IEC 62975:2021

IEC 62975 ®
Edition 1.0 2021-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Natural esters – Guidelines for maintenance and use in electrical equipment

Esters naturels – Lignes directrices pour la maintenance et l’utilisation dans les
matériels électriques
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.

IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always
committee, …). It also gives information on projects, replaced have access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and
and French, with equivalent terms in 18 additional languages.
once a month by email.
Also known as the International Electrotechnical Vocabulary

(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or
need further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.

Recherche de publications IEC - IEC online collection - oc.iec.ch
webstore.iec.ch/advsearchform Découvrez notre puissant moteur de recherche et consultez
La recherche avancée permet de trouver des publications IEC gratuitement tous les aperçus des publications. Avec un
en utilisant différents critères (numéro de référence, texte, abonnement, vous aurez toujours accès à un contenu à jour
comité d’études, …). Elle donne aussi des informations sur adapté à vos besoins.
les projets et les publications remplacées ou retirées.

Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
Le premier dictionnaire d'électrotechnologie en ligne au
Restez informé sur les nouvelles publications IEC. Just
monde, avec plus de 22 000 articles terminologiques en
Published détaille les nouvelles publications parues.
anglais et en français, ainsi que les termes équivalents dans
Disponible en ligne et une fois par mois par email.
16 langues additionnelles. Egalement appelé Vocabulaire

Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc

Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC 62975 ®
Edition 1.0 2021-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Natural esters – Guidelines for maintenance and use in electrical equipment

Esters naturels – Lignes directrices pour la maintenance et l’utilisation dans les

matériels électriques
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040.10 ISBN 978-2-8322-9222-8

– 2 – IEC 62975:2021 © IEC 2021
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 9
4 Categories of equipment . 9
5 In-service natural ester diagnostic tests . 10
6 Evaluation of natural esters in new equipment . 11
7 Evaluation of natural ester in equipment in service . 12
7.1 General . 12
7.2 Frequency of examination . 13
7.3 Testing procedures . 14
7.3.1 Field tests . 14
7.3.2 Laboratory tests . 14
7.4 Classification of in-service natural esters . 14
8 General requirements for corrective actions . 19
9 Interpretation of results. 19
9.1 General . 19
9.2 Colour and appearance . 19
9.3 Breakdown voltage . 19
9.4 Viscosity . 20
9.5 Acidity. 20
9.6 Dielectric dissipation factor (DDF) and resistivity . 20
9.7 Dissolved gas-in-oil. 21
9.8 Flash and fire points . 21
9.9 Interfacial tension (IFT) . 21
9.10 Density . 22
9.11 Pour point . 22
9.12 Additives . 22
9.13 Particle count . 22
9.14 Compatibility and miscibility of natural esters . 22
9.15 Oxidation stability . 23
9.16 Polymerization . 23
10 Sampling of natural esters from equipment . 24
Annex A (informative) Water and natural ester liquids . 25
A.1 General . 25
A.2 Water content . 25
A.2.1 General . 25
A.2.2 Water in natural esters . 25
A.3 Moisture equilibrium between liquid and solid insulation . 27
Annex B (informative) Replacement and treatments of natural esters in transformers . 30
B.1 Transformer retrofilling with natural esters . 30
B.2 Reconditioning and reclaiming . 31
B.2.1 General . 31
B.2.2 Reconditioning . 31
B.2.3 Reclaiming . 32

Annex C (informative) Use of natural ester liquids with on-load tap-changers (OLTCs) . 33
Bibliography . 35

Figure A.1 – Moisture saturation values between liquids versus temperature [10] . 27
Figure A.2 – Example of water (high concentrations) equilibrium curves for paper and
natural ester [11] . 28
Figure A.3 – Example of water (medium concentrations) equilibrium curves for paper
and liquid [11] . 29
Figure A.4 – Example of water (low concentrations) equilibrium curves for paper and
liquid [11] . 29

Table 1 – Categories of equipment . 10
Table 2 – Diagnostic tests for in-service natural esters . 11
Table 3 – Recommended limits for natural esters properties after filling in new
electrical transformers and reactors prior to energization . 12
a
Table 4 – Recommended frequency of testing . 14
Table 5 – Recommended limits for in service natural esters in transformers . 15
Table A.1 – Typical values for A and B for different insulating liquids [10] . 26
Table A.2 – Guidelines for interpreting data expressed in relative saturation . 27

– 4 – IEC 62975:2021 © IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
NATURAL ESTERS – GUIDELINES FOR
MAINTENANCE AND USE IN ELECTRICAL EQUIPMENT

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 62975 has been prepared by IEC technical committee 10: Fluids
for electrotechnical applications.
The text of this International Standard is based on the following documents:
FDIS Report on voting
10/1123/FDIS 10/1126/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC 62975:2021 © IEC 2021
INTRODUCTION
Natural esters are increasingly being used in transformers and electrical equipment employed
in electrical power generation, transmission, distribution and industrial applications.
The use of natural esters is recommended for equipment where the liquid does not remain in
continuous contact with ambient air, such as hermetically sealed units, units with closed
conservators equipped with a rubber bag (bladder) or external expansion elements (external
bag), units with a headspace having either a nitrogen blanket or a confined volume of air
(distribution transformers).
Monitoring and maintaining liquid quality are essential to ensure the reliable operation of
natural ester 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 liquid tests and
to recommend uniform criteria for the evaluation of test data.
If a certain amount of liquid 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 document is to furnish users with as broad a base of understanding of liquid
quality deterioration as is available, so that they can make informed decisions on inspection
and maintenance practices.
Unused natural ester liquids are sustainable resources and are readily available. Natural
esters are, by most regulations, deemed to be regulated and/or controlled waste. If spills
occur, the user should refer to the regulations applicable to their specific location and
requirements set by their local authorities.
This document, while 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 should be exerted in seeking the best
compromise between technical requirements and economic factors.
Application of natural ester liquids in large power transformers at this time is still relatively
limited after 20 years although a very large number of units is operating. While the collection
of operating data has allowed for the development of this document, care should be used
when applying the recommended values. Manufacturers of natural ester liquids should be
contacted with specific questions or concerns.
WARNING – This document does not purport to address all the safety problems associated
with its use. It is the responsibility of the user of this document to establish appropriate health
and safety practices and determine the applicability of regulatory limitations prior to use.
The natural esters which are the subject of this document should be handled in compliance
with local regulations and supplier’s safety datasheets.
This document is applicable to natural esters, chemicals and used sample containers. The
disposal of these items should be carried out according to local regulations regarding their
impact on the environment.
NATURAL ESTERS – GUIDELINES FOR
MAINTENANCE AND USE IN ELECTRICAL EQUIPMENT

1 Scope
This document provides procedures and guidelines that are intended for the use and
maintenance of natural ester liquid in sealed transformers and other electrical equipment.
This document is applicable to natural esters, originally supplied conforming to IEC 62770
and other applicable standards (e.g. ASTM D6871 [1] ) in transformers, switchgear and
electrical apparatus where liquid sampling is practical and where the normal operating
conditions specified in the equipment specifications apply.
At present, there is a limited amount of information available for electrical equipment other
than transformers.
This document is also intended to assist the power equipment operator to evaluate the
condition of the natural ester 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 document includes recommendations on tests and evaluation procedures and outlines
methods for reconditioning and reclaiming the liquid, when necessary.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. 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 60422:2013, Mineral insulating oils in electrical equipment – Supervision and
maintenance guidance
IEC 60475, Method of sampling insulating liquids
IEC 60567, Oil-filled electrical equipment – Sampling of gases and analysis of free and
dissolved gases – Guidance
IEC 60666, 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
___________
Numbers in square brackets refer to the bibliography.

– 8 – IEC 62975:2021 © IEC 2021
IEC 61125, Insulating liquids – Test methods for oxidation stability – Test method for
evaluating the oxidation stability of insulating liquids in the delivered state
IEC 62021-3, Insulating liquids – Determination of acidity – Part 3: Test methods for non-
mineral insulating oils
IEC 62770, Fluids for electrotechnical applications – Unused natural esters for transformers
and similar electrical equipment
IEC 62961, Insulating liquids – Test methods for the determination of interfacial tension of
insulating liquids – Determination with the ring method
ISO 2049, Petroleum products – Determination of colour (ASTM scale)
ISO 2592, Petroleum products – Determination of flash and fire points – Cleveland open cup
method
ISO 3016, Petroleum and related products from natural or synthetic sources – 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 12185, Crude Petroleum and petroleum products – Determination of density – Oscillating
U-tube method
ISO 21018-3, Hydraulic fluid power – Monitoring the level of particulate contamination of the
fluid – Part 3: Use of the filter blockage technique
ASTM D92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester
ASTM D1500, Standard Test Method for ASTM Color of Petroleum Products (ASTM Color
Scale)
ASTM D1544, Standard Test Method for Color of Transparent Liquids (Gardner Color Scale)
ASTM D3455, Standard Test Methods for Compatibility of Construction Material with Electrical
Insulating Oil of Petroleum Origin
ASTM D6922, Standard Test Method for Determination of Homogeneity and Miscibility in
Automotive Engine Oils
ASTM D7042, Standard Test Method for Dynamic Viscosity and Density of Liquids by
Stabinger Viscometer (and the Calculation of Kinematic Viscosity)
ASTM D7155, Standard Practice for Evaluating Compatibility of Mixtures of Turbine
Lubricating Oils
ASTM D7647, Standard Test Method for Automatic Particle Counting of Lubricating and
Hydraulic Fluids Using Dilution Techniques to Eliminate the Contribution of Water and
Interfering Soft Particles by Light Extinction
ASTM D7752, Standard Practice for Evaluating Compatibility of Mixtures of Hydraulic Fluids

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
– IEC Electropedia: available at http://www.electropedia.org/
– ISO Online browsing platform: available at http://www.iso.org.obp
NOTE ASTM and IEEE terminology are given in:
– ASTM D2864, Standard Terminology Relating to Electrical Insulating Liquids and Gases [2],
– IEEE C57.12.80, Standard Terminology for Power and Distribution Transformers [3].
3.1
local regulations
regulations pertinent to the particular process in the country concerned
Note 1 to entry: These regulations may be defined by local, regional or national legislation or even by 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 document to familiarize themselves with the regulations
applicable to their situation. These regulations shall refer to operational, environmental or health and safety issues.
A detailed risk assessment will usually be required.
3.2
routine tests (Group 1)
minimum tests required to monitor the liquid 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 performed to obtain further specific information about the
quality of the natural ester, and may be performed to assist in the evaluation of the natural
ester for continued use in service
3.4
special investigative tests (Group 3)
tests performed mainly to determine the suitability of the natural ester for the type of
equipment in use and to ensure compliance with environmental and operational
considerations
4 Categories of equipment
In order to consider the different user requirements, equipment has been placed in various
categories as shown in Table 1.
For practical and economic reasons, some electrical utilities may decide that their small
transformers up to 1 MVA and 36 kV are not included in this classification. It is possible that
routine monitoring programmes are not considered economical for this type of equipment.
Where a monitoring programme is required for these transformers, the guidelines given for
category C should be adequate.

– 10 – IEC 62975:2021 © IEC 2021
Table 1 – Categories of equipment
Category Type of equipment
Category O Power transformers / reactors with a nominal system voltage of 400 kV and above.
Category A Power transformers / reactors with a nominal system voltage above 170 kV and below 400 kV.
Also, power transformers of any rated voltage where continuity of supply is vital and similar
equipment for special applications operating under onerous conditions.
Category B Power transformers / reactors with a nominal system voltage above 72,5 kV and up to and
including 170 kV (other than those in Category A).
Category C Power transformers / reactors for MV/LV application e.g. nominal system voltages up to and
including 72,5 kV and traction transformers (other than those in Category A).
Category D Instrument / protection transformers with a nominal system voltage above 170 kV.
Category E Instrument / protection transformers with a nominal system voltage up to and including 170 kV.
Diverter tanks of on-load tap-changers (OLTC), including combined selector/diverter tanks (see
Category F
Annex C).
NOTE 1 Separated selector tanks of on-load tap-changers belong to the same category as the associated
transformer.
NOTE 2 Regardless of size or voltage, a risk assessment can justify condition-monitoring techniques usually
appropriate to a higher classification.
NOTE 3 Due to the very limited number of actual applications, recommended limits are not yet available for
Categories D and E. Such categories are here reported only for consistency purposes with IEC 60422.
NOTE 4 Regarding Category F, limit values are specified for breakdown voltage (BDV) and water content. For
all other parameters, the values have been adopted from the category as the associated transformer.

5 In-service natural ester diagnostic tests
Many tests may be applied to in-service natural ester liquids in electrical equipment. The tests
listed in Table 2 are considered sufficient to determine whether the condition of the in-service
natural ester is adequate for continued operation and to suggest the type of corrective action
required, if needed.
The tests are not listed in order of priority within a grouping. When more than one test method
is reported, the reference method is the first one listed and it is also reported in Clause 9. In
case of dispute this method shall be used.

Table 2 – Diagnostic tests for in-service natural esters
Sub-
Property Method
clause
a
Group 1 – Routine Colour and appearance 9.2 ISO 2049 or ASTM D1500 or ASTM D1544
Tests
Breakdown voltage 9.3 IEC 60156
Water content Annex A IEC 60814
a
Viscosity 9.4 ISO 3104 or ASTM D7042
Acidity (neutralization number) 9.5 IEC 62021-3
Dielectric dissipation factor (DDF) 9.6 IEC 60247
b
Dissolved gas-in-oil 9.7 IEC 60567
a
Group 2 – Fire point 9.8 ISO 2592 or ASTM D92
Complementary
Tests Interfacial tension (IFT) 9.9 IEC 62961
a
Density 9.10 ISO 12185 or ISO 3675 or ASTM D7042
a
Group 3 – Special
Flash point 9.8 ISO 2592 or ASTM D92
Investigative Tests
(informative) Pour point 9.11 ISO 3016
a
IEC 60666 or other suitable methods or
Additives (antioxidant) content 9.12
according to manufacturer’s advice
a
Particles (counting and sizing) 9.13 IEC 60970 or ASTM D7647 or
ISO 21018-3
a
Liquid Compatibility and miscibility 9.14 ASTM D7752 or ASTM D7155 and
ASTM D6922
Materials compatibility (see Annex B ASTM D3455
retrofilling)
a
Reference method (in case more than one method is specified).
b
DGA test is intended for hermeticity check only, based on air (N + O ) content.
2 2
6 Evaluation of natural esters in new equipment
A substantial proportion of electrical equipment is supplied to the final user, already filled with
natural ester liquids. In these cases, as the natural ester has already come into contact with
insulating and other materials, it can no longer be considered as "unused natural ester" as
defined in IEC 62770. Therefore, its properties shall be regarded as those applicable to a
natural ester in new electrical equipment prior to energization.
Natural ester properties for new equipment shall be appropriate to the category and functions
of the transformers and reactors (see Table 3).
NOTE As the characteristics of the natural ester in new equipment prior to energization are an integral part of that
equipment design, the user can request these characteristics to be better than the minimum standards suggested
in Table 3, which are based on the experience of many years of operating practice.

– 12 – IEC 62975:2021 © IEC 2021
Table 3 – Recommended limits for natural esters properties after filling
in new electrical transformers and reactors prior to energization
Property Highest voltage for equipment (kV)
72,5 kV to
≤ 72,5 kV > 170 kV
170 kV
Appearance Clear, free from sediment matter
Colour (on scale given in ISO 2049) < 2,0
Breakdown voltage (kV) > 55 > 60 > 60
a b
Water content (mg/kg) < 200 < 150 < 100

Acidity (mg KOH/g) < 0,08 < 0,08 < 0,08
Dielectric dissipation factor at 90 °C < 0,07 < 0,07 < 0,07
Density at 20 °C (g/ml) < 1
Viscosity at 40 °C (mm /s) < 50
Fire point (°C) > 300
Flash point (°C) > 250
4 c
Total gas content (dissolved gas analysis) (% or 10 µl/l) < 1,5
d
Total PCB content (mg/kg) Not detectable
a
The values are not corrected for temperature since not enough time may have elapsed to reach an equilibrium
between the natural ester and cellulose insulation.
b
For use in transformers under 72,5 kV class, the maximum water content should be agreed between supplier
and user depending upon local circumstances.
c
This limit (O + N total, only; see Table 2) is applicable only to transformers equipped with a hermetic
2 2
preservation system and without nitrogen blanked. Differences can be negotiated between customer and
manufacturer.
d
According to IEC 61619 [4] modified for esters.

7 Evaluation of natural ester in equipment in service
7.1 General
A natural ester in service is subjected to heat, oxygen, water and other catalysts, all of which
are detrimental to the properties of all insulation. In order to maintain the quality of the natural
ester in service, regular sampling and analysis shall be performed.
From an environmental point of view, visual inspection can also be used to monitor leakage
and spills of natural esters.
In case of leakage, natural esters are very prone to be oxidized, much faster than mineral oil,
forming thin films that are difficult to clean if not done in a short time.
Often the first sign of natural ester deterioration may be obtained by direct observation of the
natural ester clarity and colour through the sight glass of the conservator or sight glass of the
Buchholz relay.
The interpretation of results, in terms of the functional deterioration of the natural ester, shall
be performed by experienced personnel based on the following elements of risk management
and life cycle management:
• characteristic values for the type and family of natural ester and equipment, developed by
statistical methods;
• evaluation of trends and the rate of variation of the values for a given natural ester
property;
• typical values, for "good", "fair" and "poor" for the appropriate type and family of
equipment as are given in Table 5.
Application of natural ester liquids in power transformers is recommended for sealed
equipment only and at this time is still relatively limited compared to mineral oil. The collection
of operating data (approximately 35 000 records) from a very large, but young, population of
transformers contributed to the development of this document. The recommended values
proposed in this document shall be used carefully. Manufacturers of natural ester liquids
should be contacted in case of specific questions or concerns.
7.2 Frequency of examination
It is impossible to lay down a general rule for the frequency of examination of natural esters in
service which will be applicable to all possible situations that might be encountered.
The optimum frequency will depend on the type, function, voltage, power, construction and
service conditions of the equipment, as well as the condition of the natural esters as
determined in the previous analysis. A compromise will often have to be found between
economic factors and reliability requirements.
Much greater difficulties exist in deciding frequency of testing and permissible natural ester
deterioration levels which are acceptable for all applications of insulating liquids in relation to
differences in operating policies, reliability requirements and types of electrical system. For
example, large power companies may find the full application of these recommendations to
distribution transformers uneconomical. Conversely, the industrial user, whose activities
depend on the reliability of his power supply, may wish to institute more frequent and stricter
controls of liquids dielectric quality as a means of guarding against power failures.
By way of a guide, a suggested frequency of tests suitable for different types of equipment is
given in Table 4 but, less frequent testing may be appropriate based on life cycle analysis
(LCA) and/or life cycle management (LCM) and risk assessment (RA).
Generally, check measurements shall be carried out on the basis of the following criteria,
which apply to natural esters as other transformer dielectric liquids.
a) Characteristics may be tested periodically, at intervals as suggested in Table 4, unless
otherwise defined.
b) The frequency of examination may be increased where any of the significant properties
indicates that the liquid is in fair or poor condition, or when trend analysis indicates
significant changes.
c) The degradation of the natural ester will accelerate with increased temperature and in the
presence of oxygen (oxidation) and water (hydrolysis). Therefore, heavily loaded
transformers may need more frequent liquid-sampling and complementary testing. As a
consequence, the use of natural esters is recommended only for equipment that are not
open to the atmosphere.
d) The testing frequency shall be established by means of a cost/benefit evaluation based on
life cycle analysis and risk assessment. For some owners this approach may indicate
different testing frequencies from those indicated in Table 4. For instance, some electrical
utilities may prefer not to perform this programme on this type of equipment and small
industries may prefer to include this type of equipment even in a higher category.

– 14 – IEC 62975:2021 © IEC 2021
a
Table 4 – Recommended frequency of testing
Property Equipment category
b
O A B C F
c
Group 1 (Routine tests) – years 1 to 2 1 to 3 1 to 4 2 to 6 1 to 4
Group 2 (Complementary tests) These tests may be done periodically but less frequently than
routine tests. The frequency will depend upon the condition of
liquid, age and type. First (baseline) measurements should be
done in new or refurbished equipment prior to energization.
Group 3 (Special Investigative tests) These are very special tests that need to be done only under
special circumstances.
Group 3 (Additives) In accordance with natural esters producer recommendations.
a
These proposed periods refer to a normal routine maintenance programme. Should one or more of the
measured properties indicate that the liquid is in a fair or poor condition or if an abnormal ageing trend is
observed, these periods should be shortened according to the importance of the equipment.
b
See 7.2 d).
c
Group 1 tests shall be performed after filling or refilling the transformer, prior to energizing.

7.3 Testing procedures
7.3.1 Field tests
In some circumstances there is a need to perform tests closer to the point of sampling rather
than in the laboratory. Field tests can help the operator to achieve prompt estimation of the
natural ester condition. Field testing shall be managed by experienced operators.
7.3.2 Laboratory tests
A complete examination scheme includes all the tests listed in Table 2. These tests are
subdivided into the three groups identified as routine, complementary and investigative.
7.4 Classification of in-service natural esters
In service natural ester liquids shall be classified according to the results of the tests listed in
Table 2. The trend of these results over a period of time will add useful information for
arriving at the classification of the insulating liquid.
Insulating liquids may be classified as "good", "fair" or "poor" based on the evaluation of
properties listed in Table 2. Table 5 provides guidance to assist in this classification process.
• Good
Natural ester liquid is in normal condition; continue normal frequency of the tests.
• Fair
Natural ester liquid has some detectable degradation; increase the frequency of the tests.
• Poor
Natural ester liquid has significant degradation; schedule follow-up actions in accordance
with experienced personnel.
At the current state of art, only recommended limits for transformers are available. The limits
have been based on restricted available data and may change (especially in the categories O,
A and B) with time with increasing statistical information.
No action shall be taken on the basis of one result and one property. Repeat samples are
recommended where the result appears abnormal compared to the trend of the results
.
previously obtained
Table 5 – Recommended limits for in service natural esters in transformers
Recommended Action Limits
a b, c
Property Category Recommended Action Notes
Good Fair Poor
Colour and appearance O-A-B-C-F Clear and Darker than as new and/or As dictated by other tests Blackening is a symptom of chemical
without visible pale and/or appearance of contamination or ageing.
contamination turbidly
Turbidity is a symptom of high-water content
or material incompatibility.
Breakdown voltage (kV) O-A > 60 50 to 60 < 50 Good: Continue normal frequency of the test If OLTC symbol is unknown (delta or star or
line end) use delta limits (< 40 kV).
B > 50 40 to 50 < 40 Fair: More frequent sampling. Check other
parameters e.g. water, particle content and
C > 40 30 to 40 < 30
perhaps DDF/resistivity and acidity.
F ≥ 30 kV for < 30 kV for OLTC in star-
Poor: Recondition the liquid or, alternatively,
OLTC in star- point application
if more economical because other tests
point
indicate severe ageing, replace the insulating
< 40 kV for OLTC in delta or
application
liquid.
line-end application
≥ 40 kV for
OLTC in delta
or line-end
application
Water content (mg/kg) O-A < 100 100 to 300 > 300 Good: Continue normal frequency of the test. The values of water content shall always be
regarded together with t
...