointed out that the insulating paper
main influence the tail of the depolarization current,
while the insulating oil affects the beginning of the
current. Moisture basically has the following effects
on oil-paper insulation samples: moisture increases the
migration velocity of free charge and ion inside the
insulating paper; the hydrophilic ionic and groups easy
to combine with moisture to be charged particles, ions
deviate from the equilibrium position which led to the
polarization process enhanced. Moisture is mainly
presents in the insulating paper and has a very low
proportion in oil. Based on the above analysis, with
moisture content increases, the tail of the
depolarization curves significantly increase and the
initial phase of the curves rise overall.
1E-9
100
t/s
Fig.3 Structure of three-electrode measuring device
IV. RESULTS AND DISCUSSIONS
4.29%
3.02%
2.10%
1.15%
Paper No
different moisture contents
As shown in figure 4(b), the polarization current curve
rise with the increase of moisture, while the end of the
curves approaching steady. The polarization current
curve is composed of conduction current and different
relaxation currents, and also moisture increases the dc
conductivity. So, in the late of measure time,
relaxation currents gradually decay and the
polarization currents mainly reflect conduction
current.
B. Debye Model and Time-domain spectroscopy
Reference [14] proposed a method for solving the
extend Debye Model by depolarization current curve.
In this paper, according to this method, we solved
depolarization current curves in Figure 4(a), the
specific parameters of Debye model are shown in table
1. It can be seen from the table 1 that with the increase
of moisture content, the number of branches in the
Debye model was rising. This is because that with the
insulation
performance
dropped
significantly,
conduction current and polarization losses increased.
So the number of interface polarization increase which
is more complicated than low moisture content. So, the
number of branches of extended Debye was increased
so as to fit depolarization current curve more accurate.
It also can be seen form table 1 that the largest time
constant get larger with the increase, the reason as
mentioned above
Tab.1 Extended Debye model parameters
mc/％ 1.15
2.04
3.02
4.29
τ1/s
—
—
—
1.4
A1/A
—
—
—
5.6e-12
τ2/s
—
—
1.5
17.8
A2/A —
—
4.89e-12 7.61e-12
τ3/s
1.5
1.3
17.1
175
A3/A
2.31e-12 5.89e-12 1.13e-11 1.36e-11
τ4/s
17.5
172
173
350
A4/A 6.68e-12 8.6e-12
2.24e-11 2.15e-11
τ5/s
170
1013
1017
1510
A5/A 1.1e-11
2.56e-11 4.85e-11 5.64e-11
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2016 China International Conference on Electricity Distribution (CICED 2016)
id(t)·t/(A·s)
τ6/s
1012
1590
1593
2870
A6/A 1.3e-11
4.75e-11 1.1e-10
1.2e-10
Figure 6 is oil-paper samples’ time domain dielectric
spectroscopy, each curve in figure 6 is substantially
the same trend and at the time of 1000 seconds curves
emerged the first peak. Reference [17], [18] pointed
out that time domain dielectric spectroscopy contains
all types of polarization of oil-paper insulation. Each
kind of spectral peak represents a type of polarization,
the peak’s value represents intensity of polarization
and the spectral peak time reflects the response speed.
In theory, the time domain dielectric spectrum should
contain many peaks, but the intensity and response
time of depolarization processes are always different.
More intense and longer constant time polarization
process will cover the small, finally the time domain
dielectric spectroscopy only showed the peak of 1000s
which is the widest range and largest polarization.
Reference [14] point out that the oil mainly effect the
branches of low time constant, while the condition of
insulating paper influence the biggest time constant
branches. So, though the above analysis we can obtain
that the time constant which is longer than 1000s
present the pressboard branches, they can reflect the
polarization characteristics of pressboard under
different moisture content. But through the
depolarization current curve and the time domain
dielectric spectra can not obtain the quantitative
relation with moisture content of insulating-paper, so it
need to be further processed.
4.0x10
-8
3.0x10
-8
2.0x10
-8
1.0x10
-8
mc
mc
mc
mc
4.29%
3.02%
2.10%
1.15%
So in order to find out feature quantity of pressboard
moisture content, we selected the branches of
pressboard form the extended Debye model branches
to study the relationship between moisture and
pressboard. According to [19], this paper proposes a
new feature quantity -- Qpaper which presents the
depolarization charge quantity of pressboard, as shown
in equation (6), (7).
Qpaper  
10
100
1000
10000
1
50
10000
CICED2016 Session x
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(6)
 n1  t paper   n2
(7)
i  n1
measured values
fitting curve
t/s
40
Qpaper/10-8
Fig.6 The time domain dielectric spectroscopy of
different moisture content samples
C. Feature quantity of Insulation Paper
Depolarization current curve is closely related to
moisture content of oil-paper insulation. When we
study the moisture content of pressboard, although the
proportion of moisture content in oil is small, it is also
necessary to exclude the influence of the factors in oil.
According to (3)-(4), despite the time constant is very
low, it will affect the depolarization current curves and
the integration results. And in reality, besides moisture,
the transformer oil also contains a lot of acid and other
polar molecules produced by the aging of cellulose.
These factors will have an impact on the results.
n2
 Ai exp(t /  i )dt
In order to exclude the impact of oil on the integration
of formula (7), we selected the time constant branches
which are greater than tpaper (time constant of paper)
branches, the value of tpaper can obtain from the figure
6. Table 3 is the Qpaper of different moisture contents, it
can be seen that Qpaper with the increase of moisture
content gradually rised. This is because that water is
highly polar molecular, the increase of moisture
content makes the number of interface polarization of
pressboard rise.
Tab.2 Qpaper of samples with different moisture
contents
mc/％
1.15
2.04
3.02
4.29
Qpaper/e-8C 1.3142 10.124 22.406 41.871
In order to study the relationship between the moisture
content of pressboard and Qpaper, in this paper, the
moisture content of pressboard was fitted with Qpaper,
fitting results shown in Table 3. The results showed
that fitting Optimization is 0.98655, so the Qpaper of
pressboard and moisture content have a good linear
relationship. Therefore, the Qpaper can be used as the
characteristic parameter of moisture content of
pressboard.
Tab.3 Relationship between moisture and Qpaper
Fitting formula
R2
Y=-15.15055+12.98261x
0.98655
0.0
1
Xi’an, 10-13 Aug, 2016
30
20
10
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
mc/％
Fig.7 Fitting curve between moisture contents and
Qpaper of oil-paper
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2016 China International Conference on Electricity Distribution (CICED 2016)
V. CONCLUSION
In this paper, different moisture contents of oil-paper
insulation samples were prepared, and carried out the
PDC experiment. Though the Effects of moisture
content of samples on the different parts of the
Extended Debye model and the relationship between
time domain dielectric spectroscopy and Debye model
branches, finally we extracted feature quantity of
moisture content of pressboard. The conclusions are as
follows:
(1) The branches’ parameters of extended Debye
model can be obtained from depolarization curves,
insulating oil and pressboard affect different time
constant branches of Debye model; moisture content
mainly influence the tail of the depolarization current
curve, and with the increase of moisture content, the
tail of depolarization current curve rises obviously.
(2) The branch number of Debye model is rising and
the biggest time constant branch also get larger with
the increase of moisture content; the peak value of
time domain dielectric spectroscopy appears at the
time of 1000s which presents the intensity of
polarization of pressboard.
(3) We put forward a new feature quantity ----Qpaper.
Qpaper can exclude impact factors in oil and other aging
products which may affects the measurement result;
Qpaper has a good linear relationship with the moisture
content, so it can be used as feature quantity of
moisture content of pressboard.
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XIA Guoqiang. (1991- ), master's degree, College of Electrical
Engineering, Southwest Jiaotong University, Chengdu, Sichuan
Province, China. Research area: Power Equipment Condition
Assessment.
.
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