Paper P510
Cigré 2009
6th Southern Africa
Regional Conference
21 rue d’Artois, F-7508 PARIS
http://www.cigre.org
ENVIRONMENTAL FRIENDLY INSULATING LIQUIDS A CHALLENGE FOR POWER TRANSFORMERS
G. J. PUKEL*
Siemens Transformers Austria
GmbH & Co KG
(Austria)
R. SCHWARZ
Siemens Transformers
Austria GmbH & Co KG
(Austria)
F. BAUMANN
Siemens Transformers Austria GmbH
& Co KG
(Austria)
F. SCHATZL
Siemens Transformers
Austria GmbH & Co KG
(Austria)
A. GERSTL
Siemens Transformers Austria GmbH
& Co KG
(Austria)
SUMMARY
The most widely used liquid for electrical insulation and heat transfer in transformers is
mineral oil which is based on crude oil. Even though their technical characteristics have been
optimized over the last century to meet changing requirements, environmental performance
and their availability in the future must be addressed as petroleum resources are eventually
going to run out. Due to their excellent biodegradability properties and the fact that they can be
fabricated of renewable resources the demand for ester fluids is rising. Such environmentally
friendly insulating liquids are produced by several companies. In distribution transformers
business they have shown to be reliable in operation. Even with mineral oil in high voltage
applications, every minor change in transformer design or transformer material needs to be
vetted. Changing a crucial component such as the insulating liquid itself, the impact must be
proven most accurately. This paper illustrates issues which must be clarified in order to
successfully operate power transformers filled with these environmentally friendly and
renewable insulating liquids.
KEYWORDS
Power transformers, oil
*
Siemens Transformers Austria GmbH & Co KG, Elingasse 3, A-8160 Weiz, Austria, Email:
georg.pukel@siemens.com
1
INTRODUCTION
Many investigations have been carried out and are still in progress concerning alternative
insulating fluids [1]. These investigations are driven by research facilities at universities, but
also by liquid suppliers, utility companies and transformer manufacturers. Over the last 15
years the demanded performance and the standards for transmission equipment – such as
transformers and their insulation systems – have changed dramatically [2]. Areas of high
population density are growing more and more. To assure the rising energy demand even
large power transformers have to be placed in these areas. When power transformers, of
several hundred MVA, must be accommodated in residential tower blocks, terms like fire
point and environmental effects become more and more important.
Flash point [°C]
Fire point [°C]
Biotemp (ABB)
Envirotemp FR3 (Cooper)
MIDEL eN (M&I)
MIDEL7131 (M&I)
BecFluid 9902 (Elantas)
Powersil Fluid TR50 (Wacker)
Nytro 4000X (Nynas)
0
• naturel ester
• silicone fluid
• synthetic ester • mineral oil
50
100
150
200
250
300
350
400
Temperature in ° C
Figure 1: Typical flash and fire points of insulating fluids based on natural, synthetics and
mineral oil origins [3-9]
Next to mineral oil following types of alternative insulating liquids are capable and they are
already in common use at voltage levels up to 40 kV.
TYPES OF INSULATING LIQUIDS
MINERAL INSULATING OIL
Mineral oil is made of fossil oil and consists of hydrocarbon compounds with various bonds.
These molecule structures can be divided into paraffinic, naphthenic, aromatic and olefin
bounds (figure 2). In varying ratios these components are contained in all mineral oils [10].
The main disadvantages of transformer oil are very limited biodegradability characteristics
and the low fire point (figure 1).
Paraffin
Naphthenes
Aromatic
Olefine
Figure 2: Hydrocarbon compounds in mineral oil [11]
2
SILICONE LIQUID
Silicone fluids specially developed for transformer applications are fully synthetic coolants
and insulation fluids. Due to the high ignition temperature and the self extinguishing behavior
these liquids present a lower fire hazard than conventional transformer oil [4].
The thermal stability, even under the presence of air, is better than that of the other liquids.
The silicone fluid used is one of the polydimethylsiloxane-structured types as shown in figure
3 [12].
Figure 3: Structural formula of silicone fluid [12]
Kinematic Viscosity in mm²/s
One disadvantage is a high viscosity at higher temperatures (figure 4), so low-viscosity
silicone liquids were developed additional. Very poor lubrication properties, very limited
biodegradability characteristics and forming jelly-like bridges of silicone-oxide under arcing
are further disadvantages.
5000
1000
10
0
silicone fluid
natural ester
10
synthetic ester
mineral oil
2.5
-20 -10 0
10 20 30 40 50 60 70 80 90 100 110
Temperature in °C
Figure 4: Typical viscosity values of insulating fluids based on natural, synthetics and
mineral oil origins [3-8]
SYNTHETIC ESTERS
Synthetic esters are derived from chemicals. They are usually the product of a polyol with
synthetic or natural carboxylic acids to give structures where several acid groups are bonded
to a central polyol structure. Polyol is a molecule with more than one alcohol functional
group. The acids used are usually saturated in the chain, giving the synthetic esters a very
stable chemical structure [13].
Figure 5: Structure formula of a synthetic ester [14]
3
The viscosity of synthetic ester fluid is about four times higher as the viscosity of mineral oil
at room temperature (figure 4). Their flash and fire points are higher than those of mineral oil
(figure 1) [3, 5, 6].
NATURAL ESTER
Natural ester fluids can be broken down into saturated, single-, double and triple unsaturated
fatty acids. Saturated fatty acids are chemically stable, but have a high viscosity. Triple
unsaturated fatty acids have a lower viscosity but they are very unstable in oxidation. To
reach an acceptable value of oxidation stability of natural esters, it is necessary to add suitable
antioxidants. In addition to DBPC specific antioxidants that use complex phenols amines are
in operation. The total amount of antioxidants is limited to 1% and below due to the increase
of the conductivity to an unacceptable value. Fluids with a high percentage of single
unsaturated fatty acids have proven suitable. Plants lead to seed oils which can be
characterized by the relative quantities of fatty acids as shown in Table 1 [15, 16]. They are
highly biodegradable.
Table 1: Typical fatty acid composition of some vegetable oils [15].
Vegetable Oil
Sunflower oil, high oleic
Safflower oil, high oleic
Olive oil
Canola oil*
Corn oil
Soybean oil
Sunflower oil
Cottonseed oil
Peanut oil
Safflower oil
Unsaturated Fatty Acids Saturated Fatty Acids
in %
in %
Mono80.8
75.3
73.3
55.9
24.2
22.5
19.6
17.8
17.8
12.1
Di8.4
14.2
7.9
22.1
58.0
51.0
65.7
51.8
51.8
74.1
Tri0.2
0.6
11.1
0.7
6.8
0.2
0.2
0.4
9.2
6.1
13.2
7.9
12.7
14.2
10.5
25.8
13.6
8.5
*Low erucic acid variety of rapeseed oil; more recently canola oil containing over 75%
monounsaturate content has been developed.
Viscosity of natural esters is about four times higher than the viscosity of mineral oil. Their
flash and fire points are significantly higher compared to mineral oil [3, 6, 7, 8, 9].
COMPARISON OF ESSENTIAL PROPERTIES
The basic purposes of transformer oil are electrical insulation, cooling and lubrication to
sliding components. Very important characteristics are discharge, flashover and breakdown
behaviors.
As well as good compatibility with the other materials used in transformers. Resistance to
oxidation and chemical reaction with cellulose in the presence of moisture and temperatures
up to 100°C is a particularly essential feature for a lasting insulation system.
The different moisture behavior of the insulation liquid must also be considered for its
interaction with the impregnated cellulose. Mineral oil can only absorb water about 60 ppm at
room temperature, natural and synthetic esters can bind water many times higher than that (up
to 2700 ppm at room temperature) [17, 18].
The following issues have to be clarified before successfully using environmentally friendly
insulating liquids in power transformers:
•
•
•
Verification of the dielectric strength (for AC, BIL, SIL, DC) in respect to mineral oil,
for characteristic configurations
Material compatibility
Oxidation behavior and interaction with cellulose
4
•
•
•
•
•
•
•
•
•
Different permittivity match (oil/board)
Influence on the cooling system
Bubbling, dynamic moistening of paper/board.
Electrification
Impregnation of insulating material
Risk of thermal breakdown inside the material because of higher loss factors
Bushings, tap changer, pumps
New limits for DGA and the other fluid parameters
etc.
The comparison among insulation fluids is not only testing the oil characteristics itself, it is
also important to investigate the whole insulation system.
Standards like IEC 60156 [19] tests only the oil breakdown. To design power transformer you
need information about the whole oil/board configuration. It is essential to test the coactions
of the insulating liquid with cellulose.
INVESTIGATIONS
Various tests must be created to realistically represent the arrangement and the actual
conditions in power transformers. These tests must cover breakdown in free oil space, through
boards, and surface creepage.
A setup, combining impregnated cellulose with free oil space was built as shown in Figure 6.
The electrode shapes and dimensions where selected to get an inhomogeneous field to create
partial discharge.
a)
b)
Figure 6: a) Schematic test arrangement for an oil/board insulation system with two bare
electrodes, the point electrode is on HV, the plane electrode is grounded and b) practical
implementation of the test arrangement
Figure 7: Hermetic tight oil tank with bushing
5
Figure 7 shows the test vessel. A rig holding 8 specimens as shown in Figure 6 was placed in
a hermetically sealed oil tank with a bushing and viewing windows. Thus, the model was
impregnated in the vacuum process as in actual transformers, and protected from the outside
air and moisture.
DIELECTRIC TESTS AND PRELIMINARY RESULTS
The aim of this comparative test was to evaluate the discharge behavior of these arrangements
at various voltage stress types: AC, BIL (Basic Impulse Level) and SIL (Switching Impulse
Level).
In addition to the standard measurement equipment for the switching impulse voltage (level
and time parameter), an optical partial discharge detection system was used. This system
detects the light emission of a discharge. Examples of distinct discharge phenomena are
shown in Figure 8.
Signal of the optical system
Voltage impulse
a)
c)
No discharge
flashover
b)
d)
Partial discharge
breakdown
Figure 8a, b, c, d: Distinct discharge phenomena captured with an optical system for PD
(partial discharge) detection
Figure 8 shows the impulse voltage and the output signal of the optical system. In figure 8a no
discharge occurred.
Figure 8b shows the detection of partial discharge, while the impulse voltage does not show
any drop.
Figure 8c is an event with repeated flashover and voltage recovery.
Figure 8d is the severe sudden breakdown accompanied by a huge light emission.
For the analysis various parameters were taken into consideration:
• voltage level at beginning of pre-discharge
• time of pre-discharge before voltage breakdown
• time between start of the impulse voltage and the voltage breakdown
• time between start of the impulse voltage and the beginning of the pre-discharge
6
Predischarge before voltage breakdown
Figure 9: Time of pre-discharge before voltage breakdown
A huge advantage of the optical system is the fact that also pre-discharge (partial discharge)
can be detected easily, even if the voltage course of the voltage pulse still shows no change.
During the impulse voltage a strong transient influence of the electrical signal (conventional
PD measurement) is given, the optical system reacted however only to the partial discharge of
the experimental setup. So there is the advantage of the immunity to EMV.
As a preliminary result of these investigations a higher tendency for surface creepage for the
Midel 7131 arrangement was observed (figure 8c). Contrary the mineral oil impregnated
boards fails rather through breakdown (figure 8d).
Remarkable where the different types of traces which were found after discharge at the board
samples. Clear more visible at samples immersed in mineral oil.
The information content of the new additional parameter - named above - seems to be high.
For a solid statement we have to gain experience.
CONCLUSION
The comparison between different insulating liquids requires the collation of many different
parameters.
Many tests have already been done and valuable data has been collected.
• Esters tend more to creepage failures then mineral oils
• Innovative parameters have been recorded with an optical system
• Due to the much higher moisture capacity of esters, the interaction with cellulose has to be
considered carefully
• Experimental investigations in PD and creepage behavior are in process
• Several medium power transformers have already been tested successfully by Siemens,
more representative setups have to be created and tested to establish a base for a reliable large
power transformer design
ACKNOWLEDGMENTS
The authors would like to thank the members of the High Voltage Test Laboratory for their
assistance during the testing.
7
REFERENCES
[1] S. Tenbohlen, M. Koch, J. Baum, J. Harthun, M. Schäfer, S. Barker, R. Frotscher, D. Dohnal, P. Dyer,
“Application of Vegetable Oil-Based Insulating Fluids to Hermetically Sealed Power Transformers”
CIGRE Paris 2008, paper no. A2-102
[2] G.Balzer, F. Heil, P. Kirchesch, D. Drecher, R. Meister, C. Neumann “Evaluation of Failure Data of
HV Circuit-Breakers” (A3-305, Cigré Session 2004 Paris)
[3] Nynas, Nytro 4000X, Naphthenics Product Data Sheet 2006-12-18
[4] Technical data sheet POWERSIL® FLUID TR 50, 05.2008, http://www.wacker.com
[5] Elantas Becfluid 9902, Product Information, 07.2007
[6] M&I Materials Ltd, Product Overview Midel 7131, Technical Datasheet No 2, 01.2007
[7] M&I Materials Ltd, Product Overview Midel eN, Technical Datasheet No 2, 01.2007
[8] Cooper Power Systems, Envirotemp FR3 Fluid – Bulletin B900-00092 Product Information, June 2005
[9] ABB, Biotemp, Descriptive Bulletin 47-1050 Revised 01/02
[10] Nynas Naphthenics AB “Transformatorenöl Handbuch” www.nynas.com/naphthenics
[11] Andreas Küchler “Hochspannungstechnik 2”, Springer Verlag Berlin, Heidelberg 2005
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viscosity Siliconee Fluid” International Conference on Electrical Engineering, ICEE 2004 Sapporo
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Environmental Performance” Stuttgarter Hochspannungssymposium 2008
[14] C. Patrick McShane “Natural and Synthetic Ester Dielectric Fluids: Their Relative Environmental, Fire
Safety, and Electrical Performance” Cooper Power Systems, 2000
[15] T.V. Oommen “Vegetable Oils for Liquid-Filled Transformers” South Carolina, USA, 2002
[16] P. Boss, T.V. Oommen “New Insulating Fluids for Transformers Based in Biodegradable High Oleic
Vegetable Oil and Ester Fluid” The Institute of Electrical Engineers, London 1999
[17] H. Borsi, E. Gockenbach “Properties of Ester Liquid MIDEL 7131 as an Alternative Liquid to Mineral
Oil for Transformers Division of High Voltage Engineering Hannover, Germany, 2005
[18] M. Koch, M. Krüger, S. Tenbohlen “Moderne Verfahren zur Bestimmung des Wassergehalts in
Leistungstransformatoren” Stuttgarter Hochspannungssymposium 2008
[19] IEC 60156 “Insulating liquids – Determination of the breakdown voltage at power frequency – Test
method” Second Edition 1995-07
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