High Frequency dielectric Response of Paper/Oil
Insulation
L.E.Lundgaard1, D.Linhjell1, Ø.L.Hestad2, J-T.Borlaug2
1
2
SINTEF Energy Research, 7465 Trondheim, Norway
Faculty of natural Sciences and Technology, The Norwegian University of Science and Technology, Trondheim, Norway
Abstract- Dielectric response of oil impregnated kraft paper has
been measured in the frequency range 8 Hz to 2MHz in a custom
designed test cell. Temperature was varied between -17 and 60
°C, allowing master curves for 20 °C condition to be established
for the frequency range 10 mHz to 10 MHz. Paper types and
moisture was varied. The dielectric response showed losses in the
low frequency region dominated by conduction and in the high
frequency region, by a loss peak located around 10 MHz. The
activation energy for the two relaxation mechanisms was found
to be different: about 0. 9 eV for the conduction influenced part
and between 0.4 and 0.7 for the part influenced by the high
frequency peak. The low frequency part of the dielectric response
is very dependent on the water content, while the high frequency
part seems quite unaffected by water.
I. INTRODUCTION
The behavior of frequency dependent complex permittivity
of materials is often denoted the dielectric response, and is
explained in details by Jonscher [1, 2]. The response can be
measured either in frequency domain or in time domain as a
step response of polarization and depolarization [3], and it is
within certain restrictions possible to calculate from one
domain to the other using Fourier transform.
Within the electric power community dielectric response
measurements have become popular for diagnostic purposes
and can be used for diagnosing water content of solid
insulation both in transformers and mass impregnated cables
[4, 5, 6, 7, 8]. The techniques used this far depict the average
condition of the paper and oil insulation. Either by using
polarization and depolarization measurements blind to high
frequency effects, or using dielectric spectrometers measuring
in the frequency range below 1 kHz. Lately the occurrence of
copper sulfide which is unevenly distributed along windings
has drawn attention to more high frequency characteristics of
impregnated insulation. High frequency behavior of paper oil
insulation is also interesting for calculating pulse propagation
properties of oil insulated cables. Very little information is
available of the dielectric response behavior of oil/paper
insulation in the megahertz range.
To make a diagnosis of a composite transformer insulation
you need to know the dielectric characteristic of both the
cellulose and oil insulation, and the dependence on
temperature.
This paper aims to study the dielectric response of kraft
paper at frequencies up to 2 MHz. The main part of the study
is done on Munksjö Termo 70, a non upgraded paper at
varying moisture. Also some other paper brands were
investigated.
II. BACKGROUND
In the frequency domain the dielectric response takes the
form of a spectrum of the complex dielectric permittivity εr.
Here we skip the subscript and use ε to denote the relative
permittivity. The measured apparent complex permittivity
consists of a real part
ε ′ =1+ χ ′(ω )
and an imaginary part:
σ
ε ′′ =
+ χ ′′(ω )
ε 0ω
where χ is the complex susceptibility, σ is the conductivity and
ω the angular frequency. At low frequencies, below the range
where susceptibility plays a role, the conductivity contribute
more to the imaginary part of the measured apparent
permittivity. The apparent permittivity increases inversely
with falling frequency. In principle the real part should then
remain constant. Often the real part is found also to vary as the
inverse of the frequency. This is called low frequency
dispersion.
A dielectric spectrum is highly temperature dependent;
increasing temperature shifts the spectra towards higher
frequencies. Often the spectra plotted in a log-log plot have
the same shape only being shifted in frequency systematically
with temperature. When an Arrhenius analysis shows that this
frequency shift is proportional to e-Ea/kT, where T is the
absolute temperature and k is Boltzmann’s constant, we say
that the process is thermally activated with an activation
energy Ea. The higher the activation energy is, the stronger the
temperature dependence is. This feature gives two advantages.
First it is possible to design Master curves where temperature
shifts are used to extend the spectrum outside the limits of the
measurements. Second one gets a control of the quality of the
measurement by intercomparison of spectra from different
temperatures.
One should however be aware that there may be several
polarization mechanisms and that they could have different
activation energies.
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III.
EXPERIMENTAL TECHNIQUES
A. Test set-up
The measurements were done using a Novocontrol Alpha A
analyzer with a ZG4 4 wire impedance interface with an
output of up to 3 V. The upper frequency limit of the
instrument is 20 MHz and the lower limit depends on the
capacitance of the test object. It was operated and controlled
via software made in LabVIEW. Calibration and testing of the
equipment and software was done with proprietary software
from Novocontrol.
The test cell was made of two circular Teflon dishes with 25
mm diameter stainless steel electrodes. One electrode had a
spring system to give a high plateau pressure (1 kg/cm2) and
tight fit to the sample. The cell was sealed with o-rings, and
had a long oil-filled capillary for pressure compensation
during temperature changes. The cell was connected to the
ZG4 module in three-wire mode with three 30 cm long cables
with QN-connectors. Calibration modules were used to
compensate for the connection cables.
The test-cell was lowered into a temperature controlled bath
that was regulated down to -17 °C and up to 80 °C, earlier
described in [8]. The test cell operates without guard and
effect of stray capacitances and conduction at edges was not
compensated for. However good correlation was found to
earlier measurements where guard was used [8].
The whole set-up was controlled via a LabVIEW program
that took care of temperature setting, measurement and data
output.
sequence at 20 °C, 40 °C, 60 °C, 80 °C, 20 °C, -1 °C, -10 °C,
and 20 °C One series was also tested down to -17 °C. The
temperature was measured at the outer surface of the cell and
response measurement was started when 3 consecutive, 15
minutes interval measurements had stabilized within 0,1 °C.
One total cycle took about 48 hours.
Moisture content in paper was measured with Karl Fisher
titration of a methanol extraction from larger paper samples
treated in the same way as the tested samples.
D. Treatment of results
Fig. 1 shows the spectra of ε´ and ε´´ from one measurement
series. Attempts to combine these to one master curve gave a
poor fitting. We therefore split the spectra in two; below and
above the minimum. Then the low frequency (LF) parts and
the high frequency (HF) parts were separately fitted to master
curves using 20 °C as a reference. Activation energies were
then calculated for each of the parts.
IV. RESULTS
Fig. 2 shows the resulting HF master curve established by
moving the various curves along the frequency axis to fit with
the 20 °C spectrum. One can see that they fit quite well,
except for the 80 °C. The curve now is extended from the 2
MHz measured range to a new range up to 20 MHz. Plotting
the natural logarithm of the frequency shifts used to move the
plots from Fig. 1 into the master curve of Fig. 2 versus the
inverse temperature yields an Arrhenius plot as shown in Fig.
3. The activation energy is then found from the gradient of the
curve.
After calibration the system could be used up to 3 MHz with
minimal errors. This was verified by measuring on a sample of
0,2 mm unplasticized PVC and comparing with results from
the literature [9].
B. Objects
The test samples were made as 25 mm circular cut sheets of
kraft paper that were conditioned to a certain moisture content
and impregnated with Nynäs 10X transformer oil. Three
different paper types were used.
• Munksjö Termo 70 specifically prepared for good ageing
properties, thickness 60 µm. Two layers of paper were used
in objects.
• Munksjö Termotrans is a thermally upgraded paper, 90 µm,
and two layers were used in objects.
• Munksjö plain is a standard crude paper, thickness 142 µm,
only one layer was used in objects.
Different number of paper layers were used for the different
paper types to get samples with a suitable capacitance.
Fig. 1. Measured responses for Termo 70 with 3.4 % water content.
Upper curve shows ε´ and lower curve shows ε´´.
C. Test procedures
All objects were first heated to 80 °C for ten hours to
condition the papers and stabilize the response because of a
conditioning effect [8]. Measurements were then performed in
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Fig. 2. Master curve for high frequency part of spectra from Fig. 1. Reference
temperature is 20°. Upper curve shows ε´ and lower curve shows ε´´.
Fig. 4. Master curves for HF and LF parts of spectra for Termo 70 at varying
water content. Upper curve shows ε´ and lower curve shows ε´´.
Fig. 3. Arrhenius plot of logarithm of frequency shift for mastercurve fitting
of spectra from Fig. 1 versus inverse temperature as basis for calculation of
activation energy Ea..
This procedure was repeated for all experiments. TABLE 1
shows the activation energies found. Apparently the activation
energy is different for the low frequency and high frequency
parts.
TABLE 1
ACTIVATION ENERGY CALCULATED FOR THE LF AND HF PART OF THE SPECTRA FOR
DIFFERENT PAPER TYPESAT VARYING WATER CONTENT
Paper Type
Water content
[%]
1.2
Termo 70
Plain
Termotrans
Activation energy Ea [eV]
LF part
HF part
0.92
0.40
1.2
3.4
5.0
0.88
0.80
0.42
0.59*
0.67
0.5
0.7
0.75
0.77
0.58
0.33
*measured in the range -17 - 40 °C
Fig. 4 and Fig. 5 show the influence of the water and on
paper types on the master curves.
Fig. 5 Master curves for HF and LF parts of spectra for three different paper
types at lowest water content. Upper curve shows ε´ and lower curve shows
ε´´.
V. DISCUSSION
Earlier one has used only one master curve and activation
energy for cellulose at one moisture level [4, 6, 8] assuming
only one relaxation mechanism. All these studies have
identified water as a major influence on the master curve. It
has been proposed that the position of the minimum in the ε´´value could be used for diagnosing water content of the
cellulose [6].
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Our results clearly show that the assumption of only one
activation energy is wrong. The fact that the activation energy
and that water dependence is different for the LF part and the
HF part suggests two mechanisms, mainly conduction for the
LF part and a relaxation mechanism for the HF part. The high
frequency peak is wider than a Debye-relaxation, suggesting
multiple synergetic effects.
frequency region being influenced by conductive phenomenon
and being sensitive to water content. Another in the high
frequency region with a resonance peak in the 10-20 MHz
region. The high frequency resonance peak is little influenced
by water.
REFERENCES
The LF part seems to be mainly influenced by conduction
mechanism, but the fact that the real component of the
permittivity ε´ changes is not in line with this. If it had been
pure conduction behavior it should not have influenced the
real component. This behavior is usually called low frequency
dispersion. Possibly this could be explained by cellulose being
a porous material where the oil at low frequencies will short
circuit the spaces between the cellulose fibers thus reducing
the virtual thickness of the sample.
It is no surprise that the conduction dominated LF part is
influenced by the water content. Water will increase
dissociation, increase the number of charge carriers (ions) and
increase conductivity in the oil that surrounds the cellulose
fibers. The activation energy found here being 0.8-0.92 eV is
somewhat smaller than the 0.98 – 1.08 eV values found earlier
[8], and shows that these values are uncertain. It is larger than
the values earlier found for oil (about 0.5 eV), showing that it
is not only dominated by the oil. Since the measurements were
done without guard, the low value could be a sign of some
influence of conduction in the oil around the object. Also the
variation with water content was within 25 % of what found
earlier [8].
The relaxation is not water dependent and seems to be more
related to the cellulose fibers themselves. The results shown in
Fig. 4 did not reveal the relaxation peak clearly.
Measurements down to -17 °C extended the master curve a bit
more than showed in Fig. 4, and suggest that it will be found
in the 10-20 MHz range. To get a better identification of the
peak we need either to improve the high frequency limit of the
set-up or make measurements at lower temperatures.
[1] A.K. Jonscher: ” Dielecttric Relaxation in Solids”,
Chelsea Dielectric press Ltd, 33 Lynwood Road, London,
1983.
[2] A.K. Jonscher: ”Universal Relaxation Law”, Chelsea
Dielectric Press Ltd, 33 Lynwood Road, London, 1996
[3] Cigre SC TF D1.01.09 (S.Gubanski), “Dielectric
Response Methods for Diagnostics of Power
transformers”, Brochure No. 254, Paris 2004
[4] U.Gäfvert, G.Frimpong and J.Fuhr, “Modelling of
Dielectric Measurements on Power Transformers”, Paper
15:1.2, CIGRÉ session, Paris, August 1998.
[5] C.Ekanayake, “Diagnosis of Moisture in Transformer
Insulation”, Thesis, Chalmers Univ. of Technology,
Gothenburg, Sweden, Jan. 2006.
[6] R.Neimanis: “On Estimation of Moisture Content in Mass
Impregnated Distribution Cables”. Thesis, Royal Institute
of Technology (KTH), Stockholm, Sweden, 2000.
[7] V.d.Houhanessian “Measurement and Analysis of
Dielectric Response in Oil-Paper Insulation Systems”,
Thesis, Swiss Federal Institute of Technology, Zürich,
Switzerland, 1998.
[8] D.Linhjell, L.Lundgaard, U.Gäfvert:„Dielectric Response
of Mineral Oil Impregnated Cellulose and the Impact of
Aging”, IEEE TDEI
[9] N.G.McCrum, B.E.Read, G.Williams: ”Anelastic and
Dielectric Effects in Polymeric Solids – Dover Edition”,
Dover Publications Inc, 31 East 2nd Street, Mineola, NY,
11501,1991
The comparison of the different cellulose types does not
support any clear conclusion. The HF and LF parts do not fit
as well as in Fig. 4. The water content was a bit different, but
cannot explain the large difference between Termo 70 and
Termotrans. The discontinuation seen in ε´´ for plain paper
does also appear in the ε´ value. This is clearly an artifact and
believed to stem from the curve fitting. For this paper also the
gradient of the LF part is different. This sample had only one
paper layer making oil channel bridging through the paper
possible. The basic shape is the same for all paper types, but
more work is needed to sort out potential differences between
different paper qualities.
VI. CONCLUSION
It is shown that the dielectric response of oil impregnated
cellulose is influenced by two mechanisms. One in the low
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