2022 Electrical Insulation Conference (EIC), Knoxville, TN, USA, 19-23 June 2022
2022 IEEE Electrical Insulation Conference (EIC) | 978-1-6654-8023-9/22/$31.00 ©2022 IEEE | DOI: 10.1109/EIC51169.2022.9833175
Vegetable fluids: the reason of the new trend
Fabio Scatiggio
Giorgio Campi
A&A Fratelli Parodi
A&A Fratelli Parodi
Genoa, Italy
fscatiggio@fratelliparodi.it
Genoa, Italy
gcampi@fratelliparodi.it
Abstract—: In recent time the use of natural esters (NE), as well
synthetic esters (SE), are growing up much faster than expected.
Despite some little drawbacks (oxidation stability, viscosity) the
advantages in terms of transformer life extension, reduced risk of
fire, biodegradability and sustainability have resulted in esters
being applied in all types of electrical equipment. Even without
mention the conventional and historical application for
distribution, large power transformers with no limit in terms of
voltage or power have been installed in many countries, railway
traction transformers and offshore wind turbine are today filled
with NE or SE only, SF6 in instrument transformer is
progressively replaced by NE.
Focus of this paper is to understand the reason of the current
market new “green” orientation. Performances like oxidation
stability, gassing, water tolerance, self-extinguishing and smoke
emission in case of fire.
Keywords—natural esters, high-oleic sunflower, fire risk,
sustainability, life extension, paper, stray gassing, transformers.
formulations of new esters and careful evaluations of new
liquids in new transformer applications results in a wider
acceptance of ester liquids in power transformers. There were
individual pilot projects presented by the industry in the past,
including prototype transformers for transmission grid in power
ratings up to 300 MVA and with voltage rating up to 400 kV [1].
Based on such successful pilot projects, the transmission
companies or other users of large power transformers consider
project specifications with broader use of natural ester liquids.
For example, the national Italian Transmission System Operator
has already installed dozens of autotransformers 250 MVA, 380
kV immersed in natural esters and is currently refurbishing its
fleet where certain proportion of the transformers rated for 250
MVA, 380 kV (1050 kV BIL) will use the natural ester liquid
[2, 3]. The transformers would benefit not only from the
improved fire safety performance and reduced environmental
footprint but additionally, would have improved overloadability.
I. INTRODUCTION – NATURAL ESTERS
Mineral oil has been considered as the most preferred liquid
insulation over the past 140 years for power transformers. With
its strong credentials as an efficient dielectric and cooling
medium coupled with low cost, it sounded unlikely that any
alternative liquid could significantly replace the mineral oil, at
least in the short time. Anyway, a totally different scenario
appeared, at the beginning in developed countries and later
worldwide, where the growing interest in fire safe, eco-friendly,
and sustainable materials could fast tip the balance in favour of
vegetable esters. Additionally, the current electrical market due
to the intermittency of the renewable energies (photovoltaic and
wind energy) and due to the high fluctuating energy demand (as
during the Covid pandemic) requires more flexible and resilient
transformers able to be overloaded occasionally, but also
permanently. The use of natural esters coupled with solid
insulating materials of higher performance (like aramid Nomex®
insulation) provides the best solution for overcoming such
complexity.
This could be achieved by the combination of the natural
ester liquid with higher class insulation system in transformer
windings
Additionally, more recently, natural ester has been also
applied to instrument transformers with the aim of creating a
totally “green” substation [5].
II. BENEFITS OF THE NATURAL ESTERS
Natural esters can be obtained from dozens of different
vegetable crops but the most largely used are soybean, rapeseed,
and high-oleic sunflower. Natural esters for electrical
application results as a mixture of some different long chain fatty
acids; the latter ones can be fully saturated (no carbon-carbon
doble-bonds), some of them are mono-unsaturated (one carboncarbon doble bond), and some others are poly-unsaturated
(multiple carbon-carbon double bonds) which encompass also
the so-called Ω-3. The balance between the different fatty acids
affects the final properties of electric natural esters, mainly
oxidation stability, viscosity, and stray gassing. In Figure 1 the
fatty acid composition of Paryol Electra 7426 based on high
oleic sunflower is shown, where the towering contribution of
oleic acid, which is monounsaturated (C18:1) and the limited
contribution of linolenic acid, which is poly-unsaturated (C18:3)
can be perceived.
In this sense, many renewable energy production plants
require transformers that use biodegradable and sustainable
liquids. They are also becoming more and more popular solution
in urban substations due to the safety of these liquids in case of
fire, as compared to traditional mineral oils.
The use of natural esters has grown exponentially even if
initially it was limited to small distribution and industrial
transformers only. The continuous improvements in
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Technical Brochure 296 [8]. Figure 3 shows the results of stray
gassing test at 120°C:
100
Fatty acid composition (%)
82,16
• after 16 hours which simulates the ramp up,
• after 164 hours simulating the plateau,
• after 164 + degassing + 16 hours demonstrating the
recrudescence after degassing.
80
%
60
40
0,03
4,67
0,15
0,25
20
0,05
0,04
Stray Gassing
0,05
0,03
2,96
0,90
8,07
0,04
1400
0
1200
C2H6
ppm
1000
0
III. CHEMICAL EVALUATION: OXIDATION STABILITY AND
STRAY GASSING
200
Oxidation Stability by Rapid-Oxy
160°C
150
100
50
0
20
Soybean 1
20,1
Soybean 2
31
Rapeseed
PARYOL
ELECTRA 7426
(high-oleic
sunflower NE)
90
67
5 20
Paryol
Paryol
Paryol Soya 16 Soya 164 Soya Rapeseed Rapeseed Rapeseed
Electra 16 Electra Electra
164+D+16
16
164
164+D+
164 164+D+16
16
There are three suggested factors that improve cellulose life
when immersed in natural esters.
187,2
1.
The high-level water tolerance of esters which is one
or two orders of magnitude greater than that of mineral
oils. But this pathway by itself cannot explain how the
paper insulation gets dry, because the cellulose has
much greater affinity for moisture than insulating
liquids.
2.
The hydrolytic protection because water reacts with the
natural ester forming long-chain fatty acids, harmless
to paper, producing a continuous dehydration process
of the cellulose.
3.
The cellulose trans-esterification, otherwise, the longchain fatty acids react with hydroxyls (-OH)
termination of cellulose promoting the paper
hydrophobicity [10].
103,2
37,8
2
263
IV. SOLID INSUALTION PROTECTION AND TRANSFORMER LIFE
EXTENSION
53,3
53,3
2
93
As it is well known, the stray gassing may be also influenced
by the presence of the additives especially the metal passivators
[9].
140°C
95,8
114
34
388
332
Figure 3: Hydrogen (H2) and ethane (C2H6) generated during the stray gassing
tests at 120°C
As a matter of fact, the peculiar composition of high oleic
sunflower natural ester improves its oxidation stability, which
results in being superior of soybean and rapeseed and almost
comparable with the uninhibited mineral oils, as demonstrated
by the accelerated tests conducted by the Rapid-Oxy technique.
The test procedure was chosen for reducing the test time
rather than IEC 61225-C modified by IEC 62770, which takes
48 hours. The used instrument was a Rapid-Oxy 100
manufactured by Anton Paar. A small sample of liquid (5 ml)
was arranged in a special closed vessel made by stainless steel.
The vessel was closed and pressurized with pure oxygen at 700
kPa and heated at 140°C. The stop criterion was the time until
pressure drop of 10% from Pmax. The test was repeated at
160°C, too. Results shown in Figure 2 are the average values, in
minutes, of three replications.
866
630
600
200
Figure 1: Fatty acid composition (%) of high-oleic sunflower Paryol Electra
7426.
803
800
400
minutes
H2
1200
0,29
0,24
0,05
30,7
Uninhibited
mineral oil
These three factors are the base for a common assumption
that ester liquids improve long-term thermal performance of
cellulose papers. Various studies on this topic are summarized
in Informative Annex C of IEC 60076-14 [11]. However, those
studies are mainly based on one specific natural ester liquid from
one supplier and may not be representative for all different
liquids available today for the industry. For that reason, there are
more and more studies available that analyze insulation systems
with different liquids of different origin and compositions [12].
Figure 2: Oxidation stability by Rapid-Oxy100 tests at 140°C (orange) and
160°C (blue)
There are many demonstrations of the direct correlation
between the Ω-3 acids content and the ethane generation (C2H6)
at relatively low temperature in absence of electrical field (the
so-called stray gassing) [6, 7]. To confirm such studies, we
carried out some stray gassing tests, in according with CIGRE
The measured improvement of cellulose thermal
performance presented in those studies is not always the same.
This may result in standardization committee removing soon the
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typical guidance on cellulose in ester performance from the
industry standards. Instead, the evaluation of insulation systems
that combine given liquids with given solid insulation materials
may be suggested.
This assumption has been confirmed by real fire experiment
on field [14] were the time to ignition of a distribution
transformer filled with mineral oil and with natural ester was
respectively 12 and 27 minutes as reported in Figure 4.
Extensive studies following the procedure defined in IEEE
Std. C57.100 [13] combining aramid DuPont™ Nomex® 910
immersed in high-oleic sunflower natural ester A&A Parodi
Paryol Electra 7426 are in progress and will be available soon.
Temperature (°C)
500
V. SELF-EXTINGUISHING LIQUIDS AND FIRE RISK
Fires caused by insulating liquid combustion in case of
transformer breakdown have ever been considered as a primary
source of risk in electrical installations, especially for urban
distribution lines, where the consequences of catastrophic events
(fire and explosion) could involve the operators and even the
population.
NE
Paryol Electra
7426
10
20
Heating time (min)
VI. CONCLUSION
After more than 20 years (Figure 5) of experience nowadays
natural esters cannot be considered as an alternative to mineral
oils only. NEs belong to new paradigm of insulating fluids with
superior performances in terms of biodegradability, lowflammability, self-extinguishing behaviour, low gassing, and
life extension.
Energy to
raise F.P.
Estimation of transformers filled with NE
0.92
165
355
°C
75
265
KJ/Kg·K
1.860
2.080
KJ/l
2.500.000
250.000
Units per Year
0.88
°C
30
Figure 4: Time to ignition of transformer filled with Parodi Paryol Electra
7426 (NE) and mineral oil.
300.000
Kg/l
MO
Mineral Oil
123
507
2.000.000
200.000
1.500.000
150.000
1.000.000
100.000
Cumulative Units
Fluid
Type
Specific heat
capacity at
80°C
200
0
TABLE I: FIRE PROPERIES OF LIQUIDS
Temperature
gap from
90°C
300
0
The today unanimously recognized best solution for fire
prevention is the replacement of mineral oil with natural or
synthetic ester. These fluids in addition to their high fire-points
are not harmful for humans, flora, fauna and waterborne.
Fire
point
400
100
Askarel (PCB diluted in trichlorobenzene) has been used for
some decades as “ultimate” solution, a sort of universal panacea,
for fully eradicate the fire risk. As well-known this fluid is not
an option today because, due to its carcinogenicity, it is
worldwide banned.
Density
Fire ignition
500.000
50.000
0
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
0
By itself, as reported in Table , the specific heat capacity is
higher for the natural ester than the mineral oil, that means one
needs to provide more heat to increase the temperature by 1
Kelvin degree for the natural ester than for the mineral oil.
Additionally, one needs to provide more heat to the natural ester
compared to the mineral oil to take the fluid to its fire point,
since the gap from the max top oil temperature (90°C) is greater.
For instance, the natural ester needs a heat energy of 507 KJ/l
whereas mineral oil needs just 123 KJ/l to take the fluid to its
fire point. This essentially means that the time to ignition will be
longer for the natural ester compared to a conventional mineral
oil.
Units per year
Cumulative
Figure 5: Global market trend for transformers fille with NE
Currently the value of environmental sustainability is and
must become even more a duty for all industrial applications
including the electrical one. This is a sector of industry that,
more than many others, has always been ready to acknowledge
and adopt technical innovations while respecting social ethics.
It is desirable that all local fire safety legislations will soon
incorporate the obvious advantages offered by natural esters,
translating them into regulatory simplifications, such as the
reduction of distances, the elimination of fire-barriers and of the
deluge systems in case of spillage.
Moreover, in case of fire, the natural ester generates less heat
compared to the mineral oil during combustion, respectively 39
MJ/kg and 46 MJ/kg. As a result of lower heat of combustion,
the natural ester will not support the fire to the same extent as
the mineral oil.
All of which would speed up green transition of the electrical
industry.
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[7]
ACKNOWLEDGMENT
The authors sincerely thank the DuPont team for the support
and the contribution in the preparation of this paper.
[8]
[9]
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