IEC 60422:2013

IEC 60422 ®
Edition 4.0 2013-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Mineral insulating oils in electrical equipment – Supervision and maintenance
guidance
Huiles minérales isolantes dans les matériels électriques – Lignes directrices
pour la maintenance et la surveillance

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IEC 60422 ®
Edition 4.0 2013-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Mineral insulating oils in electrical equipment – Supervision and maintenance

guidance
Huiles minérales isolantes dans les matériels électriques – Lignes directrices

pour la maintenance et la surveillance

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 29.040.10 ISBN 978-2-83220-560-0

– 2 – 60422  IEC:2013
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Properties and deterioration/degradation of oil . 10
5 Oil tests and their significance . 11
5.1 General . 11
5.2 Colour and appearance . 12
5.3 Breakdown voltage . 12
5.4 Water content . 12
5.4.1 General . 12
5.4.2 Water in oil . 12
5.4.3 Water content in the oil/paper-system . 14
5.4.4 Interpretation of results. 15
5.5 Acidity . 15
5.6 Dielectric dissipation factor (DDF) and resistivity . 15
5.7 Inhibitor content and oxidation stability . 18
5.7.1 Oxidation stability . 18
5.7.2 Monitoring of uninhibited oils . 18
5.7.3 Monitoring of inhibited oils . 18
5.8 Sediment and sludge . 18
5.9 Interfacial tension (IFT) . 19
5.10 Particle count . 19
5.11 Flash point . 19
5.12 Compatibility of insulating oils . 20
5.13 Pour point . 20
5.14 Density . 20
5.15 Viscosity. 20
5.16 Polychlorinated biphenyls (PCBs) . 21
5.17 Corrosive sulphur . 21
5.18 Dibenzyl disulphide (DBDS) . 22
5.19 Passivator . 22
6 Sampling of oil from equipment . 22
7 Categories of equipment . 23
8 Evaluation of mineral insulating oil in new equipment . 23
9 Evaluation of oil in service . 24
9.1 General . 24
9.2 Frequency of examination of oils in service . 25
9.3 Testing procedures . 26
9.3.1 General . 26
9.3.2 Field tests. 26
9.3.3 Laboratory tests . 27
9.4 Classification of the condition of oils in service . 27
9.5 Corrective action . 27

60422  IEC:2013 – 3 –
10 Handling and storage . 32
11 Treatment . 33
11.1 WARNING . 33
11.2 Reconditioning . 34
11.2.1 General . 34
11.2.2 Reconditioning equipment . 35
11.2.3 Application to electrical equipment . 36
11.3 Reclaiming . 37
11.3.1 General . 37
11.3.2 Reclaiming by percolation . 37
11.3.3 Reclaiming by contact . 38
11.3.4 Renewal of additives . 38
11.4 Decontamination of oils containing PCBs . 38
11.4.1 General . 38
11.4.2 Dehalogenation processes using sodium and lithium derivatives . 38
11.4.3 Dehalogenation processes using polyethylene glycol and potassium
hydroxide (KPEG) . 39
11.4.4 Dehalogenation in continuous mode by closed circuit process . 39
12 Replacement of oil in electrical equipment . 39
12.1 Replacement of oil in transformers rated below 72,5 kV and in switchgear
and associated equipment . 39
12.2 Replacement of oil in transformers rated 72,5 kV and above . 39
12.3 Replacement of oil in electrical equipment contaminated with PCB . 40
13 Passivation . 40
Annex A (informative) Evaluating water in oil and insulation. 41
Annex B (informative) Particles . 43
Annex C (informative) Test method for determination of sediment and sludge . 44
Bibliography . 45

Figure 1 – Example of variation in saturation water content with oil temperature and
acidity for insulating oil originally conforming to IEC 60296 . 14
Figure 2 – Example of variation of resistivity with temperature for insulating oils . 17
Figure A.1 – Typical correction factors . 41

Table 1 – Tests for in-service mineral insulating oils . 11
Table 2 – Categories of equipment . 23
Table 3 – Recommended limits for mineral insulating oils after filling in new electrical
equipment prior to energization . 24
a
Table 4 – Recommended frequency of testing . 26
Table 5 – Application and interpretation of tests (1 of 4) . 28
Table 6 – Summary of typical actions . 32
Table 7 – Conditions for processing inhibited and/ or passivator containing mineral
insulating oils . 35
Table A.1 – Guidelines for interpreting data expressed in per cent saturation . 42
Table B.1 – Typical contamination levels (particles) encountered on power transformer
insulating oil as measured using IEC 60970 . 43

– 4 – 60422  IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MINERAL INSULATING OILS IN ELECTRICAL EQUIPMENT –
SUPERVISION AND MAINTENANCE GUIDANCE

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,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
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.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
International Standard IEC 60422 has been prepared by IEC technical committee 10: Fluids
for electrotechnical applications.
This fourth edition cancels and replaces the third edition, published in 2005, and constitutes a
technical revision.
The main changes with respect to the previous edition are as follows:
This new edition represents a major revision of the third edition, in order to bring in line this
standard with latest development of oil condition monitoring, containing new limits for oil
parameters, suggested corrective actions in the tables and new test methods.
The action limits for all oil tests have been revised and changes made where necessary to
enable users to use current methodology and comply with requirements and regulations
affecting safety and environmental aspects.

60422  IEC:2013 – 5 –
In addition, this standard incorporates changes introduced in associated standards since the
third edition was published.
The text of this standard is based on the following documents:
FDIS Report on voting
10/894/FDIS 10/896/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
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 web site 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.
The contents of the corrigendum of December 2013 have been included in this copy.

– 6 – 60422  IEC:2013
INTRODUCTION
Insulating mineral oils are used in electrical equipment employed in the generation,
transmission, distribution and use of electrical energy, so that the amount of oil in service,
worldwide, amounts to hundreds of millions of kilograms.
Monitoring and maintaining oil quality is essential to ensure the reliable operation of oil-filled
electrical equipment. Codes of practice for this purpose have been established by electrical
power authorities, power companies and industries in many countries.
A review of current experience reveals a wide variation of procedures and criteria. It is
possible, however, to compare the value and significance of standardized oil tests and to
recommend uniform criteria for the evaluation of test data.
If a certain amount of oil deterioration (by degradation or contamination) is exceeded, there is
inevitably some erosion of safety margins and the question of the risk of premature failure
should be considered. While the quantification of the risk can be very difficult, a first step
involves the identification of potential effects of increased deterioration. The philosophy
underlying this standard is to furnish users with as broad a base of understanding of oil
quality deterioration as is available, so that they can make informed decisions on inspection
and maintenance practices.
Unused mineral oils are limited resources and should be handled with this in mind. Used
mineral oils are, by most regulations, deemed to be controlled waste. If spills occur this may
have a negative environmental impact especially if the oil is contaminated by persistent
organic pollutants such as polychlorinated biphenyls (PCBs).
This International Standard, whilst technically sound, is mainly intended to serve as a
common basis for the preparation of more specific and complete codes of practice by users in
the light of local circumstances. Sound engineering judgement will have to be exerted in
seeking the best compromise between technical requirements and economic factors.
Reference should also be made to instructions from the equipment manufacturer.
General caution
This International Standard does not purport to address all the safety problems associated
with its use. It is the responsibility of the user of this standard to establish appropriate health
and safety practices and determine the applicability of regulatory limitations prior to use.
The mineral oils and oil additives which are the subject of this standard should be handled
with due regard to personal hygiene. Direct contact with the 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. For more information, refer to the safety data sheet
provided by the manufacturer. Some of the tests specified in this standard involve the use of
processes that could lead to a hazardous situation. Attention is drawn to the relevant standard
for guidance.
Environment
This standard is applicable to mineral oils, chemicals and used sample containers.
Attention is drawn to the fact that, at the time of writing this standard, some mineral oils in
service are known to be contaminated to some degree by PCBs.

60422  IEC:2013 – 7 –
Because of this, safety countermeasures should be taken to avoid risks to workers, the public
and the environment during the life of the equipment, by strictly controlling spills and
emissions. Disposal or decontamination of these oils should be carried out strictly according
to local regulations. Every precaution should be taken to prevent release of mineral oil into
the environment.
– 8 – 60422  IEC:2013
MINERAL INSULATING OILS IN ELECTRICAL EQUIPMENT –
SUPERVISION AND MAINTENANCE GUIDANCE

1 Scope
This International Standard gives guidance on the supervision and maintenance of the quality
of the insulating oil in electrical equipment.
This standard is applicable to mineral insulating oils, originally supplied conforming to
IEC 60296, in transformers, switchgear and other electrical apparatus where oil sampling is
reasonably practicable and where the normal operating conditions specified in the equipment
specifications apply.
This standard is also intended to assist the power equipment operator to evaluate the
condition of the oil and maintain it in a serviceable condition. It also provides a common basis
for the preparation of more specific and complete local codes of practice.
The standard includes recommendations on tests and evaluation procedures and outlines
methods for reconditioning and reclaiming oil and the decontamination of oil contaminated
with PCBs.
NOTE The condition monitoring of electrical equipment, for example by analysis of dissolved gases, furanic
compounds or other means, is outside the scope of this standard.
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.
IEC 60156, Insulating liquids – Determination of the breakdown voltage at power frequency –
Test method
IEC 60247, Insulating liquids – Measurement of relative permittivity, dielectric dissipation
factor (tan δ) and d.c. resistivity
IEC 60296:2012, Fluids for electrotechnical applications – Unused mineral insulating oils for
transformers and switchgear
IEC 60475, Method of sampling liquid dielectrics
IEC 60666:2010, Detection and determination of specified additives in mineral insulating oils
IEC 60814, Insulating liquids – Oil-impregnated paper and pressboard – Determination of
water by automatic coulometric Karl Fischer titration
IEC 60970, Insulating liquids – Methods for counting and sizing particles
IEC 61125:1992, Unused hydrocarbon based insulating liquids – Test methods for evaluating
the oxidation stability
60422  IEC:2013 – 9 –
IEC 61619, Insulating liquids – Contamination by polychlorinated biphenyls (PCBs) – Method
of determination by capillary column gas chromatography
IEC 62021-1, Insulating liquids – Determination of acidity – Part 1: Automatic potentiometric
titration
IEC 62021-2, Insulating liquids – Determination of acidity – Part 2: Colourimetric titration
IEC 62535:2008, Insulating liquids – Test method for detection of potentially corrosive sulphur
in used and unused insulating oils
IEC 62697-1:2012, Test methods for quantitative determination of corrosive sulfur compounds
in unused and used insulating liquids - Part 1: Test method for quantitative determination of
dibenzyldisulfide (DBDS)
ISO 2049, Petroleum products – Determination of colour (ASTM scale)
ISO 2719, Determination of flash point – Pensky-Martens closed cup method
ISO 3016, Petroleum products – Determination of pour point
ISO 3104, Petroleum products – Transparent and opaque liquids – Determination of kinematic
viscosity and calculation of dynamic viscosity
ISO 3675, Crude petroleum and liquid petroleum products – Laboratory determination of
density – Hydrometer method
ISO 4406:1999, Hydraulic fluid power – Fluids – Method for coding the level of contamination
by solid particles
EN 14210, Surface active agents – Determination of interfacial tension of solutions of surface
active agents by the stirrup or ring method
ASTM D971, Standard Test Method for Interfacial Tension of Oil Against Water by the Ring
Method
ASTM D1275:2006, Standard Test Method for Corrosive Sulfur in Electrical Insulating Oils
DIN 51353: Testing of insulating oils; Detection of corrosive sulphur; Silver strip test
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
local regulations
regulations pertinent to the particular process in the country concerned
Note 1 to entry: Such regulations may be defined by local, regional or national legislation or even the owner or
operator of the equipment itself. They are always to be considered as the most stringent of any combination
thereof. It is the responsibility of each user of this standard to familiarize themselves with the regulations
applicable to their situation. Such regulations may refer to operational, environmental or health and safety issues.
A detailed risk assessment will usually be required.

– 10 – 60422  IEC:2013
3.2
routine tests (Group 1)
minimum tests required to monitor the oil and to ensure that it is suitable for continued service
Note 1 to entry: If the results obtained from these tests do not exceed recommended action limits usually no
further tests are considered necessary until the next regular period for inspection but, under certain perceived
conditions, complementary tests may be deemed prudent.
3.3
complementary tests (Group 2)
additional tests, which may be used to obtain further specific information about the quality of
the oil, and may be used to assist in the evaluation of the oil for continued use in service
3.4
special investigative tests (Group 3)
tests used mainly to determine the suitability of the oil for the type of equipment in use and to
ensure compliance with environmental and operational considerations
3.5
reconditioning
process that eliminates or reduces gases, water and solid particles and contaminants by
physical processing only
3.6
reclamation
process that eliminates or reduces soluble and insoluble polar contaminants from the oil by
chemical and physical processing
3.7
PCB decontamination
process that eliminates or reduces PCB contamination from mineral oil
4 Properties and deterioration/degradation of oil
The reliable performance of mineral insulating oil in an insulation system depends upon
certain basic oil characteristics that can affect the overall performance of the electrical
equipment.
In order to accomplish its multiple roles of dielectric, coolant and arc-quencher, the oil needs
to possess certain properties, in particular:
• high dielectric strength to withstand the electric stresses imposed in service
• sufficiently low viscosity so that its ability to circulate and transfer heat is not impaired
• adequate low-temperature properties down to the lowest temperature expected at the
installation site
• resistance to oxidation to maximize service life
In service, mineral oil degrades due to the conditions of use. In many applications, insulating
oil is in contact with air and is therefore subject to oxidation. Elevated temperatures
accelerate degradation. The presence of metals, organo-metallic compounds or both may act
as a catalyst for oxidation. Changes in colour, the formation of acidic compounds and, at an
advanced stage of oxidation, precipitation of sludge may occur. Dielectric and, in extreme
cases, thermal properties may be impaired.
In addition to oxidation products, many other undesirable contaminants, such as water, solid
particles and oil-soluble polar compounds can accumulate in the oil during service and affect
its electrical properties. The presence of such contaminants and any oil degradation products
are indicated by a change of one or more properties as described in Table 1.

60422  IEC:2013 – 11 –
Deterioration of other constructional materials, which may interfere with the proper functioning
of the electrical equipment and shorten its working life, may also be indicated by changes in
oil properties.
5 Oil tests and their significance
5.1 General
A large number of tests can be applied to mineral insulating oils in electrical equipment. The
tests listed in Table 1 and discussed in 5.2 to 5.19 are considered sufficient to determine
whether the condition of the oil is adequate for continued operation and to suggest the type of
corrective action required, where applicable. The tests are not listed in order of priority.
Table 1 – Tests for in-service mineral insulating oils
a
Property Group Subclause Method
Colour and appearance 1 5.2 ISO 2049
Breakdown voltage 1 5.3 IEC 60156
Water content 1 5.4 IEC 60814
Acidity (neutralization value) 1 5.5 IEC 62021-1 or
IEC 62021-2
Dielectric dissipation factor (DDF) and resistivity 1 5.6 IEC 60247
b
Inhibitor content 1 5.7.3 IEC 60666
Sediment 2 5.8 Annex C of this standard
Sludge
c
Interfacial tension (IFT) 2 5.9 ASTM D971
EN 14210
c
Particles (counting and sizing) 2 5.10 IEC 60970
c
Oxidation stability 3 5.7 IEC 61125
d
Flash point 3 5.11 ISO 2719
d
Compatibility 3 5.12 IEC 61125
d
Pour point 3 5.13 ISO 3016
d
Density 3 5.14  ISO 3675
d
Viscosity 3 5.15 ISO 3104
Polychlorinated biphenyls (PCBs) 3 5.16 IEC 61619
c
Corrosive sulphur 3 5.17 IEC 62535
ASTM D1275, Method B
DIN 51353
Dibenzyl disulfide (DBDS) content 3 5.18 IEC 62697-1
b
Passivator content 3 5.19 Annex B of
IEC 60666:2010
a
Group 1 are routine tests, Group 2 are complementary tests, Group 3 are special investigative tests.
b
Restricted to inhibited and or passivated oils.
c
Only needed under special circumstances, see applicable subclause.
d
Not essential, but can be used to establish type identification.

– 12 – 60422  IEC:2013
5.2 Colour and appearance
The colour of an insulating oil is determined in transmitted light and is expressed by a
numerical value based on comparison with a series of colour standards. It is not a critical
property, but it may be useful for comparative evaluation. A rapidly increasing or a high colour
number may be an indication of oil degradation or contamination.
Besides colour, the appearance of oil may show cloudiness or sediment, which may indicate
the presence of free water, insoluble sludge, carbon particles, fibres, dust, or other
contaminants.
5.3 Breakdown voltage
Breakdown voltage is a measure of the ability of oil to withstand electric stress and has
primary importance for the safe operation of electrical equipment. It is strongly dependent on
the sampling temperature (5.4.3 and 5.4.4).
Dry and clean oil exhibits an inherently high breakdown voltage. Free water and solid
particles, the latter particularly in combination with high levels of dissolved water, tend to
migrate to regions of high electric stress and reduce breakdown voltage dramatically. The
measurement of breakdown voltage, therefore, serves primarily to indicate the presence of
contaminants such as water or particles. A low value of breakdown voltage can indicate that
one or more of these are present. However, a high breakdown voltage does not necessarily
indicate the absence of all contaminants.
The values of breakdown voltage are only significant when the oil has been sampled at the
operating temperature of the transformer. Samples taken at < 20 °C may give an optimistic
view of the state of the transformer when analysed at room temperature. The breakdown
voltage of spare units that have been long out of service and are again energized should be
monitored more often until the transformer has reached a steady state.
5.4 Water content
5.4.1 General
Depending on the amount of water, the temperature of the insulating system and the status of
the oil, the water content of insulating oils influences
• the breakdown voltage of the oil,
• the solid insulation,
• the ageing tendency of the liquid and solid insulation.
The water content in the liquid and solid insulation thus has a significant impact on the actual
operating conditions and the lifetime of the transformer.
There are two main sources of water increase in transformer insulation:
• ingress of moisture from the atmosphere;
• degradation of insulation.
Water is transferred in oil filled electrical equipment by the insulating liquid. Water is present
in oil in a dissolved form and may also be present as a hydrate adsorbed by polar ageing
products (bonded water). Particles, such as cellulose fibres may bind some water.
5.4.2 Water in oil
), given in mg/kg, depends on the condition of the oil, the
The solubility of water in oil (W
s
temperature and type of oil. The absolute water content (W ) is independent of the
abs
60422  IEC:2013 – 13 –
temperature, type and condition of the oil and the result is given in mg/kg. W can be
abs
measured according to IEC 60814. The relative water content (W ) is defined by the ratio
rel
W /W and the result is given in per cent. The relative water content can be evaluated by
abs s
use of a suitable method such as that in BS 6522 [1] or on-line by means of capacitive
sensors [2]. Water solubility (W ) should be determined at the same temperature as that of
s
the oil sample when taken. By way of a guide, the condition of cellulosic insulation in relation
to oil percentage saturation is given in Table A.1.
At water contents in oil above the saturation level, i.e. when W > W (or W > 100 %), the
abs s rel
excess water cannot remain dissolved and free water may be seen in the form of cloudiness
or droplets.
Usually, the temperature is determined directly in the oil stream of the sample taken. In cases
where top oil indicator readings or corrections for ONAN (natural oil or natural air) or OFAF
(forced oil or forced air) cooling mode are used, this should be explicitly noted.
The water content in oil is directly proportional to the relative water concentration (relative
saturation) up to the saturation level. The temperature dependence of the solubility of water in
oil (W ) is expressed by:
S
(−B /T )
W = W e
s oil
(1)
where T is the temperature of the oil at the point of sampling in Kelvin and W and B are
0il
constants that are similar for many transformer oils but may be different for some products,
mainly due to differences in aromatic content. Where present, some free water may transfer
into dissolved water at elevated temperatures.
As oils become very oxidized with increasing amounts of polar ageing by-products, their water
solubility characteristics, which are also dependent on the type of the oil, also increase. The
solubility of water in very aged oils may be much higher than that in unused oils (Figure 1).
Each oil should be considered separately and no universal formula is available.

—————————
Figures in square brackets refer to the bibliography.

– 14 – 60422  IEC:2013
0 10 20 30 40 50 60
Oil temperature during operation  (°C)
Saturation water content in unused oil (log W = 7,0895-1567/T)
s
Typical saturation water content in oxidized oil with acidity of 0,3 mg KOH/g
IEC  2406/12
Figure 1 – Example of variation in saturation water content with oil temperature and
acidity for insulating oil originally conforming to IEC 60296
5.4.3 Water content in the oil/paper-system
Transformers are dried during the manufacturing process until measurements or standard
practices yield a moisture content in the cellulosic insulation of less than 0,5 % to 1,0 %
depending upon purchaser's and manufacturer's requirements. After the initial drying, the
moisture content of the insulation system increases depending on the environmental and/or
operating conditions.
In a transformer, the total mass of water is distributed between the paper and the oil such that
the bulk of the water is in the paper. Small changes in temperature significantly change the
dissolved water content of the oil but only slightly change the water content of the paper.
When oil in a transformer is operating at a constant, relatively elevated temperature for a long
period, thermodynamic equilibrium between water absorbed by cellulose and water dissolved
in oil is closely approached. This equilibrium is temperature dependent so that at elevated
temperatures more water diffuses from the paper into the oil. However, if the oil temperature
is not high enough, such equilibrium is not reached because of the lower rate of diffusion of
water to the oil from the cellulose insulation.
Saturation water content of oil  (mg/kg)

60422  IEC:2013 – 15 –
The determination of the water content in the paper of a transformer by the measurement of
the water in oil has been frequently described, but practical results are often not in line with
the theoretical predictions. The drying process of the paper may not take out as much water
as calculated.
All calculations and correlations of the water content in oil and the water content in the
oil/paper-system depend on the equilibrium state between the insulating oil and the oil/paper-
system and vice versa. The equilibrium is influenced by many factors, such as the difference
in the temperature between oil and the cellulose/oil-system. The calculation of the water
content of the paper/pressboard by determination of the water in the oil has been examined in
several studies and publications (see Annex A).
5.4.4 Interpretation of results
Breakdown voltage and water content are strongly interrelated. Both of them are temperature
dependent, therefore it is most informative to measure them at different transformer
temperatures in order to obtain a reliable assessment of humidity in the combined oil-paper
insulation system. The interpretation of water content in oil is strongly related to the sampling
temperature determined by measuring the temperature directly in the oil stream. In cases
where the top oil temperature indicator (OTI) temperature corrections for ONAN or OFAF
cooling mode are used, this should be explicitly noted.
For transformers with a relatively steady load, a normalizing calculation of the water content
for 20 °C may be helpful for trending. The procedure is described in Annex A.
5.5 Acidity
The acidity (neutralization value) of oil is a measure of the acidic constituents or contaminants
in the oil.
The acidity of a used oil is due to the formation of acidic oxidation products. Acids and other
oxidation products will, in conjunction with water and solid contaminants, affect the dielectric
and other properties of the oil. Acids have an impact on the degradation of cellulosic materials
and may also be responsible for the corrosion of metal parts in a transformer.
The rate of increase of acidity of oil in service is a good indicator of the ageing rate. The
acidity level is used as a general guide for determining when the oil should be replaced or
reclaimed.
Generally, inhibited oil should show no significant increase in acidity from its original value
provided that the inhibitor is present in sufficient amount.
5.6 Dielectric dissipation factor (DDF) and resistivity
These parameters are very sensitive to the presence of soluble polar contaminants, ageing
products or colloids in the oil. Changes in the levels of the contaminants can be monitored by
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