IEC 60599:2015
(Main)Mineral oil-filled electrical equipment in service - Guidance on the interpretation of dissolved and free gases analysis
Mineral oil-filled electrical equipment in service - Guidance on the interpretation of dissolved and free gases analysis
IEC 60599:2015 describes how the concentrations of dissolved gases or free gases may be interpreted to diagnose the condition of oil-filled electrical equipment in service and suggest future action. This standard is applicable to electrical equipment filled with mineral insulating oil and insulated with cellulosic paper or pressboard-based solid insulation. Information about specific types of equipment such as transformers (power, instrument, industrial, railways, distribution), reactors, bushings, switchgear and oil-filled cables is given only as an indication in the application notes. This standard may be applied, but only with caution, to other liquid-solid insulating systems. This third edition cancels and replaces the second edition published in 1999 and Amendment 1:2007. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) revision of 5.5, 6.1, 7, 8, 9, 10, A.2.6, A.3, A.7;
b) addition of new sub-clause 4.3;
c) expansion of the Bibliography;
d) revision of Figure 1;
e) addition of Figure B.4.
Matériels électriques remplis d'huile minérale en service - Lignes directrices pour l'interprétation de l'analyse des gaz dissous et des gaz libres
IEC 60599:2015 décrit comment les concentrations de gaz dissous ou de gaz libres peuvent être interprétées pour diagnostiquer l'état des matériels électriques remplis d'huile en service et pour proposer une intervention ultérieure. La présente norme s'applique aux matériels électriques remplis d'huile minérale isolante et isolés par des isolants solides constitués de papier ou de carton cellulosiques. Des informations relatives aux types spécifiques de matériels tels que les transformateurs (de puissance, de mesure, industriels, ferroviaires, de distribution), les réactances, les traversées, les appareillages de connexion et les câbles à huile sont données, à titre informatif seulement, dans les notes d'application (voir Annexe A). La présente norme peut être appliquée, mais uniquement avec prudence, à d'autres systèmes d'isolation liquide-solide. Cette troisième édition annule et remplace la deuxième édition parue en 1999 et l'Amendement 1:2007. Cette édition constitue une révision technique. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) révision de 5.5, 6.1, 7, 8, 9, 10, A.2.6, A.3, A.7 ;
b) ajout d'un nouveau paragraphe 4.3;
c) enrichissement de la Bibliographie;
d) révision de la Figure 1;
e) ajout de la Figure B.4.
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IEC 60599 ®
Edition 3.0 2015-09
REDLINE VERSION
INTERNATIONAL
STANDARD
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Mineral oil-filled electrical equipment in service – Guidance on the interpretation
of dissolved and free gases analysis
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IEC 60599 ®
Edition 3.0 2015-09
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Mineral oil-filled electrical equipment in service – Guidance on the interpretation
of dissolved and free gases analysis
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.220.99; 29.040.10; 29.180 ISBN 978-2-8322-2907-1
– 2 – IEC 60599:2015 RLV © IEC 2015
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and abbreviations . 9
3.1 Terms and definitions . 9
3.2 Abbreviations . 11
3.2.1 Chemical names and symbols formula . 11
3.2.2 General abbreviations . 11
4 Mechanisms of gas formation . 12
4.1 Decomposition of oil . 12
4.2 Decomposition of cellulosic insulation . 12
4.3 Stray gassing of oil . 12
4.4 Other sources of gas . 13
5 Identification of faults . 13
5.1 General . 13
5.2 Dissolved gas compositions . 13
5.3 Types of faults . 14
5.4 Basic gas ratios . 14
5.5 CO /CO ratio . 16
5.6 O /N ratio . 16
2 2
5.7 C H /H ratio . 17
2 2 2
5.8 C hydrocarbons . 17
5.9 Evolution of faults . 17
5.10 Graphical representations . 17
6 Conditions for calculating ratios . 18
6.1 Examination of DGA values . 18
6.2 Uncertainty on gas ratios . 18
7 Application to free gases in gas relays . 19
8 Gas concentration levels in service . 20
8.1 Probability of failure in service . 20
8.1.1 General . 20
8.1.2 Calculation methods . 21
8.2 Typical concentration values . 21
8.2.1 General . 21
8.2.2 Calculation methods . 21
8.2.3 Choice of normality percentages . 22
8.2.4 Alarm concentration values . 22
8.3 Rates of gas increase . 22
9 Recommended method of DGA interpretation (see Figure 1) . 23
10 Report of results . 23
Annex A (informative) Equipment application notes . 25
A.1 General warning . 25
A.2 Power transformers . 25
A.2.1 Specific sub-types . 25
A.2.2 Typical faults . 26
A.2.3 Identification of faults by DGA . 26
A.2.4 Typical concentration values . 27
A.2.5 Typical rates of gas increase . 28
A.2.6 Specific information to be added to the DGA report (see Clause 10) . 29
A.3 Industrial and special transformers . 30
A.3.1 Specific sub-types . 30
A.3.2 Typical faults . 30
A.3.3 Identification of faults by DGA. . 30
A.3.4 Typical concentration values . 30
A.4 Instrument transformers . 31
A.4.1 Specific sub-types . 31
A.4.2 Typical faults . 31
A.4.3 Identification of faults by DGA . 31
A.4.4 Typical concentration values . 32
A.5 Bushings . 32
A.5.1 Specific sub-types . 32
A.5.2 Typical faults . 32
A.5.3 Identification of faults by DGA . 33
A.5.4 Typical concentration values . 33
A.6 Oil-filled cables . 34
A.6.1 Typical faults . 34
A.6.2 Identification of faults by DGA . 34
A.6.3 Typical concentration values . 34
A.7 Switching equipment . 34
A.7.1 Specific sub-types . 34
A.7.2 Normal operation . 34
A.7.3 Typical faults . 35
A.7.4 Identification of faults by DGA . 35
A.8 Equipment filled with non-mineral fluids . 36
Annex B (informative) Graphical representations of gas ratios (see 5.10) . 37
Bibliography . 41
Figure 1 – Flow chart . 24
Figure B.1 – Graphical representation 1 of gas ratios (see [3]) . 37
Figure B.2 – Graphical representation 2 of gas ratios (see [4] of annex C) . 38
Figure B.3 – Graphical representation 3 of gas ratios – Duval's triangle 1 for
transformers, bushings and cables (see [4]) . 39
Figure B.4 – Graphical representation 4 of gas ratios – Duval's triangle 2 for OLTCs
(see A.7.2) . 40
Table 1 – Abbreviations .
Table 1 – DGA interpretation table . 15
Table 2 – Simplified scheme of interpretation . 15
Table 3 – Ostwald solubility coefficients for various gases in mineral insulating oils . 20
Table A.1 – Typical faults in power transformers . 26
Table A.2 – Ranges of 90 % typical gas concentration values observed in power
transformers (all types), in µl/l . 27
– 4 – IEC 60599:2015 RLV © IEC 2015
Table A.3 – Typical rates of gas increase for power transformers Ranges of 90 %
typical rates of gas increase observed in power transformers (all types), in µl/l/year . 29
Table A.4 – Examples of 90 % typical concentration values observed on a typical
individual networks (all types of transformers). 30
Table A.5 – Typical faults in instrument transformers . 31
Table A.6 – Ranges of 90 % typical concentration values observed in instrument
transformers . 32
Table A.7 – Maximum admissible values for sealed instrument transformers. 32
Table A.8 – Typical faults in bushings . 33
Table A.9 – Simplified interpretation scheme for bushings . 33
Table A.10 – 95 % typical concentration values in bushings . 34
Table A.11 – Ranges of 95 % typical concentration values observed on cables . 34
Table A.12 – Typical faults in switching equipment . 35
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MINERAL OIL-IMPREGNATED FILLED ELECTRICAL EQUIPMENT
IN SERVICE – GUIDANCE ON THE INTERPRETATION
OF DISSOLVED AND FREE GASES ANALYSIS
FOREWORD
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– 6 – IEC 60599:2015 RLV © IEC 2015
International Standard IEC 60599 has been prepared by IEC technical committee 10: Fluids
for electrotechnical applications.
This third edition cancels and replaces the second edition published in 1999 and
Amendment 1:2007. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) revision of 5.5, 6.1, 7, 8, 9, 10, A.2.6, A.3, A.7;
b) addition of new sub-clause 4.3;
c) expansion of the Bibliography;
d) revision of Figure 1;
e) addition of Figure B.4.
The text of this standard is based on the following documents:
FDIS Report on voting
10/967/FDIS 10/973/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 website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
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 publication using a colour printer.
INTRODUCTION
Dissolved and free gas analysis (DGA) is one of the most widely used diagnostic tools for
detecting and evaluating faults in electrical equipment filled with insulating liquid. However,
interpretation of DGA results is often complex and should always be done with care, involving
experienced insulation maintenance personnel.
This International Standard gives information for facilitating this interpretation. The first
edition, published in 1978, has served the industry well, but had its limitations, such as the
absence of a diagnosis in some cases, the absence of concentration levels and the fact that it
was based mainly on experience gained from power transformers. The second edition
attempted to address some of these shortcomings. Interpretation schemes were based on
observations made after inspection of a large number of faulty oil-filled equipment in service
and concentrations levels deduced from analyses collected worldwide.
– 8 – IEC 60599:2015 RLV © IEC 2015
MINERAL OIL-IMPREGNATED FILLED ELECTRICAL EQUIPMENT IN
SERVICE – GUIDANCE ON THE INTERPRETATION
OF DISSOLVED AND FREE GASES ANALYSIS
1 Scope
This International Standard describes how the concentrations of dissolved gases or free
gases may be interpreted to diagnose the condition of oil-filled electrical equipment in service
and suggest future action.
This standard is applicable to electrical equipment filled with mineral insulating oil and
insulated with cellulosic paper or pressboard-based solid insulation. Information about
specific types of equipment such as transformers (power, instrument, industrial, railways,
distribution), reactors, bushings, switchgear and oil-filled cables is given only as an indication
in the application notes (see Annex A).
This standard may be applied, but only with caution, to other liquid-solid insulating systems.
In any case, the indications obtained should be viewed only as guidance and any resulting
action should be undertaken only with proper engineering judgment.
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 60050-191:1990, International Electrotechnical Vocabulary – Chapter 191: Dependability
and quality of service (available at http://www.electropedia.org)
IEC 60050-192:2015, International Electrotechnical Vocabulary – Part 192: Dependability
(available at http://www.electropedia.org)
IEC 60050-212:2010, International Electrotechnical Vocabulary – Part 212: Electrical
insulating solids, liquids and gases (available at http://www.electropedia.org)
IEC 60050-604:1987, International Electrotechnical Vocabulary – Chapter 604: Generation,
transmission and distribution of electricity – Operation
(available at http://www.electropedia.org)
IEC 60475, Method of sampling insulating liquids
IEC 60567:1992 2011, Guide for the sampling of gases and of Oil-filled electrical equipment
and for the – Sampling of gases and analysis of free and dissolved gases – Guidance
IEC 61198:1993, Mineral insulating oils – Methods for the determination of 2-furfural and
related compounds
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions, some of which are
based on IEC 60050-191, IEC 60050-192, IEC 60050-212 and IEC 60050-604, apply.
3.1.1
fault
unplanned occurrence or defect in an item which may result in one or more failures of the item
itself or of other associated equipment
NOTE – In electrical equipment, a fault may or may not result in damage to the insulation and failure of the
equipment.
[SOURCE: IEC 60050-604:1987, 604-02-01]
3.1.2
non-damage fault
fault which does not involve repair or replacement action at the point of the fault
Note 1 to entry: Typical examples are self-extinguishing arcs in switching equipment or general overheating
without paper carbonization or stray gassing of oil.
[SOURCE: IEC 60050-604:1987, 604-02-09]
3.1.3
damage fault
fault that involves repair or replacement action at the point of the fault
[SOURCE: IEC 60050-604:1987, 604-02-08]
3.1.4
incident
an event related to an internal fault which temporarily or permanently disturbs the normal
operation of an equipment
event of external or internal origin, affecting equipment or the supply system and which
disturbs its normal operation
Note 1 to entry: For the purposes of the present standard “incidents” are related to internal faults.
Note 2 to entry: For the purposes of the present standard typical examples of “incidents” are gas alarms,
equipment tripping or equipment leakage.
[SOURCE: IEC 60050-604:1987, 604-02-03]
3.1.5
failure
the termination loss of ability of an item to perform a as required function
Note 1 to entry: In electrical equipment, failure will result from a damage fault or incident necessitating outage,
repair or replacement of the equipment, such as internal breakdown, rupture of tank, fire or explosion.
[SOURCE: IEC 60050-192:2015, 192-03-01]
3.1.6
electrical fault
partial or disruptive discharge through the insulation
– 10 – IEC 60599:2015 RLV © IEC 2015
3.1.7
partial discharge
electric discharge that only partially bridges the insulation between conductors. It may occur
inside the insulation or adjacent to a conductor
Note 1 to entry: A partial discharge may occur inside the insulation or adjacent to a conductor.
NOTE 2 – X-wax is a solid material which is formed from mineral insulating oil as a result of electrical discharges
and which consists of polymerized fragments of the molecules of the original liquid [IEV 212-07-24, modified].
Comparable products may be formed from other liquids under similar conditions.
Note 2 to entry: Scintillations of low energy on the surface of insulating materials are often described as partial
discharges but should rather be considered as disruptive discharges of low energy, since they are the result of
local dielectric breakdowns of high ionization density, or small arcs, according to the conventions of physics.
NOTE 3 – Sparking of low energy, for example because of metals or floating potentials, is sometimes described as
partial discharge but should rather be considered as a discharge of low energy.
Note 3 to entry: For the purposes of this standard the following consideration may also be added:
– Corona is a form of partial discharge that occurs in gaseous media around conductors that are remote from
solid or liquid insulation. This term shall not be used as a general term for all forms of partial discharges
– As a result of corona discharges, X-wax, a solid material consisting of polymerized fragments of the molecules
of the original liquid, can be formed.
[SOURCE: IEC 60050-212:2010, 212-11-39]
3.1.8
discharge (disruptive)
passage of an arc following the breakdown of the insulation
Note 1 to entry: The term "sparkover" (in French: "amorçage") is used when a disruptive discharge occurs in a
gaseous or liquid dielectric.
The term "flashover" (in French: "contournement") is used when a disruptive discharge occurs over the surface of a
solid dielectric surrounded by a gaseous or liquid medium.
The term "puncture" (in French: "perforation") is used when a disruptive discharge occurs through a solid dielectric.
Note 2 to entry: Discharges are often described as arcing, breakdown or short circuits. The following other
specific terms are also used in some countries:
– sparkover (discharge through the oil);
– puncture (discharge through the solid insulation);
– flashover (discharge at the surface of the solid insulation);
– tracking (the progressive degradation of the surface of solid insulation by local discharges to form conducting or
partially conducting paths);
– sparking discharges that, in the conventions of physics, are local dielectric breakdowns of high ionization density
or small arcs.
NOTE 2 – Depending on the amount of energy contained in the discharge, it will be described as a discharge of low
or high energy, based on the extent of damage observed on the equipment (see 5.2).
[SOURCE: IEC 60050-604:1987, 604-03-38]
3.1.9
thermal fault
excessive temperature rise in the insulation
Note 1 to entry: Typical causes are
– insufficient cooling;
– excessive currents circulating in adjacent metal parts (as a result of bad contacts, eddy currents, stray losses
or leakage flux);
– excessive currents circulating through the insulation (as a result of high dielectric losses), leading to a thermal
runaway;
– overheating of internal winding or bushing connection lead;
– overloading.
3.1.10
typical values of gas concentrations
gas concentrations normally found in the equipment in service that have no symptoms of
failure, and that are overpassed exceeded by only an arbitrary percentage of higher gas
contents (for example 10 % (see 8.2.1))
Note 1 to entry: Typical values will differ in different types of equipment and in different networks, depending on
operating practices (load levels, climate, etc.).
Note 2 to entry: Typical values, in many countries and by many users, are quoted as "normal values", but this
term has not been used here to avoid possible misinterpretations.
3.2 Abbreviations
3.2.1 Chemical names and symbols formula
Name Symbol Formula
Nitrogen N
Oxygen O
Hydrogen H
Carbon monoxide CO
Carbon dioxide CO
Methane CH
Ethane C H
2 6
Ethylene C H
2 4
Acetylene C H
2 2
NOTE Acetylene and ethyne are both used for C H ; ethylene and ethene are both used for C H
2 2 2 4
3.2.2 General abbreviations
D1 discharges of low energy
D2 discharges of high energy
DGA dissolved gas analysis
CIGRE Conférence Conseil International des Grands Réseaux Électriques
PD corona partial discharges
S analytical detection limit
T1 thermal fault, t <300 °C
T2 thermal fault, 300 °C
T3 thermal fault, t >700 °C
T thermal fault
D electrical fault
TP thermal fault in paper
ppm parts per million by volume of gas in oil, equivalent to µl(of gas)/l(of oil).
See IEC 60567:2011, 8.7, note 1.
OLTC on load tap changer
– 12 – IEC 60599:2015 RLV © IEC 2015
4 Mechanisms of gas formation
4.1 Decomposition of oil
Mineral insulating oils are made of a blend of different hydrocarbon molecules containing CH ,
and CH chemical groups linked together by carbon-carbon molecular bonds.
CH
Scission of some of the C-H and C-C bonds may occur as a result of electrical and thermal
faults, with the formation of small unstable fragments, in radical or ionic form, such as
• •
• • •
H , CH , CH , CH or C (among many other more complex forms), which recombine rapidly,
3 2
through complex reactions, into gas molecules such as hydrogen (H-H), methane (CH -H),
ethane (CH -CH ), ethylene (CH = CH ) or acetylene (CH ≡ CH). C and C hydrocarbon
3 3 2 2 3 4
gases, as well as solid particles of carbon and hydrocarbon polymers (X-wax), are other
possible recombination products. The gases formed dissolve in oil, or accumulate as free
gases if produced rapidly in large quantities, and may be analysed by DGA according to
IEC 60567.
Low-energy faults, such as partial discharges of the cold plasma type (corona discharges),
favour the scission of the weakest C-H bonds (338 kJ/mol) through ionization reactions and
the accumulation of hydrogen as the main recombination gas. More and more energy and/or
higher temperatures are needed for the scission of the C-C bonds and their recombination
into gases with a C-C single bond (607 kJ/mol), C=C double bond (720 kJ/mol) or C≡C triple
bond (960 kJ/mol), following processes bearing some similarities with those observed in the
petroleum oil-cracking industry.
Ethylene is thus favoured over ethane and methane above temperatures of approximately
500 °C (although still present in lower quantities below). Acetylene requires temperatures of
at least 800 °C to 1 200 °C, and a rapid quenching to lower temperatures, in order to
accumulate as a stable recombination product. Acetylene is thus formed in significant
quantities mainly in arcs, where the conductive ionized channel is at several thousands of
degrees Celsius, and the interface with the surrounding liquid oil necessarily below 400 °C
(above which oil vaporizes completely), with a layer of oil vapour/decomposition gases in
between. Acetylene may still be formed at lower temperatures (<800 °C), but in very minor
quantities. Carbon particles form at 500 °C to 800 °C and are indeed observed after arcing in
oil or around very hot spots.
Oil may oxidize with the formation of small quantities of CO and CO , which can accumulate
over long periods of time into more substantial amounts.
4.2 Decomposition of cellulosic insulation
The polymeric chains of solid cellulosic insulation (paper, pressboard, wood blocks) contain a
large number of anhydroglucose rings, and weak C-O molecular bonds and glycosidic bonds
which are thermally less stable than the hydrocarbon bonds in oil, and which decompose at
lower temperatures. Significant rates of polymer chain scission occur at temperatures higher
than 105 °C, with complete decomposition and carbonization above 300 °C (damage fault).
Mostly Carbon monoxide and dioxide, as well as water, is formed, in much larger quantities
than by oxidation of oil at the same temperature, together with minor amounts of hydrocarbon
gases, furanic and other compounds. The latter Furanic compounds can be are analysed
according to IEC 61198, and used to complement DGA interpretation and confirm whether or
not cellulosic insulation is involved in a fault. CO and CO formation increases not only with
temperature but also with the oxygen content of oil and the moisture content of paper.
4.3 Stray gassing of oil
Stray gassing of oil has been defined by CIGRE [6] as the formation of gases in oil heated to
moderate temperatures (<200 °C). H , CH and C H may be formed in all equipment at such
2 4 2 6
___________
Numbers in square brackets refer to the Bibliography.
temperatures or as a result of oil oxidation, depending on oil chemical structure. Stray
gassing is a non-damage fault. It can be evaluated using methods described in reference [6]
and [12].
NOTE Stray gassing of oil has been observed in some cases to be enhanced by the presence in oil of a metal
passivator or other additives.
4.4 Other sources of gas
Gases may be generated in some cases not as a result of faults in the equipment, but through
rusting or other chemical reactions involving steel, uncoated surfaces or protective paints.
Hydrogen may be produced by reaction of steel and galvanized steel with water, as long as
oxygen is available from the oil nearby. Large quantities of hydrogen have thus been reported
in some transformers that had never been energized. Hydrogen may also be formed by
reaction of free water with special coatings on metal surfaces, or by catalytic reaction of some
types of stainless steel with oil, in particular oil containing dissolved oxygen at elevated
temperatures. Hydrogen, acetylene and other gases may also be formed in new stainless
steel, absorbed during its manufacturing process, or produced by welding, and released
slowly into the oil.
Hydrogen may also be formed by the decomposition of the thin oil film between overheated
*
core laminates at temperatures of 140 °C and above (see [1] of annex C).
Internal transformer paints, such as alkyd resins and modified polyurethanes containing fatty
acids in their formulation, may also form gases.
Gases may also be produced, and oxygen consumed, by exposure of oil to sunlight or may be
formed during repair of the equipment.
These occurrences, however, are very unusual, and can be detected by performing DGA
analyses on new equipment which has never been energized, and by material compatibility
tests. The presence of hydrogen with the total absence of other hydrocarbon gases, for
example, may be an indication of such a problem.
NOTE The case of gases formed at a previous fault and remnant in the transformer is dealt with in 5.4.
5 Identification of faults
5.1 General
Any gas formation in service, be it minimal, results from a stress of some kind, even if it is a
very mild one, like normal temperature ageing. However, as long as gas formation
concentration is below typical values and not significantly increasing, it should not be
considered as an indication of a "fault", but rather as "the result of typical gas formation" (see
Figure 1). Typical values are specific for each kind of equipment.
5.2 Dissolved gas compositions
Although the formation of some gases is favoured, depending on the temperature reached or
the energy contained in a fault (see 4.1), in practice mixtures of gases are almost always
obtained. One reason is thermodynamic: although not favoured, secondary gases are still
formed, albeit in minor quantities. Existing thermodynamic models derived from the petroleum
industry, however, cannot predict accurately the gas compositions formed, because they
correspond to ideal gas/temperature equilibria that do not exist in actual faults. Large
temperature gradients also occur in practice, for instance as a result of oil flow or vaporization
___________
*
Figures in square brackets refer to the bibliography in annex C.
– 14 – IEC 60599:2015 RLV © IEC 2015
along a hot surface. This is particularly true in the case of arcs with power follow-through,
which transfer a lot of heat to the oil vapour/decomposition gas layer between the arc and the
oil, probably explaining the increasing formation of ethylene observed in addition to acetylene.
In addition, existing thermodynamic models do not apply to paper that turns irreversibly to
carbon above 300 °C.
5.3 Types of faults
Internal inspection of hundreds of faulty equipment has led to the following broad classes of
visually detectable faults:
– partial discharges (PD) of the cold plasma (corona) type, resulting in possible X-wax
deposition on paper insulation, or of the sparking type, inducing pinhole, carbonized
perforations (punctures) in paper, which, however, may not be easy to find;
– discharges of low energy (D1), in oil or/and paper, evidenced by larger carbonized
perforations through paper (punctures), carbonization of the paper surface (tracking) or
carbon particles in oil (as in tap changer diverter operation); also, partial discharges of the
sparking type, inducing pinhole, carbonized perforations (punctures) in paper, which,
however, may not be easy to find;
– discharges of high energy (D2), in oil or/and paper, with power follow-through, evidenced
by extensive destruction and carbonization of paper, metal fusion at the discharge
extremities, extensive carbonization in oil and, in some cases, tripping of the equipment,
confirming the large current follow-through;
– thermal faults, in oil or/and paper, below 300 °C if the paper has turned brownish (T1), and
above 300 °C if it has carbonized (T2);
– thermal faults of temperatures above 700 °C (T3) if there is strong evidence of
carbonization of the oil, metal coloration (800 °C) or metal fusion (>1 000 °C).
Table 1 – Abbreviations
PD Partial discharges
D1 Discharges of low energy
D2 Discharges of high energy
T1 Thermal fault, t < 300 °C
T2 Thermal fault, 300 °C < t < 700 °C
T3 Thermal fault, t > 700 °C
5.4 Basic gas ratios
Each of the six broad classes of faults leads to a characteristic pattern of hydrocarbon gas
composition, which can be translated into a DGA interpretation table, such as the one
recommended in Table 1 and based on the use of three basic gas ratios:
C H CH C H
2 2 4 2 4
C H H C H
2 4 2 2 6
Table 1 applies to all types of equipment, with a few differences in gas ratio limits depending
on the specific type of equipment.
Table 1 – DGA interpretation table
Case Characteristic fault
C H CH C H
2 2 4 2 4
C H
H C H
2 4 2 2 6
a
PD Partial discharges (see notes 3 and 4) NS
<0,1 <0,2
D1 Discharges of low energy 0,1 – 0,5
>1 >1
D2 Discharges of high energy 0,6 – 2,5 0,1 – 1
>2
a a
T1
Thermal fault t <300 °C NS >1 but NS <1
T2 1 – 4
Thermal fault 300 °C < t <700 °C <0,1 >1
b
T3
Thermal fault t >700 °C <0,2 >1 >4
NOTE 1 In some countries, the ratio C H /C H is used, rather than the ratio CH /H . Also in some countries,
2 2 2 6 4 2
slightly different ratio limits are used.
NOTE 2 – The above ratios are significant and should be calculated only if at least one of the gases is at a
concentration and a rate of gas increase above typical values (see clause 9).
NOTE 2 Conditions for calculating gas ratios are indicated in 6.1 c).
NOTE 3 CH /H <0,2 for partial discharges in instrument transformers. CH /H <0,07 for partial discharges in
4 2 4 2
bushings.
NOTE 4 Gas decomposition patterns similar to partial discharges have been reported as a result of the
decomposition stray gassing of thin oil film between overheated core laminates at temperatures of 140 °C and
above oil (see 4.3 and [1] of annex C).
a
NS = Non-significant whatever the value.
b
An increasing value of the amount of C H may indicate that the hot spot temperature is higher than
2 2
1 000 °C.
Typical examples of faults in the various types of equipment (power transformers, instrument
transformers, etc.), corresponding to the six cases of Table 1, may be found in Tables A.1,
A.5, A.8 and A.12.
Some overlap between faults D1 and D2 is apparent in Table 1, meaning that a dual
attribution of D1 or D2 must be given in some cases of DGA results. The distinction between
D1 and D2 has been kept, however, as the amount of energy in the discharge may
significantly increase the potential damage to the equipment and necessitate different
preventive measures. Tabl
...
IEC 60599 ®
Edition 3.0 2015-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Mineral oil-filled electrical equipment in service – Guidance on the interpretation
of dissolved and free gases analysis
Matériels électriques remplis d'huile minérale en service – Lignes directrices
pour l'interprétation de l'analyse des gaz dissous et des gaz libres
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IEC 60599 ®
Edition 3.0 2015-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Mineral oil-filled electrical equipment in service – Guidance on the interpretation
of dissolved and free gases analysis
Matériels électriques remplis d'huile minérale en service – Lignes directrices
pour l'interprétation de l'analyse des gaz dissous et des gaz libres
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220.99; 29.040.10; 29.180 ISBN 978-2-8322-2899-9
– 2 – IEC 60599:2015 © IEC 2015
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and abbreviations . 9
3.1 Terms and definitions . 9
3.2 Abbreviations . 11
3.2.1 Chemical names and formulae . 11
3.2.2 General abbreviations . 11
4 Mechanisms of gas formation . 11
4.1 Decomposition of oil . 11
4.2 Decomposition of cellulosic insulation . 12
4.3 Stray gassing of oil . 12
4.4 Other sources of gas . 12
5 Identification of faults . 13
5.1 General . 13
5.2 Dissolved gas compositions . 13
5.3 Types of faults . 13
5.4 Basic gas ratios . 14
5.5 CO /CO ratio . 15
5.6 O /N ratio . 16
2 2
5.7 C H /H ratio . 16
2 2 2
5.8 C hydrocarbons . 16
5.9 Evolution of faults . 16
5.10 Graphical representations . 17
6 Conditions for calculating ratios . 17
6.1 Examination of DGA values . 17
6.2 Uncertainty on gas ratios . 17
7 Application to free gases in gas relays . 18
8 Gas concentration levels in service . 19
8.1 Probability of failure in service . 19
8.1.1 General . 19
8.1.2 Calculation methods . 20
8.2 Typical concentration values . 20
8.2.1 General . 20
8.2.2 Calculation methods . 20
8.2.3 Choice of normality percentages . 20
8.2.4 Alarm concentration values . 21
8.3 Rates of gas increase . 21
9 Recommended method of DGA interpretation (see Figure 1) . 21
10 Report of results . 22
Annex A (informative) Equipment application notes . 24
A.1 General warning . 24
A.2 Power transformers . 24
A.2.1 Specific sub-types . 24
A.2.2 Typical faults . 24
A.2.3 Identification of faults by DGA . 25
A.2.4 Typical concentration values . 25
A.2.5 Typical rates of gas increase . 26
A.2.6 Specific information to be added to the DGA report (see Clause 10) . 27
A.3 Industrial and special transformers . 27
A.3.1 Specific sub-types . 27
A.3.2 Typical faults . 27
A.3.3 Identification of faults by DGA. . 27
A.3.4 Typical concentration values . 27
A.4 Instrument transformers . 28
A.4.1 Specific sub-types . 28
A.4.2 Typical faults . 28
A.4.3 Identification of faults by DGA . 29
A.4.4 Typical concentration values . 29
A.5 Bushings . 30
A.5.1 Specific sub-types . 30
A.5.2 Typical faults . 30
A.5.3 Identification of faults by DGA . 30
A.5.4 Typical concentration values . 31
A.6 Oil-filled cables . 31
A.6.1 Typical faults . 31
A.6.2 Identification of faults by DGA . 31
A.6.3 Typical concentration values . 31
A.7 Switching equipment . 32
A.7.1 Specific sub-types . 32
A.7.2 Normal operation . 32
A.7.3 Typical faults . 32
A.7.4 Identification of faults by DGA . 32
A.8 Equipment filled with non-mineral fluids . 33
Annex B (informative) Graphical representations of gas ratios (see 5.10) . 34
Bibliography . 38
Figure 1 – Flow chart . 23
Figure B.1 – Graphical representation 1 of gas ratios (see [3]) . 34
Figure B.2 – Graphical representation 2 of gas ratios . 35
Figure B.3 – Graphical representation 3 of gas ratios – Duval's triangle 1 for
transformers, bushings and cables(see [4]) . 36
Figure B.4 – Graphical representation 4 of gas ratios – Duval's triangle 2 for OLTCs
(see A.7.2) . 37
Table 1 – DGA interpretation table . 14
Table 2 – Simplified scheme of interpretation . 15
Table 3 – Ostwald solubility coefficients for various gases in mineral insulating oils . 19
Table A.1 – Typical faults in power transformers . 25
Table A.2 – Ranges of 90 % typical gas concentration values observed in power
transformers, in µl/l . 26
– 4 – IEC 60599:2015 © IEC 2015
Table A.3 – Ranges of 90 % typical rates of gas increase observed in power
transformers (all types), in µl/l/year . 26
Table A.4 – Examples of 90 % typical concentration values observed on individual
networks . 28
Table A.5 – Typical faults in instrument transformers . 29
Table A.6 – Ranges of 90 % typical concentration values observed in instrument
transformers . 29
Table A.7 – Maximum admissible values for sealed instrument transformers. 30
Table A.8 – Typical faults in bushings . 30
Table A.9 – Simplified interpretation scheme for bushings . 31
Table A.10 – 95 % typical concentration values in bushings . 31
Table A.11 – Ranges of 95 % typical concentration values observed on cables . 32
Table A.12 – Typical faults in switching equipment . 32
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MINERAL OIL-FILLED ELECTRICAL EQUIPMENT
IN SERVICE – GUIDANCE ON THE INTERPRETATION
OF DISSOLVED AND FREE GASES ANALYSIS
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
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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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 60599 has been prepared by IEC technical committee 10: Fluids
for electrotechnical applications.
This third edition cancels and replaces the second edition published in 1999 and
Amendment 1:2007. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) revision of 5.5, 6.1, 7, 8, 9, 10, A.2.6, A.3, A.7;
b) addition of new sub-clause 4.3;
c) expansion of the Bibliography;
d) revision of Figure 1;
e) addition of Figure B.4.
– 6 – IEC 60599:2015 © IEC 2015
The text of this standard is based on the following documents:
FDIS Report on voting
10/967/FDIS 10/973/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 website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
INTRODUCTION
Dissolved and free gas analysis (DGA) is one of the most widely used diagnostic tools for
detecting and evaluating faults in electrical equipment filled with insulating liquid. However,
interpretation of DGA results is often complex and should always be done with care, involving
experienced insulation maintenance personnel.
This International Standard gives information for facilitating this interpretation. The first
edition, published in 1978, has served the industry well, but had its limitations, such as the
absence of a diagnosis in some cases, the absence of concentration levels and the fact that it
was based mainly on experience gained from power transformers. The second edition
attempted to address some of these shortcomings. Interpretation schemes were based on
observations made after inspection of a large number of faulty oil-filled equipment in service
and concentrations levels deduced from analyses collected worldwide.
– 8 – IEC 60599:2015 © IEC 2015
MINERAL OIL-FILLED ELECTRICAL EQUIPMENT
IN SERVICE – GUIDANCE ON THE INTERPRETATION
OF DISSOLVED AND FREE GASES ANALYSIS
1 Scope
This International Standard describes how the concentrations of dissolved gases or free
gases may be interpreted to diagnose the condition of oil-filled electrical equipment in service
and suggest future action.
This standard is applicable to electrical equipment filled with mineral insulating oil and
insulated with cellulosic paper or pressboard-based solid insulation. Information about
specific types of equipment such as transformers (power, instrument, industrial, railways,
distribution), reactors, bushings, switchgear and oil-filled cables is given only as an indication
in the application notes (see Annex A).
This standard may be applied, but only with caution, to other liquid-solid insulating systems.
In any case, the indications obtained should be viewed only as guidance and any resulting
action should be undertaken only with proper engineering judgment.
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 60050-191:1990, International Electrotechnical Vocabulary – Chapter 191: Dependability
and quality of service (available at http://www.electropedia.org)
IEC 60050-192:2015, International Electrotechnical Vocabulary – Part 192: Dependability
(available at http://www.electropedia.org)
IEC 60050-212:2010, International Electrotechnical Vocabulary – Part 212: Electrical
insulating solids, liquids and gases (available at http://www.electropedia.org)
IEC 60050-604:1987, International Electrotechnical Vocabulary – Chapter 604: Generation,
transmission and distribution of electricity – Operation (available at
http://www.electropedia.org)
IEC 60475, Method of sampling insulating liquids
IEC 60567:2011, Oil-filled electrical equipment – Sampling of gases and analysis of free and
dissolved gases – Guidance
IEC 61198, Mineral insulating oils – Methods for the determination of 2-furfural and related
compounds
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions, some of which are
based on IEC 60050-191, IEC 60050-192, IEC 60050-212 and IEC 60050-604, apply.
3.1.1
fault
unplanned occurrence or defect in an item which may result in one or more failures of the item
itself or of other associated equipment
[SOURCE: IEC 60050-604:1987, 604-02-01]
3.1.2
non-damage fault
fault which does not involve repair or replacement action at the point of the fault
Note 1 to entry: Typical examples are self-extinguishing arcs in switching equipment or general overheating
without paper carbonization or stray gassing of oil.
[SOURCE: IEC 60050-604:1987, 604-02-09]
3.1.3
damage fault
fault that involves repair or replacement action at the point of the fault
[SOURCE: IEC 60050-604:1987, 604-02-08]
3.1.4
incident
event of external or internal origin, affecting equipment or the supply system and which
disturbs its normal operation
Note 1 to entry: For the purposes of the present standard “incidents” are related to internal faults.
Note 2 to entry: For the purposes of the present standard typical examples of “incidents” are gas alarms,
equipment tripping or equipment leakage.
[SOURCE: IEC 60050-604:1987, 604-02-03]
3.1.5
failure
loss of ability to perform as required
Note 1 to entry: In electrical equipment, failure will result from a damage fault or incident necessitating outage,
repair or replacement of the equipment, such as internal breakdown, rupture of tank, fire or explosion.
[SOURCE: IEC 60050-192:2015, 192-03-01]
3.1.6
electrical fault
partial or disruptive discharge through the insulation
3.1.7
partial discharge
electric discharge that only partially bridges the insulation between conductors
Note 1 to entry: A partial discharge may occur inside the insulation or adjacent to a conductor.
– 10 – IEC 60599:2015 © IEC 2015
Note 2 to entry: Scintillations of low energy on the surface of insulating materials are often described as partial
discharges but should rather be considered as disruptive discharges of low energy, since they are the result of
local dielectric breakdowns of high ionization density, or small arcs, according to the conventions of physics.
Note 3 to entry: For the purposes of this standard the following consideration may also be added:
– Corona is a form of partial discharge that occurs in gaseous media around conductors that are remote from
solid or liquid insulation. This term shall not be used as a general term for all forms of partial discharges
– As a result of corona discharges, X-wax, a solid material consisting of polymerized fragments of the molecules
of the original liquid, can be formed.
[SOURCE: IEC 60050-212:2010, 212-11-39]
3.1.8
discharge (disruptive)
passage of an arc following the breakdown
Note 1 to entry: The term "sparkover" (in French: "amorçage") is used when a disruptive discharge occurs in a
gaseous or liquid dielectric.
The term "flashover" (in French: "contournement") is used when a disruptive discharge occurs over the surface of a
solid dielectric surrounded by a gaseous or liquid medium.
The term "puncture" (in French: "perforation") is used when a disruptive discharge occurs through a solid dielectric.
Note 2 to entry: Discharges are often described as arcing, breakdown or short circuits. The following other
specific terms are also used in some countries:
– tracking (the progressive degradation of the surface of solid insulation by local discharges to form conducting
or partially conducting paths);
– sparking discharges that, in the conventions of physics, are local dielectric breakdowns of high ionization
density or small arcs.
[SOURCE: IEC 60050-604:1987, 604-03-38]
3.1.9
thermal fault
excessive temperature rise in the insulation
Note 1 to entry: Typical causes are
– insufficient cooling;
– excessive currents circulating in adjacent metal parts (as a result of bad contacts, eddy currents, stray losses
or leakage flux);
– excessive currents circulating through the insulation (as a result of high dielectric losses), leading to a thermal
runaway;
– overheating of internal winding or bushing connection lead;
– overloading.
3.1.10
typical values of gas concentrations
gas concentrations normally found in the equipment in service that have no symptoms of
failure, and that are exceeded by only an arbitrary percentage of higher gas contents (for
example 10 % (see 8.2.1))
Note 1 to entry: Typical values will differ in different types of equipment and in different networks, depending on
operating practices (load levels, climate, etc.).
Note 2 to entry: Typical values, in many countries and by many users, are quoted as "normal values", but this
term has not been used here to avoid possible misinterpretations.
3.2 Abbreviations
3.2.1 Chemical names and formulae
Name Formula
Nitrogen N
Oxygen O
Hydrogen H
Carbon monoxide CO
Carbon dioxide CO
Methane CH
Ethane C H
2 6
Ethylene C H
2 4
Acetylene C H
2 2
NOTE Acetylene and ethyne are both used for C H ; ethylene and ethene are both used for C H
2 2 2 4
3.2.2 General abbreviations
D1 discharges of low energy
D2 discharges of high energy
DGA: dissolved gas analysis
CIGRE Conseil International des Grands Réseaux Électriques
PD corona partial discharges
S analytical detection limit
T1 thermal fault, t <300 °C
T2 thermal fault, 300 °C
T3 thermal fault, t >700 °C
T thermal fault
D electrical fault
TP thermal fault in paper
ppm parts per million by volume of gas in oil, equivalent to µl(of gas)/l(of oil). See
IEC 60567:2011, 8.7, note 1.
OLTC on load tap changer
4 Mechanisms of gas formation
4.1 Decomposition of oil
Mineral insulating oils are made of a blend of different hydrocarbon molecules containing CH ,
CH and CH chemical groups linked together by carbon-carbon molecular bonds.
Scission of some of the C-H and C-C bonds may occur as a result of electrical and thermal
faults, with the formation of small unstable fragments, in radical or ionic form, such as
• •
• • •
H , CH , CH , CH or C (among many other more complex forms), which recombine rapidly,
3 2
through complex reactions, into gas molecules such as hydrogen (H-H), methane (CH -H),
ethane (CH -CH ), ethylene (CH = CH ) or acetylene (CH ≡ CH). C and C hydrocarbon
3 3 2 2 3 4
gases, as well as solid particles of carbon and hydrocarbon polymers (X-wax), are other
possible recombination products. The gases formed dissolve in oil, or accumulate as free
gases if produced rapidly in large quantities, and may be analysed by DGA according to
IEC 60567.
– 12 – IEC 60599:2015 © IEC 2015
Low-energy faults, such as partial discharges of the cold plasma type (corona discharges),
favour the scission of the weakest C-H bonds (338 kJ/mol) through ionization reactions and
the accumulation of hydrogen as the main recombination gas. More and more energy and/or
higher temperatures are needed for the scission of the C-C bonds and their recombination
into gases with a C-C single bond (607 kJ/mol), C=C double bond (720 kJ/mol) or C≡C triple
bond (960 kJ/mol), following processes bearing some similarities with those observed in the
petroleum oil-cracking industry.
Ethylene is thus favoured over ethane and methane above temperatures of approximately
500 °C (although still present in lower quantities below). Acetylene requires temperatures of
at least 800 °C to 1 200 °C, and a rapid quenching to lower temperatures, in order to
accumulate as a stable recombination product. Acetylene is thus formed in significant
quantities mainly in arcs, where the conductive ionized channel is at several thousands of
degrees Celsius, and the interface with the surrounding liquid oil necessarily below 400 °C
(above which oil vaporizes completely), with a layer of oil vapour/decomposition gases in
between. Acetylene may still be formed at lower temperatures (<800 °C), but in very minor
quantities. Carbon particles form at 500 °C to 800 °C and are indeed observed after arcing in
oil or around very hot spots.
Oil may oxidize with the formation of small quantities of CO and CO , which can accumulate
over long periods of time into more substantial amounts.
4.2 Decomposition of cellulosic insulation
The polymeric chains of solid cellulosic insulation (paper, pressboard, wood blocks) contain a
large number of anhydroglucose rings, and weak C-O molecular bonds and glycosidic bonds
which are thermally less stable than the hydrocarbon bonds in oil, and which decompose at
lower temperatures. Significant rates of polymer chain scission occur at temperatures higher
than 105 °C, with complete decomposition and carbonization above 300 °C (damage fault).
Carbon monoxide and dioxide, as well as water, is formed, together with minor amounts of
hydrocarbon gases, furanic and other compounds. Furanic compounds are analysed
according to IEC 61198, and used to complement DGA interpretation and confirm whether or
not cellulosic insulation is involved in a fault. CO and CO formation increases not only with
temperature but also with the oxygen content of oil and the moisture content of paper.
4.3 Stray gassing of oil
Stray gassing of oil has been defined by CIGRE [6] as the formation of gases in oil heated to
moderate temperatures (<200 °C). H , CH and C H may be formed in all equipment at such
2 4 2 6
temperatures or as a result of oil oxidation, depending on oil chemical structure. Stray
gassing is a non-damage fault. It can be evaluated using methods described in reference [6]
and [12].
NOTE Stray gassing of oil has been observed in some cases to be enhanced by the presence in oil of a metal
passivator or other additives.
4.4 Other sources of gas
Gases may be generated in some cases not as a result of faults in the equipment, but through
rusting or other chemical reactions involving steel, uncoated surfaces or protective paints.
Hydrogen may be produced by reaction of steel and galvanized steel with water, as long as
oxygen is available from the oil nearby. Large quantities of hydrogen have thus been reported
in some transformers that had never been energized. Hydrogen may also be formed by
reaction of free water with special coatings on metal surfaces, or by catalytic reaction of some
types of stainless steel with oil, in particular oil containing dissolved oxygen at elevated
temperatures. Hydrogen, acetylene and other gases may also be formed in new stainless
steel, absorbed during its manufacturing process, or produced by welding, and released
___________
Numbers in square brackets refer to the Bibliography.
slowly into the oil. Internal transformer paints, such as alkyd resins and modified
polyurethanes containing fatty acids in their formulation, may also form gases.
Gases may also be produced, and oxygen consumed, by exposure of oil to sunlight.
These occurrences, however, are very unusual, and can be detected by performing DGA
analyses on new equipment which has never been energized, and by material compatibility
tests. The presence of hydrogen with the total absence of other hydrocarbon gases, for
example, may be an indication of such a problem.
NOTE The case of gases formed at a previous fault and remnant in the transformer is dealt with in 5.4.
5 Identification of faults
5.1 General
Any gas formation in service, be it minimal, results from a stress of some kind, even if it is a
very mild one, like normal temperature ageing. However, as long as gas concentration is
below typical values and not significantly increasing, it should not be considered as an
indication of a "fault", but rather as the result of typical gas formation (see Figure 1). Typical
values are specific for each kind of equipment.
5.2 Dissolved gas compositions
Although the formation of some gases is favoured, depending on the temperature reached or
the energy contained in a fault (see 4.1), in practice mixtures of gases are almost always
obtained. One reason is thermodynamic: although not favoured, secondary gases are still
formed, albeit in minor quantities. Existing thermodynamic models derived from the petroleum
industry, however, cannot predict accurately the gas compositions formed, because they
correspond to ideal gas/temperature equilibria that do not exist in actual faults. Large
temperature gradients also occur in practice, for instance as a result of oil flow or vaporization
along a hot surface. This is particularly true in the case of arcs with power follow-through,
which transfer a lot of heat to the oil vapour/decomposition gas layer between the arc and the
oil, probably explaining the increasing formation of ethylene observed in addition to acetylene.
In addition, existing thermodynamic models do not apply to paper that turns irreversibly to
carbon above 300 °C.
5.3 Types of faults
Internal inspection of hundreds of faulty equipment has led to the following broad classes of
visually detectable faults:
– partial discharges (PD) of the cold plasma (corona) type, resulting in possible X-wax
deposition on paper insulation;
– discharges of low energy (D1), in oil or/and paper, evidenced by larger carbonized
perforations through paper (punctures), carbonization of the paper surface (tracking) or
carbon particles in oil (as in tap changer diverter operation); also, partial discharges of the
sparking type, inducing pinhole, carbonized perforations (punctures) in paper, which,
however, may not be easy to find;
– discharges of high energy (D2), in oil or/and paper, with power follow-through, evidenced
by extensive destruction and carbonization of paper, metal fusion at the discharge
extremities, extensive carbonization in oil and, in some cases, tripping of the equipment,
confirming the large current follow-through;
– thermal faults, in oil or/and paper, below 300 °C if the paper has turned brownish (T1), and
above 300 °C if it has carbonized (T2);
– thermal faults of temperatures above 700 °C (T3) if there is strong evidence of
carbonization of the oil, metal coloration (800 °C) or metal fusion (>1 000 °C).
– 14 – IEC 60599:2015 © IEC 2015
5.4 Basic gas ratios
Each of the six broad classes of faults leads to a characteristic pattern of hydrocarbon gas
composition, which can be translated into a DGA interpretation table, such as the one
recommended in Table 1 and based on the use of three basic gas ratios:
C H CH C H
2 2 4 2 4
C H H C H
2 4 2 2 6
Table 1 applies to all types of equipment, with a few differences in gas ratio limits depending
on the specific type of equipment.
Table 1 – DGA interpretation table
Case Characteristic fault
C H CH C H
2 2 4 2 4
C H H C H
2 4 2 2 6
a
PD Partial discharges (see notes 3 and 4) NS <0,1 <0,2
D1
Discharges of low energy >1 0,1 – 0,5 >1
D2 Discharges of high energy 0,6 – 2,5 0,1 – 1
>2
a
a
T1 NS
Thermal fault t <300 °C >1 but NS <1
T2 1 – 4
Thermal fault 300 °C < t <700 °C <0,1 >1
b
T3 Thermal fault t >700 °C <0,2 >1 >4
NOTE 1 In some countries, the ratio C H /C H is used, rather than the ratio CH /H . Also in some countries,
2 2 2 6 4 2
slightly different ratio limits are used.
NOTE 2 Conditions for calculating gas ratios are indicated in 6.1 c).
NOTE 3 CH /H <0,2 for partial discharges in instrument transformers. CH /H <0,07 for partial discharges in
4 2 4 2
bushings.
NOTE 4 Gas decomposition patterns similar to partial discharges have been reported as a result of stray
gassing of oil (see 4.3).
a
NS = Non-significant whatever the value.
b
An increasing value of the amount of C H may indicate that the hot spot temperature is higher than
2 2
1 000 °C.
Typical examples of faults in the various types of equipment (power transformers, instrument
transformers, etc.), corresponding to the six cases of Table 1, may be found in Tables A.1,
A.5, A.8 and A.12.
Some overlap between faults D1 and D2 is apparent in Table 1, meaning that a dual
attribution of D1 or D2 must be given in some cases of DGA results. The distinction between
D1 and D2 has been kept, however, as the amount of energy in the discharge may
significantly increase the potential damage to the equipment and necessitate different
preventive measures. Table 1 applies to transformers. For switching equipment see Clause
A.7 and reference [8] in the bibliography.
NOTE Combinations of gas ratios that fall outside the range limits of Table 1 and do not correspond to a
characteristic fault of this table can be considered a mixture of faults, or new faults that combine with a high
background gas level (see 6.1).
In such a case, Table 1 cannot provide a diagnosis, but the graphical representations given in Annex B can be
used t
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