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Dissolved Gas Analysis and its interpretation
techniques for Power Transformers
Adam Junid, Simin Li, Lingfeng Ni
October 21, 2008
Abstract
This report is an investigation on Dissolved Gas Analysis and its interpre-
tation techniques for power transformers. Aspects covered in this report
include sampling method guidelines, interpretation techniques and stan-
dards, results-based guidelines and recommendations. A case study is
used to demonstrate interpretation.
1
Acknowledgements:
We thank:
� Dr Toan Phung for his help and explanations on ELEC9712 (High VoltageSystem) topics
� Zulkarnain Muhamad and Ahmad Zarir Makhtar from Petronas Gas Berhad(CUF-Kerteh) for sharing practical DGA data and photos with us
� Yee Yen Fu and Nor Azhar Saad from Asean Bintulu Fertilizer for sharingDGA reports with us
2
Acronyms and Abbreviations
ANSI
American National Standards Institute, www.ansi.org
AS
Australian Standard, www.standards.org.au
ASTM
American Society for Testing and Materials
BS
British Standards
H2
Hydrogen
C2H2
Acetylene
C2H4
Ethylene
CH4
Methane
C2H6
Ethane
CM
Condition Monitoring
DG
Dissolved Gas
DGA
Dissolved Gas Analysis
3
HPLC
High Performance Liquid Chromatography [Duval et al, 1977]
LTC
Load Tap Changer
OLTC
On-Load Tap Changer
PD
Partial Discharge
ppm
Parts per million
TCG
Total Combustible Gas
TDCG
Total Dissolved Combustible Gas
4
Glossary
Condition Monitoring
The monitoring a parameter of condition in machinery, such that a signi�cantchange is indicative of a developing failure. It is a component of predictivemaintenance [Wikipedia, 2008b]
Dissolved Gas Analysis
A method of measuring dissolved gases in transformer oil in order to:
1. Deduce the operating condition of the transformer
2. Estimate a transformer's future safe operating range
3. Estimate probability of transformer failure
Fault Gases
Hydrogen, Methane, Ethane, Ethylene, Acetylene, Carbon Monoxide, CarbonDioxide; which are used in DGA to diagnose transformer faults. The degreeof hydrogen unsaturation of the molecule correlates to the amount of energyreleased by the fault [Duval, 1989]
Fuller's Earth
A nonplastic clay or claylike earthy material used to purify mineral oil [Wikipedia, 2008d]
Furan
A colorless, �ammable, highly volatile liquid with a boiling point of about 31.4oC; produced by thermal decomposition of cellulose [Wikipedia, 2008g]
Hot Metal Gases
Ethylene(C2H4), Ethane (C2H6), Methane (CH4); produced when transformeroil contacts hot metal [Jakob et al, 2003]
On-Load Tap Changer
A Tap Changer that can be adjusted either manually or automatically while thetransformer is supplying power to loads
Pyrolysis
Thermal Decomposition [Wikipedia, 2008f]
5
(Beta) Scission
The initial step in the chemistry of thermal cracking of hydrocarbons and theformation of free radicals [Wikipedia, 2008i]
Tap Changer
A mechanism for selecting a desired number of transformer windings to be used
X-Wax
Solid particles of carbon and other hydrocarbon products produced by trans-former heat, partial discharge or arcing. Excessive X-Wax buildup along paperinsulation increases its dielectric dissipation factor and may result in excessiveheating and increased risk of fault occurence [IEC 60599, 2007, 4.1, 5.8].
6
Contents
1 Introduction 121.1 Concepts . . . . . . . . . . . . . . . . . . . . . . 12
1.2 Applications . . . . . . . . . . . . . . . . . . . . 16
1.3 History . . . . . . . . . . . . . . . . . . . . . . . 17
2 Sampling 182.1 Guidelines . . . . . . . . . . . . . . . . . . . . . 18
2.2 O�ine sampling . . . . . . . . . . . . . . . . . 18
2.3 Online sampling . . . . . . . . . . . . . . . . . 19
2.3.1 Manual sampling while the transformeris on . . . . . . . . . . . . . . . . . . . . 19
2.3.2 Continuous automated online sampling 19
2.4 Sampling periodicity . . . . . . . . . . . . . . . 19
2.5 DGA inaccuracy due to poor sampling tech-nique . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Gas extraction and measurement 213.1 Extraction methods . . . . . . . . . . . . . . . 21
3.2 Measurement equipment calibration . . . . . 21
3.3 DGA report guidelines . . . . . . . . . . . . . 21
4 Interpretation techniques 234.1 Guidelines . . . . . . . . . . . . . . . . . . . . . 23
4.1.1 Roger's Ratio method . . . . . . . . . 24
4.1.2 IEC 60599 Ratios method . . . . . . . 25
4.1.3 IEEE (�Key Gas�) and DornenburgRatios . . . . . . . . . . . . . . . . . . . 26
4.1.4 Duval Triangle Method . . . . . . . . . 28
4.1.5 Single ratios . . . . . . . . . . . . . . . 30
4.2 Relative diagnostic accuracies of DGA inter-pretation techniques . . . . . . . . . . . . . . . 30
4.3 DGA inaccuracies due to low DG concentra-tions and lab equipment error margins . . . . 31
4.4 Application to OLTCs . . . . . . . . . . . . . . 33
7
4.5 Predicting fault locations within the trans-former based on DGA results . . . . . . . . . 34
4.6 Headspace Fault Gas . . . . . . . . . . . . . . 34
5 Case Study 355.1 Pre-trip DG . . . . . . . . . . . . . . . . . . . 35
5.2 Post-trip DG . . . . . . . . . . . . . . . . . . . 35
5.3 Duval Triangle method applied . . . . . . . . 36
5.4 Comparison with Youngblood (Cinergy) ratios 37
6 Future developments in DGA 396.1 Online continuous monitoring . . . . . . . . . 39
6.2 New analysis methods . . . . . . . . . . . . . . 40
6.3 New DGA interpretation techniques for biode-gradeable oil . . . . . . . . . . . . . . . . . . . . 40
7 Conclusion 42
8
List of Figures
1. Figure 1: A chemcal bond diagram showing how (naphtenic) mineral oil,under heat or arcing, breaks down to form hydrocarbon gases
2. Figure [IEC 60599, 2007]: Scission energies required to produce particularcarbon-carbon bonds in fault gases
3. Figure 3: Table of DGA Fault Gases, in order of energy required to pro-duce them
4. Figure 4: Relative gas production with respect to oil temperature
5. Figure 5: Relative gas solubility with respect to temperature
6. Figure 6: Tables of dissolved Fault Gas ratios which indicate pyrolysis dueto overheating, partial discharge and arcing
7. Figure 7: Transformer oil sampling setup at drain point
8. Figure 8: Typical DGA sampling periodicity by utilities
9. Figure 10: Table of key gases used for particular DGA interpretations
10. Figure 9: Sampling syringe and 3-way valve set
11. Figure 11: Diagnostics using the Roger's Ratio approach
12. Figure 12: A �owchart approach to Roger's Ratio diagnostics
13. Figure 13: IEC 60599 Dissolved Gas Ratio Diagnostics
14. Figure 14: Simpli�ed diagnostics table for IEC 60599 interpretation
15. Figure 15: IEEE DGA diagnostic guidelines
16. Figure 16: IEEE de�nitions for transformer condition based on TDCG
17. Figure 17: IEEE recommendations for transformer operation and mainte-nance action, based on TDCG
18. Figure 18: The Duval Triangle
19. Figure 19: Comparison table of DGA interpretation technique accuracy
20. Figure 20: Plot showing DG reading errors increase as DG concentrationsget lower for factory (�) and routine (-) testing
21. Figure 21: Table showing variations in laboratory reading errors for mediumand low DG concentrations
22. Figure 22: DG concentration limits for transformers and separate OLTCtanks requiring attention
9
23. Figure 23: Cinergy Monthly Watch Criteria for DG in LTCs
24. Figure 24: Youngblood (Cinergy) 90th percentile Fault Gas Ratios
25. Figure 31: Graphical representation of IEC 60599 DGA ratios
26. Figure 25: 11kV pre-trip DG concentrations
27. Figure 26: 11kV post-trip DG concentrations
28. Figure 27:11kV DG concentration percentages
29. Figure 29: CUF-K 11kV transformer incident ratios (2008) compared toYoungblood (Cinergy) 90th percentile Fault Gas Ratios
30. Figure 28: Duval Triangle applied to case study
31. Figure 30: Proposed graphical interpretation of IEC 60599 C2H4C2H6
ratioapplied to biodegradeable oil
32. Figure 32: IEC 60599 DGA interpretation technique �owchart
10
Standards and Recommended Practice
1. ANSI C57.104-1978: Guide for the detection and determination of gener-ated gases in oil-immersed transformers and their relation to the service-ability of the equipment
2. IEEE Std C57.104-1991: IEEE Guide for the Interpretation of Gases Gen-erated in Oil-Immersed Transformers
3. IEC 60567, 2006, Oil-�lled electrical equipment � Sampling of gases andof oil for analysis of free and dissolved gases � Guidance
4. IEC 60599-1999: The Interpretation of Gases in Transformer and OtherOil-�lled Electrical Equipment in Service
11
1 Introduction
Dissolved Gas Analysis (DGA) is a method of diagnosing faults1 in electricalequipment having oil as a conductor insulator [Wikipedia, 2008a]. It may also bea good indicator of the likelihood [IEC 60599, 2007, 8.1.1] of transformer failure[Duval, 1989, Figures 8 and 9]. DGA interpretation techniques have become sosensitive and accurate that it has now become standard practice in transformeracceptance [Waukesha Electric, 2004], maintenance programs involving onlineCM [Wong, 2000, Osztermayer at al, 2003], and post-mortem equipment failurestudies [Schroeder, 2000].
This report focuses speci�cally on DGA2 and its interpretation techniques foroil-insulated power transformers, including oil at tap changers3. It also presentssampling method guidelines, gas extraction practice, maintenance recommen-dations4 based on interpretation techniques, and case studies.
1.1 Concepts
Transformer insulating oil, whether para�nic, naphthenic or aromatic containdissolved gases.
1It is also used in transformer factory acceptance tests, both before, during and after aheat run [IEC 60567, 2006, "Introduction"]
2Analysis of Furans, although not part of this report because they are considered �uids[Wikipedia, 2008g] at standard temperature and pressure, are useful in predicting buildup ofX-Wax at paper insulation that could possibly lead a future fault [IEC 60599, 2007, 5.8]. Oiladditives such as oxidation inhibitors, and the e�ect of copper sludge has on oil conductivityand tan δ are also not covered here.
3Winding and tap-changer oil should ideally be contained separately to ease troubleshoot-ing and fault location [Kramer, 2008, Reinhausen Grp, 2006].
4See Figure 17
12
Figure 1: A chemcal bond diagram showing how (naphtenic) mineral oil,under heat or arcing, breaks down to form hydrocarbon gases (Taken from[Jakob, 2008, Slide 12])
The decomposition5 (Figure 1) of transformer oil6 under heat or arcing result inmore Fault Gases7 produced8 and dissolved9 in the oil than were there originally[Blackburn, 2008]10. Both the amount and types of Fault Gases produced are
5Also referred to as �thermal breakdown� [Wikipedia, 2008a, "Faults"]6There may be accompanying paper insulation breakdown (also referred to as cellulose
breakdown, or pyrolysis [Wikipedia, 2008f]), which result in Furanic compounds produced inthe oil, which can be interpreted using IEC 61198 to estimate the extent cellulose involvmentin the fault [IEC 60599, 2007, 4.2].
7Often referred to as DGA �Fault Gases� (see Figure 3)8Gas volume production is nearly quadratic with voltage, and linear with arc duration9Until saturation limits are reached [Blackburn, 2008, Slide 5]
10X-wax may also be produced [IEC 60599, 2007, 4.1: Decomposition of Oil]
13
Bond type Scission energy required
C −H 338 kJ/moleC − C 607 kJ/moleC = C 720 kJ/mole
Figure 2: Scission energies required to produce particular carbon-carbon bondsin fault gases [IEC 60599, 2007, 4.1].
related to the energy produced by the fault (Figure 2).The quantifying and historical trending of these Fault Gas ratios with respectto industry-wide interpretation guidelines are the basis of DGA interpretation[Jakob et al, 2003].
�Key� Fault Gas Indication
Hydrogen H2 Partial Discharge, Heating, ArcingEthyleneC2H4, Ethane C2H6, Methane CH4 Hot metal
Acetylene C2H2 ArcingCarbon Monoxide CO, Carbon Dioxide CO2 Cellulose insulation degradation
Figure 3: Table of DGA Fault Gases, in order of energy required to producethem (Taken from [Jakob et al, 2003, Table 2])
By having baseline records and trending of DG content, transformer operatorsand owners would have a good indication how much their transformer oil hasdeteriorated, either due to age, overheating (see Figure 4), arcing or externalwater ingress.
14
Figure 4: Relative gas production with respect to oil temperature (Taken from[Blackburn, 2008, Slide 13])
DGA is typically done by:
1. Taking an oil sample,
2. Quantifying gas content by using techniques such as liquid chromatogra-phy [Wikipedia, 2008e], and
3. Correlating the results11 with known standards and Fault Gas12 ratio13
guidelines to gauge the transformer's internal condition
11This may include headspace Fault Gas results (see Section 4.6), should the suspect faultbe large enough to produce large enough Fault Gas volume
12The degree of hydrogen unsaturation of the molecule correlates to the amount of energyreleased by the fault [Duval, 1989]
13Gas ratios are used because Fault Gas solubility with respect to oil temperature relativeto one another appears reasonably constant (see Figure 5). Thus DGA samples may be takeneven when the transformer has cooled and their relative ratios would remain similar.
15
Figure 5: Relative gas solubility with respect to temperature (Taken from[Blackburn, 2008, Slide 6])
1.2 Applications
DGA concepts are applied to assess oil which insulates conductors, such astransformer cores and windings, and oil-insulated cables. By using DGA tech-niques, information about the equipment from which the oil sample was takenfrom can be deduced, e.g. has there been overheating (see Figure 414), has therebeen internal arcing within the oil (see Figure 6), has there been atmosphericingress. Such DGA interpretations would then be applied to estimate:
1. The transformer's most recent operating condition,
2. How much derating (or uprating) it should (or could) be safely operatedat in its remaining condition, and
3. How soon the next transformer oil change, �ltering, Fuller's Earth treat-ment or inspection should be.
14There are also other sources of gas production, such as from newly facbricated steel andvia reaction of steel with water in the oil [IEC 60599, 2007, 4.3]
16
Pyrolysis Ratios
Ethylene C2H4 60%Ethane C2H6 20%Methane CH4 20%
PD Ratios
Hydrogen H2 85%Methane CH4 5%Ethane C2H6 5%
Arcing Ratios
Acetylene C2H2 30%Hydrogen H2 60%Methane CH4 5%Ethylene C2H4 5%
Figure 6: Tables of dissolved Fault Gas ratios which indicate pyrolysis due tooverheating, partial discharge and arcing (Taken from [Blackburn, 2008, Slide16])
1.3 History
DGA has been used for transformer routine monitoring since the late 1960s[Duval, 1989]. In 2003, it was estimated that about a million DGA tests weredone annually at laboratories worldwide [Duval et al, 2003]15.
15This averages to more than 10 DGAs per minute, or > 1 DGA every 6 seconds
17
2 Sampling
2.1 Guidelines
Transformer operators may obtain gas samples from the gas relays (or Buchholzrelays), but for more accurate and early diagnosis [Wikipedia, 2008a, "Usage"],DGA by careful oil sampling, gas extraction16 [Duval et al, 2003] and chro-matography is recommended. In all cases, sampling should not endanger theoperation of the equipment [IEC 60567, 2006, 4.1].
2.2 O�ine sampling
ASTM D3613 requires that transformer oil sampling be taken via a syringe[POA, 2007] and stopcock system [Duval, 1989] from a mineral-oil insulatedtransformer's drain point to ensure no oil contact with air.
Figure 7: Transformer oil sampling setup at drain point (Taken from[Alamo Transformer, 2008])
To minimise air ingress, it is important that the syringe not be pulled forcefully,i.e. the transformer oil's natural gravity �ow should be allowed to work the oilinto the syringe [NTT, 2001a].
16Internationally recognized standards for gas extraction from oil samples are IEC 60567(see Section 3.1) and ASTM D3612.
18
2.3 Online sampling
2.3.1 Manual sampling while the transformer is on
Some oil-sampling contractors can obtain oil-samples from live transformers forDGA [IET, 2008], subject to transformer owner clearance of such practice.
2.3.2 Continuous automated online sampling
There are also manufacturers o�ering automated online sampling equipment forundissolved gases [Kelman, 2005-8]. However the gases such equipment detectfrom the headspace or Buchholz (gas) relay and are not dissolved, limiting therapidity of this CM diagnostic.
Continuous automated online sampling of DG o�ers cutting edge DGA CMaccuracy and diagnostics. This is addressed in Section 6.1.
2.4 Sampling periodicity
For new, overhauled, repaired, or newly oil-�ltered transformers, a recommendedmonthly sampling periodicity may be increased or decreased, depending onwhether DGA results show that gas content is stabilizing [Wikipedia, 2008a,"Monitoring"]. Once the DGA concentrations have acceptably stabilized, thetransformer operator may commence monitoring every one or two years. Someexamples of DGA periodicity practiced are as follows:
Operator Periodicity
NGC UK (1998) Annually [Esp et al, 1998]Petronas CUF-K (2008) Annually [Makhtar et al, 2008]Petronas MCOT (2004) Pending oil dielectric test per 2 years [Makhtar et al, 2008]
Shell (2000) Pending dielectric test per 4 yrs; nonsealed units [Shell, 2000]Syprotec (1995) Annually [Gibeault et al, 1995]Transgrid (2008) Annually [Transgrid, 2007]
Figure 8: Typical DGA sampling periodicity by utilities
2.5 DGA inaccuracy due to poor sampling technique
DGA readings may be skewed by poor sampling technique, which the samplingoperator must guard against, e.g.:
1. Contaminated oil sample containers [IET, 2008]
19
2. Not allowing some oil to �ow to clean the drain valve piping before takinga sample [NTT, 2001a]17
3. Exposing the oil to sunlight [IEC 60567, 2006, 4.1]
4. Bubble in the syringe sample, in which case resampling is recommended[IEC 60567, 2006, 4.2.2]
5. Pre-existing oxygen in the sample [IEC 60599, 2007, 4.3]
Figure 9: Sampling syringe and 3-way valve set (Taken from [NTT, 2008b])
17Oil at the bottom of the tank may also be more susceptible to contaminants & di�erentDG levels [Ward, 2003]
20
3 Gas extraction and measurement
3.1 Extraction methods
Gas extraction methods speci�ed in IEC 60567 include [IEC 60567, 2006, "Scope"]:
� Extraction by vacuum
� Extraction by displacement of the dissolved gases by bubbling a carriergas through the oil sample (stripping)
� Extraction by partitioning of gases between the oil sample and a smallvolume of the carrier gas at the head space
3.2 Measurement equipment calibration
Chromatography equipment accuracy is veri�ed and corrected by measuringgas content from calibrated gassed oil samples, known as gas-in-oil samples.Trained and competent personnel are required to maintain the calibration ofDGA extraction and chromatography equipment, which require re-calibration[IEC 60567, 2006, 8.6] prior to each sample and linearity checks [IEC 60567, 2006,9.1] every 6 months or following changes in apparatus or operating conditions.Given that di�erent labs have been known to report di�erent DGA quantities
for the sample oil sample, each DGA lab report should include the accuracymargins of the lab equipment used [Duval et al, 2005].
3.3 DGA report guidelines
IEC 60559 recommends that DGA interpretation reports should contain thefollowing [IEC 60599, 2007, 10.]:
1. Method of DGA
2. DG lab equipment sensitivity thresholds and accuracy
3. Transformer date of commissioning, rated voltage and power, sealed orvented, OLTC type, make and model
4. Oil volume, sampling date and location
5. Special incidents just before the sampling, e.g. tripping, alarm, repair,degassing, outage
6. Previous DGA results on the transformer
7. Indication of typical DG values for the equipment, inlcuding �healthy� and�fault� values, and identi�cation of the previous fault types
8. Recommended action, e.g.:
21
(a) Revised oil sampling frequency
(b) Furanic compound analysis if CO2CO < 3
(c) Other tests, inspections or maintenance
22
4 Interpretation techniques
4.1 Guidelines
DGA interpretation techniques take into account the amounts of �Key Gases�[Blackburn, 2008, "Analysis"] (see Figure 10) found in oil samples (in ppm) andtheir relative ratios to each other to arrive at conclusions about the transformercondition. These techniques include empirical approaches such as Roger's Ra-tios (IEEE - 1978)18, IEC 60599 Ratios [Blackburn, 2008], IEEE 57.104 Ratios[Jakob, 2008]19, and graphical methods such as the Duval Triangle.
Key Fault Gases Interpretation applied
C2H4, C2H6, CH4, H2 Thermal Fault: Overheated oilCO2, CO Thermal Fault: Overheated cellulose
H2, CH4, C2H6, C2H4 Electrical Fault, PD in OilH2, C2H2, C2H2, CH4, C2H6 Arcing In Oil
Figure 10: Table of key gases used for particular DGA interpretations (Takenfrom [Blackburn, 2008, "Analysis"])
The following gas ratio methods work best with diaphragm or hermeticallysealed transformers. Kan et al has also noted its applicability to nitrogen sealedtransformers with N2 replenishment, because release of fault gases are infrequentenough to treat the gas mixtures as being in equilibrium within the transformer[Kan et al, 1995, "CO2/CO value for transformers with gas space above oil",]20.It would be less accurate21 for transformers with open type conservators because:
1. The di�usion rate of H2 would be faster than the heavier Fault Gases
2. O2entering the transformer would a�ect the insulation paper's character-istics in terms of CO and CO2 retention [Kan et al, 1995, "Cases whereCO/CO2 method does not apply",].
To obtain data related to the transformer's most recent fault, the most recentDGA results should have gas concentrations from previous DGA results sub-tracted from it [IEC 60599, 2007, 6.1].
18Roger's Ratios originated from the Doernenburg method [Serveron, 2007, p8]19Also known as the �Key Gas Method� [Serveron, 2007, Table 3, p4]20Although it has been argued that the headspace gases are a�ected by headspace purging
[Woolley et al, 2994]21IEC60599 mentions there is no agreement yet on adjustment techniques to account for
gases esacping to the atmophere for ventilated transformers [IEC 60599, 2007, 6.1]
23
4.1.1 Roger's Ratio method
Based on ratios of particular fault gases, a diagnostics summary using Roger'sRatios [Rogers, 1975]22 is as follows23:
C2H6CH4
C2H4C2H6
C2H2C2H4
Diagnosis
R < 10 R < 1 R < 0 Normal deteriorationR < 1 R < 1 R < 0.5 PD activityR < 1 R < 1 R < 0.5 Heating to 150 oCR > 1 R < 1 R < 0.5 Heating 150 - 300 oCR < 1 1 < R < 3 R < 0.5 Winding circulating current and overheatingR < 1 R > 3 R < 0.5 Overheated contactsR < 1 R < 1 0.5 < R < 3 Transient arcingR < 1 R > 1 R > 0.5 Transient arcing at power frequencyR < 1 R > 3 R > 3.0 Continuous sparkingR < 1 R < 1 R > 0.5 PD and tracking
Figure 11: Diagnostics using the Roger's Ratio approach (Taken from[Blackburn, 2008, Slide 20])
A manually faster approach to Roger's Ratio diagnostics may be done using thefollowing Roger's Ratio �owchart [Jakob, 2008, Slide 26]:
22Also found in Table 7 of ANSI/IEEE C57.104-197823For DGA diagnosis involving single ratios of CO2
CO, see Section 4.1.5.
24
Figure 12: A �owchart approach to Roger's Ratio diagnostics (Taken from[Jakob, 2008, Slide 26])
4.1.2 IEC 60599 Ratios method
IEC 60599 has a similar diagnostic approach, using dissolved gas ratios [IEC 60599, 2007,5.3 (Table 2), 9, Figure 1]24:
The IEC 60599 ratios in Figure 13 above should only apply if one of the gasconcentrations exceeds typical values [IEC 60599, 2007, 5.3, Table 2 and 6.1c].The ratios may also be reproduced in a twin square, graphical format (Figure31, Appendix). In some cases, no diagnosis may be forthcoming from the above
table, in which case a simpli�ed diagnostics table may be applied:
24IEC 60599 [Duval et al, 2001] also speci�es that: (a) Faults are considered active if DGrates continue to rise at 10% per month for hermetically sealed transformers [IEC 60599, 2007,8.4]; and (b) Higher DG rates of increase such as 50% per week are considered very serious.
25
Figure 13: IEC 60599 Dissolved Gas Ratio Diagnostics (Taken from[Miranda et al, 2005, Table II])
Case C2H2C2H4
CH4H2
C2H4C2H6
Partial Discharge < 0.2Discharge (Arcing) > 0.2Thermal fault < 0.2
Figure 14: Simpli�ed diagnostics table for IEC 60599 interpretation[IEC 60599, 2007, Table 3].
For TDCG baseline acceptance criteria, IEC 60599 allows transformer opera-tors to set their own thresholds, based on forensic evidence [IEC 60599, 2007,8.2]. IEC 60599 also presents an overall �owchart for its DGA interpretation,reproduced in Figure 32, Appendix.
4.1.3 IEEE (�Key Gas�) and Dornenburg Ratios
IEEE 57.104 also has a set of ratios for DG and headspace gas25 diagnosticsthat borrows from the Dornenburg method26:
25Fault Gases from may help asses the size of a recent large fault [IEC 60567, 2006, "In-troduction"]. Headspace Fault Gas analysis is addressed more completely in IEC 60599[IEC 60599, 2007, 7.]which uses Ostwald coe�cients to convert headspace Fault Gas con-centrations to a DG equivalent.
26Found in Tables 5 and 6 of ANSI/IEEE 57.104-1978
26
Figure 15: IEEE DGA diagnostic guidelines (Taken from [Jakob, 2008, Slide29])
In addition, based on TDCG, IEEE 57.104 includes condition de�nitions (Figure16) and maintenance guidelines (Figure 17).
Figure 16: IEEE de�nitions for transformer condition based on TDCG (Takenfrom[Jakob, 2008, Slide 33])
DG concentration ratios below the above threshold values do not necessarilymean no fault is present, such results merely mean that the concentration valuesare not high enough to be able to estimate probabilities of an incipient fault[IEC 60599, 2007, 8.2.1, 8.3].
27
Figure 17: IEEE recommendations for transformer operation and maintenanceaction, based on TDCG (Taken from [Jakob, 2008, Slide 34])
For new transformer oil TDCG baseline acceptance criteria, both IEC andIEEE guidelines allow the transformer operators to set their own thresholds[Jakob, 2008, Slide 31].
4.1.4 Duval Triangle Method27
A popular [Bandhopadhyay, 2006, Field et al, 2002] method that has been notedto be more accurate [Serveron, 2007, Table 7, p10]28 than the above three,the Duval triangle29 provides DGA fault diagnosis simply based on relativepercentages of CH4, C2H4 and C2H2, where the fault codes are [Delta-X, 2008]:
� PD : Partial discharge
27Found in Appendix B of IEC 60599-1991 [Delta-X, 2008]28Caveats to this accuracy [Delta-X, 2008] : (a) Because extremely low levels are related to
decreases in detection accuracy, detected gases involved in the triangle should be reasonablyabove the detection limit; (b) Existing gases that were detected in the original oil sample(during baseline sampling or before a suspect fault) should be subtracted out from the samplepercentages to be substituted into the triangle.
29Developed by Michael Duval of Hydro Quebec [Bandhopadhyay, 2006]
28
� T1 : Low-range thermal fault (below 300 oC)
� T2 : Medium-range thermal fault (300-700 oC)
� T3 : High-range thermal fault (above 700 oC)
� D1 : Low-energy electrical discharge
� D2 : High-energy electrical discharge
� DT : Indeterminate - thermal fault or electrical discharge
Figure 18: The Duval Triangle (Taken from [Blackburn, 2008, Slide 22])
29
4.1.5 Single ratios
CO2CO
30, O2N2
31 and C2H2H2
32 ratios may indicate paper insulation involvement infaults, excessive heating or contamination by OLTC oil respectively [Serveron, 2007,p11]. However, the diagnosis accuracy of CO2
CO and O2N2
have been disputed.
The CO2CO ratio is particularly nebulous33 because of its temperature depen-
dency and CO2's higher absorption rate into insulation paper relative to COat higher temperatures [Kan et al, 1994, 5. Conclusion]. For example, one nor-mal CO2
CO range has been de�ned as > 7, with CO2CO < 3 [IEC 60599, 2007, 5.4:
CO2/CO ratio]indicating severe overheating of paper insulation [Jakob, 2008,Slide 28]. However, studies by Kan et al have proposed that incipient faults mayactually be occuring at CO2
CO ratios < 10 because CO2 gets absorbed into insulat-ing the paper [Kan et al, 1995] at higher temperatures, depriving DGA samplesof a representative CO2 concentration. In addition to the CO2
CO temperature de-
pendence skew, Kan et al also noted that a CO2CO ratio method: (a) cannot apply
accurately to new transformer oil for the �rst six months because of a lack ofchemical equilibrium, and (b) cannot apply accurately to naturally circulated oilbecause of the greater likelihood of localised hotspots [Kan et al, 1995, "Caseswhere CO2/CO method does not apply",].
When applying the O2N2
ratio, it must be noted that ransformers that have notbeen Nitrogen-purged may have signi�cant amount of Oxygen already inside[Woolley et al, 2994]. A O2
N2ratio of < 3 may indicates excessive oxygen con-
sumption within the transformer [IEC 60599, 2007, 5.5].
IEC 60599 also noted that transformers with breahing apparatus (i.e. non-hermetically sealed transformers) may have CO2 coming in from external airthat could falsify diagnostic results [IEC 60599, 2007, 5.4].
In short, DGA involving these single ratios of CO2CO and O2
N2must be supplemented
by one of the methods mentioned in Section 4.1.
4.2 Relative diagnostic accuracies of DGA interpretationtechniques
Case studies using the IEEE �Key Gases� approach have shown that [Serveron, 2007,p5]:
1. There can be high rates of incorrect diagnosis
30This ratio has been noted to vary depending on transformer model and its operationalmode [Failhauer et al, 2006]. Due to a lack of diagnostic consistency or distinguishable pat-
terns, the CO2CO
ratio was removed from a data mining technique by Esp et al [Esp et al, 1998].31Transformers that have not been Nitrogen-purged may have signi�cant amount of Oxygen
already inside [Woolley et al, 2994]32 C2H2
H2ratios higher than 2 or 3 in the main tank indicates oil contamination from the
OLTC tank [IEC 60599, 2007, 5.5]33It has also been shown that CO2 levels in headspace can �uctuate by up to 500ppm per
day [Ward et al, 2000], depending on transformer loading.
30
2. Some gas combinations do not �t into the speci�ed range of values andthus a diagnosis of the fault type cannot be given
Studies of DGA technique relative accuracies have shown that the Duval Tri-angle method has relatively good consistency and accuracy when taking into ac-count cases that are �undiagnosable� by the other methods [Muhamad et al, 2007].
Method % No Diagnoses % Wrong Diagnoses
IEEE Key Gas Method 0 58IEEE Rogers Ratios 33 35Dornenburg Ratios 26 3
IEC Basic Gas Ratios 15 8IECDuval Triangle 0 4
Figure 19: Comparison table of DGA interpretation technique accuracy (Takenfrom [Duval et al, 2005, Table 3])
Although the Dornenburg method appears more accurate than the Rogers,Thang et al noted that [Thang et al, 2000]:
� Although Dornenburg's method appears relatively more accurate, it is alsomore susceptible to a non-interpretation result.
� Although the Duval Triangle o�ers relative greater accuracy, it forces theuser into a transformer fault diagnosis because it has no area to accountfor gase ratios due to operational aging. Thus the Duval triangle shouldonly be applied after the gases have been scrutinized for normalcy.
4.3 DGA inaccuracies due to low DG concentrations andlab equipment error margins
IEC 60567 [IEC 60567, 2006, Table 5], IEC 60599 [IEC 60599, 2007, 6.2] andstudies by Duval et al have noted that DG volume reading inaccuracies increaseat very low concentrations of DG [Duval et al, 2005]:
31
Figure 20: Plot showing DG reading errors increase as DG concentrations getlower for factory (�) and routine (-) testing (Taken from [Duval et al, 2005,Figure 1])
However, correction factors may be applied to minimise this [Duval et al, 2005].Duval et al also noted that DG reading errors for controlled medium and low DGconcentration oil samples varied signi�cantly from laboratory to laboratory34:
At medium gas concentrations At low gas concentrations
Best lab ±3% ±22%Average ±15% ±30%Worst lab ±65% ±64%
Figure 21: Table showing variations in laboratory reading errors for mediumand low DG concentrations (Taken from [Duval et al, 2005, Table 2])
34IEC 60567 [IEC 60567, 2006, 9.3]mentions methods of how to minimize this, includingthe storage of oil samples in fridges
32
To reduce the likelihoods of false readings resulting in incorrect DGA diag-nosis35, both IEC 60567 [IEC 60567, 2006, 10] and Duval et al recommendthat DGA laboratories publish their lab's accuracy �gures on all test reports[Duval et al, 2005].
4.4 Application to OLTCs
The DG production pattern in an OLTC oil tank is di�erent from that of themain tank. For example, more Acetylene (C2H2
H2) is produced36 (see Section
4.1.5). DG concentration limits for �untight OLTCs� that may require attentionhave been presented by Blackburn [Blackburn, 2008, Slide 14]:
Fault Gas Transformer OLTC
Hydrogen (H2) 60 - 150 75 - 150Carbon Monoxide (CO) 540 - 900 400 - 850Carbon Dioxide (CO2) 5100 - 13000 5300 - 12000
Methane (CH4) 40 - 110 35 - 130Ethane (C2H6) 50 - 90 50 - 70Ethylene (C2H4) 60 - 280 110 - 250Acetylene (C2H2) 3 - 50 80 - 270
Figure 22: DG concentration limits for transformers and separate OLTC tanksrequiring attention (Taken from [Blackburn, 2008, Slide 14])
Guidelines also exist from CM consultants (Figure ) as well as CM contractors,who provide model-speci�c [Jakob et al, 2008, Slides 21-22]DG concentrationlimits for OLTCs.
LTC type Hydrogen H2 Acetylene C2H2 Ethylene C2H4
Vented > 1500 ppm > 1000 ppm > 1000 ppmSealed > 5000 ppm > 9000 ppm > 12000 ppmVacuum > 10 ppm > 5 ppm > 100 ppm
Figure 23: Cinergy Monthly Watch Criteria for DG in LTCs (Taken from[Jakob et al, 2008, Slide 19])
35Incorrect DGA diagnosis would also result in unnecessary downtime and resources wastedto inspect a transformer [Duval et al, 2005]
36 C2H2H2
ratios higher than 2 or 3 in the main tank indicates oil contamination from the
OLTC tank [IEC 60599, 2007, 5.5]
33
Youngblood et al of Cinergy have also provided LTC DGA ratio limits basedon empirical data [Youngblood, 2003]:
Gas ratio LimitEthylene(C2H4)Acetylene(C2H2)
0.3378Ethylene(C2H4)
Acetylene(C2H2)+Hydrogen(H2)0.500
Ethylene(C2H4)+Ethane(C2H6)+Methane(CH4)Acetylene(C2H2)+Hydrogen(H2)
0.9157Ethane(C2H6)Methane(CH4)
0.2067Ethylene(C2H4)Ethane(C2H6)
4.83
Figure 24: 90th percentile Fault Gas Ratios (Taken from [Jakob et al, 2003])
4.5 Predicting fault locations within the transformer basedon DGA results
In general, once DGA DG concentration and ratio limits indicate an increas-ing faults within the transformer, operators will arrange for the transformershutdown and inspection. Often the inspection may take weeks because thefault may not be visually apparent upon detailed winding and core inspection[Makhtar et al, 2008]. Given that equivalent heat areas of transformer cores
and winding di�er, some work has been done by Zama et al on estimating thelikelihood of the fault lying in the windings or the core, based on C2H4
C2H6ratioss
[Zama et al, 2008, Part F].
4.6 Headspace Fault Gas
Analysis of headspace gas collected from transformer Buchholz or gas relaysfor analysis [Wikipedia, 2008c] are technically not part of DGA, since theyare not dissolved, and are a�ected by headspace purging [Woolley et al, 2994]and solubility equilibrium �uctutations that may only stabilise after 15 hours[Ward et al, 2000]. However, excess Fault Gases from this headspace may behelpful in assessing the size of a recent fault [IEC 60567, 2006, "Introduction"].Headspace Fault Gas analysis is partially addressed in Figure 15, and morecompletely in IEC 60599 [IEC 60599, 2007, 7.]which uses Ostwald coe�cientsto convert headspace Fault Gas concentrations to a DG equivalent.
34
5 Case Study
A case study involving an 11kV transformer37 trip at in 2008 is referred [Makhtar et al, 2008].Pre and post trip DG results are presented, and an interpretation suggested viaapplication of the Duval Triangle method.
5.1 Pre-trip DG
DG concentrations from a sample taken on 28 March 2008 are as follows:
Figure 25: 11kV pre-trip DG concentrations (Taken from [Makhtar et al, 2008]).
5.2 Post-trip DG
DG concentrations from a sample taken on 8 July 2008 after a trip incident areas follows:
37Transformer is sealed but not Nitrogen packed, and has on o�-circuit LTC in the same oilcompartment as the transformer main winding [Makhtar et al, 2008].
35
Figure 26: 11kV post-trip DG concentrations (Taken from[Makhtar et al, 2008]).
5.3 Duval Triangle method applied
Tabulating the above results,
Gas 4ppm %
Methane (CH4) 546-2=544 34.4Ethylene (C2H4) 430 27.1Acetylene (C2H2) 607 38.4
Figure 27: 11kV DG concentration percentages (Taken from[Makhtar et al, 2008]).
and plotting the intersects,
36
Figure 28: Duval Triangle applied to case study (Taken from[Makhtar et al, 2008]).
we obtain a Duval Triangle that suggests a D2 Fault: High energy arcing withpower follow-through, and possible carbonization and metal fusion [IEC 60599, 2007,5.2].
5.4 Comparison with Youngblood (Cinergy) ratios
The post-incident DG concentration ratios are compared to failure probabilityratios given by Youngblood (Cinergy):
37
Gas ratio Limit Pre-incident Post-incident Pfailure
Ethylene(C2H4)Acetylene(C2H2)
0.3378 NILNIL 0.708 210%
Ethylene(C2H4)Acetylene(C2H2)+Hydrogen(H2)
0.500 0 0.878 176%Ethylene(C2H4)+Ethane(C2H6)+Methane(CH4)
Acetylene(C2H2)+Hydrogen(H2)0.9157 0.004 0.205 22.3%
Ethane(C2H6)Methane(CH4)
0.2067 0 0.051 24.7%Ethylene(C2H4)Ethane(C2H6)
4.83 0 15.36 318%
Figure 29: CUF-K 11kV transformer incident ratios (2008) comparedto Youngblood (Cinergy) 90th percentile Fault Gas Ratios (Taken from[Jakob et al, 2003] and [Makhtar et al, 2008])
38
6 Future developments in DGA
Some future developments in DGA include [Blackburn, 2008, Slide 33]:
� Online continuous monitoring that is more reliable and a�ordable
� New analysis methods involving gases with C3 bonds
� New DGA interpretation techniques for biodegradeable oil
6.1 Online continuous monitoring
Although not yet widespread, online DGA monitoring systems have been inuse38 since 1980 [Yamada et al, 1981]. Advantages include [Lindgren, 2003, Side20]:
1. Theoretically more accurate diagnosis, including better resolution of ratesof DG, e.g. to discriminate between faults and contamination by OLTCoil
2. The ability to uprate individual transformers and run it closer to thermallimits
3. Since lab DGA often contains air, online monitoring will have much betterresolution of actual air leaks in the transformer
4. Possible capture of DG at higher temperature solubility limits, thus negat-ing the e�ects of DGA inaccuracy at low DG concentration levels (seeFigure 20)
5. Lower-cost online or portable dissolved hydrogen concentration monitorsbeing used to decide whether a complete DGA is necessary [Belanger et al, 1977]
Some disadvantages are that [Duval et al, 2003, "On-Line Monitoring Devices"]:
1. Many of the online DGA monitors mentioned in [Duval et al, 2003, TableVI] do not detect all Fault Gases, i.e. they detect mainly H2, C2H2, CHX ,CO , CO2 which �ag arcing faults well, but are less e�ective for detectinglow and medium overheating
2. Being mostly located outdoors, they may have have poorer maintenance,resulting in poorer accuracy relative to indoor DGA lab equipment
3. Detection responses for online DGA monitors utilising fuel cells are di�er-ent for each Fault Gas
4. Engineers are still challenged to produce online DGA monitoring devicesthat are accurate, reliable and economical
Online DGA systems should use one of the more accurate extraction [Duval et al, 2003]and diagnostic techniques mentioned in Section 4.1 [Serveron, 2007, p11-12].
38Duval in 2003 estimated that about 18000 online DGA monitors had been installed world-wide [Duval et al, 2003].
39
6.2 New analysis methods
The DGA interpretation techniques mentioned in Section 4 utilize DG ratiosinvolving C1 and C2 bonds. However, IEC 60599 mentions that more preciseanalysis methods are possible if the DG ratios involving C3 bonds are scruti-nized as well [IEC 60599, 2007, 5.7]. This implies that methods such as theDuval Triangle (Section 4.1.4) could be modi�ed and re�ned to involve an anal-ogous, square shaped percentage-ratio-reference chart, given enough forensicDGA data [IEC 60599, 2007, 8.1.2] to verify the chart's demarcations and diag-nostic accuracy.
6.3 New DGA interpretation techniques for biodegrade-able oil
Studies have shown that biodegradeable oils tend to release about three timesmore ethane (C2H6) than mineral oil during thermal heating tests. This wouldresult in IEC 60599 DGA interpretations having to be modi�ed for biodegrade-able oils, in particular, the C2H4
C2H6ratio:
40
Figure 30: Proposed graphical interpretation of IEC 60599 C2H4C2H6
ratio appliedto biodegradeable oil (Taken from [IEC 60599, 2007, Annex B] and edited tore�ect ethane production data from [Muhamad et al, 2008, Fig. 8]).
41
7 Conclusion
DGA methods have been used since the 1980s to gauge transformer conditionfor:
1. factory acceptance tests,
2. site acceptance tests (e.g. for new oil),
3. predictions of remaining time to next transformer maintenance, and
4. post-mortem fault analysis
DGA sampling involves careful preparation of oil samples to ASTM D3613.
The extraction of gas from oil requires calibrated lab equipment compliant toIEC 60567.
The actual DGA interpretation technique may rely on many di�erent methodsmentioned in IEC and IEEE standards; the Duval Triangle method being oneof the more consistent methods.
Some future developments in DGA include [Blackburn, 2008, Slide 33]:
� Online continuous monitoring that is more reliable and a�ordable
� New analysis methods involving gases with C3 bonds
� New DGA interpretation techniques for biodegradeable oil
42
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49
Appendix A
Graphical representation of IEC 60599 DGA ratios39
Figure 31: Graphical representation of IEC 60599 DGA ratios (Taken from[IEC 60599, 2007, Annex B])
39Gas volume production is nearly quadratic with voltage, and linear with arc duration
50
Appendix B
IEC 60599 DGA interpretation �owchart
Figure 32: IEC 60599 DGA interpretation technique �owchart (Taken from[IEC 60599, 2007, Figure 1])
51
