EN 60422:2006

STANDARDMineralna izolacijska olja v električni opremi – Napotki za nadzorovanje in vzdrževanje (IEC 60422:2005)Mineral insulating oils in electrical equipment – Supervision
and maintenance guidance (IEC 60422:2005)©
Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljenoReferenčna številkaSIST EN 60422:2006(en)ICS29.040.10

EUROPEAN STANDARD EN 60422 NORME EUROPÉENNE
EUROPÄISCHE NORM May 2006
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2006 CENELEC -
All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60422:2006 E
ICS 29.040.10; 29.130
English version
Mineral insulating oils in electrical equipment -
Supervision and maintenance guidance (IEC 60422:2005)
Huiles minérales isolantes
dans les matériels électriques - Lignes directrices pour la maintenance
et la surveillance (CEI 60422:2005)
Richtlinien zur Überwachung und Wartung von Isolierölen auf Mineralölbasis
in elektrischen Betriebsmitteln (IEC 60422:2005)
This European Standard was approved by CENELEC on 2006-04-04. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.

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Foreword The text of document 10/636/FDIS, future edition 3 of IEC 60422, prepared by IEC TC 10, Fluids for electrotechnical applications, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 60422 on 2006-04-04. The following dates were fixed: – latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement
(dop)
2007-02-01 – latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow)
2009-05-01 Annex ZA has been added by CENELEC. __________ Endorsement notice The text of the International Standard IEC 60422:2005 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following notes have to be added for the standards indicated: IEC 60567 NOTE Harmonized as EN 60567:2005 (not modified). IEC 60599 NOTE Harmonized as EN 60599:1999 (not modified). IEC 61198 NOTE Harmonized as EN 61198:1994 (not modified). __________

- 3 - EN 60422:2006
Annex ZA
(normative)
Normative references to international publications with their corresponding European publications
The following referenced documents are indispensable for the application 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.
NOTE
When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
Publication Year Title EN/HD Year
IEC 60156 - 1) Insulating liquids - Determination of the breakdown voltage at power frequency - Test method EN 60156 1995 2)
IEC 60247 - 1) Insulating liquids - Measurement of relative permittivity, dielectric dissipation factor (tan d) and d.c. resistivity EN 60247 2004 2)
IEC 60296 - 1) Fluids for electrotechnical applications - Unused mineral insulating oils for transformers and switchgear EN 60296 + corr. September 2004 2)2004
IEC 60475 - 1) Method of sampling liquid dielectrics - -
IEC 60666 - 1) Detection and determination of specified anti-oxidant additives in insulating oils HD 415 S1 1981 2)
IEC 60814 - 1) Insulating liquids - Oil-impregnated paper and pressboard - Determination of water by automatic coulometric Karl Fischer titration EN 60814 1997 2)
IEC 60970 - 1) Methods for counting and sizing particles in insulating liquids - -
IEC 61125 - 1) Unused hydrocarbon-based insulating liquids - Test methods for evaluating the oxidation stability - -
IEC 61619 - 1) Insulating liquids - Contamination by polychlorinated biphenyls (PCBs) - Method of determination by capillary column gas chromatography EN 61619 1997 2)
IEC 62021-1 - 1) Insulating liquids - Determination of acidity Part 1: Automatic potentiometric titration EN 62021-1 2003 2)
ISO 2049 - 1) Petroleum products - Determination of colour (ASTM scale) EN 12049 1996 2)
ISO 2719 - 1) Petroleum products and lubricants - Determination of Flash Point Pensky-Martens closed cup method EN ISO 2719 2002 2)
ISO 3016 - 1) Petroleum Oils - Determination of pour point - -
1) Undated reference.
2) Valid edition at date of issue.

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Publication Year Title EN/HD Year ISO 3104 - 1) Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity EN ISO 3104 1996 2)
ISO 3675 - 1) Crude petroleum and liquid petroleum products - Laboratory determination of density - Hydrometermethod ISO 3675 1998 2)
ASTM D971-99a 2004 Standard test method for interfacial tension of oil against water by the ring method - -

NORME INTERNATIONALECEIIEC INTERNATIONAL STANDARD 60422Troisième éditionThird edition2005-10 Huiles minérales isolantes
dans les matériels électriques – Lignes directrices pour la maintenance
et la surveillance
Mineral insulating oils in electrical equipment – Supervision and maintenance guidance
Pour prix, voir catalogue en vigueur For price, see current catalogue IEC 2005
Droits de reproduction réservés

Copyright - all rights reserved 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'éditeur. 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 the publisher. International Electrotechnical Commission,
3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, SwitzerlandTelephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch
Web: www.iec.ch CODE PRIX PRICE CODE X Commission Electrotechnique InternationaleInternational Electrotechnical Commission

60422  IEC:2005 – 3 –
CONTENTS FOREWORD.7 INTRODUCTION.11
1 Scope.13 2 Normative references.13 3 Terms and definitions.15 4 General remarks.17 4.1 General caution.17 4.2 Environment.17 5 Properties and deterioration/degradation of oil.19 6 Oil tests and their significance.19 6.1 General.19 6.2 Colour and appearance.21 6.3 Breakdown voltage.21 6.4 Water content.23 6.5 Acidity.29 6.6 Dielectric Dissipation Factor (DDF) and resistivity.29 6.7 Inhibitor content and oxidation stability.35 6.8 Sediment and sludge.37 6.9 Interfacial tension (IFT).37 6.10 Particle count.37 6.11 Flash-point.39 6.12 Compatibility of insulating oils.39 6.13 Pour-point.41 6.14 Density.41 6.15 Viscosity.41 6.16 Polychlorinated biphenyls (PCBs).41 6.17 Corrosive sulphur.41 7 Sampling of oil from equipment.43 8 Categories of equipment.43 9 Evaluation of mineral insulating oil in new equipment.45 10 Evaluation of oil in service.47 10.1 General.47 10.2 Frequency of examination of oils in service.47 10.3 Testing procedures.51 10.4 Classification of the condition of oils in service.51 10.5 Corrective action.53 11 Handling and storage.63 12 Treatment.65 12.1 General.65 12.2 Reconditioning.65 12.3 Reclaiming.73 12.4 Decontamination of oils containing PCB.75

60422  IEC:2005 – 5 –
13 Replacement of oil in electrical equipment.77 13.1 Replacement of oil in transformers rated below 72,5 kV and in switchgear and associated equipment.77 13.2 Replacement of oil in transformers rated 72,5 kV and above.77 13.3 Replacement of oil in electrical equipment contaminated with PCB.77
Annex A (informative)
Sampling temperature below 20 ºC.79 Annex B (informative)
Particles.81 Annex C (informative)
Test method for determination of sediment and sludge.83
Bibliography.85
Figure 1 – Example of the variation in saturation water content with oil temperature
and acidity for insulating oil originally conforming to IEC 60296.25 Figure 2 – Typical correction factors.29 Figure 3 – Example of variation of resistivity with temperature for insulating oils.33
Table 1 – Tests for mineral insulating oils.21 Table 2 – Categories of equipment.43 Table 3 – Recommended limits for mineral insulating oils after filling in new electrical equipment prior to energization.45 Table 4 – Recommended frequency of testing(1).49 Table 5 – Application and interpretation of tests.55 Table 6 – Corrective actions.63 Table 7 – Conditions for processing inhibited mineral insulating oils.67 Table A.1 – Guidelines for interpreting data expressed in percent saturation.79 Table B.1 – Examples of contamination levels (particles) encountered on
power transformer insulating oil as measured by laser counting (ISO 4406).81

60422  IEC:2005 – 7 –
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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with an IEC Publication. 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 third edition cancels and replaces the second edition, published in 1989, and constitutes a technical revision. The main changes with regard to the previous edition are as follows: This standard has been revised to take into account changes in oil and equipment technology and to have due regard for the best practices currently in use world-wide.

60422  IEC:2005 – 9 –
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. This guidance incorporates changes introduced in associated standards since the publication of the second edition. The text of this standard is based on the following documents: FDIS Report on voting 10/636/FDIS 10/641/RVD
Full information on the voting for the approval of this guide 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 maintenance result 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.

60422  IEC:2005 – 11 –
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 then 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 (PCB). The guidelines given in this standard, whilst technically sound, are 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.

60422  IEC:2005 – 13 –
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, and used 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 assists 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.
This standard includes recommendations on tests and evaluation procedures and outlines methods for reconditioning and reclaiming oil and the decontamination of oil contaminated with PCB. 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 referenced documents are indispensable for the application 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 60296: Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear IEC 60475: Method of sampling liquid dielectrics IEC 60666: Detection and determination of specified anti-oxidant additives in insulating oils IEC 60814: Insulating liquids – Oil-impregnated paper and pressboard – Determination of water by automatic coulometric Karl Fischer titration IEC 60970: Methods for counting and sizing particles in insulating liquids

60422  IEC:2005 – 15 –
IEC 61125: Unused hydrocarbon-based insulating liquids – Test methods for evaluating the oxidation stability 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 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 ASTM D971-99a: 2004 Standard test method for interfacial tension of oil against water by the ring method 3 Terms and definitions For the purposes of this document, the following terms and definitions apply.
3.1
distribution transformer small transformer, usually less than 2 MVA, used by electrical utilities for the distribution of electricity 3.2
industrial or strategic transformer transformer that, irrespective of its rated power and voltage, is used by industry in general and has a critical role in the proper functioning of such industry 3.3
local regulations regulations pertinent to the particular process in the country concerned NOTE They may be defined by local, regional or national legislation or even the owner or operator of the equipment itself. They should always be considered as the most stringent of any combination thereof. It is the responsibility of all users of this standard to familiarize themselves with the regulations applicable to their situation. They may refer to operational, environmental or health and safety issues. Usually detailed risk assessment will be required. 3.4
routine tests (Group 1) minimum tests required to monitor the oil and to ensure that it is suitable for continued service NOTE 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.

60422  IEC:2005 – 17 –
3.5
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 3.6
special investigative tests (Group 3) 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 3.7
reconditioning process that eliminates water and solid particles and contaminants by physical processing only 3.8
reclamation process that eliminates soluble and insoluble polar contaminants from the oil by chemical and physical processing 3.9
PCB decontamination process by which PCB contamination may be removed from mineral oil 4 General remarks 4.1 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 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. 4.2 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 guide, some mineral oils in service are known to be contaminated to some degree by PCBs. Because of this, safety countermeasures shall 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 shall be carried out strictly in accordance with local regulations. Every precaution should be taken to prevent release of mineral oil into the environment.

60422  IEC:2005 – 19 –
5 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 role 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 maximise 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 degradation product of oil are indicated by a change of one or more properties as described in Table 1. 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. 6 Oil tests and their significance 6.1 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 6.2 to 6.10 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.

60422  IEC:2005 – 21 –
Table 1 – Tests for mineral insulating oils Property Groupd Subclause Method Colour and appearance 1 6.2 ISO 2049 Breakdown voltage 1 6.3 IEC 60156 Water content 1 6.4 IEC 60814 Acidity (neutralization value) 1 6.5 IEC 62021-1 Dielectric dissipation factor (DDF) or resistivity 1 6.6 IEC 60247 Inhibitor content c 1 6.7 IEC 60666 Sediment and sludge 2 6.8
Annex C Interfacial tension (IFT) a 2 6.9 ASTM D971-99a Particles (particle count) 2 6.10 IEC 60970 Oxidation stability a 3 6.7.4 IEC 61125 Flash-point b 3 6.11 ISO 2719 Compatibility b 3 6.12 IEC 61125 Pour-point b 3 6.13 ISO 3016 Density b 3 6.14 ISO 3675 Viscosity b 3 6.15 ISO 3104 Polychlorinated biphenyls (PCBs) 3 6.16 IEC 61619 Corrosive sulphur a 3 6.17 DIN 51353 [5]1 a Only needed under special circumstances, see applicable subclause. b Not essential, but can be used to establish type identification. c Restricted to inhibited oils.
d Group 1 are routine tests, group 2 are complementary tests, group 3 are special investigative tests.
6.2 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 sediments, which may indicate the presence of free water, insoluble sludge, carbon, fibres, dirt, or other contaminants. 6.3 Breakdown voltage Breakdown voltage is a measure of the ability of oil to withstand electric stress. 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. ——————— 1 References in square brackets refer to the bibliography.

60422  IEC:2005 – 23 –
6.4 Water content 6.4.1 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 and – 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 cellulose and oil. 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 bond some water. 6.4.2 Water in oil The solubility of water in oil, given in mg/kg, depends on the condition of the oil, the temperature and type of oil. The absolute water content (Wabs) is independent of the temperature, type and condition of the oil and the result is given in mg/kg. Wabs can be measured according to IEC 60814. The relative water content (Wrel) is defined by the ratio Wabs/water solubility and the result is given in percent. Water solubility should be determined at the same temperature as the oil has been taken. 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 (WS) is expressed by
WS = WOil e(–B/T) (1) where T is the temperature of the oil at the point of sampling in Kelvin and W0il and B are constants that are similar for many transformer oils but may be different for some products, mainly due to differences in aromatic contents. At elevated temperatures, some amount of hydrated water may transfer into dissolved water. 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.

60422  IEC:2005 – 25 –
250 Oil temperature during operation
°CSaturation water content of oil
mg/kg Saturation water content in unused oil
(log Ws = 7,0895 – 1 567/T) Typical saturation water content in oxidised oil
with acidity of 0,3 mg KOH/g 200 150 100 50 0 0 10 20 30 40 50 60 IEC
1694/05
Figure 1 – Example of the variation in saturation water content with oil temperature
and acidity for insulating oil originally conforming to IEC 60296 6.4.3 Water content in the oil/paper-system Transformers are dried during the manufacturing process until measurements or standard practices would 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 water is in the paper. Small changes in temperature significantly modify the dissolved water content of the oil but only slightly modify the water content of the paper.

60422  IEC:2005 – 27 –
When oil in a transformer is working 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 an equilibrium is not reached because of the lower rate of diffusion of water to the oil from the cellulose insulation. 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. Many factors such as the difference in the temperature between oil and the cellulose/oil-system influence the equilibrium. The calculation of the water content of the paper/pressboard by determination of water in the oil has been examined in several studies and publications [2, 4, 6]. 6.4.4 Interpretation of results 6.4.4.1 General The tools presented in this subclause for interpreting the results are applicable only if the following conditions are fulfilled: – equilibrium exists between oil and paper; – no abnormal ingress of water (leaks); – presence of paper in the equipment; – absence of free water. 6.4.4.2 Sampling temperature at or above 20 ºC For the proper interpretation of moisture content, the analytical result of water content of the oil at a given sampling temperature needs to be corrected to that at a defined temperature. For practical reasons, the defined temperature is set at 20 °C, since below 20 °C the rate of diffusion of water is too slow to achieve equilibrium in operational equipment The correction formula, as demonstrated by several independent studies (Figure 2), is:
()s04,0e24,2tf−= (2) where f is the correction factor and ts is the oil sampling temperature in Celsius. NOTE 1 Corrected values are valid only for comparing results obtained at different oil temperatures. Actual values of water in oil at sampling points are the measured values, not the corrected ones. NOTE 2 This formula is not applicable to temperatures below 20 °C.

60422  IEC:2005 – 29 –
0,00 1,00 2,00 20 30 40 50607080 90100ts °CCorrection factor IEC
1695/05
Figure 2 – Typical correction factors EXAMPLE Measured water content 10 mg/kg Sampling temperature 40 °C Correction factor (from formula 2) 0,45 Corrected dissolved water content (10 × 0,45) 4,5 mg/kg
6.5 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. 6.6 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 measurement of these parameters even when contamination is so slight as to be near the limit of chemical detection. Acceptable limits for these parameters depend largely upon the type of equipment. However, high values of dielectric dissipation factor, or low values of resistivity, may deleteriously affect the power factor and/or the insulation resistance of the electrical equipment.

60422  IEC:2005 – 31 –
There is generally a relationship between DDF and resistivity, with resistivity decreasing as DDF increases. It is normally not necessary to conduct both tests on the same oil and generally DDF is found to be the more common test. Resistivity and DDF are temperature dependent and Figure 3 illustrates typical changes of resistivity with temperature for insulating oils that are virtually free from solid contamination and water. Useful additional information can be obtained by measuring resistivity or DDF at both ambient temperature and a higher temperature such as 90 °C.
In the case of VHV and UHV instrument transformers, special attention shall be paid to DDF as it has been reported that a higher value of DDF may lead to thermal runaway leading to transformer failure. Oils classified as ‘good’ (see 10.4) will have characteristics similar to curves A and B in Figure 3 and will result in satisfactory test results being obtained at both the higher and lower temperatures. Oils classified as ‘poor’ (see 10.4) will have characteristics similar to curve C and will result in a satisfactory test result at 90 °C coupled with an unsatisfactory value at the lower temperature. This is an indication of the presence of water or degradation/deterioration products precipitable in the cold without any significant amount of chemical degradation or general contamination. Unsatisfactory results at both temperatures indicate a greater extent of contamination and that it may not be possible to restore the oil to a satisfactory condition by reconditioning. The measurement of resistivity is also considered to be of value for monitoring oils in service, as it has been shown to be reasonably proportional to oxidation acids and to be affected by undesirable contaminants such as metal salts and water. Other compounds present in used oils, which can affect resistivity, include aldehydes, ketones and alcohols. An increase in temperature reduces the resistivity, as also does water when precipitated at low temperature due to the saturation point being reached.
NOTE It has been observed in instrument transformers that some types of oil may experience a huge increase in DDF after a very short oxidation time, leading to failure of the equipment. It is therefore recommended to measure the DDF of the unused oil after subjecting it to a short oxidation period according to IEC 61125 method C to verify that the oil is suitable for this application.

60422  IEC:2005 – 33 –
1 000 500 200 50 20 10 5 2 1
0,5 0,2 100 0 20 406080100 120 Oil temperature
°CResistivity
GΩ·m BACIEC
1696/05
Line A: Dry oil having a resistivity of 60 GΩ⋅m at 20 °C Line B: Dry oil having a resistivity of 200 GΩ⋅m at 20 °C Line C: Wet oil that is 100 % saturated at 35 ºC NOTE In transformers in service, the behaviour of line C is unlikely to occur in the windings, but rather along tank walls or other very cold surfaces. Figure 3 – Example of variation of resistivity with temperature for insulating oils

60422  IEC:2005 – 35 –
6.7 Inhibitor content and oxidation stability 6.7.1 Oxidation stability The ability of mineral electrical insulating oil to withstand oxidation under thermal stress and in the presence of oxygen and a copper catalyst is called oxidation stability. It gives general information about the life expectancy of the oil under service conditions in electrical equipment. The property is defined as resistance to formation of acidic compounds, sludge and compounds influencing the Dielectric Dissipation Factor (DDF) under given conditions. For oils complying with IEC 60296, these conditions are detailed in IEC 61125 method C and the limits of acceptable performance in IEC 60296. The property depends mainly on the refining process and how it is applied to a given feedstock. Refined mineral oils contain, to a varying degree, natural compounds acting as oxidation inhibitors. These are known as natural inhibitors. Oils containing only natural inhibitors are designated as uninhibited oils.
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