82396840 DGA and Duval Triangle

7/22/2019 82396840 DGA and Duval Triangle

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Dissolved gas analysis

and the Duval Triangle

by Michel Duval


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-DGA is for Dissolved Gas Analysis.

-Still today, DGA is probably the most powerful toolfor detecting faults in electrical equipment in

service.

-Over one million DGA analyses are performed

each year by more than 400 laboratoriesworldwide.


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-Gases in oil always result from the decompositionof electrical insulation materials (oil or paper), as a

result of faults or chemical reactions in theequipment.

-for example, oil is a molecule of hydrocarbons,i.e., containing hydrogen and carbon atoms,linked by chemical bonds (C-H, C-C).

-some of these bonds may break and formH*,CH3*, CH2* and CH*radicals.


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All these radicals then recombine to form the faultgases observed in oil:


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-in addition to these gases, the decomposition ofpaper produces CO2, CO and H2O, because of thepresence of oxygen atoms in the molecule ofcellulose:


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Hydrogen H2

Methane CH4

Ethane C2H6

Ethylene C2H4

Acetylene C2H2

Carbon monoxide CO

Carbon dioxide CO2

Oxygen O2

Nitrogen N2

The main gases analyzed by DGA


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-some of these gases will be formed in larger orsmaller quantities depending on the energy contentof the fault.

-for example, low energy faults such as coronapartial discharges in gas bubbles, or lowtemperature hot spots, will form mainly H2 andCH4.


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-faults of higher temperatures are necessary toformlarge quantities of C2H4.

-and finally, it takes faults with a very high energycontent, such as in electrical arcs, to formlarge

amounts of C2H2.

-by looking at the relative proportion of gases

in the DGA results it is possible to identify thetype of fault occurring in a transformer inservice.


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2.Discharges of low energy (D1)

-typical examples are partial discharges of thesparking-type, inducing pinholes or carbonizedpunctures in paper.

-or low-energy arcing, inducing carbonizedperforations or surface tracking of paper, orcarbon particles in oil.


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3.Discharges of high energy (D2)

-typical examples are high energy arcing,flashovers and short circuits, with power follow-through, resulting in extensive damage to paper,

large formation of carbon particles in oil, metalfusion, tripping of the equipment or gas alarms .


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4.Thermal faults of temperatures < 300 C (T1)

Faults T1 are evidenced by paper turning:-brown (> 200 C).

-black or carbonized (> 300 C).Typical examples are overloading, blocked oilducts, stray flux in beams


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5.Thermal faults of temperatures between 300 and

700C (T2)

Faults T2 are evidenced by :

-carbonization of paper.-formation of carbon particles in oil.

Typical examples are defective contacts or welds,

circulating currents.


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6.Thermal faults of temperatures > 700C (T3)

Faults T3 are evidenced by :-extensive formation of carbon particles in oil.-metal coloration (800 C) or metal fusion(> 1000 C).

Typical examples are large circulating currents

in tank and core, short circuits in laminations.


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The first one was the Dornenburg method inSwitzerland in the late 1960s, then the Rogersmethod in UK in the mid 1970s.

Variations on these methods have later beenproposed by the IEC (60599) and IEEE.

Several diagnosis methods have been proposedto identify these faults in service.


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Depending on the values of these gas ratios, codesor zones are defined for each type of fault.

One drawback of these methods is that nodiagnosis can be given in a significant number of

cases, because they fall outside the defined zones.

All these methods use 3 basic gas ratios: (CH4/H2,C2H2/C2H4 and C2H6/C2H4).


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Another method used by IEEE is the so-called key-

gas method, which looks at the main gas formedfor each fault, e.g, C2H2 for arcing.

One drawback of this method is that it oftenprovides wrong diagnoses.


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Finally, there is the Triangle method, which was

developed empirically in the early 1970s, and isbased on the use of 3 gases (CH4, C2H4 and C2H2)corresponding to the increasing energy levels of gasformation.

One advantage of this method is that it alwaysprovides a diagnosis, with a low percentage of

wrong diagnoses.


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Comparison of diagnosis methods.

% Unresolveddiagnoses

% Wrongdiagnoses

% Total

Key gases 0 58 58

Rogers 33 5 38

Dornenburg 26 3 29

IEC 15 8 23

Triangle 0 4 4


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However, many people are not quite familiar withthe use of triangular coordinates, so I will try toexplain that in more detail today.

The triangle representation also allows to easilyfollow graphically and visually the evolution of faultswith time.


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The triangle method.


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The triangle method plots the relative % of CH4,C

2H

4and C

2H

2on each side of the triangle, from

0% to 100%.

The 6 main zones of faults are indicated in the

triangle, plus a DT zone (mixture of thermal andelectrical faults).


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Question: how corona PDs, which form a lot of H2,can be identified in the Triangle without using thisgas ?

Answer: in such faults, CH4 is formed in smalleramounts than H2 (typically 10 to 20 times less),but it can still be measured easily by DGA.


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Answer: because CH4 provides better overalldiagnoses for all types of faults.

Another question: in the Triangle, why not useH2 rather than CH4 to represent low energyfaults ?

A possible explanation (?): H2 diffuses muchmore rapidly than hydrocarbon gases from

transformer oil. This will affect gas ratios using H2but not those using hydrocarbon gases.


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So, how to use the triangle ?

First calculate: CH4 + C2H4 + C2H2 = 300 ppm.

If for example the DGA lab results are:

CH4 = 100 ppmC2H4 = 100 ppmC2H2 = 100 ppm


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Then calculate the relative % of each gas:relative % of CH4 = 100 / 300 = 33,3 %relative % of C2H4 = 100 / 300 = 33,3 %

relative % of C2H4 = 100 / 300 = 33,3 %

These values are the triangular coordinates tobe used on each side of the triangle.

To verify that the calculation was done correctly,the sumof these 3 values should always give100%, and should correspond to only one point

in the triangle.


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Each DGA analysis received from the lab willalways give only one point in the triangle.

The zone in which the point falls in the Triangle willidentify the fault responsible for the DGA results.


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The calculation of triangular coordinates can easily

be done manually, or with the help of a smallalgorithm or software.

Errors are often made when developing such an

algorithm, so check it first with the free softwareavailable from [email protected].


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For those familiar with computer graphics, it is also

possible to develop a software displaying the pointand the fault zones graphically in the triangle.

Several commercial software are available for thatpurpose, e.g., from Serveron, Kelman or Delta-XResearch in Canada.


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.The Triangle, being a graphical method, allowsto easily follow the evolution of faults with time,for instance froma thermal fault to a potentiallymuch more severe fault such as D2.


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.


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Fault zones in the triangle have been defined byusing a large number of cases of faultytransformers in service which had been inspected

visually.


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Cases of faults PD and D1

tracking;U sparking;{ small arcing.


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Cases of faults D2


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circulating currents ;{ laminations ;U bad contacts

Cases of thermal faults in oil only


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{ brownish paper ; carbonized paper ;U not mentioned

Cases of thermal faults in paper


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A popular ratio used for that purpose is the CO2 /

CO ratio.

If the CO2 / CO ratio is < 3, this is a strong

indication of a fault in paper, either a hot spotor electrical arcing.

A fault in paper is generally considered as moreserious than a fault in oil only, because paper isoften placed in a HV area (windings, barriers).


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The CO2 / CO ratio, however, is not very accurate,because it is also affected by the background of

CO2 and CO coming from oil oxidation.

The amounts of furans in oil may also be used insome cases to confirm paper involvement,however, the interpretation of results is often

difficult.


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.Other useful gas ratios:

-C2H2/ H2 : a ratio > 3 in the main tank indicatescontamination by the LTC compartment

-O2/ N2: a decrease of this ratio indicates excessiveheating (< 0.3 in breathing transformers).


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.

Gassing not related to faults in service:

-Catalytic reactions on metal surfaces: formationof H2 only.

-Straygassing of oil: the unexpected gassing of

oil at relatively low temperatures (80 to 200 C).


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Stray gassing after 16hours of test at 120C,

in ppm:

.

Oil H2 CH4 C2H4 C2H6 C2H2 CO CO2

Non-stray gassing 3 1 - - - 3 43

Strongly stray gassing 1088 172 11 27 - 500 1880

in ppm


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It has been found at CIGRE that stray gassing:

. -may interfere with DGA diagnoses in serviceonly in the case of the most stray gassing oils,or under overloading conditions.

- will not interfere with diagnoses during factorytests.


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.


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Now, a critical look at DGA results coming fromthe laboratory.

DGA labs are not perfect. Like everyone elsethey will sometimes make mistakes, and someare not as accurate as we expect them to be.

Laboratory accuracy, however, has a direct

effect on diagnosis accuracy and on diagnosisuncertainty.


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The accuracy of the averagelab has been foundby CIGRE to be 15% at medium (routine) gasconcentration levels (> 10 ppmfor hydrocarbons).

Accuracy will thus fall to ~ 30% at 6 ppm, and

100% near the detection limit.

Accuracy decreases rapidly as gas concentrationdecreases, following approximately the equation:

15% 2 ppm(detection limit).


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Effect of laboratory accuracy (15% and 30%,respectively) on DGA diagnosis uncertainty.


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When an area of uncertainty crosses several faultzones in the triangle, a reliable diagnosis cannotbe given.

Lab accuracies worse than 30% in general willprovide unreliable or totally wrong diagnoses.


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Diagnosis uncertainty corresponding to labaccuracies of 15, 30, 50 and 75 %:


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Accuracy of laboratories at medium gas concentrations


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Accuracy of laboratories at low gas concentrations


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Users should ask their DGA labs to indicate theaccuracy of their DGA results, to be able to

calculate the uncertainty on the diagnoses.

To verify the accuracy of routine DGA analyses,users should also from time to time send to thelab a blindsample of gas-in-oil standard.


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Such gas-in-oil standards are now availablecommercially, e.g., from Morgan Schaffer inCanada

They can also be prepared by the laboratory,

following procedures or concepts described inIEC 60567 or ASTM D3612.


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Inaccurate DGA results, whatever their cost, low

or high, are a waste of money since they cannotbe used reliably.

Furthermore, they may lead to wrong diagnoses,with possibly serious consequences for the

equipment.


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A similar investigation is presently underway at

CIGRE TF15 to evaluate the accuracy of on-lineand portable gas monitors.


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A recommendation of CIGRE and the IEC is that

DGA diagnosis should be attempted only if gasconcentrations or rates of gas increase in oil arehigh enough to be considered significant.

Low gas levels may be due to contamination oraging of insulation, not necessarily to an actual

fault.

Gas levels in service


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Also, there is always a small level of gases in

service, and it would not be economically viableto suspect all pieces of equipment.

It is better to concentrate on the upper percentileof the transformer population with the highestgas levels.


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This is the philosophy behind the use of 90%typical concentrations and 90% typical rates ofincrease, in order to concentrate maintenance

efforts on the 10% of the population most at risk.

A lot of work has been done recently at CIGRE

and the IEC in these areas, and a consensusreached on typical values observed in serviceworldwide.


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Ranges of 90 % typical concentration valuesfor power transformers, in ppm:

C2H2 H2 CH4 C2H4 C2H6 CO CO2

All transformers 50-

150

30-

130

60-

280

20-

90

400-

600

3800-

14000

No OLTC 2-20

Communicating

OLTC

60-280


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Ranges of 90 % typical rates of gas increasefor power transformers, in ppm/year:

C2H2 H2 CH4 C2H4 C2H6 CO CO2

All transformers 35-

132

10-

120

32-

146

5-

90

260-

1060

1700-

10,000

No OLTC 0-4

Communicating

OLTC

21-37


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90% typical values are within the same range onall networks, with some differences related tothe individual loading conditions, equipmentused, weather, etc.


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Influence of some parameters on typical values:

-Typical values are significantly higher in youngequipment (suggesting there are some unstablechemical bonds in new oil and paper ?).-A bit higher in very old equipment.

-Significantly lower in instrument transformers.-Higher in shell-type and shunt reactors(operating at higher temperatures ?).

-Not affected by oil volume (suggests that largerfaults are formed in larger transformers ?).


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When DGA results are above typical values:

-a diagnosis may be attempted to identify the

fault producing these gases.

-the equipment should not be considered at risk.

-however, the equipment should be monitoredmore frequently by DGA.


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The typical values surveyed by CIGRE are rangesof values observed worldwide on a large numberof networks.

Each individual network should preferably

calculate its own specific typical values.

To calculate typical concentration values, the

cumulative number of analyses should be drawnas a function of concentration, for each gas.


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Cumulative number of DGA analyses, in %vs. gas concentration, in ppm

T = the 90% typicalconcentration value


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As long as DGA values in service remain relativelyclose to typical values, there is no reason to beconcerned by the condition of the transformer.

To evaluate how much at risk a transformer maybecome above typical values, the probability offailure in service (PFS) has to be examined.

PFS has been defined as the number of DGAanalyses followed by a failure-related event(e.g., tripping, fault gas alarm, fire, etc), dividedby the total number of analyses, at a given gasconcentration.

Probability of having a failure-related event ( PFS, % )


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90 98 99 Norm, in %vs. the concentration of C2H2 in ppm

100 300 400 ppm

PFS, in %


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The PFS remains almost constant below andabove the 90% typical value, until it reaches aninflexion point on the curve (pre-failure value).

DGA monitoring should be done more and morefrequently as gas concentrations increase fromtypical to pre-failure value.


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Pre-failure values were found by CIGRE to besurprisingly close on different networks,

H2 CH4 C2H4 C2H6 C2H2 CO

240-

1320

270-

460

700-

990

750-

1800

310-

600

984-

3000

(in ppm)

This suggests that failure occurs when a criticalamount of insulation is destroyed.


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In-between typical and pre-failure values, specificalarm values can be defined, depending on thetolerance to risk of the maintenance personnel,also on the maintenance budget available.

For example, higher alarm values may be usedwhen the maintenance budget is low, and loweralarm values in the case of strategic equipment.

Summary of typical, alarm and pre-failure values:


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Summary of typical, alarm and pre failure values:

Concentration

Time


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82396840 DGA and Duval Triangle

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