Ageing Behaviour of Mineral Oil and Ester Liquids: a
I. Fofana1, 2, A. Bouaïcha1, 2, M. Farzaneh2 and J. Sabau3
1Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME),
University of Quebec at Chicoutimi, Qc, Canada
2International Research Centre on Atmospheric Icing and Power Network Engineering (CenGivre),
University of Quebec at Chicoutimi, Qc, Canada
3InsOil Canada Ltd, Calgary, AB, Canada
Abstract- There is a general agreement that in service
conditions the quality of insulating fluids gradually deteriorates
under the impact of electrical, thermal and chemical stress. Two
ASTM methods are used to monitor the deterioration of liquid
insulation step by step. A comparison is made between the
performances of a commercial based mineral oil and ester fluids.
The results obtained using a laboratory grade spectrophotometer
and a ratio-turbidimeter, indicate that the absorbance increases by a
significant and easily observable margin with ageing rate. Under the
same aging condition, synthetic ester ages considerably slower than
mineral oil; natural ester showing an intermediate behaviour.
It is now an established fact that the ability of the insulating
liquid to serve as an effective insulator and coolant is an
important factor in determining the current condition of a
While in service, both the liquid and solid insulation of
windings undergo a slow but steady decay process under the
impact of electrical, thermal, mechanical and chemical
stresses. Recent studies have shown that the gassing of oil has
an important side effect. The breakdown of hydrocarbon
chains generates not only soluble gases, as it is currently
believed, but also colloidal suspensions [1-3]. These by-
products are deleterious to the transformer and favour further
oxidation of the oil. The chemical aggressiveness of oxygen
contributes to the formation of soluble oxidation products as
well as insoluble sludge which are detrimental to the solid
insulation. Both types of unnoticed substances are irreversibly
retained by cellulose insulation stopping heat being dissipated.
The sludge acts as barrier to the flow of heat from the fluid to
the cooling unit and from the core to the coils to the cool oil
In today’s economic climate, it is important to know the
condition, by means of suitable diagnostic tests, of the liquid
impregnated paper usable as primary insulation in
transformers. The development of several new laboratory
testing procedures for insulating liquids over the past years has
been the rewarding result of a cooperative research project
designed to extend the life expectancy of aging power
transformers by eliminating the causes of premature
deterioration. Assessing the outcome of oxidation reactions
solely by measuring the organic acidity and interfacial tension
of oil (both more than thirty years old), the foreseeable
formation of the colloidal sludge and x-waxes was ignored .
Previously published work demonstrates the suitability of
substituting synthetic ester fluid (introduced in the 70s to
replace mineral oil in transformers as a safer alternative where
fire safety and protection of the environment are primary
considerations) for mineral oil in liquid-paper insulation
systems [6-8]. This comparative study was undertaken to
better understand and quantify the dissolved decay products in
mineral oil, natural and synthetic ester fluids following
accelerated ageing procedure in laboratory.
As long as chemical bonds inside the molecule chains are
not broken, the formations of decay products are impeded.
The sources of energy capable of splitting of a covalent bond
made up of a pair of electrons are three fold:
? The strong electro-magnetic field that triggers the free
electron injection process in the liquid insulation [2, 9].
? The thermal energy generated by the active parts.
? Finally, the aggressiveness of dissolved oxygen.
A. Electrical stress
Free electrons (e-) are the primary source of energy for the
breakdown of vulnerable covalent bonds (approximately 4 eV
≈ 386 kJ mol-1). Electrons escape from the conduction band of
the metal conductor and are emitted from its surface,
especially during very short but frequent commutation voltage
surges . The free electrons injected into the liquid insulation
are accelerated by the electric field. The collision of a fast
inelastic, leading to very different results:
(i) Elastic collision:
(ii) Inelastic collision:
fast) with a molecule M may be either elastic or
fast + Mslow → e−
less fast + Mfaster.
fast + Mslow → e−
slower + M* (excited molecule)
slower + M+ + e−
Whereas stable molecules reaching their singlet excitation
level (M*) usually release the absorbed energy as a quantum
of harmless fluorescent light (hν):
M* → M + hν,
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vulnerable molecules (R-R’) decompose and generate a pair of
free radicals (R• and R’•).
R-R’ + hν ? R• + R’•
B. Thermal Stress
Heat produced by the magnetic core and the windings is
another contributing factor to the liquid decaying.
In normal operating conditions the temperature of oil is
usually below 100°C. In such circumstances mainly the
rotational and vibrational energies of hydrocarbon chains are
excited, while the split of a covalent bond takes place only
when an electronic excitation level is reached. However,
weakly bonded molecules can occasionally accumulate
sufficient energy to reach the electronic excitation level and
decompose to form, initially, two free radicals which can then
react as discussed above.
In addition, high temperature accelerates oxidation process,
increases the mobility of free radicals generated by the gassing
of oil, and thus promotes the random secondary chemical
reactions that precede the formation of decay products.
C. Chemical Stress
Oxygen molecule, which is “diradical” specie, has very
special electronic features. Since its ground state is a triplet,
the first two excited states of oxygen are singlet states that
have very long lifetimes [3, 10]. The dissolved oxygen content
of an oil sample, taken from the tank of a free-breathing
transformer, shows values, between 5,000 and 40,000 ppm
. The omnipresence of oxygen (with the catalytic effect of
copper), heat and water will promote insulating liquid
oxidation even under ideal conditions.
The oxidation process is a complex phenomenon involving
free radical reactions. A few possible reactions can be found in
R-H + X. → R. + XH
R. + O2 → R-O-O.
ROO. + RH → ROOH + R.
ROOH thermal energy R-O. + OH.
RO. + RH → ROH + R.
OH. + RH → H2O + R.
(X. = any chemical with one or more unpaired electrons,
including O2, capable of abstracting a hydrogen atom from a
When the concentration of dissolved oxygen is high as it is
in a free breathing transformer, the electronic excitation
energy of the oxygen molecule can be transferred to
vulnerable molecules. The oxidation accelerated by
temperature, moisture or other chemicals, results in peroxides
which dissociate to form free radicals. This later can act as
initiators for chain reactions involving free radicals and liquid
D. Decay products
All broken molecules and “knocked-out” hydrogen atoms
are free radicals and hence paramagnetic. The paramount
importance of free radicals in the physical organic chemistry
of insulating oils has been underscored by Tanaka .
As the population of free radicals increases, some gaseous or
liquid fractions may capture a free electron and form an ion.
R• + e- ? R-
The accumulation of such ionized molecules increases the
dissipation factor of oil-paper insulation. Alternatively, large
free radicals may combine, leading to the formation of large
colloidal compounds having a molecular weight between
500 and 600 (sludge that is a solid phase). When two large
free radicals couple their unpaired electrons to generate a
similar insoluble hydrocarbon without oxygen in the
middle, the decay product formed has the generic name of
x-wax. The increase in free radicals concentration raises
random chemical reactions between free radicals and soluble
and insoluble oil-born decay products are the outcome (Figure
Fig. 1. Decay process of oil paper insulation in free breathing power
The terminal stage of the liquid deterioration process is
sludge or x-waxes and acid in sufficient quantity to impair its
heat transfer and dielectric properties. The result of the
reactions causes the oil to change color from bright yellow
to amber. Colloidal suspensions and acids, absorbed by the
large surface of paper, attack on the cellulose fibres and metals
forming metallic soaps, lacquers, aldehydes, alcohols and
ketones . Sludge appears faster in heavily loaded, hot
running and abused transformers causing shrinkage of the
insulation through leaching out varnishes and cellulose
Even though in service conditions the amount of soluble and
insoluble impurities is growing day by day, their amount is not
reason for concern for several years. During this time DGA
(Dissolved Gas Analysis) results are far below acceptable
upper limits whilst the TAN (Total Acid Number) is very
small and the IFT (Interfacial Tension) is high. Unfortunately,
the paper insulation relentlessly adsorbs these trace impurities
on its large surface. Therefore, the induction period is
misleading. Previous investigations have shown that
traditional analytical methods, such as TAN and IFT are not
accurate enough to detect these decay products during the
induction period of the oxidation process . Consequently,
more accurate, laboratory testing procedures were developed
by ASTM [12, 13].
The ASTM D 6802  method is based upon the
observation that in the range of visible spectrum all brands of
new insulating liquids are almost completely transparent to a
monochromatic beam of light. On the contrary, when the fluid
contains decay products, the absorbance curve, as determined
by a scanning spectrophotometer, significantly shifts to longer
wavelengths. The numerical integration of the area below
these absorbance curves permits the relative content of
Dissolved oxidation Decay Products (DDP: peroxides,
aldehydes, ketones and organic acids) in the fluid samples.
The ASTM D 6181  method is an accurate optical
laboratory technique developed to quantitatively determine the
amount of microscopic solid suspension that may exist in both
new and in-service fluids. Increasing turbidity signifies
increasing fluid contamination. Other turbidity sources, such
as water droplets or gas bubbles, are eliminated .
The results shown in Fig. 2 [2, 3] between new, slightly
oxidized, oxidized and heavily oxidized oil samples,
convincingly proves that unlike TAN and IFT this modern
laboratory technique is able to monitor step-by-step the
dissolved decay process of mineral insulating oils.
Fig. 2. Absorbance curves of new, slightly oxidized, oxidized and heavily
oxidized oils: DDF=Dielectric Dissipation factor, T = turbidity, TAN =
acidity, Area= numerical integration of the area below these absorbance
curves [2, 3].
III. AGEING PROCEDURE
Because the general life is much longer than the research
duration, the investigation was based on studying the aging
effect with increased speed. The ageing procedure is done by
placing insulation specimens in a convection oven at 100°C
and aging them for an extended specific period, with air
(oxygen) inlet . Typical transformer proportions of copper,
aluminium, zinc and iron (each 3 g/l) were added to oil-paper
insulation during the aging process in order to simulate the
possible oxidation processes. The samples were aged for
extended periods varying between 250 to 3100 hours which
simulate long time service condition due to the used severe
It should be noticed that MO always represents Mineral Oil,
NE for Natural Ester, SE for Synthetic Ester while DF stands
for Dissipation Factor. Thus “MO – 2000 h for example,
means the dissipation factor of mineral oil sample with an
aging duration of 2000 h. An example of the frequency scans
of the Dissipation Factor (DF) of the three fluid samples is
given in Fig. 3 while data at line frequency are provided in
Fig. 3. Frequency scans of the dissipation Factor for laboratory-aged fluid
Fig. 4. Dissipation Factor for laboratory-aged fluid samples at line frequency
as a function of ageing duration.
The measurements were performed with the Insulation
Diagnostic Analyser IDA200 using the liquid test cell type
2903 for liquid insulants by Tettex. Fig. 4 reflects the
differences between new, slightly oxidized and heavily
oxidized fluid samples; the DF almost increases during the
accelerated aging tests.
The relative content of DDP in an aged fluid samples was
numerically characterized from the absorbance curves
according to ASTM D6802). The computed results are
summarized in Fig. 5.
Out of this Figure, it can be observed that the rate of DDP
increase in mineral oil is much higher than that of synthetic
ester (5-7 times higher than in synthetic ester fluid after 3,100
hours aging). Natural ester shows an intermediate behaviour.
The amount of microscopic solid suspension in both new and
in aged fluid samples was also determined and summarized in
Fig. 6, according to ASTM D6181. Again, the same
observation as for the DDP applies.
Fig. 5. Relative content of the Dissolved Decay Products as function of ageing Download full-text
duration for different fluids.
The solid suspension content inside mineral oil increases 3-5
hundreds times than in ester fluids after 3,100 hours aging.
Even though the DF values obtained for mineral oil are lower
than those of esters (Figs. 3 and 4), the amount of decay
products is retentively high.
Fig. 6. Turbidity as function of ageing duration for different fluids.
Actually, oxidation process inside oil is controlled or slowed
down by preventive maintenance procedures by incorporating
oxidation inhibitors or antioxidants with the aim of
interrupting and terminating the free radical process of
oxidation. Inhibitors used in transformer oils offer stability for
a limited time, the so-called the ‘induction period’ after which
the oil oxidizes at the normal uninhibited rate. The
commercial MO used in these investigations contains 0.08%
of oxidation inhibitor content. From the obtained results, this
phenomenon seems to happen after 1000 hours aging duration.
Figs. 5 and 6 might be related to this striking phenomenon.
The oxidation stability (or capability to resist oxidation) of
synthetic ester is to be emphasized.
Because the insulating properties of the fluid are also
affected by contaminants from the solid materials in the
transformer dissolving in the oil, the authors are currently
engaged in additional investigations on the liquids samples
when aged with the solid insulation.
Most of the decay products that progressively damage the
properties of oil-paper insulation in power transformers result
from secondary chemical reactions between decomposed
molecule under the impact of electrical, chemical and thermal
Fast, inexpensive and reliable laboratory testing procedures
developed by ASTM (D 6802 and D 6181) have been used to
monitor decay products as trace impurities. These testing
procedures can also be used to assess the capability of any
fluid sample to resist oxidation, including reclaimed oils
effectiveness. From the investigations with the available
commercial fluids, we find that synthetic ester ages
considerably slower than the natural ester dielectric fluid and
mineral oil under the same aging condition; the natural ester
showing an intermediate behaviour. Even though the DF
values obtained for mineral oil are lower than those of esters,
the amount of decay products is retentively high.
As part of an overall maintenance strategy, these tests can
help taking restorative measures before deterioration reaches a
point where failure of the transformer is inevitable.
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