Mineral oil-impregnated electrical equipment in service - Guide to the interpretation of dissolved and free gases analysis
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 suggests future action. 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. May be applied 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 judgement.
In Betrieb befindliche, mit Mineralöl imprägnierte elektrische Geräte - Leitfaden zur Interpretation der Analyse gelöster und freier Gase
Matériels électriques imprégnés d'huile minérale en service - Guide pour l'interprétation de l'analyse des gaz dissous et des gaz libres
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. 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 spécifiques aux différents types de matériels tels que transformateurs (de puissance, de mesure, industriels, ferroviaires, de distribution), réactances, traversées, appareillage de coupure et câbles à l'huile sont données, à titre informatif seulement, dans les note d'application. Peut être appliqué, mais avec prudence, à d'autres systèmes d'isolation liquide-solide.
Električna oprema, impregnirana z mineralnim oljem, v delovanju – Vodilo za tolmačenje rezultatov analize raztopljenih in prostih plinov (IEC 60599:1999)
Standards Content (Sample)
SIST EN 60599:1999
SIST HD 397 S1:1998
Mineral oil-impregnated electrical equipment in service - Guide to the interpretation ofdissolved and free gases analysis
In Betrieb befindliche, mit Mineralöl imprägnierte elektrische Geräte - Leitfaden zurInterpretation der Analyse gelöster und freier Gase
Matériels électriques imprégnés d'huile minérale en service - Guide pour l'interprétationde l'analyse des gaz dissous et des gaz libres
Ta slovenski standard je istoveten z: EN 60599:1999
29.040.10 Izolacijska olja Insulating oils
SIST EN 60599:1999 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.---------------------- Page: 1 ----------------------
Matériels électriques imprégnés d’huile minérale
en service –
Guide pour l’interprétation de l’analyse des gaz
dissous et des gaz libres
Mineral oil-impregnated electrical equipment
in service –
Guide to the interpretation of dissolved
and free gases analysis
IEC 1999 Droits de reproduction réservés Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in
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Pour prix, voir catalogue en vigueur
For price, see current catalogue
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60599 © IEC:1999 – 3 –
FOREWORD ................................................................................................................... 5
INTRODUCTION ............................................................................................................. 7Clause
1 Scope .......................................................................................................................9
2 Normative references ................................................................................................ 9
3 Definitions and abbreviations..................................................................................... 9
4 Mechanisms of gas formation.................................................................................... 15
5 Identification of faults ................................................................................................ 17
6 Conditions for calculating ratios................................................................................. 27
7 Application to free gases in gas relays....................................................................... 29
8 Gas concentration levels in service............................................................................ 31
9 Recommended method of DGA interpretation (figure 1) ............................................. 37
10 Report of results ....................................................................................................... 37
Annex A (informative) Equipment application notes .......................................................... 43
Annex B (informative) Graphical representation of gas ratios ........................................... 63
Annex C (informative) Bibliography .................................................................................. 69
Figure 1 – Flow chart ....................................................................................................... 41
Figure B.1 – Graphical representation 1 of gas ratios ....................................................... 63
Figure B.2 – Graphical representation 2 of gas ratios ....................................................... 65
Figure B.3 – Graphical representation 3 of gas ratios – Duval's triangle............................ 67---------------------- Page: 3 ----------------------
60599 © IEC:1999 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
MINERAL OIL-IMPREGNATED ELECTRICAL EQUIPMENT IN SERVICE –
GUIDE TO THE INTERPRETATION OF DISSOLVED AND
FREE GASES ANALYSIS
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the twoorganizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representationfrom all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearlyindicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for anyequipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The 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 forelectrotechnical applications.
This second edition cancels and replaces the first edition published in 1978. This secondedition constitutes a technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
Full information on the voting for the approval of this standard can be found in the report onvoting indicated in the above table.
Annexes A, B and C are for information only.
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60599 © IEC:1999 – 7 –
Dissolved and free gas analysis (DGA) is one of the most widely used diagnostic tools for
detecting and evaluating faults in electrical equipment. However, interpretation of DGA results
is often complex and should always be done with care, involving experienced insulationmaintenance personnel.
This guide 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. This second edition attempts to address some of
these shortcomings. Interpretation schemes are based on observations made after inspection
of a large number of faulty oil-filled equipment in service and concentrations levels deducedfrom analyses collected worldwide.
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60599 © IEC:1999 – 9 –
MINERAL OIL-IMPREGNATED ELECTRICAL EQUIPMENT IN SERVICE –
GUIDE TO THE INTERPRETATION OF DISSOLVED AND
FREE GASES ANALYSIS
This International Standard is a guide describing how the concentrations of dissolved gases or
free gases may be interpreted to diagnose the condition of oil-filled electrical equipment inservice and suggest future action.
This guide 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).
The Guide may be applied only with caution to other liquid-solid insulating systems.
In any case, the indications obtained should be viewed only as guidance and any resultingaction should be undertaken only with proper engineering judgment.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this International Standard. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and parties to
agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. Members ofIEC and ISO maintain registers of currently valid International Standards.
IEC 60050(191):1990, International Electrotechnical Vocabulary (IEV) – Chapter 191: Depen-dability and quality of service
IEC 60050(212):1990, International Electrotechnical Vocabulary (IEV) – Chapter 212: Insulatingsolids, liquids and gases
IEC 60050(604):1987, International Electrotechnical Vocabulary (IEV) – Chapter 604: Generation,transmission and distribution of electricity – Operation
IEC 60567:1992, Guide for the sampling of gases and of oil from oil-filled electrical equipmentand for the analysis of free and dissolved gases
IEC 61198:1993, Mineral insulating oils – Methods for the determination of 2-furfural andrelated compounds
3 Definitions and abbreviations
For the purpose of this International Standard, the following definitions, some of them based onIEC 60050(191), IEC 60050(212) and IEC 60050(604) apply:
an unplanned occurrence or defect in an item which may result in one or more failures of theitem itself or of other associated equipment [IEV 604-02-01]
NOTE – In electrical equipment, a fault may or may not result in damage to the insulation and failure of theequipment.
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60599 © IEC:1999 – 11 –
a fault which does not involve repair or replacement action at the point of the fault[IEV 604-02-09]
NOTE – Typical examples are self-extinguishing arcs in switching equipment or general overheating without papercarbonization.
a fault which involves repair or replacement action at the point of the fault
[IEV 604-02-08, modified]
an event related to an internal fault which temporarily or permanently disturbs the normaloperation of an equipment [IEV 604-02-03, modified]
NOTE – Typical examples are gas alarms, equipment tripping or equipment leakage.
the termination of the ability of an item to perform a required function [IEV 191-04-01]
NOTE – In the 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.3.1.6
a partial or disruptive discharge through the insulation
a discharge which only partially bridges the insulation between conductors. It may occur insidethe insulation or adjacent to a conductor [IEV 212-01-34, modified]
NOTE 1 – Corona is a form of partial discharge that occurs in gazeous media around conductors which are remote
from solid or liquid insulation. This term is not to be used as a general term for all forms of partial discharges.
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 3 – Sparking of low energy, for example because of metals or floating potentials, is sometimes described aspartial discharge but should rather be considered as a discharge of low energy.
the passage of an arc following the breakdown of the insulation [IEV 604-03-38, modified]
NOTE 1 – Discharges are often described as arcing, breakdown or short circuits. The more specific following termsare also used:
– 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 orpartially conducting paths);
– sparking discharges which, in the conventions of physics, are local dielectric breakdowns of high ionizationdensity 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).---------------------- Page: 7 ----------------------
60599 © IEC:1999 – 13 –
excessive temperature rise in the insulation
NOTE – Typical causes are
– insufficient cooling,
– excessive currents circulating in adjacent metal parts (as a result of bad contacts, eddy currents, stray losses orleakage flux),
– excessive currents circulating through the insulation (as a result of high dielectric losses), leading to a thermalrunaway,
– overheating of internal winding or bushing connection lead.
typical values of gas concentrations
gas concentrations normally found in the equipment in service which have no symptoms of
failure, and which are overpassed by only an arbitary percentage of higher gas contents, forexample 10 % (see 8.2.1)
NOTE 1 – Typical values will differ in different types of equipment and in different networks, depending on operatingpractices (load levels, climate, etc.).
NOTE 2 – Typical values, in many countries and by many users, are quoted as "normal values", but this term hasnot been used here to avoid possible misinterpretations.
3.2.1 Chemical names and symbols
Carbon monoxide CO
Carbon dioxide CO
Ethane C H
Ethylene C H
Acetylene C H
3.2.2 General abbreviations
DGA: Dissolved gas analysis
CIGRE: Conférence Internationale des Grands Réseaux Électriques
S: Analytical detection limit
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60599 © IEC:1999 – 15 –
4 Mechanisms of gas formation
4.1 Decomposition of oil
Mineral insulating oils are made of a blend of different hydrocarbon molecules containingCH , 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• •• ••
HC,,H CH,CHorC (among many other more complex forms), which recombine rapidly,
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 hydrocarbon3 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 analyzed by DGA according toIEC 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/mole) 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/mole), C=C double bond (720 kJ/mole) or C≡C triple
bond (960 kJ/mole), following processes bearing some similarities with those observed in thepetroleum 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 accumulateover 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. Mostly carbon
monoxide and dioxide, as well as water, are formed, in much larger quantities than by oxidation
of oil at the same temperature, together with minor amounts of hydrocarbon gases and furanic
compounds. The latter can be analyzed 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 themoisture content of paper.
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60599 © IEC:1999 – 17 –
4.3 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 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 may also be
formed in new stainless steel, absorbed during its manufacturing process, or produced bywelding, and released slowly into the oil.
Hydrogen may also be formed by the decomposition of the thin oil film between overheatedcore laminates at temperatures of 140 °C and above (see  of annex C).
Gases may also be produced by exposure of oil to sunlight or may be formed during repair ofthe equipment.
Internal transformer paints, such as alkyd resins and modified polyurethanes containing fattyacids in their formulation, may also form gases.
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, forexample, 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 5.3.5 Identification of faults
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 is below
typical values, it should not be considered as an indication of a "fault", but rather as "typicalgas formation" (see figure 1).
5.1 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 which 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, which turns irreversibly tocarbon above 300 °C.
Figures in square brackets refer to the bibliography in annex C.
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60599 © IEC:1999 – 19 –
5.2 Types of faults
Internal inspection of hundreds of faulty equipment has led to the following broad classes ofvisually 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, carbonizedperforations (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) orcarbon particles in oil (as in tap changer diverter operation);
– 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 thelarge current follow-through;
– thermal faults, in oil or/and paper, below 300 °C if the paper has turned brownish (T1), andabove 300 °C if it has carbonized (T2);
– thermal faults of temperatures above 700 °C (T3) if there is strong evidence ofcarbonization 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
5.3 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 onerecommended in table 2 and based on the use of three basic gas ratios:
CH CH CH
22 4 24
CH H CH
2 4 2 2 6
Table 2 applies to all types of equipment, with a few differences in gas ratio limits dependingon the specific type of equipment.
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60599 © IEC:1999 – 21 –
Table 2 – DGA interpretation table
CH CH CH
22 4 24
Case Characteristic fault
CH H CH
2 4 2 2 6
PD Partial discharges NS <0,1 <0,2
(see notes 3 and 4)
D1 Discharges of low energy >1 0,1 – 0,5 >1
D2 Discharges of high energy 0,6 – 2,5 0,1 – 1 >2
T1 Thermal fault NS >1 but <1
t < 300 °C NS
T2 Thermal fault <0,1 >1 1 – 4
300 °C < t < 700 °C
T3 Thermal fault <0,2 >1 >4
t > 700 C
NOTE 1 – In some countries, the ratio C H /C H is used, rather than the
2 2 2 6
ratio CH /H . Also in some countries, 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 3 – CH /H <0,2 for partial discharges in instrument transformers.
CH /H <0,07 for partial discharges in bushings.
NOTE 4 – Gas decomposition patterns similar to partial discharges have
been reported as a result of the decomposition of thin oil film between over-
heated core laminates at temperatures of 140 °C and above (see 4.3 and 
of annex C).
NS = Non-significant whatever the value.
An increasing value of the amount of C H may indicate that the hot spot
temperature is higher than 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 2, may be found in tables A.1, A.5,A.7 and A.11.
Some overlap between faults D1 and D2 is apparent in table 2, 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.
NOTE – Combinations of gas ratios which fall outside the range limits of table 2 and do not correspond to a
characteristic fault of this table may be considered a mixture of faults, or new faults which combine with a highbackground gas level (see 6.1).
In such a case, table 2 cannot provide a diagnosis, but the graphical representations given in annex B may be usedto visualize which characteristic fault of table 2 is closest to the case.
The less detailed scheme of table 3 may also be used in such a case in order to get at least a rough distinction
between partial discharges (PD), discharges (D) and thermal fault (T), rather than no diagnosis at all.---------------------- Page: 12 ----------------------
60599 © IEC:1999 – 23 –
Table 3 – Simplified scheme of interpretation
CH CH CH
22 4 24
CH H CH
2 4 2 2 6
5.4 CO /CO ratio
The formation of CO and CO from oil-impregnated paper insulation increases rapidly with
temperature. Incremental (corrected) CO /CO ratios less than 3 are generally considered as an
indication of probable paper involvement in a fault, with some degree of carbonization.
In order to get reliable CO /CO ratios in the equipment, CO and CO values should be2 2
corrected (incremented) first for possible CO absorption from atmospheric air, and for the
CO and CO background values (see 6.1 and clause 9), resulting from the ageing of cellulosic
insulation, overheating of wooden blocks and the long term oxidation of oil (which will be
strongly influenced by the availability of oxygen caused by specific equipment constructiondetails and its way of operation).
Air-breathing equipment, for example, saturated with approximately 10 % of dissolved air, may
contain up to 300 μl/l of CO coming from the air. In sealed equipment, air is normally excluded
but may enter through leaks, and CO concentration will be in proportion of air present.
When excessive paper degradation is suspected (CO /CO < 3), it is advisable to ask for a
furanic compounds analysis or a measurement of the degree of polymerization of papersamples, when this is possible.
5.5 O /N ratio
Dissolved O and N may be found in oil, as a result of contact with atmospheric air in the2 2
conservator of air-breathing equipment, or through leaks in sealed equipment. At equilibrium,
taking into account the relative solubilities of O and N , the O /N ratio in oil reflects air2 2 2 2
composition and is close to 0,5.
In service, this ratio may decrease as a result of oil oxidation and/or paper ageing, if O is
consumed more rapidly than it is replaced by diffusion. Factors such as the load and
preservation system used may also affect the ratio, but ratios less than 0,3 are generallyconsidered to indicate excessive consumption of oxygen.
5.6 C H /H ratio
2 2 2
In power transformers, on load tap changer (OLTC) operations produce gases corresponding
to discharges of low energy (D1). If some oil or gas communication is possible between the
OLTC compartment and the main tank, or between the respective conservators, these gases
may contaminate the oil in the main tank and lead to wrong diagnoses. The pattern of gas
decomposition in the OLTC, however, is quite specific and different from that of regular D1s inthe main tank.
---------------------- Page: 13 ----------------------
60599 © IEC:1999 – 25 –
C H /H ratios higher than 2 to 3 in the main tank are thus considered as an indication of2 2 2
OLTC contamination. This can be confirmed by comparing DGA results in the main tank, in the
OLTC and in the conservators. The values of the gas ratio and of the acetylene concentration
depend on the number of OLTC operations and on the way the contamination has occurred(through the oil or the g