Dielectric Characterization of Power Transformers Oils
Dielectric Characterization of Power Transformers Oils
Dielectric Characterization of Power Transformers Oils
igh power transformers are principally of two types: hermetic and air breathing. The hermetic transformer will always have a great advantage over the air-breathing type as long
as it remains air proof. The vast majority of air-breathing transformers are equipped with an exterior expansion vessel (necessary for this volumetric change) called a conservator, which is
connected to the transformer main body. When directly exposed
to the atmosphere, insulating liquids absorb humidity, thus diminishing their dielectric strength. As moisture is absorbed by
the liquid material, the insulation capability of the insulating paper is also reduced. Therefore, to prevent inhalation of moisture
from the atmosphere into the conservator, a dehydrating (silica
gel) breather is fitted at the end of the air inlet pipe of the conservator. The air goes in and out automatically via the dehydrating
agent. The dehydrating breather contributes to the safe and reliable operation of the transformer. If there are defects in the breathing system filter, the insulating liquid will absorb moisture from
the atmosphere. The ratio of moisture uptake depends on the oil
type used, as well as the surrounding atmospheric conditions,
i.e., relative air humidity and temperature. Therefore, it is useful
to predict the dielectric behavior of the insulation liquid under
standard outdoor conditions as well as the moisture uptake of the
insulating liquids under some selected conditions [1].
C. T. Dervos, C. D. Paraskevas, P.
Skafidas
School of Electrical and Computer Engineering,
National Technical University of Athens 9, Iroon
Polytechniou Str, Zografou 157 80, Athens, Greece
P. Vassiliou
School of Chemical Engineering, National Technical
University of Athens 9, Iroon Polytechniou Str,
Zografou 157 80, Athens, Greece
11
Fire properties
Density, 23 C [kg/m 3 ]
856
Flash point [ C]
Density, 90 C [kg/m 3 ]
810
Flame point [ C]
[ C]
Pour point
40
Toxicity
slightly toxic
Biodegradability
Water solubility,
Self ignition
130 135
[10 3 kJ/kg]
46
[ C]
330
high
20 C
[ppm]
45
650
Electrical properties (at 23 C)
Heat transfer
Cinematic viscosity, 20 C [mm 2 /s]
16
[kV]
2.3
0.135
0.125
Specific heat,
20 C
[kJ/kg.K]
>60
2.2
>10 10 4
>100 10 12
1.85
are their wide availability and low cost. Mineral oils have a relatively low permittivity, but exhibit a low flash point and are slightly
toxic. They also have the disadvantage of endangering the environment in case of a transformer leakage or fire event [3].
The initially employed transformer liquids were based on
mineral oil products, but later synthetic oils based on polychlorinated biphenyls (PCBs) were also adopted for certain field applications. Because of their low flammability, PCBs were used extensively for insulating and cooling electrical equipment, such as
transformers and capacitors. In the mid 1970s, a series of EPA
regulations restricted the manufacture, import, export, use, transportation, and disposal of PCBs [5]. Today, PCB insulating liquids in HV components are not installed. Current law requires
that many, but not all, existing PCB-containing electrical transformers be retired. However, in many cases, the operating units
may still have mineral oils contaminated with very low PCB concentrations that improve their overall electrical performance.
Currently, PCB additives have maximum allowable concentrations of the order of 50 ppm and are expected to be reduced further to 2 ppm by the end of the year 2010 [6], [7].
When searching substitutes for PCBs, ecological considerations raised the concern of searching for incombustible and nontoxic insulating liquids, and, as a result, the ester liquids [2], silicone fluids [4], and vegetable oils were proposed [8], [9]. However, their relatively high cost and low availability has limited
their use only to special transformer applications. Note that ester
liquids, silicone fluids, and vegetable oils belong to the high fire
point (HFP) materials, also known as less flammable liquids.
An HFP liquid must have a minimum fire point of 300C [10].
However, all these HFP ecological oils are characterized by their
high humidity absorbance, having room temperature saturation
levels of 740 ppm compared with the 45 ppm of mineral oils.
12
Combustion heat
150 175
(1)
where C is the capacitance with the material under test between the parallel electrodes, and C0 is the vacuum capacitance
(without the material). Let C = rCo and G = Cor, then,
(2)
where
r* = r jr
(3)
The relative complex permittivity (r*) is a dimensionless quantity, which compares the complex permittivity (*) of a material
to the permittivity of the free space (o = 8.854 1012 F/m).
r* = */ o
(4)
ASTM
method
69 kV and
below
Above 69 kV
through 288 kV
345 kV and
above
ASTM D 3487
D877
26
26
26
Dielectric breakdown,
@ 0.04 gap (kV min.)
D1816
23
26
26
20
Dielectric breakdown,
@ 0.08 gap (kV min.)
D1816
34
45
45
35
Interfacial tension,
(dynes/cm min.)
D971
24
26
30
40
Neutralization number,
(mg KOH/g max.)
D974
0.2
35
0.2
D1583
25
Relative Density
D1298
0.840 0.900**
Color
D1500
0.5 8***
0.1
20
0.03
15
0.91
0.5 max
*After: InterNational Electrical Testing Association (NETA), Maintenance Testing Specifications for Electrical Power Distribution Equipment and
Systems, 2001.
** The data are mean values obtained from many manufacturers.
***Color is not always a reliable guide to product quality and should not be used indiscriminately in product specifications (ASTM D1500).
13
Experimental Setup
For the purpose of this work, all samples were mineral-insulating oils employed for 150-kV power transformers. Measurements were performed on both types: 1) synthetic mixtures of
brand-new oil samples with varying humidity content and 2) asreceived samples from field-operating power transformers. The
tested representative samples are described in Table 3.
Oil samples C, D, E, F, and G fall within the acceptable limits
of all the physicochemical tests given in Table 2 and, therefore,
are considered as acceptable insulating liquids for oil-filled electrical equipment. Oil samples A and B would require dehydration before employment in field operations. The purpose of employing various water concentrations in samples A and B was to
study the effect of free water content on oil permittivity. In practice, this may simulate water droplet formation produced by high
moisture-containing oil when it cools down.
Figure 3. The experimental setup used for the multi-frequency, complex permittivity evaluation of transformer insulating oils at
different temperatures. It includes 1) a high precision LCR measuring unit, 2) the measuring cell located in the environmental
chamber, and 3) the temperature-controlled environmental chamber. All units are computer controlled.
cluded that good quality oil samples will be characterized by low
r values (practically measured in the range of 2.2) and low r
values (and, therefore, tan, which in practice is measured in the
range of 10-3 to 10-4 at all frequencies). In the absence of polarization processes, these results should be temperature independent, implying that the examined samples are high purity oils
being entirely free of contaminants. For the purpose of this study,
measurements were performed at three different temperatures
(20oC, 35oC, and 50oC).
According to the experimentally obtained results, as the temperature of the material increases, the real part of the relative
Sample description
complex permittivity (r) tends to decrease slightly. Additionally, r is frequency independent (Figures 5a, 6a). The temperature effect on tan is shown in Figures 4a and b. Here, for any oil
sample, the highest losses are always encountered at the highest
temperatures. Therefore, temperature increase induces the worstcase operating electrical conditions (i.e., highest breakdown probabilities). Throughout this work, the same scaling has been used
for all corresponding graphs for comparison purposes.
16
a
r/
TABLE 4. Relation between the water content and
T
50 oC.
ratios. Examined temperature range 20 o C
Sample ID
45 ppm
16 ppm
a humidity level of 13 ppm, exhibits an almost perfect permittivity response (i.e., low r, low r/, and low tan values).
According to this, although all of the examined samples originating from field-operating power transformers fulfill the ASTM
requirements of all of the physicochemical tests given in Table 2,
they tend to exhibit significant variations among their permittivity responses. These can be investigated by the temperature-dependent dielectric spectroscopy in the frequency domain. Permittivity changes are created by the polarizable/ionizable aging
byproducts within the insulating liquid. During the oil degradation process, gases will evolve as the hydrocarbon chains breakdown, leaving large free radicals at the liquid interface [15]. The
collision of such free radicals usually generate large colloidal
decay products with an average molecular weight of 450 to 550
that are no longer soluble in oil and precipitate as sludge or ash
[12]. The most frequently detected gases by gas chromatography
(GC) in transformer mineral oils are O2, N2, H2, CH4, CO, CO2,
C2H6, C2H4, C2H2, C3H8, and other liquid hydrocarbons [15], [16],
[22]. Limitations of dissolved gas analysis (DGA) techniques
(ASTM D3612) may arise by the fact that the gases in the oil can
be dynamically either evolving or absorbing. Therefore, measured concentrations will be the net effect of two competitive
reactions of the traced specific gases.
The proposed temperature-dependent dielectric spectroscopy
acquires overall information concerning the electrical energy loss
and storage by the insulating oil, thus allowing for systematic
differentiation monitoring among the operational transformer oils.
Conclusions
The monitoring of the complex permittivity of transformer
oils, as a function of frequency and temperature, may provide
insight information concerning the state of the insulation within
the components. In this work, it has been investigated as a quality control method, providing service life estimations of power
transformers. The formation of databases recording polarization
effects in a wide frequency and temperature range may be used
effectively as a working tool for service engineers. Reliable insulation monitoring will reduce accidents in aged HV transform-
17
ers, some of which have been in operation since the 1960s. These
transformers always impose an environmental threat because of
possible accidental leaks or fires, especially when containing
traces of PCBs.
Acknowledgments
The authors thank the Public Power Corporation of Greece
for supporting this project. Special thanks to field engineers A.
Dratsas and N. Stefanou for providing the oil samples.
References
[1] B. H. Ward, A survey of new techniques in insulation monitoring
of power transformers, IEEE Elect. Insul. Mag., vol. 17, no.3, pp.
1623, 2001.
[2] I. Fofana, V. Wasserberg, H. Borsi, and E. Gockenbach, Retrofilling
conditions of high-voltage transformers, IEEE Elect. Insul. Mag.,
vol. 17, no. 2, pp. 1730, 2001.
[3] H. Borsi, Esterfluessigkeit Midel 7131 als Ersatz fuer Mineraloel
in Transformatoren, Elektrizitaetswirtschaft, Jg. 93, vol. 24, pp.
15231528, 1994.
[4] I. Fofana, H. Borsi, and E. Gockenback, Fundamental investigations on some transformer liquids under various outdoor conditions, IEEE Trans. Dielect. Elec. Insul. vol. 8, no. 6, pp. 1040
1047, 2001.
[5] J. Harte, C. Holdren, R. Schneider, and C Shirley, Toxics A to Z: A
Guide to Everyday Pollution Hazards. Los Angeles, CA: University of California Press, 1991.
[6] Official Journal of European Communities, Commission Decision of 16 January 2001, establishing two reference methods of
measurement for PCBs pursuant to Article 10(a) of Council Directive 96/59/EC on the disposal of polychlorinated biphenyls and
polychlorinated terphenyls (PCBs/PCTs): Article 2: PCBs in insulating liquids, L23/31, 25.1.2001.
[7] EPA, PCB Regulatory Improvement Act, Oct. 1988. H. Kanbe
and M. Shibuya, Solvent cleaning of pole transformers containing PCB contaminated insulating oil, Waste Manage., vol. 21, pp.
371380, 2001.
[8] T. V. Oommen, Vegetable oils for liquid-filled transformers, IEEE
Elect. Insul. Mag., vol. 18, no. 1, pp. 611, 2002.
[9] T. V. Oommen, et al., Biodegradable transformer fluid from high
oleic vegetable oils, Doble Conf. Paper, Apr. 1999.
[10] H. Borsi, Dielectric behavior of silicone and ester fluids for use in
distribution transformers, IEEE Trans. Elec. Insul., vol. 26, no. 4,
pp. 755762, 1991.
[11] V. T. Morgan, Effects of frequency, temperature, compression, and
air pressure on the dielectric properties of a multilayer stack of dry
Kraft paper, IEEE Trans. Dielect. Elect. Insul., vol. 5, no 1, pp.
125131, Feb. 1998.
[12] R. Ferguson, A. Lobeiras, and J. Sabau, Suspended particles in
the liquid insulation of aging power transformers, IEEE Elect. Insul.
Mag., vol. 18, no. 4, pp. 1723, 2002.
[13] M. Kanno, N. Oota, T. Suzuki, and T. Ishii, Changes in ECT and
dielectric dissipation factor of insulating oils due to aging in oxygen, IEEE Trans. Dielect. Elec. Insul., vol. 8, no. 6, pp. 1048
1053, 2001.
[14] M. A. A. Wahab, M. M. Hamada, A. G. Zeitoun, and G. Ismail,
Novel modeling for the prediction of aged transformer oil characteristics, Elsevier Science S.A. Electric Power Systems Research,
vol. 51, pp. 6170, 1999.
[15] M. Duval, and A. dePablo, Interpretation of gas-in-oil analysis
using new IEC publication 60599 and IEC TC 10 databases, IEEE
Elect. Insul. Mag., vol. 17, no. 2, pp. 3141, 2001.
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[16] V. G. Arakelian, Effective diagnostics for oil-filled electrical equipment, IEEE Elect. Insul. Mag., vol. 18, no. 6, pp. 2638, 2002.
[17] M. P. Goetz, Permittivity measurements of frequency dependent
electronic materials, Hewlett-Packard Application Note 59641506E, CA, 1995.
[18] American Standards for Testing Materials, ASTM D150, 1998. Test
methods for AC loss characteristics and permittivity (dielectric
constant) of solid electrical insulation materials.
[19] W. S. Zaengl, Dielectric spectroscopy in time and frequency domain for HV power equipment, Part I: Theoretical considerations,
IEEE Elect. Insul. Mag., vol. 19, no 5, pp. 519, 2003.
[20] CIGRE Task Force 15.01.09, Dielectric response methods for diagnostics of power transformers, IEEE Elect. Insul. Mag., vol. 19,
no. 3, pp. 1218, 2003.
[21] J. E. Castle, T. B. Whitfield, and M. Ali, The transport of copper
through oil-impregnated paper insulation in electrical current transformers and bushings, IEEE Elect. Insul. Mag., vol. 19, no. 1, pp.
2529, 2003.
[22] M. Duval, A review of faults detectable by gas-in-oil analysis in
transformers, IEEE Elect. Insul. Mag., vol. 18, no. 3, pp. 817,
2002.
sion Systems and Materials Technology of the School of Electrical and Computer Engineering (NTUA, Greece). The main topic
related to this work is the insulation diagnosis of oil-filled electrical equipment.
19