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A comparison of the PDIV characteristics of
ester and mineral oils
Carlos Gustavo Azcárraga Ramos1, Andrea Cavallini2 and Ugo Piovan3
Article originally published on the IEEE Electrical Insulation Magazine, Sept.-Oct. 2014
Abstract
The voltage-withstand properties from the point of view of partial discharge inception voltage (PDIV) of ester oils are comparable or even
better to those of mineral oils. Ester oils could therefore replace mineral oils as insulation in high voltage transformers.
Instituto de Investigaciones Eléctricas
Universidad de Bologna, Italia
3
Trafoexperts GmbH, Uster, Suiza
1
2
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Introduction
The growing price of mineral oils (MOs) over the last 15 years, and environmental and safety concerns, have promoted research on alternative insulating
fluids. Ester oils (EOs), which were used initially in transformer insulation [1],
have again attracted attention; both synthetic and natural liquids have been
investigated thoroughly, in the search for suitable substitutes for MOs. EOs
have interesting properties(CIGRE WG A2-35, 2010). Their most attractive
properties are those concerning flammability, biodegradability and hygroscopicity, a property that favors the drying of solid insulation, thus ensuring that
depolymerization of cellulose is slowed down.
EOs also have drawbacks. They can be oxidized easily, so that additives are necessary, and their use is generally limited to non-breathing transformers. Their
viscosities are larger than those of MOs, so that replacing MOs with EOs without adjusting the oil gaps between adjacent pressboard barriers requires caution.
Also, their pour points are higher, which could limit their use in cold climates.
Finally, streamer propagation under positive lightning impulses is faster than in
MO, leading to lower lightning impulse breakdown voltages.
Their advantages promoted the use of EOs mostly in medium voltage transformers (generally below 35 kV)(Bertrand, 2012). Their use in high voltage
(HV) transformers is hampered by the limited experience of manufacturers.
The higher electric stress levels in HV transformers make detailed knowledge
of the dielectric properties of the insulating oil essential. Partial discharge inception voltages (PDIVs) under ac or lightning impulse, and under corona
and creepage discharges, are also relevant (Sokolov et al., 1999).
In this paper, we report a comparison between an MO and an EO based on
soy seeds, both of commercial grade. Partial discharge tests were carried out in
order to characterize fully the dielectric behavior of both oils. The measured
data were treated statistically in order to deal with uncertainty. The experimental results are compared with the available body of knowledge and the results
of FEM simulations.
Sample preparation
Before testing, virgin MO an EO oil samples were:
1. Filtered using a filter with 2 µm pores
2. Dehydrated and degassed for 24 hours at ambient temperature and a pressure 0.1 mbar.
The effectiveness of the drying process was evaluated using a commercial Coulometric titrator. Following the above procedures and using a magnetic stirrer
during oil drying, final water concentrations of 2-3 and 20-22 ppm were obtained for MO and EO respectively.
Pressboard (PB) samples were dehydrated and degassed using the following
procedure(ASTM D2413-99, 2009):
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1. The samples were placed in an open vessel that
was heated in an oven at 115 °C for 24 hours at
atmospheric pressure.
2. After closing the vessel, it was connected to the
vacuum pump, maintaining a pressure less than
0.5 mbar for 48 hours.
3. Without breaking the vacuum, dry insulating
oil was added to the vessel until the samples
were completely immersed.
4. The temperature of the vessel was hold at 60 °C
for 48 hours before breaking the vacuum.
5. After removing the vessel from the oven, the
samples were extracted and stored in a dessicator for 24 hours, allowing them to cool to room
temperature.
Test procedure
The PDIV measurements were performed for corona
(point/plane electrodes in oil) and creepage (point/
plane electrodes laid on a board sheet) discharges.
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All tests were randomized in order to avoid illusory correlation. Thus corona PDIV tests were performed following the gap sequence 20, 5, 40 and
10 mm. This procedure was followed to ensure that
the effects of progressive, unnoticed changes in the
components, e.g., needle tip losing its sharpness,
board deteriorating as a result of burning marks, oil
absorbing humidity from the atmosphere, were not
attributed to unidirectional changes in gap length.
Measurement data were fitted to the 2-parameter Weibull distribution. Alternative distributions,
i.e., the Gaussian and the 3-parameter Weibull
distribution, were also investigated. However, the
Gaussian distribution did not fit the data convincingly, while the 3-parameter Weibull distribution,
which is much more difficult to implement, did
not yield significantly better fits than the 2-parameter Weibull distribution. The scale parameter a
(63.2% probability percentile), and the 10th percentile, B10, with their 95% confidence intervals,
are shown in the figures next. B10 is reported since,
for design purposes, it is better to compare oils at
low unreliability levels (Wang et al., 2011). The
Weibull shape parameter b is not reported, since an
indirect measure of data dispersion can be obtained
from the relationship between a and B10, i.e., the
closer a and B(10), the higher b.
PDIV tests are not at present completely specified in the relevant standards
(IEC 61294, 1993). The existing specifications should be reviewed considering
that: a) new detector types have become available and b) tests in the laboratory can be carried out with much greater sensitivities than tests in the field.
As an example, IEC 61294 (IEC 61294, 1993) dating back to 1993, specifies
that PDIV is attained when the PD magnitude exceeds 100 pC while raising
the voltage at a rate of 1 kV/s. This prescription is reasonable in the field, but
seems rather conservative in the laboratory, where sensitivity can be much better than 100 pC. Indeed, Wang and coworkers (Wang et al., 2011) found that
(IEC 61294, 1993) leads to PDIV values which are much larger than those
obtained using more accurate procedures, e.g., better sensitivity, lower rate of
voltage increase.
Considering the above, the PDIV measurements reported here were made
following a procedure which departs significantly from that specified in (IEC
61294, 1993). PDs were detected using a commercial UWB (40 MHz) detector with the capability of displaying PD pulse waveforms. PDs were coupled
through a 50 resistor in series with the test sample and a 100 pF capacitor. The
sensitivity of the detection circuit was better than 5 pC. The following procedure was used to measure PDIV:
1. Increase the voltage at 1 kV/min till PD inception (a clear PD pattern or a
recognizable PD pulse should be recorded).
2. Reduce the voltage to extinguish PD activity, i.e., to the partial discharge
extinction voltage (PDEV).
3. Wait 5 minutes at the PDEV to allow positive and negative ions remaining
after the previous PD activity to recombine.
4. Start increasing the voltage from 2kV below the PDEV in steps of 100 V
or 250 V (depending on the gap size), waiting 300 s at each step.
5. Record PD pulses and patterns during each voltage step.
6. While testing at the next voltage level, check for the presence of PD pulses
in the record for the previous voltage level (corona PD can occur but is
extremely rare).
7. If PD pulses are found, stop the test and record the PDIV.
In all cases, ten PDIV measurements were carried out at each test gap. The
results presented below show that the variance of the experimental data is low,
suggesting that the procedure minimizes measurement randomness.
a. Corona in oil
A hermetic Teflon test cell equipped with a micrometric adjustment device was
used to measure PDIV at gap lengths of 5, 10, 20 and 40 mm gaps, with
point-to-plane geometry electrodes immersed in MO or EO. The high voltage
electrodes of the test cell were tungsten needles 0.5 mm in diameter and 5 mm
long, supplied by Fine Science Tools Inc. The tip radius was less than 5µm.
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b. Creeping discharges
The test setup consisted of a point/plane electrode configuration laid on a
board sheet. The vessel was insulated from the ground to minimize the effect
of neighboring objects at ground potential. In order to control exactly the distance between the tip and the ground electrode, and be able to change the
contact point between tip and board if burning marks appeared, a movable
sample holder was constructed. The adjustable arm of the HV electrode was
positioned in such a way that the needle was at an angle of 30° with respect to
the horizontal plane (pressboard sheet). In order to maintain the same contact
forces between the needle and the pressboard in all experiments, the body of
the HV electrode was hinged to the frame of the test cell. In this way, the
weight of the electrode became the contact force and was therefore constant.
The LV electrode was moved to obtain the required test gaps (5, 10, 20 and
40 mm). Between tests the pressboard was moved in the transverse direction
in order to minimize surface damage (tracking burns). However, no tracking
damage was observed during the short time required to complete the tests.
A note on needle electrodes
As stated above, the needle electrodes were tungsten steel needles supplied by
Fine Science Tools Inc. The field at the needle tip is given by
2
Etip ~ V•
ln (4 • d/r + 1) • r
Where r is the tip radius, d is the gap length, and
V is the applied voltage (Mason, 1955). As the
current associated with PD activity can blunt the
needle tip, a 1 k resistor was inserted in series with
the tip. The tips were regularly inspected using an
optical microscope, and the needles were replaced
if signs of degradation were observed. In addition,
the accumulated data were examined for evidence
of trends, possibly due to electrode wear after every
ten PDIV measurements.
Inspection of needles from different production
batches showed that the variation in tip radius
was small within the same batch, but significantly larger between batches. So, in order to achieve
stable and consistent results, only comparative tests
should be carried out, using needles from the same
production batch.
Experimental results
Corona in oil
Corona in oil tests showed that EO has larger
PDIVs than MO (figure 1). The confidence intervals (confidence g=95%) show that the EO data
are characterized by a larger variance than the MO
data.
Creeping discharges
PDIV a and B10 and their confidence intervals
(g=95%) are presented in figure 2. In this case the
EO outperformed MO for short gaps, but not for
longer gaps. The variance of the data is small for
both oils.
Figure 3 shows a comparison of the PDIV results
for corona and creepage discharges.
Discussion
Figure 1. Statistics of partial discharge inception voltage (PDIVs) of EO and
MO measured at different gap lengths and using point-plane electrodes.
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As stated above, the PDIVs of EO are higher than
those of MO under corona and creeping discharges. In order to explain this observation, it is instructive to focus on inception fields, rather than
inception voltages, since the former provides more
insight into the inherent insulating properties of
the oils and the board/oil interfaces. The expression for the electric field at the needle tip (1) was
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derived under reasonable assumptions, e.g., needle
assumed to be a perfectly conducting hyperboloid, space charge effects neglected (Mason, 1955).
However, it is not applicable when the needle
is laid on a board sheet. We therefore carried out
FEM simulations using COMSOL Multiphysics.
In order to make the problem tractable, the radius
of the needle tip was taken as 5 mm, and the electrical properties of the materials shown in table 1
were adopted. These properties were measured in
the laboratory and compared with some relevant
references (Schultz and Küchler, 1998). Charge
injection from the needle tip into the oil was not
included in the simulations.
The calculated PD inception fields under corona
discharge are shown in table 2. Since the permittivity of the oil does not influence the solution, the
fields obtained by applying 1 V are the same for
MO and EO. It follows that the relationship between the measured PDIVs for MO and EO will
be the same as the relationship between the corresponding calculated PD inception fields. Thus EO
performs better than MO at all gap lengths. This
result might be explained in terms of the difference
between the ionization energies of the chemical
species formed in the two oils. However, the distribution of chemical species and their ionization energies are not known precisely. Another factor that
should be taken into account is the larger electrical conductivity of EO with respect to MO. Since
space charge injected from the needle tip into the
oil during the negative half-cycle of the supply voltage may have played an important role in PD inception, it is possible that the greater conductivity
of the EO prevented the buildup of space charge,
thus reducing the field at the needle tip when the
tip voltage became positive.
The results of similar simulations with the board
present are presented in table 3. It will be seen that
the electric fields for 1 kV applied are larger than
those obtained without the board. The MO electric fields are about twice those of the EO, which
partly explains why EO tends to have higher PDIV
values. The difference can be attributed to the fact
that EO and the board have similar permittivities.
The inception field values are higher than those
obtained in the absence of pressboard, showing
that the presence of the board increases the voltage
withstand capability of both oils.
Figure 2. Statistics of creeping partial discharge inception voltage (PDIV) for
EO and MO measured at different gap lengths, and using point (needle)-plane electrodes with the needles lying on a board sheet.
Figure 3. PDIVs for MO and EO measured for corona PD (without board)
and for creepage PD (with board).
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Relative
permittivity
Material
MO
2.2
MO impregnated board
3.5
EO
3.2
EO impregnated board
4.1
Permittivity
mismatch
Conductivity (S/m)
3.8×10-13
59%
12.4×10-15
3.3×10-12
28%
17.1×10-15
Table 1. Material properties for fem simulations.
Gap (mm)
Electric Field
(kV/mm) at 1kV
PDIV (kV)
Inception field (kV/mm)
MO and EO
MO
EO
MO
EO
5
51.9
8.6
9.2
450
480
10
44.5
9.4
11.0
420
490
20
36.9
11.0
12.8
400
470
40
29.4
14.2
16.7
420
490
Table 2. Estimated partial discharge inception fields - without board.
Gap (mm)
Electric Field
(kV/mm) at 1kV
PDIV (kV)
Inception field (kV/mm)
MO
EO
MO
EO
MO
EO
5
154.4
82.4
4.4
12.3
680
1010
10
134.0
71.4
9.9
13.7
1330
980
20
113.2
60.4
14.6
16.4
1650
990
40
90.9
48.7
20.0
19.4
1820
950
Table 3. Estimated partial discharge inception fields - with board.
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However, the results are somehow contradictory,
since the inception field for MO increases by a factor of almost three in going from short to long gaps,
while the EO inception field is almost constant.
Considering the inception fields and their confidence intervals, one might ask whether the changes
shown in Tables 2 and 3 in going from short to long
gaps are significant. Figure 4 shows that the inception fields are approximately constant, except for
the case of MO under creepage PD.
The observation that corona inception fields, i.e.,
measured using point-plane electrodes and without board, are generally lower than those measured
when the needle is placed in contact with pressboard could be explained in terms of the structure
of the pressboard/oil interface. It is assumed that
the transition between oil and pressboard occurs
through a mixture of pressboard fibers projecting
into the oil. Mitchinson et al (Mitchinson et al.,
2010) presented a microscope image of the pressboard/oil interface, confirming the presence of the
board fibers protruding into the oil. These authors
suggested that the interface could be more resistant to PD inception than pure oil, due to Van der
Waals forces which would prevent oil molecules
directly in contact with the pressboard from drifting freely, thereby forming a zone within which a
gradual transition from bound to free oil molecules
occurs (no-slip layer).
In order to explain the difference between MO and
EO inception fields as functions of gap length for
creepage discharges, it is instructive to study the
electric vector field near the needle tip. Figure 5
shows that, for a 5 mm gap, there are significant
differences between the field vectors, but not for a
40 mm gap. In particular, at 5 mm the field in EO
tends to be parallel to or directed into the board,
whereas in MO the field tends to be directed from
the needle into the oil. At 40 mm the electric field
tends to be parallel to or directed into the board
in both oils. This difference suggests different PD
inception mechanisms between EO and MO at 5
mm in at short gaps, e.g., in EO PDs might be initiated at weak points of the interface, but in MO in
the oil. At 40 mm PDs might be initiated at weak
points of the interface in both oils. This suggestion
is consistent with the PD pattern and PD pulse
sequences at short gaps reported in fure 6; MO
shows a pattern typical of bubble discharges with
PD pulse bursts (Pompili et al., 2008; Pompili et
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Figure 4. Electric field at inception for MO and EO measured for corona PD (without board) and for creepage PD (with board). Confidence intervals with 95% probability are also
shown).
Figure 5. Electric field distribution near the needle tip for MO
(upper) and EO (lower).
Figure 6. PD pattern and discharge behavior close to PDIV for a 5 mm gap.
al., 2009), whereas EO shows a pattern typical of
surface discharges (Cavallini et al., 2010).
It should be noted that, because of the structure of
the interface (board fibers separated by oil), and the
smaller permittivity mismatch between the board
fibers and EO compared to that between the board
fibers and MO, the electric field in the EO will
be smaller than the electric field in the MO. The
observation that EO performs better than MO in
corona inception measurements then leads to the
conclusion that, for creepage discharges, larger inception fields are to be expected for EO. However, the experimental results show that MO performs
better at long gaps. Any proposed explanation of this discrepancy must take
into account the microscopic structure of the oil/board interface. In particular,
it may be that the Van der Waals forces in the no-slip layer are stronger within
the MO/board interface than within the EO/board interface.
Our present description of the oil/pressboard interactions is still incomplete.
Some authors (Mitchinson et al., 2010; Dai et al., 2010) have emphasized that
initiation of creeping PD on a fully dried pressboard surface is very unlikely; wet pressboard sheet usually has a lower PDIV. Thus a reduction of PDIV
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Figure 7. Oil humidity before and after PDIV tests, without and with board.
from 38 kV to 28 kV was observed when the relative humidity of pressboard was increased from less
than 0.5% to 2.5%, for a gap of 40 mm (Dai et al.,
2010). In (Zainuddin et al., 2012) it is speculated
that the leakage current flowing in pressboard can
(a) heat the oil in the pores, causing gassing and,
eventually, inception of PD in the pores, and (b)
give rise to phenomena similar to dry band arcing
when PD within the pores is sufficiently energetic to dry the pressboard further and release gases.
Since ester oils are highly hygroscopic they contribute to drying of pressboard during the impregnation phase and beyond, thereby preventing high
current densities in the board. These effects were
confirmed experimentally by measuring moisture
content of the oil at the beginning and end of the
PDIV tests for corona (without board) and creepage (with board) discharges. A period of approximately 24 hr was required to complete measurements at one gap length for each oil. These data are
presented in figure 7. It will be seen that the moisture content of both oils did not increase appreciably for tests performed without pressboard sheets
(the observed increase is due to diffusion from the
atmosphere into the oil). However, the increase is
much more marked for EO for tests performed
using a pressboard sheet. These observations show
that moisture extraction continues after drying and
impregnation, and can help to ensure reliable longterm behavior of the insulation.
Conclusions
EO has proved also to be a viable substitute for
MO from the point of view of PDIV at least when
new, dried and highly purified fluids are used for
the comparison. The smaller permittivity mismatch
between pressboard and EO, compared to that between pressboard and MO, leads to a more uniform field distribution at the oil/pressboard interface. We have observed that EO has higher PDIVs
than MO for corona and creepage discharges.
However, our understanding of the oil/pressboard
interface is incomplete. There is a lack of standardization in PDIV measurements, particularly
in the case of creepage discharges. Since industry
is striving to provide new insulating oils for use at
increased transmission voltages, improving electrical testing techniques and the understanding of the
oil/pressboard interface is important.
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Acknowledgments
Carlos Azcarraga gratefully acknowledges the Instituto de Investigaciones Eléctricas for study leave
and the Mexican Science Council (CONACYT)
for financial support of his graduate studies.
References
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no. 6, pp. 1511–1518, 2009.
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August 2010.
H. Zainuddin, P.L. Lewin, and P. M. Mitchinson. Characteristics of Leakage Current During Surface Discharge at the Oil-Pressboard Interface. In Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Montreal, CA., 2012, pp. 483-486.
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ASTM D2413 - 99(2009). Standard Practice for Preparation of
Insulating Paper and Board Impregnated with a Liquid Dielectric.
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IEC 61294 (Ed.1) – 1993. Insulating Liquids - Determination of
the Partial Discharge Inception Voltage (PDIV) - Test Procedure.
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CARLOS GUSTAVO AZCÁRRAGA RAMOS
[carlos.azcarraga@iie.org.mx]
PhD in Electrical Engineering from the University of Bologna (2011- 2014). MSc degree in Electrical Engineering at the Mexican National Polytechnic Institute (2002-2004, Cum Laude). BSc
degree in Electromechanical Engineering from the Zacatepec Institute of Technology (1991-1995,
honors). After pursuing the AIT program in 1997, Carlos Azcárraga was signed by the Electrical
Equipment Department of the IIE to work in the development of state of the art diagnostic techniques for power transformers and substation equipment. He has represented the IIE in several
technical forums, including Cigré SC A3 High Voltage Equipment. He is an author and coauthor
of more than 30 technical papers related to high voltage insulation assessment. He has been a
Professor at the Universidad Cuauhnahuac, Universidad Morelos and at the Postgraduate Center
of the IIE. His current research interests include diagnostic techniques of high voltage systems,
electromagnetic transients, partial discharge physics and numerical electromagnetics.
ANDREA CAVALLINI
Andrea Cavallini received the MSc and PhD degrees in electrical engineering from the University
of Bologna. From 1995 to 1998 he was a researcher at Ferrara University. Since 1998 he has been
associate professor at the University of Bologna. His research work relates to endurance modeling
and diagnostics of insulation systems. He is interested in the physics of partial discharge (PD)
phenomena, e.g., physical/stochastic modeling of PD phenomena, the effects of the waveshape
and frequency of the applied voltage, and the position of defects, on PD phenomena. He is also
involved in more applied research concerning PD identification in different types of apparatus,
e.g., cables, rotating machines, transformers, GIS with different types of insulation, e.g., polymeric, epoxy/mica, paper/oil, gas. From 2004 to 2011 he was the Italian representative in the
SC D1 (Materials and Emerging Technologies) of Cigrè, convener of Cigrè WG D1.43 (Rotating
machine insulation voltage endurance under fast, repetitive voltage transients) and Administrative
committee member of the IEEE Dielectric and Electric Insulation Society (DEIS). He is currently
chair of the IEEE DEIS Educational Committee.
UGO PIOVAN
Ugo Piovan is the director and owner of Trafoexperts GmbH, Switzerland, a consulting company
which he founded in 2011. He was previously Technical Leader of Weidmann Electrical AG, and
prior to that, Technical Manager of TAMINI Transformers. He is an expert in ultra high voltage
(up to 1200 kV AC) and ultra high current (up to 120 kA) transformers. He is active in international bodies such as CIGRE and IEC, with a particular interest in standards for converter transformers (both industrial and HVDC). In 2012 he was honored with the IEC 1906 award for his
contribution to the development of IEC standards on converter transformers. Ugo holds a MSc in
Electrical Power Engineering from the Polytechnic of Milan.
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