April 4, 2005
IEEE PC57.104 D12
Draft Guide for the Interpretation of Gases in Oil
Immersed Transformers
Sponsored by the
Transformers Committee
of the
IEEE Power Engineering Society
Copyright © 2004 2005 by the Institute of Electrical and Electronics Engineers, Inc.
Three Park Avenue
New York, New York 10016-5997, USA
All rights reserved.
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such, this document is subject to change. Permission is hereby granted for IEEE Standards Committee
participants to reproduce this document for purposes of IEEE standardization activities only. Prior to
submitting this document to another standard development organization for standardization activities,
permission must first be obtained from the Manager, Standards Licensing and Contracts, IEEE Standards
Activities Department. Other entities seeking permission to reproduce portions of this document must
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April 4, 2005
IEEE PC57.104 D12
Introduction
(This introduction is not part of PC57.104, Draft Guide for the Interpretation of Gases in Oil Immersed
Transformers. )
This revised IEEE guide for the interpretation of gases generated in operating oil-immersed transformers
presents to the operators and manufacturers of oil filled transformers an improved routine surveillance oil
sampling program and procedures for detecting the presence of combustible gases in the equipment during
service that indicate faults which if not detected in the very early or incipient stages may eventually lead to
failure of the transformer. The detection methods presented here frequently provide the first available
indications of a malfunction. They employ consensus guidelines that allow operators of transformers of any
age which are absent any significant gas data or operators of transformers with established gas databases to
ascertain by surveillance sampling whether the transformer is generating normal quantities of gas,
increasingly abnormal quantities in the caution range, or dangerous quantities in the warning range. A
procedure is given for the statistical development of surveillance norms from a gas database and a detection
procedure is given for transformers that employ norms from units with identical design and service
conditions. Sampling intervals and operating guidelines are suggested for each range and generating rate. A
Key gas screening procedure is given to permit an estimate of a gas generating mechanism, called a fault, in
the caution range between normal and warning levels. A ratio procedure is also provided for refined
diagnoses in the warning range which provides empirical but reliable diagnoses of internal problems and the
use of graphical trends is suggested to further refine the diagnoses. The revised guide suggests preference
for the statistical development of detection norms jointly acceptable to user and manufacturer for units
under warranty and between user and insurer for units beyond manufacturing warranty. These revisions
provide the transformer user with an earlier and clearer picture of his operating options and shorten the time
required to evaluate the current status and make more timely and appropriate operating decisions.
This Guide was prepared by the Insulating Fluids Subcommittee of the IEEE Power Engineering Society. At
the time this Guide was completed, the Working Group had the following membership:
F.W Heinrichs, Chair
S. McNelly, Secretary
J. Corkran
A. Darwin
F. Gryszkiewicz
J. Goudie
T. Haupert
F. Jakob
J. Kelly
D. Kim
R. Ladroga
J. Lackey
S. Lindgren
T. Lundquist
S. McNelly
P. McShane
T.V. Oommen
T. Prevost
T. Rouse
J. Smith
The following persons were members of the balloting group that approved this document for submission to
the IEEE Standards board as a recommended practice.
Copyright © 2004 2005 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
ii
April 4, 2005
IEEE PC57.104 D12
Contents
Introduction............................................................................................................................................................ ii
1. Overview.............................................................................................................................................................5
1.1 Scope.......................................................................................................................................................... 65
1.2 Limitations....................................................................................................................................................6
2. References...........................................................................................................................................................6
3. Definitions ..........................................................................................................................................................7
4. General decomposition theory...........................................................................................................................8
4.1 Cellulose decomposition .......................................................................................................................... 88
4.2 Mineral oil decomposition........................................................................................................................ 98
5.0 Combustible gas generation in equipment ......................................................................................................9
5.1 Thermal fault ............................................................................................................................................. 99
5.2 Thermal faults involving cellulose......................................................................................................... 109
5.3 Electrical faults ....................................................................................................................................... 109
5.4 Sampling and laboratory procedures ........................................................................................................ 10
5.5 Variability and precision of analysis ....................................................................................................1110
6. Detection and operating guidelines.............................................................................................................1110
6.1 Detection procedure for transformers in Type 1 category....................................................................... 12
6.2 Detection procedure for Type 2 transformers......................................................................................1413
7. Diagnosis of transformer faults ..................................................................................................................1514
7.1 Analysis in the caution surveillance range ...........................................................................................1615
7.2 Diagnosis in the warning surveillance range........................................................................................1615
7.3 Diagnosis of cellulose decomposition.................................................................................................1716
8.0 Trend and graphical analysis .....................................................................................................................1816
8.1 Construction of a 3 D graph .................................................................................................................1817
9.0 Detection and analysis of gases in gas space, relay or other fixed or portable devices..........................1918
Annex A (Normative) TCG detection and operating procedure....................................................................2119
Annex B (Normative) TCG diagnosis of fault type .......................................................................................2220
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April 4, 2005
IEEE PC57.104 D12
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April 4, 2005
IEEE PC57.104 D12
Draft Guide for the Interpretation of Gases
Generated in Oil Immersed Transformers
1. Overview
The detection of certain gases generated in an operating mineral oil-filled transformer in service is frequently
the first available indication of abnormalities called faults that may eventually lead to failure if not
corrected. Electrical faults such as arcing, low energy sparking and partial discharge (previously called
corona) are described. Thermal faults such as severe overloading, cooling pump equipment failure, and
overheating in the insulation system are described. These conditions occurring singly, or as several events
can result in decomposition of the insulating materials and the formation of various combustible gases
dissolved in the insulating oil, in the inert gas space above the oil, or in gas collecting devices.
The procedure for detection of faults for transformers of any age or design but with no significant gas
history, defined as Type 1 transformers, is given in clause 6.1. Detection procedures for transformers that
have been operating long enough to have a gas history or database, defined as Type 2 transformers, are given
in 6.2. Detection in Type 1 or Type 2 transformers begins with regular surveillance sampling and analysis to
determine whether the gas concentrations have reached normal, caution, or warning surveillance ranges.
Guidelines for typical gas concentrations in the normal, caution and warning surveillance ranges for type 1
transformers are offered in Table 1. For Type 2 transformers where sufficient data exists, statistically
derived norms are preferred and. They should ultimately be agreed upon by the manufacturer and user in
transformers under warranty or by manufacturer, insurer, or transformer operators beyond warranty as
appropriate. The increasing order of the surveillance ranges reflect fault severity. Surveillance involves
regular oil sampling or other testing and monitoring procedures, which are subject to errors. The norms
found in Table 1 for Type 1 transformers are current reported values based on industry experience.
Special applications and service requirements may be considered in establishing the initial sampling interval.
For instance, operators of small transformers in very critical applications may choose shorter sampling
intervals than they would for larger units in a less demanding application. However, the guide suggests a
preference for the statistical development of norms as soon as sufficient data is available. In any case,
detection and the resulting adjustment of sampling interval and operating procedure requires an evaluation
of the quantities of generated gases present and determination of their rates of generation. A Key gas
diagnostic screening procedure is given for use in the caution range. A Ratio procedure for diagnosis in the
warning range is given to indicate the possible fault mechanism and it’s source. These procedures may
reveal the source of the disturbance based on an empirical relationship between generated combustible gas
species and fault types. Trends and graphical analysis may further refine diagnoses.
It should be noted that normal operation of the transformer might also result in the formation of small
quantities of combustible gases. In fact, it is possible for some transformers to operate throughout their
useful life with even larger quantities of combustible gases present although it is not a normal occurrence.
Proper application of the step-by-step procedures described in this guide can provide the operator with the
earliest indication of a developing problem and positive useful information on the serviceability of his
equipment.
Copyright © 2004 2005 IEEE. All rights reserved.
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5
April 4, 2005
IEEE PC57.104 D12
1.1 Scope
This guide applies to operating mineral oil immersed transformers that are newly energized and have no
previous combustible gas history; and to operating mineral oil immersed transformers that have been
operating long enough to develop a combustible gas database. The guide addresses: General Theory,
Sampling and Laboratory Procedures, Variability and Precision Detection, Operating Guidelines,
Development of Norms, Diagnostic Techniques, Fixed and Portable Monitoring Devices, Normative
Annexes.
1.2 Limitations
The scientific principles of hydrocarbon and cellulose decomposition and the analytical procedures described
in this guide are well established. However, application of these principles to the determination of faults in
transformers was established by empirical studies relating measured combustible gas concentrations with
observed disturbances and failures. The empirical nature of these studies confers a degree of variability and
uncertainty that must be recognized particularly in numerical fault diagnosis. The variables involved are
equipment type, the materials involved in the fault, and the extraction and analytical procedures themselves.
Equipment variables are transformer design, location, service and operating temperatures. Material variables
are solubility related to oil, degree of saturation and the types of materials involved at the fault. Analytical
variables associated with the sampling, extraction and measuring process affect to some degree the
confidence in diagnostic procedures especially between laboratories. Also, gas concentrations below 10
times the minimum detection level may invalidate Ratio ratio (numerical) diagnosis. Replicate samples are
recommended before developing an operating decision or fault diagnosis. Because of this inherent
variability, it is difficult to obtain a consensus on guidelines for detection norms or diagnostic Ratiosratios.
Exact fault types or degrees of fault intensity may not be inferred from a single gas concentration, or even
after repeated sampling. Rather, development of concentration trends will give more reliable insight into
developing faults. In addition, diagnosis of the primary or failure-initiating fault from a sample taken after
failure is invalid because of the saturation quantities of all gas species that are generated by the heat
secondary effectsreleased from of the transformer failure itself.
In the light of these variables, this guide is offered as an advisory document, providing guidelines to assist
the transformer operator and manufacturer in deciding on the status and continued operation of a
transformer that exhibits combustible gas formation.
This guide applies only to operating transformers in service. It does not include transformers where arcing
switch contacts or expulsion tubes are exposed to the main oil volume. No attempt should be made to relate
these guidelines to new transformers during factory temperature rise tests. Finally, the operators’
development of a combustible gas database accompanied by actual confirmation of predicted fault by failure
investigation will allow each individual operator to establish his own norms unique to his size and type of
transformer and operating conditions.
2. References
This guide shall be used in conjunction with the following standards. When the following standards are
superseded by an approved version, the revision shall apply.
ASTM D2945-90 (2003) e1, Standard Test Method for Gas Content of Insulating Oils1
1
ASTM publications are available from the American Society for Testing and Materials, 100 Barr Harbor
Drive, West Conshohocken, PA 19428-2959, USA (http://www.astm.org/).
Copyright © 2004 2005 IEEE. All rights reserved.
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April 4, 2005
IEEE PC57.104 D12
ASTM D3284-1999, Standard Test Method for Combustible Gases in the Gas Space of Electrical Apparatus
Using Portable Meters
ASTM D3305-95 94 (1999) e1, Standard Practice for Sampling Small Gas Volume in a Transformer
ASTM D3612-2002, Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by
Gas Chromatography
ASTM D3613-1998, Standard Practice for Sampling Insulating Liquids for Gas Analysis and Determination
of Water Content
ASTM D 2759-2000, Standard Practice for Sampling Gas from a Transformer under Positive Pressure
ASTM D5837-1999 e1, Standard Test Method for Furanic Compounds in Electrical Insulating Liquids by
High-Performance Liquid Chromatography (HPLC)
IEC 60599-1999, Mineral oil impregnated electrical equipment in service - Guide to the interpretation of
dissolved and free gases analysis2
IEEE Std C57.12.80-2002, Terminology for Power and Distribution Transformers3
3. Definitions
3.1 Caution surveillance range: Combustible gas concentrations above the normal surveillance range and
below the warning surveillance range.
3.2 Dissolved gas analysis (DGA): The extraction, detection and analysis of gases dissolved in oil.
3.3 Fault diagnosis: A numerical diagnostic procedure using Rogers ratios to determine specific
transformer fault types for combustible gas concentrations at or above the warning surveillance range.
3.4 Key gas: A characteristic combustible gas generated in oil filled transformers that can be used for
qualitative determination of fault types based on gases, which are typical or predominant at various
temperatures.
3.5 Minimum detection level (MDL): The lowest level at which a gas can be detected in oil; described in
ASTM D3612.
3.6 n% Probability norm: A statistically determined value, which signifies the probability that n% of all
normally operating transformers have individual or total combustible gas concentrations at or below that
value.
3.7 Normal surveillance range: A combustible gas concentration level at or below which individual or
total combustible gas concentrations in transformers are considered normal.
2
IEC publications are available from the Sales Department of the International Electrotechnical
Commission, Case Postale 131, 3, rue deVarembé, CH-1211, Genève 20, Switzerland/Suisse
(http://www.iec.ch/). IEC publications are also available in the United States from the Sales Department,
American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, USA
(http://www.ansi.org/).
3
IEEE documents in the reference list are available from the Institute of Electrical and Electronics
Engineers, Service Center, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA.
Copyright © 2004 2005 IEEE. All rights reserved.
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April 4, 2005
IEEE PC57.104 D12
3.8 Partial discharge (PD): An electric discharge which only partially bridges the Insulation insulation
between conductors, and which may or may not occur adjacent to a conductor.
NOTES:
(1) Depending on intensity, Partial discharges are often accompanied by emission of light, heat, or sound, separately
or in combination. The term “ Corona “ has also been used to describe partial discharges. This is a non-preferred term
since it has other non-related meanings.Partial discharges occur when the local electric-field intensity exceeds the
dielectric strength of the dielectric involved, resulting in local ionization and breakdown. Depending on intensity, partial
discharges are often accompanied by emission of light, heat, sound, and radio influence voltage (with a wide frequency
range).
(2) The relative intensity of partial discharge can be observed at the transformer terminals by measurement of the
apparent charge (coulombs). However, the apparent charge (terminal charge) should not be confused with the actual
charge transferred across the discharging element in the dielectric, which in most cases cannot be ascertained.
Partial discharge tests using the radio influence voltage techniques, which are responsive to the apparent
terminal charges, are generally used for measurement of relative discharge intensity.
(3) Partial discharges can also be detected and located using sonic techniques.
(4) “Corona” has also been used to describe partial discharges. This is a non-preferred term since it has other
unrelated meanings.
3.9 Screening diagnosis: A diagnostic procedure using Key gases to obtain a preliminary estimate of lowlevel incipient fault development in the caution surveillance range.
3.10 Total combustible gas (TCG): The sum (%) of all combustible gases including carbon monoxide and
excluding oxygen reported as a % percent of the transformer gas space.
3.11 Total dissolved combustible gas (TDCG): The sum of all combustible gases that are dissolved in the
insulating oil.
3.12 Type 1 Service category: Type 1 Transformers have never received been sampled for DGA before or
do not have statistically significant gas databases.
3.13 Type 2 Service category: Type 2 Transformers have had previous DGA tests or have a gas database
large enough for statistical derivation of surveillance norms. Type 2 transformers may also have been
assigned surveillance norms because of their similarity to other Type 2 transformers in design and loading.
3.14 Warning surveillance range: Combustible gas concentrations above the Caution surveillance range.
Combustible gas concentrations within this range generally can result in failure of the transformer and
should result in immediate investigation.
4. General decomposition theory
The principal causes of gas formation within an operating transformer are thermal and/or electrical
disturbances such as conductor losses and electrical disturbances and other temperature heat producing
effects, such as conductor loses, that produce combustible gases from mineral oil, and cellulosic
decomposition. Generally, there is little or no heat associated with low energy partial discharges where
decomposition gases are formed principally by low-level ionization. Mineral oil decomposition is the
predominant mechanism utilized in the detection and diagnostic procedures applied to transformers.
4.1 Cellulose decomposition
The thermal decomposition of oil-impregnated cellulose insulation produces carbon oxides (CO, CO2) and
some hydrogen or methane (H2, CH4) from the oil (note: CO2 is not a combustible gas). The rate at which
they are produced depends exponentially on the temperature and directly on the volume of material at that
temperature. A large, heated volume of insulation at moderate temperature may produce about the same
quantity of gas as a smaller volume at a higher temperature.
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IEEE PC57.104 D12
4.2 Mineral oil decomposition
Mineral transformer oils are mixtures of many different hydrocarbon molecules, and the decomposition
processes for these hydrocarbons in thermal or electrical faults are complex. The fundamental steps are the
breaking of carbon-hydrogen and carbon-carbon bonds by the heat evolved from the fault mechanism.
Active hydrogen atoms and hydrocarbon fragments are formed. These free radicals can combine with each
other to form gases, molecular hydrogen, methane, ethane, etc., or can recombine to form new, condensable
molecules.
Further decomposition and rearrangement processes lead to the formation of products such as ethylene and
acetylene and, in the extreme, to modestly hydrogenated carbon in particulate form. These processes are
dependent on the presence of individual hydrocarbons, on the distribution of energy and temperature in the
neighborhood of the disturbance, and on the time during which the oil is thermally or electrically stressed.
These are chemical transformations; therefore, the specific degradations of the transformer oil hydrocarbon
ensembles and the fault conditions may not be predicted reliably from purely chemical kinetic
considerations. An alternative approach is to assume that all hydrocarbons in the oil are decomposed into
predictable products and that each product is in equilibrium with all the others. In addition, the presence of
certain metals and coatings can have a catalytic effect on mineral oil decomposition.
It is well known that combustible gases are formed by the thermal decomposition of mineral oil. And the
proportions of certain combustible gases are unique for a specific temperature of decomposition. These
relationships, coupled with evidence that thermal faults in a transformer generally produce lower
temperatures in the oil than electrical or arcing faults are the basis for the empirical development of the fault
diagnostic procedures given in this guide. Some examples of these relationships are: the presence of
methane suggesting a relatively low energy discharge or thermal fault; and the presence of acetylene
suggesting that a high-energy arc has occurred. The thermodynamic approach has limits; it must assume an
idealized but nonexistent isothermal equilibrium in the region of a fault, and there is no provision for
dealing with multiple faults in a transformer. Much work has been done to correlate these predictions from
thermodynamic models with the actual behavior of transformers.
5.0 Combustible gas generation in equipment
All transformers generate gases to some extent at normal operating temperatures. But occasionally gasgenerating abnormalities known as faults occur within an operating transformer such as a local or general
overheating, electrical problems, or a combination of these. These types of faults produce characteristic
gases that are generally combustible. Their concentrations are determined by extraction and Gas
Chromatography Chromatography. Detection of combustible gases dissolved in the oil or found in the inert
gas space or relay may indicate the existence of any one, or a combination of thermal or electrical faults.
Furthermore, the ratios of certain gases have been found to suggest fault types using the diagnostic
procedures in 7.0.
Interpretation by the individual gases can become difficult when there is more than one fault, or when one
type of fault progresses to another type, such as an electrical problem developing from a thermal one or vice
versa. Analytical laboratories should report the MDL for their particular extraction procedures and
equipment. Gas concentrations greater than ten times MDL are preferred for reliable numerical ratio
diagnoses. Finally, operators should avoid attempts to assign greater significance to gas measurements than
justified by the natural variability of the generating and analytical processes. While new logical procedures
may reduce diagnostic uncertainty, increased sampling and observation of gas generating trends should also
enhance the correlation between Fault and numerical Diagnosis. Carefully following the rules guidelines
and procedures given here will provide valuable assistance to the operator and manufacturer.
5.1 Thermal fault
At oil temperatures from 150 ºC to 300 ºC relatively large quantities of the low molecular weight gases,
such as hydrogen (H2) and methane (CH4), and trace quantities of the higher molecular weight gases ethylene
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April 4, 2005
IEEE PC57.104 D12
(C2H4) and ethane (C2H6) are produced. by poor cooling or stray losses in winding or leads; between core
laminations or in core, tank or supporting structures. As the decomposition temperature in mineral oil
increases from 300 ºC to 700 ºC, the hydrogen concentration exceeds that of methane and is accompanied by
significant quantities of higher molecular weight gases, first ethane and then ethylene. Beyond 700 ºC (the
upper end of the thermal fault range), increasing quantities of hydrogen and ethylene and traces of acetylene
(C2H2) may be produced. In general, non-electrical thermal faults produce temperatures below 700 ºC;
however, welding on oil filled equipment or oily surfaces produces acetylene due to the very high
temperature. Also, thermal faults may evolve into electrical faults, which may be a source of error in
diagnosis.
5.2 Thermal faults involving cellulose
The thermal decomposition of cellulose paper and solid insulation produces mostly carbon monoxide (CO),
carbon dioxide (CO2), and water even at normal operating temperatures. Also, moisture generated by
cellulose decomposition will accelerate further decomposition especially in sealed units. Gaseous byproducts of cellulose decomposition are found at normal operating temperatures in the transformer.
Diagnosis of thermal faults involving cellulose is given in this guide.
5.3 Electrical faults
Electrical faults range in energy and temperature from intermittent low energy partial discharges to steady
discharges of high energy (arcing). As the discharge progresses from low energy to higher energy, the
acetylene and ethylene concentrations rise significantly.
5.3.1 Partial discharges (PD)
The very low temperature of partial discharges (150 ºC to 300 ºC) produces mainly hydrogen, with lesser
quantities of methane and trace quantities of acetylene. Note that this temperature range also produces some
thermal decomposition of oil and cellulose. The gases produced at these low temperatures may suggest
either an electrical or thermal fault or both.
5.3.2 Low and high energy discharge (arcing)
As the intensity of the electrical discharge reaches arcing or continuing discharge proportions, producing
temperatures from 700 ºC to 1800 ºC, the quantity of acetylene becomes pronounced. Note that these
temperatures are also produced by welding, complicating diagnosis.
5.4 Sampling and laboratory procedures
Establishing a reference point (baseline) for statistical norms for gas concentrations in new and repaired
transformers in service, and developing a routine surveillance sampling program are critical elements in the
application of this guide. ASTM Standard methods are utilized to obtain oil or gas samples for extraction
and analysis by gas chromatography. Monitoring devices are available which directly and continuously or
periodically extract and measure combustible gas components.
5.4.1 Surveillance sampling
Regular surveillance monitoring of combustible gas concentrations in operating transformers can begin
anytime. Generally, daily or weekly sampling is recommended after start -up followed by monthly or longer
intervals, which may vary depending on application and individual system requirements. After detection of
increasing amounts of generated gas, shorter sampling intervals are suggested. When a possible source of
gas generation has been determined, good engineering judgment should be applied to determine sampling
interval and operating procedure.
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April 4, 2005
IEEE PC57.104 D12
5.4.2 Sampling and analytical method
All samples of oil from electrical apparatus for DGA should be taken in accordance with ASTM D3613.
TCG samples from the gas space above the oil may be taken in accordance with ASTM D 2759 or D3305 if
the gas volume is small. The combustible gases contained in oil samples for DGA are extracted from the oil
and analyzed by gas chromatography per ASTM D3612. TCG samples of the gas from the inert gas space
may be analyzed per ASTM D3612 or equivalent industry standards. (See also Annex A, B). The syringe or
cylinder method is preferred for all oil or gas samples.
5.5 Variability and precision of analysis
Minimum detectable levels of gas concentration measurements for gas species dissolved in oil are given in
ASTM D3612. ASTM D3612 states the MDL as 5 for H2, 1 for hydrocarbons, and 25 for carbon oxides.
Gas concentrations near MDL have a high probability of large error. Gas concentrations at ten times MDL
or greater have better precision.typical inter-lab precision of approximately 5% and typical intra -lab
precision about 10% which should be recognized when establishing a DGA program. Equal intrainterlaboratory precision is recommended when analysis is performed by more than one laboratory. Wide
variations in gas concentrations within a single sample are cause for concern.
6. Detection and operating guidelines
Much information has been acquired on the utilization of combustible gas data from surveillance oil
samples for detecting and determining incipient fault conditions in operating transformers. This guide seeks
to assist the operator in applying this gas data to Type 1 transformers per clause 6.1 and Type 2 transformers
per clause 6.2 following a step-by-step process described in the following Flow Chart Figure 1.
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April 4, 2005
IEEE PC57.104 D12
FIGURE 1 – Flow chart
MAINTENANCE
Derive 90%
Probability
Norms 6.2.1
Sample Compare to
Sample Compare to
Sample Compare to
Database Norms 6.2.1
Range Guidelines
Table 1, 6.1
Assigned Norms 6.2.2
Normal Range
RANGE DETERMINED
Table 1
Key Gas
Procedure
Table 3, 7.1.1
Caution Range
Warning Range
Key Gas
Procedure
Table 1
Table 1
Table 3, 7.1.1
Generation Rate 6.1.1
Ratio Diagnosis
Determine Sampling Interval
and Operating Procedure
Table 4, 7.2.1
Table 2, 6.1.2
Trend & Graphics 8.0
Database
ACTION & OPERATING DECISION
The Flow Chart process begins with taking oil samples containing dissolved gases from Type 1 or Type 2
transformers during routine maintenance surveillance. The samples are sent to the laboratory where
dissolved gases are extracted and analyzed. The gas concentrations from Type 1 transformers are compared
per clause 6.1 to the guidelines in Table 1 to determine their surveillance range. Alternatively, the
concentrations from Type 2 transformers are compared per clause 6.2 to statistically derived norms or
assigned norms from similar units and their corresponding surveillance range is determined. Gas generation
rates are determined per clause 6.1.1 and the range and rate information is applied to Table 2 to determine
the guidelines for new sampling intervals and operating procedures.
6.1 Detection and operating guidelines for Type 1 transformersDetection
procedure for transformers in Type 1 category
Referring to Flow Chart Fig. 1, the operators of Type 1 transformers only, for which no previous gas data
exists, must initially rely on Table 1 for guidance that indicates whether their transformer is operating in the
Copyright © 2004 2005 IEEE. All rights reserved.
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12
April 4, 2005
IEEE PC57.104 D12
normal surveillance range or in the caution or warning surveillance ranges. The Table 1 guidelines used for
this purpose were surveyed from many laboratories, manufacturers and operators representing industry
practice for normally operating transformers. The Type 1 transformer is then assigned to the particular
surveillance range within which either TDCG or any one or more component gas concentrations fall. For
example, if TDCG is >700 mgµL/kg L (ppm) or C2H2 is 4 mgµL/kg L (ppm), the transformer is in the
caution surveillance range.
TABLE 1 - Guidelines for surveillance range for Type 1 transformers having no previous
combustible gas testsa
Generated
Gases
Surveillance
range
Normal
µmLg/kg
(ppm)
L
Caution
mgµL/kg
(ppm)
L
Warningb
mgµL/kg
(ppm)
L
Methane
Acetylene
Ethylene
Ethane
Carbon
Monoxide
TOTALa
Hydrogen
H2
CH4
C2H2
C2H4
C2H6
CO
TDCG
<100
<120
<2
<50
<65
<350
<700
100 to
700
120 to
400
2 to
5
50 to
100
65 to
100
350 to
570
700 to
1900
>700
>400
>5
>100
>100
>570
>1900
a
Total of all combustible gases
The numbers shown in Table 1 are in µL/L (ppm) of oil volumetrically. They
are typical for oil filled transformers of various size and design; representative of U.S. and European reported
experiences. Small distribution transformers and voltage regulators may contain combustible gases because of the
operation of internal expulsion fuses or load break switches. For this reason, Table 1 does not apply to these apparatus
or any other apparatus in which load break switches operate under oil in the main tank.
b
Any component or their total gas concentrations in warning range indicates a severe problem generally requiring
immediate intervention or removal.
6.1.1 Combustible gas generation rate
When any combustible gas is detected in any surveillance range, it is essential to determine whether the
source is active or passive by taking a second sample St at T days later and computing the generation rate Rs
as a % per day of the first sample So. As a rule of thumb, a generating rate exceeding 0.5% per day for any
single component or their sum total suggests a possible problem; generation rates exceeding 3% per day
suggest an abnormality exists. The surveillance range determined from Table 1 and the generation rate
determined from Eq.1 are then applied to Table 2 (clause 6.1.2) to determine the suggested operating
guidelines with the exception of C2H2, where a generation rate > MDL is cause for concern and daily
surveillance.

 100 
Rs = ( St − So )
 T
 So 

(1)
Where:
Rs
So
St
T
is the Rate (% of So per day)
is the first sample (ppm)
is the second sample (ppm)
is the time in days
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IEEE PC57.104 D12
6.1.2 Determining the operator guidelines from surveillance range and generation rate
Table 2 gives suggested operator guidelines for TDCG in the normal, caution and warning ranges found
from Table 1 for given rate limits. The gas concentrations for individual components can be substituted for
TDCG in column 2, Table 2 to determine operator guidelines from just the components.
TABLE 2 - Guidelines for surveillance range and generation rate
a
Suggested Operator Guidelines
Surveillance
range
TDCG
µLmg/kg L
(ppm)
Generation
rate Rs
(%/day)
Normal
< 700
== > 0.3
=<= 0.5
Monthly
< 0.3
Normal
surveillance
=7
Daily
=3<7
Weekly
> 0.5 < 3
Monthly
=7
Daily
Caution
700 to 1900
Operating Procedure
Caution check load
dependence
Continue normal operation
Caution; Screening diagnosis;
check load dependence.
Advise manufacturer or
insurer.
Extreme caution: Diagnostics
plan outage; Advise
<7
Weekly
manufacturer or insurer.
a
The numbers shown in Table 2 are in µL/L (ppm) of oil volumetrically. They are typical for oil
filled transformers of various size and design; representative of U.S. and European reported
experiences. Small distribution transformers and voltage regulators may contain combustible gases
because of the operation of internal expulsion fuses or load break switches. For this reason, Table 2
does not apply to these apparatus or any other apparatus in which load break switches operate under
oil in the main tank.
Warning
> 1900
Sampling
Interval
Transformers with TDCG in the normal surveillance range with generating rates > 0.3% per day, have
reduced sampling intervals, therefore stricter operating procedures are suggested. If their generating rates are
< 0.3% per day the operator may continue with normal surveillance sampling and operating procedure. In
addition, with rates < 0.3% per day, normal transformers may have their normal maintenance sampling
intervals lengthened.
Transformers with TDCG in the caution or warning surveillance range will have reduced sampling intervals
and stricter operating procedures.
6.2 Detection procedure for Type 2 transformers
Referring again to the Flow Chart, Fig. 1, transformers with previous combustible gas history may have
databases that are sufficient for statistical derivation of 90% probability norms for individual components or
their total and operators can then establish their own equivalent to Table 1. A procedure for determining
norms from a database is given in clause 6.2.1. The norms derived in this manner from a database must
reflect data from fault-free equipment. The newly derived 90% probability norms may be substituted for the
normal surveillance range values given in Table 1. The caution and warning surveillance range norms will
be proportional to those in Table 1. These new surveillance range norms and the generation rates calculated
for the given sample gas concentrations are then applied in Table 2 to determine the suggested operating
guidelines and surveillance sampling intervals.
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IEEE PC57.104 D12
Both the statistically derived range values and the proportional caution and warning surveillance ranges are
used as trial values which are refined by continued experience and should be agreed upon by user,
manufacturer and insurer as appropriate. Transformers with statistically insignificant numbers of tests in the
database should continue to be evaluated per clause 6.1. Although a database of similar transformers is
preferred for developing the most accurate surveillance range norms, absent a statistically sufficient
population of similar transformers, the user may choose to use the norms from Table 1 as a reference to
general industry experience. Special cases may occur where the equipment operator, insurer and
manufacturer agree on values called assigned norms that have been assigned to a particular type of
equipment or service based on experience. Procedures for utilizing assigned norms are given in clause 6.2.2.
6.2.1 Procedure for statistical development of surveillance range norms from a gas
database
a.
A Statistically sufficient number of normal, fault free, transformers of similar design and service are
collected into a database.
b.
Values for each component Combustible gas (H2, CH4, C2H2, C2H4, C2H6, CO) and their total (TDCG)
determined from each transformer in the database are tabulated and sorted in decreasing order. A
separate Frequency Distribution Chart (probability chart) is then constructed for each component and
TDCG for all the transformers in the database using well-known statistical procedures such as those
available in most spreadsheet programs. From the plotted frequency distribution for each component,
the 90% probability norm for that component is the unique 90% probability value for the database and
it signifies that 90% of all the normal transformers in this database have values at or below that value.
c.
You now can construct a table similar to Table 1 in this guide for your own particular case using the
derived 90% norms for the normal surveillance range guidelines. The caution and warning values in
your new table will be proportional to those in Table 1.
This method of deriving individual norms from a transformer population will yield values, which are
statistically representative of that population and preferable to the survey values in Table 1. Continued
analysis of the database will refine the estimated norms and increase the users confidence in their reliability.
The reliability of derived norms depends on the number of transformers in the database.
6.2.2 Procedure using assigned norms
Referring to the Flow Chart Fig.1, the step-by-step procedure may employ assigned norms which are values
for the normal, caution and warning surveillance ranges that have been developed as part of a proprietary
surveillance plan; either from data developed within the operator’s company or from recommendations from
outside contractorsexperts. For example, banks of transformers of duplicate design and loading may all
utilize the same set of surveillance norms based on sound engineering judgment and agreement between
user, manufacturer and insurer.
Assigned norms are substituted for the values in Table 1. Then sample gas concentrations from transformers
with assigned norms are utilized to determine the surveillance ranges and calculated generation rates which
are then applied to Table 2 in the same manner as statistically determined norms in clause 6.2.1.
7. Diagnosis of transformer faults
Referring to the Flow Chart, Fig.1, after completing the detection procedures in clause 6, the final steps of
this process are to validate the findings using the Key gas method in the caution surveillance range and the
Ratio diagnosis. The results and data from these investigations are then placed in a database record for each
unit to enhance the reliability of subsequent tests.
When the concentrations or generating rates of any single dissolved combustible gas component or the sum
of the components (TDCG) extracted from the oil or gas space of an operating transformer rise into the
caution range, the operator should begin to consider the possible sources of these gases and their generation
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IEEE PC57.104 D12
progress by utilizing analysis in the caution range per clause 7.1. When the values reach the warning range,
fault diagnosis per clause 7.2 employs an array of Ratios attributed to Rogers et al given in Table 4.
Diagnosis of cellulose decomposition per clause 7.3 may also be applied. These diagnostic procedures can
allow the operator to focus his subsequent investigation, utilizing load studies or thermal imaging to
pinpoint thermal problems in the windings or cooling system or partial discharge monitoring to pinpoint
electrical trouble. Generally, thermal faults produce lower temperatures and than electrical faults except for
low-level partial discharges, which produce higher temperatures. The diagnostic Ratios used in Table 4 are:
Ratio 1 (R1) = CH4 / H2
Ratio 2 (R2) = C2H2 / C2H4
Ratio 3 (R3) = C2H4 / C2H6
Graphical 2 or 3 dimensional plots of the three Rogers ratios provide an excellent physical representation of
the limit envelopes circumscribing each fault type. Ratio diagnosis applies to gases extracted from the oil.
For diagnoses of components extracted from the transformer inert gas space or relays see Annex B.
7.1 Analysis in the caution surveillance range
In addition to increased surveillance for generation rate and trends, transformers with TDCG in the caution
range may receive preliminary or screening diagnosis by the Key gas procedure.
7.1.1 Key gas procedure
The Key gas procedure permits a tentative determination of possible fault types empirically determined from
their unique gas species. The Key gas method may also be useful for benchmarks in the normal range and
may also help to confirm diagnoses in the warning range. Table 3 relates fault types with typical proportions
of their Key gas indicators.
Table 3 – Key gases
KEY GAS
FAULT TYPE
TYPICAL PROPORTIONS OF GENERATED
COMBUSTIBLE GASES
Ethylene
(C2H4)
Thermal Oil
Predominantly Ethylene with smaller proportions of Ethane,
Methane, and Hydrogen. Traces of Acetylene at very high fault
temperatures.
CarbonMonoxide
(CO)
Thermal Oil and
Cellulose
Predominantly Carbon Monoxide with much smaller quantities of
Hydrocarbon Gases in same proportions as Thermal faults in oil
alone.
Hydrogen
(H2)
Electrical Low
Energy P.D.
Predominantly Hydrogen with small quantities of Methane and
traces of Ethylene and Ethane.
Hydrogen and
Acetylene
(H2, C2H2)
Electrical High
Energy (arcing)
Predominantly Hydrogen and Acetylene with minor traces of
Methane, Ethylene, and Ethane. Also Carbon Monoxide if
cellulose is involved.
7.2 Diagnosis in the warning surveillance range
When the dissolved gas concentrations and generating rates progress from the caution into the warning
range, they are high enough to minimize the effects of sampling and analytical error. In the warning range,
the Ratio ratio diagnostic procedures given in clause 6.2.17.2.1 are significant and provide relatively
accurate fault diagnosis. The fault types defined in clauses 45.1, 45.2, and 45.3 were empirically derived
from considerable experience of several European investigators who correlated gas diagnoses on many units
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IEEE PC57.104 D12
with fault types subsequently associated with observed disturbances or failures. Trends per clause 8.0 can
confirm Ratio ratio diagnosis. The ratios of CO2/CO in clause 67.3 can provide diagnostic information on
cellulosic degradation associated with thermal faults.
7.2.1 Rogers Ratio diagnosis
This procedure utilizes the three ratios R1, R2, and R3. When gas concentrations yield a ratio outside the
Table 4 limits for any given fault type, applying the Key gas method and graphical analysis of gas generating
trends can help to clarify the situation.
Ratio diagnosis begins with extraction of the combustible gases from the oil sample and chromatographic
analysis per clause 5.4.2.
Ratios are calculated from the reported gas concentrations. Then the Ratios ratios are compared to values in
Table 4 providing fault diagnosis.
Table 4 - Diagnostic ratios for fault determination
CASE
C2H2 (R2)
C2H4
CH4 (R1)
H2
C2H4 (R3)
C2H6
0
< 0.01
< 0.1
< 1.0
Normal
1
=>1.0
=>0.1 < 0.5
=> 1.0
Low energy Discharge
(Partial Discharge)
2
=> 0.6 < 3.0
=> 0.1 < 1.0
=> 2.0
High energy Discharge
3
< 0.01
=> 1.0
< 1.0
Low Temp.
(<300 °C)
4
< 0.10
=> 1.0
=>1.0 < 4.0
Suggested Diagnosis
Thermal
Thermal (300 °C< - 700
°C)
5
< 0.2
=> 1.0
=> 4.0
Thermal (> 700 °C)
Case 0 = Normal Unit
Case 1 = Low Energy Discharge
Case 2= High Energy Discharge
Case 3 = Low Temp. Thermal (< 300°C)
Case 4 = Thermal (300°C - 700°C)
Case 5 = Thermal (> 700°C)
7.3 Diagnosis of cellulose decomposition
The ratio of CO2/CO may sometimes be used as an indicator of the thermal decomposition of cellulose.
This ratio for normal cellulosic decomposition is usually between 7 and 10. Also, CO2/CO ratios below 3
and or significantly greater than 10, suggest excessive thermal decomposition. The magnitudes of the
concentrations of CO2 and CO should exceed 5000 ppm and 500 ppm respectively, in order to improve the
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IEEE PC57.104 D12
certainty factor. Ratios are sensitive to minimum values. In addition to the CO2/CO ratio, the Furanic
Series per ASTM D5837 is also an indicator of cellulose decomposition.
8.0 Trend and graphical analysis
Trends in the generation of individual gas concentrations, key gases, generation rate, or Ratios may be used
to evaluate the temporal development of faults. Graphical analysis of trends may help to indicate when
faults progress from relatively benign thermal faults into more critical electrical faults. It is strongly
suggested that the report of a possible fault by either the Key gas procedure, or Ratio diagnosis, should
include a numerical value for an increasing or decreasing trend. The trend for the magnitude of CO to
increase and CO2 to decrease indicates advancing degradation of cellulose insulation.
Graphical analysis may be applied using two-dimensional (2D) or three-dimensional (3D) plots of the values
for the pertinent ratios. Two-dimensional plots are used to determine the correlation between the ratio
CH4/H2 on the x axis vs. the ratio C2H2/C2H4 on the y axis; or the ratio CH4/H2 on the x axis vs. the ratio
C2H4/C2H6 on the y axis with the coordinates for the Rogers ratio limits for particular cases.
However, the 3D plots are more revealing. Three-dimensional graphical analysis may be accomplished by
developing a three-dimensional composite plot of all five Rogers fault case envelopes from Table 4. A
simpler alternative is to develop separate envelopes for each of the five Rogers fault cases. Then gas ratios
from a transformer sample may be plotted for either the 2D or 3D representations. A repeated result within
a specific fault region or volume confirms a diagnosis. A trend for the results to move toward the
boundaries of the envelopes suggests another fault or more than one fault exists. For example, in the 3D
case say the three calculated ratios for a particular analysis converge at a point within a Case 3 fault case
envelope (Low temperature Thermal fault). Then, if the solution points for successive samplings show a
trend for the diagnosis to move out toward the boundaries of the Case 3 envelope, the conclusion may be
drawn that the fault is changing in intensity or it may be progressing to another fault type. An example
would be the progression of a thermal fault case into an electrical fault case that suggests the movement of
gas bubbles from a thermal fault location into nearby areas of high electrical stress.
8.1 Construction of a 3 D graph
The construction of a 3D Ratio graph is accomplished by plotting the limit envelopes from Table 4 for ratio
R1 vs. R3 (on the xy plane); ratio R1 vs. R2 (on the xz plane); and ratios R3 vs. R2 (on the yz plane) using
appropriate scales. All 5 cases from Table 4 may be shown together on the same graph, or each case may be
graphed separately as in Figs. 2a and 2b for simplicity.
Then the actual values of the ratios from the transformer sample test data are located within their respective
envelopes.
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IEEE PC57.104 D12
Figure 2 – 3D ratio graph
Fig. 2a, Case 3
Fig. 2b, Case 1
R3
R3
Xb
Za
Xa
Yb
Ya
R1
R2
Low Temperature Thermal Fault
Zb
R2
R1
Low Energy Discharge
Fig. 2a is an example of a 3D Rogers Ratio envelope for a Low Temperature Thermal fault (Case 3 – Table
4). The line Xa represents the interval between 1.0, the minimum Table 4 value of R1, and 4.0 an arbitrary
maximum limit (see note) used for plotting the envelope. The line Ya represents the interval between 0, the
arbitrary minimum (see note) value of R3, and its Table 4 limit 1.0. The line Za represents the interval
between 0, the arbitrary minimum value of R2, and its Table 4 limit 0.10.
Fig. 2b is an example of a 3D Rogers Ratio envelope for a low energy discharge fault (Case 1 – Table 4).
The lines Xb, Yb, and Zb represent the Table 4 intervals for plotting the Case 1 envelope. Arbitrary values
of 4.0 were used for maximum R2 and R3 limits for plotting the envelope.
Note: In order to describe a graphical envelope an arbitrarily chosen maximum limit was chosen for a Table 4 ratio that
does not contain a minimum limit. Zero values are not obtained with ratios.
9.0 Detection and analysis of gases in gas space, relay or other fixed or
portable devices
The on-site detection and determination of combustible gases in an operating transformer, using a portable
or on-line combustible gas meter can be the earliest and most easily obtained indication of a thermal or
electrical fault. And it may form the basis for further testing.
Other portable or on-line devices are useful for periodic testing of operating transformers without removing
them from service. These devices normally accept a small volume of oil or gas depending on the technology
used. They will provide a measurement of hydrogen or composite values of hydrogen and carbon monoxide,
or in the case of a portable GC, a measurement of all combustible gases.
Another type of gas monitoring system samples the transformer gas space at fixed intervals to provide a
measure of individual combustible gases. Periodically, a sample of the oil is pumped from the transformer
into the monitoring system that extracts the gases and passes them through a micro GC for measurement.
This system uses helium as the carrier gas so careful attention should be given to ensure proper operation.
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Gases collected in the gas space or gas relays can be sampled per clause 5.4. ASTM Method, D-3612, can
determine the individual components of the gas samples. Since the sample is already in the gas phase the
extraction step in the ASTM method is not applicable.
Annex A describes a TCG procedure for evaluating the transformer operating procedure given TCG
percentages and generating rates. The procedure in Annex A is similar to the DGA procedure in Table 2
clause 6.1.2.
Combustible gases may be explosive. Strict precautions should be observed when sampling directly from the
transformer.
Annex B describes procedures for converting concentrations from gases from relays or sampled from gas
spaces into dissolved gas-in-oil equivalents that can then be diagnosed per clause 7.2.
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Annex A (Normative)
TCG detection and operating procedure
When combustible gases are first found in the transformer gas space, sampling is repeated to determine the
rate of increase of % TCG per day. Then the % combustibles (TCG) and the rate of increase (%/day) are
applied to Table A1 to determine guidelines for sampling intervals and operating procedures in the normal,
caution and warning surveillance ranges similar to the procedures for dissolved gas. Table A1 utilizes
values for TCG % limits and generation rates which can be used to determine the surveillance ranges and the
suggested sampling intervals and operating guidelines similar to Table 2 in clause 6.1.2.
Table A1 – TCG surveillance ranges and suggested guidelines
Surveillance
Range
Warning
Caution
Normal
a
b
c
Suggested Operator Guidelines
TCG %
Generation
Rate
% TCG / day
>5
.> .03
Daily
< .03
Weekly
> 0.03
Weekly
.01 to .03
Monthly
< 0.01
Quarterly
>0.01
Monthly
Cautionc
< 0.01
Normal
Surveillance
Continue Normal
operation
.5 to 5
< .5
Sampling
interval
Operating
procedure
Extreme Caution
DGA per 7.2 a
Caution DGA per
7.1 b
Plan outage; advise manufacturer or insurer.
Check load dependence; advise manufacturer or insurer.
Check load dependence.
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Annex B (Normative)
TCG diagnosis of fault type
The gas sample taken from the transformer gas space may be separated into its components using gas
chromatography. Each component concentration multiplied by its Ostwald coefficient in Table B1 is the
dissolved gas equivalent. The dissolved gas equivalents may then be applied per the diagnostic procedure in
clause 7.2. Note that the transformer should be considered to be in the warning surveillance range for TCG
diagnosis by the equivalent DGA concentrations to be considered accurate.
Table B1 – Ostwald coefficient for determining dissolved gas equivalent values
Ostwald Coeff.a
Gas
a
(25°C)
H2
0.0558
CH4
0.438
C2H2
1.22
C2H4
1.76
C2H6
2.59
CO
0.133
Coefficients are for 0.885 density oil at STP. Coefficients for different densities may be calculated
per ASTM D3612 procedure.
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22