IEC TR 62874:2015

IEC TR 62874 ®
Edition 1.0 2015-05
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Guidance on the interpretation of carbon dioxide and 2-furfuraldehyde as
markers of paper thermal degradation in insulating mineral oil

Guide pour l’interprétation du dioxyde de carbone et du 2-furfuraldéhyde
comme marqueurs de la dégradation thermique du papier dans de l’huile
minérale isolante
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IEC TR 62874 ®
Edition 1.0 2015-05
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Guidance on the interpretation of carbon dioxide and 2-furfuraldehyde as

markers of paper thermal degradation in insulating mineral oil

Guide pour l’interprétation du dioxyde de carbone et du 2-furfuraldéhyde

comme marqueurs de la dégradation thermique du papier dans de l’huile

minérale isolante
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040.10 ISBN 978-2-8322-3085-5

– 2 – IEC TR 62874:2015 © IEC 2015
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Significance . 8
3.1 General . 8
3.2 Thermal and mechanical degradation of paper . 8
3.2.1 General . 8
3.2.2 Impact of temperature . 8
3.2.3 Impact of humidity and oxygen . 9
3.3 Symptoms of paper ageing in insulating oil . 10
3.3.1 General . 10
3.3.2 Volatile by-products . 11
3.3.3 Soluble by-products . 11
3.3.4 Insoluble by-products . 11
3.4 Operational parameters influencing paper thermal ageing . 11
3.5 Role of oil type and condition . 12
3.6 Fault conditions that may affect thermal ageing . 12
3.7 Maintenance operations that may affect thermal ageing indicators . 13
3.7.1 General . 13
3.7.2 Effects of oil reconditioning . 13
3.7.3 Effects of oil reclamation . 13
3.7.4 Effects of oil change . 13
4 Monitoring protocol . 14
4.1 General . 14
4.2 Parameters . 14
4.2.1 Basic monitoring . 14
4.2.2 Complementary monitoring . 14
4.3 Recommended testing frequencies . 14
5 Typical values of paper ageing symptoms . 15
5.1 General . 15
5.2 Families of equipment . 15
6 Estimation of paper thermal degradation and ageing rate . 16
6.1 General approach . 16
6.2 Practice . 16
7 Actions . 17
Annex A (informative) Typical values tables . 19
A.1 General warning . 19
A.2 2-FAL typical values . 19
A.2.1 General . 19
A.2.2 Family: GSU (generation step-up units) . 19
A.2.3 Family: network transmission units . 20
A.2.4 Family: large distribution units . 20
A.2.5 Family: industrial distribution units . 20
A.2.6 Family: LVDC units . 21

A.3 Carbon dioxide typical values. 21
A.3.1 General . 21
A.3.2 Family: GSU (generation step-up units) . 21
A.3.3 Family: network transmission units . 21
A.3.4 Family: large distribution units . 22
A.3.5 Family: industrial distribution units . 22
A.3.6 Family: LVDC units . 22
Bibliography . 23

Figure 1 – Schematic diagram showing rate of ageing k, depending on different ageing
mechanisms . 9
Figure 2 – Relationship between mechanical properties of insulating paper and paper
degree of polymerization (DP) [5] . 10
Figure 3 – Example of flow-chart for the estimation of paper degradation conditions . 17

Table A.1 – 2-FAL typical values for GSU transformers, filled with uninhibited mineral
oil (based on a population of 1 860 units) . 19
Table A.2 – 2-FAL typical values for GSU transformers, filled with inhibited mineral oil
(based on a population of 176 units) . 19
Table A.3 – 2-FAL typical values for network transmission transformers, filled with
uninhibited mineral oil (based on a population of 2 845 units) . 20
Table A.4 – 2-FAL typical values for large distribution transformers, with open
breathing conservator, filled with uninhibited mineral oil (based on a population of
7 107 units) . 20
Table A.5 – 2-FAL typical values for large distribution transformers, with sealed
conservator, filled with uninhibited mineral oil (based on a population of 288 units) . 20
Table A.6 – 2-FAL typical values for industrial distribution transformers, filled with
uninhibited mineral oil (based on a population of 3 885 units) . 20
Table A.7 – 2-FAL typical values for LVDC transformers, filled with uninhibited mineral
oil (based on a population of 360 units) . 21
Table A.8 – CO typical values for GSU and excitation transformers, filled with
uninhibited mineral oil (based on a population of 1 098 units) . 21
Table A.9 – CO typical values for network transmission transformers, filled with
uninhibited mineral oil (based on a population of 435 units) . 21
Table A.10 – CO typical values for large distribution transformers, filled with
uninhibited mineral oil (based on a population of 7 291 units) . 22
Table A.11 – CO typical values for industrial distribution transformers, filled with
uninhibited mineral oil (based on a population of 4 556 units) . 22
Table A.12 – CO typical values for LVDC transformers, filled with uninhibited mineral
oil (based on a population of 273 units) . 22

– 4 – IEC TR 62874:2015 © IEC 2015
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GUIDANCE ON THE INTERPRETATION OF CARBON DIOXIDE
AND 2-FURFURALDEHYDE AS MARKERS OF PAPER THERMAL
DEGRADATION IN INSULATING MINERAL OIL

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 62874, which is a Technical Report, has been prepared by IEC technical committee
10: Fluids for electrotechnical applications.
This bilingual version (2016-01) corresponds to the English version, published in 2015-05.
The text of this Technical Report is based on the following documents:

Enquiry draft Report on voting
10/903/DTR 10/917A/RVC
Full information on the voting for the approval of this Technical Report can be found in the
report on voting indicated in the above table.
The French version of this Technical Report has note been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC TR 62874:2015 © IEC 2015
INTRODUCTION
The cellulosic solid insulation of transformers and other electrical apparatus is subject to
thermal degradation during their operational lifetime. This results in a progressive loss of
paper’s mechanical properties, such as tensile strength, which are related to the duration of
the technical life of the equipment [3,4] .
During its thermal degradation process (also called “ageing” in this Technical Report),
cellulose forms several by-products, some of which may be detected by means of insulating
oil’s chemical analysis [1,2]. The concentration and rate of increase of those by-products can
be used as a tool to estimate the progress of paper thermal degradation in transformers and
other electrical apparata in service.
For this reason, IEC technical committee 10 has prepared this Technical Report for the
monitoring of insulating oil parameters related to cellulose ageing and the interpretation of
results, as a guidance to the thermal degradation evaluation of insulating paper.
This Technical Report is based on the evaluation of cellulose ageing by-products content in
insulating oil, and their rate of formation during the life of the oil-immersed electrical
equipment. Statistical reference values reported in Annex A of this Technical Report are
based on data collected by TC10. The final report of CIGRE WG D1.01.TF13 [7] was taken as
a source of information concerning mechanisms and parameters influencing the formation of
furanic compounds.
NOTE Methods for the estimation of actual degree of polymerization (DP) values of paper, which are widely
available in literature, were not applied within this Technical Report. This is due to the fact that a number of
different models have been developed and reported, and they often lead to different results. Moreover, the
applicability of those models has not been sufficiently proven by comparison with field experience to be included
into an IEC standard.
Health and safety
This Technical Report does not purport to address all the safety problems associated with its
use. It is the responsibility of the user of the Technical Report to establish appropriate health
and safety practices and determine the applicability of regulatory limitations prior to use.
The mineral oils which are the subject of this Technical Report should be handled with due
regard to personal safety and hygiene. Direct contact with eyes may cause slight irritation. In
the case of eye contact, irrigation with copious quantities of clean running water should be
carried out and medical advice sought.
Some of the tests specified in this Technical Report involve the use of processes that could
lead to a hazardous situation. Attention is drawn to the relevant standard for guidance.
Environment
This Technical Report involves mineral oils, chemicals and used sample containers. The
disposal of these items should be carried out in accordance with current national legislation
with regard to the impact on the environment. Every precaution should be taken to prevent the
release into the environment of mineral oil.
___________
Figures in square brackets refer to the Bibliography.

GUIDANCE ON THE INTERPRETATION OF CARBON DIOXIDE
AND 2-FURFURALDEHYDE AS MARKERS OF PAPER THERMAL
DEGRADATION IN INSULATING MINERAL OIL

1 Scope
IEC TR 62874, which is a Technical Report, provides guidance for the estimation of
consumed thermal life of transformers' cellulosic insulators, through the analysis of some
compound dissolved in the insulating mineral oil. A comparison between analytical results of
2-furfural (2-FAL) and carbon oxides and their correspondent typical values estimated for
different families of equipment gives information on the estimated thermal degradation of
papers.
The ageing rate of insulating papers can be evaluated, in short time ranges (e.g. 1 year), by
regularly monitoring 2-FAL and carbon oxides content in the oil and by comparing them to
typical rates of increase.
A statistical approach for the estimation of paper thermal degradation, and the evaluation of
ageing rate is given.
Typical values for concentrations and rates of increase of the parameters related to paper
ageing were extrapolated from a statistical database collected, and are reported in Annex A.
They may be used as a rough guide, but they should not be considered as threshold values.
This Technical Report is only applicable to transformers and reactors filled with insulating
mineral oils and insulated with Kraft paper. The approaches and procedures specified should
be taken as a practical guidance to investigate the thermal degradation of cellulosic
insulation, and not as an algorithm to calculate the actual degree of polymerization (DP) of
papers.
The paper thermal life evaluation protocol described in this Technical Report applies to
mineral oil impregnated transformers and reactors, insulated with Kraft paper. Any equipment
filled with insulating liquids other than mineral oil (i.e. esters, silicones) or insulated with solid
materials other than Kraft paper (i.e. TUP – thermally upgraded Kraft paper, synthetic
polymers) is outside of the scope of this Technical Report.
This Technical Report is applicable to equipment that has been submitted to a regular
monitoring practice during the service, and for which maintenance and fault history is known.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
None.
– 8 – IEC TR 62874:2015 © IEC 2015
3 Significance
3.1 General
This Technical Report describes a statistical approach to paper thermal life evaluation. This
means that all typical values are obtained from populations of transformers belonging to the
same family for technical characteristics and application (see Annex A).
The approach used to collect statistical data, described in 6.1, can be applied by utilities or
owners having a large population of units, to calculate individual reference values related to a
specific family of transformers or reactors. This is very important because different population
of transformers (i.e. operating in different climates or under different operational conditions)
may have different typical values.
NOTE For an extensive survey on furanic compounds as markers for diagnosis of paper insulation degradation,
see CIGRE Brochure 494/2012 [7].
3.2 Thermal and mechanical degradation of paper
3.2.1 General
There are main factors: design and materials, contaminants in the insulation system and
operational conditions, that will determine the ageing of a transformer [1,2]. For the solid
insulation – paper and pressboard – it means a combination of mechanical and dielectric
performance, which are interlinked and synergetic. For a transformer, in the context of
thermal ageing, it is the mechanical strength of the paper that matters. The ageing of paper
results in a decreased mechanical strength and is assumed to reduce the ability of the
transformer to withstand short circuit stress. This, however, has not been statistically
demonstrated, yet.
Tensile strength, elongation and folding strength all decay with time, and more quickly at
higher temperatures.
The mechanical performance of cellulosic insulation is given in terms of tensile index or
degree of polymerization (DP), which are strongly influenced by ageing. The DP value is an
average value of chain lengths of the cellulose molecules given as a number of glucose rings
in a cellulose chain. It is measured through measurement of the viscosity of a paper solution,
according to IEC 60450 [8].
It is more convenient to perform DP than tensile index, because of the limited amount of
paper accessible for tests; therefore it is widely used for the evaluation of the cellulosic
ageing status.
There are three main processes of degradation:
– hydrolysis;
– oxidation;
– pyrolysis.
3.2.2 Impact of temperature
Temperature affects the rate of degradation. This fact is reflected in IEC 60076-7 [3] and
IEEE Std C57.91 [4] transformer loading guides.
IEC 60076-7 [3] suggests in accordance with Montsinger that the life of a transformer can be
described according to Equation (1):
−p×θ
Life duration = e (1)
where:
p is a constant (a value of 6 is suggested in the range 80 °C to 140 °C)
θ is the temperature in degrees Celsius.
This is a simplified version of Arrhenius law used in IEEE Std C57.91 [4].
Since a precise end-of-line criterion for a transformer is not really available, IEEE and IEC
standards use an approach where ageing rate is considered. This is the inverse of lifetime –
in Montsinger form:
p×θ
×e
Rate of ageing = constant (2)
The constant in Equation (2) is dependent on many parameters, e.g. original quality of
cellulosic products as well as environmental parameters (moisture content and oxygen in the
system). A graphical representation of these influences is shown in Figure 1.
Pyrolysis
Mixed mechanism –
non linear plot
Hydrolysis
Oxidation
1/T
IEC
Figure 1 – Schematic diagram showing rate of ageing k,
depending on different ageing mechanisms
3.2.3 Impact of humidity and oxygen
Humidity and oxygen ingress (oxidation) have an important impact on the ageing of Kraft
paper. This means not only that the mechanical strength of paper rapidly decreases under
ingress of moisture and air, but practically causes an increasing contamination of the
combined liquid-solid insulation under these conditions. It is a consequence of the
degradation products formed from oil and paper leading to a further degradation.
During the ageing of the combined cellulosic and oil insulation many by-products are formed –
carbon oxides, water, acids, sludge and furanic compounds. Many of these degradation
products, e.g. furanic compounds, are soluble in oil and stable enough to be used as
diagnostic markers. Furanics are formed by dehydration reactions following hydrolysis of the
cellulose and hemicellulose as well as by oxidative pyrolysis of cellulose. Their analytical
determination is well known and reliable (see IEC 61198 [12]).
In a transformer all these processes – hydrolysis, oxidation and pyrolysis – act
simultaneously, resulting in a non-linear mechanism (see Figure 1). Which process will
dominate depends on the temperature and the operational parameters. In fact the application
of one activation energy, although often practiced, is very difficult because of the complexity
of the degradation processes.
Ln k
– 10 – IEC TR 62874:2015 © IEC 2015
3.3 Symptoms of paper ageing in insulating oil
3.3.1 General
The ageing of paper can be detected by direct investigation on the paper or by the
measurement of by-products dissolved in the oil.
Cellulose degradation mainly affects the mechanical properties (tensile strength, elongation,
burst strength, double fold strength, etc.) of paper (see Figure 2), but a direct measure of
those parameters requires the sampling of a large amount of paper, which is normally
impossible during the operational lifetime of a transformer. However, the relationship between
the mechanical indexes and the degree of polymerization (DP) is well known. Degradation of
paper does not significantly affect its resistance to the compression forces mostly and
continuously applied to transformer windings through clamping.
0,5
Severe ageing
Moderate ageing
0 0,5 1
Relative degree of polymerisation
IEC
Figure 2 – Relationship between mechanical properties of
insulating paper and paper degree of polymerization (DP) [5]
The DP value is the average number of glycoside rings in the cellulose polymer; in the native
cellulose DP may be as high as or more than 10 000 units but after the purification process
and other treatments the DP value of the electrical Kraft paper decreases to around
1 000 units (typical value: 1 200).
DP is measured in accordance with IEC 60450 [8], through measurement of the specific
viscosity of a very small amount of paper dissolved in cupri-ethylene-diamine (CuED). From
this measurement the intrinsic viscosity of solution is deduced and from this, using the
Martin’s formula, the DP value is easily calculated.
By-products of aged paper may be classified as volatile, soluble and insoluble, and are
dependent on the specific decomposition process: pyrolysis, hydrolysis or oxidation.
Relative value of indicated parameter

3.3.2 Volatile by-products
Carbon oxides (CO and CO ) are the ultimate products of cellulose degradation and are
measured with dissolved gas analysis (DGA) in accordance with IEC 60567 [9]. It must be
taken into account that both CO and CO can be generated from oil oxidation as well.
Water can reach several per cent of the paper weight. Most of the water formed is adsorbed
and retained in the solid insulation of the transformer and only a little part is dissolved in oil.
Of course another contribution to total water is the ingress of moisture from atmosphere.
The detection of water in oil is performed in accordance with IEC 60814 [10].
3.3.3 Soluble by-products
A large number of oil soluble compounds (acids, alcohols, etc.) are generated from paper
degradation. The most commonly used compound for diagnosis is 2-furfural (2-FAL) and its
related compounds:
– 5 hydroxymethyl 2-furfural (5-HMF)
– 2 furfurylalcohol (2-FOL)
– 2 acetylfuran (2-ACF)
– 5 methyl 2-furfural (5-MEF).
The detection of 2-FAL and related compounds in oil is performed in accordance with
IEC 61198 [12].
In the same way as with water, a relevant amount of generated furanic compounds is retained
in the bulk of the paper. The ratio between the concentration of furanic compounds in the
paper and in the oil differs for each single compound, and is affected by temperature.
Increased temperature forces the equilibrium of furanic compounds to a higher concentration
in the oil. Decreased temperature forces the equilibrium of furanic compounds to a higher
concentration in the paper, especially in the case of dry paper.
Paper humidity and type of paper also influence the oil-to-paper concentration ratio of furanic
compounds. A wet paper tends to retain a larger amount of furanic compounds, thus reducing
the oil-to-paper concentration ratio.
Furanic compounds are not highly stable, and may be degraded by oxidation, mostly in oils
with high oxygen content. Decay in the concentration of 2-FAL was observed in transformers
during their operation, due to its inherent instability.
Acid compounds may be formed either by cellulose and/or oil oxidation, and a high acidity
often accompanies other symptoms of paper ageing.
3.3.4 Insoluble by-products
Severe paper ageing can finally lead to the fragmentation of polymeric cellulose, and small
paper fibres can be detached from the paper mass.
The cellulose fibres can be detected as particles present in insulating oil, in accordance with
IEC 60970 [11].
3.4 Operational parameters influencing paper thermal ageing
In addition to transformer hours of service as a key parameter in defining “real age” of paper
insulation in normal working regimes, other operational parameters such as load, type of
cooling and transformer sub-type have a major influence on paper thermal ageing. The nature
of the oil may also be a fundamental parameter for the estimation of paper thermal ageing.

– 12 – IEC TR 62874:2015 © IEC 2015
The high-load of a transformer, implying elevated operating temperatures, promotes the paper
thermal degradation process, observed with some types of transformers that are often
overloaded (shunt reactors, HVDC, generator step-up (GSU) in thermal power plants (TPP),
high voltage inter-tie transmission transformers).
The type of cooling, in terms of cooling media (water or air) and type of flow applied (forced or
natural convection), affects efficiency of heat removal, thus influencing the rate of paper
thermal degradation. The most efficient cooling can be achieved by applying water as coolant
in forced oil flow.
For example, it was observed in most cases that the degree of paper degradation with GSU
transformers in hydro power plants (HPP) is lower than with thermal power plant GSU units,
having a similar service duration. These findings are correlated to different types of cooling
(OFWF versus ONAF and OFAF), hours of service and loading history of HPP and TPP units
[6].
Among different transformer sub-types, air-breathing transformers are subjected to more
intensive paper degradation than sealed ones, due to higher oxygen and moisture content.
Elevated concentrations of oxygen and water accelerate the paper degradation process.
Since the paper degradation process is temperature driven, every environmental and
operational condition that may affect the temperature can also modify the degradation rate of
the solid insulation. An elevated environmental temperature or a high loading can thus
increase the rate of paper degradation, resulting in a sudden increase of 2-FAL, CO , CO and
other by-products.
3.5 Role of oil type and condition
Oil type may affect the ageing rate of paper. Inhibited oils show a lower tendency to form
acidity, and the oxidation process is slackened; the effect of oxygen in the paper oxidation
process is reduced.
Transformers impregnated with inhibited oil may show a lower content of 2-FAL if compared
with units insulated with an uninhibited oil, even if showing the same degree of polymerization
(DP) of the paper.
The effect of passivators (triazole derivates) in the ageing of celluloses is still not well
defined. By definition, metal passivators may induce a lower rate of the oil degradation
process by deactivating the copper catalyst in oxidation processes, therefore slowing down
the paper degradation process, but influence of metal passivators on 2-FAL concentration in
the oil may not be straightforward. Some laboratory studies have shown that papers
impregnated with oils to which a passivator has been added, may have a lower tendency to
form furanic compounds; this may lead to optimistic estimation of ageing in presence of
triazolic passivators.
The ageing condition of the oil may also affect the partition of furanic compounds between
solid and liquid insulation; acidic oils may result in an increased 2-FAL concentration in oil,
due to its augmented capability to extract polar compounds from the paper.
3.6 Fault conditions that may affect thermal ageing
In transformers where the degradation mechanism may be either thermal or electrical, the rate
of paper degradation may increase rapidly as a consequence of significant temperature rise.
High energy thermal and electrical faults involving excessive currents circulating through the
insulation and large current follow-through lead to extensive destruction and carbonization of
paper.
In presence of local thermal degradation due to a fault, the estimation of the paper’s
consumed thermal life may become very difficult, since the extension of the paper volume

involved is unknown, and temperature may have strong variations even over a short time.
Investigations on the presence of thermal faults through DGA should always accompany
thermal life evaluation, to avoid misleading conclusions.
High energy electrical faults (discharges of high energy) usually involve a very small volume
of paper, so that the contribution to the detected concentration of furanic compounds is
negligible. In case of discharges with paper involved, a sharp increase of carbon oxides is
observed, rather than an noticeable increase of 2-FAL. The formation of cellulose by-product
has not been found to be related to partial discharges.
3.7 Maintenance operations that may affect thermal ageing indicators
3.7.1 General
Maintenance operations on the oil may affect (partially or totally) parameters used as
indicators of cellulose thermal ageing (see 3.3). Their effects should be taken into account
during the estimation of the total 2-FAL concentration, and in evaluating the rate of increase
of the thermal ageing indicators.
3.7.2 Effects of oil reconditioning
Oil reconditioning may reduce 2-FAL concentration in oil, depending on the duration/efficiency
of the treatment.
Oil reconditioning normally does not significantly affect 2-FAL, gas and moisture
concentration in cellulose. On-line degassing or long-term reconditioning may reduce moisture
in paper.
The equilibrium of 2-FAL distribution between oil and paper is restored in a time depending on
temperature, cooling and oil circulation.
Dissolved gases and water dissolved in oil are mostly removed by vacuum degassing.
In the 6 months following a reconditioning, the rates of increase of 2-FAL, dissolved gases
and moisture should not be considered as an indicator of increased ageing rate, the
equilibrium being forced thermodynamically through the increase of concentration in the oil.
3.7.3 Effects of oil reclamation
Reclaiming the oil has major effects on the concentration of 2-FAL. Furanic compounds are
polar and they are almost completely removed by fuller’s earth and other adsorbing media.
After an oil reclamation the trend of furanic compounds should be carefully recorded (with
frequent sampling) to monitor the increase of 2-FAL, taking into account new equilibrium
conditions.
NOTE For effects of reclamation on dissolved gases and moisture see 3.7.2
3.7.4 Effects of oil change
Oil change has major effects on the concentration of 2-FAL, as well. All the by-products
dissolved in the oil are removed. Nevertheless, after an oil change a new equilibrium between
solid and liquid insulation is dependent on temperature, cooling and oil circulation.
After an oil replacement the trend of furanic compounds should be carefully recorded (with
frequent sampling) to monitor the increase of 2-FAL, taking into account new equilibrium
conditions.
NOTE For effects of oil change on dissolved gases and moisture see 3.7.2

– 14 – IEC TR 62874:2015 © IEC 2015
4 Monitoring protocol
4.1 General
A regular monitoring of parameters related to thermal ageing of cellulose is strictly required
for the estimation of paper ageing condition and its rate of thermal degradation. No evaluation
should be done and no action should be taken on the basis of a single determination.
Evaluation based on a few samples close to the end of the operational lifetime will not lead to
reliable conclusions; the approach for the estimation of paper thermal degradation repo
...