practicing dga - diagnóstico dga

DIAGNOSIS GASESDISSOLVED GAS ANALYSIS

(DGA) is the single most comprehensive asset condition assessment

and management tool for oil-fi lled transformers. DGA offers

advanced detection of incipient fault conditions leading to almost

all of the failure modes listed below.

Chart Source: William H. Bartley, P.E.

The Hartford Steam Boiler Inspection and Insurance Co.

METHODSOIL COLLECTION

Manual Collection - A small volume of oil is collected for laboratory

analysis and transferred into a gas-tight container from a dedicated

fi tting and then transported to the laboratory. ASTM method D3613

details procedures for oil sample handling.

On-Line Collection - In the case of On-Line Transfomer Monitors

a small volume of oil is continuously circulated through the monitor

and then returned to the transformer. The circulating oil is sampled

and analyzed for gas content. On-Line Monitors offer a closed-loop

repeatable oil collection process.

GAS EXTRACTION

Dissolved gases are present in transformer oil at concentrations from

less than 1 part-per-million (ppm) up to a few percent of oil volume.

ASTM method D3612 specifi es three ways to separate the relatively

small amount of dissolved gases from the oil.

METHOD A – Introduce the oil sample into a pre-evacuated known

volume. The evolved gases are compressed to atmospheric pressure

and the total volume measured. The gases are then analyzed by gas

chromatography.

METHOD B – Sparging the oil with a carrier gas on a stripper column

containing a high surface area bead. The gases are then fl ushed

from the stripper column into a gas chromatograph for analysis.

METHOD C – Bring an oil sample in contact with a gas phase

(headspace) in a closed vessel purged with argon. The dissolved

gases contained in the oil are then equilibrated in the two phases in

contact under controlled conditions (in accordance with Henry’s law).

At equilibrium, the headspace is over pressurized with argon and the

content of a loop is fi lled by the depressurization of the headspace

against the ambient atmospheric pressure. The gases contained in

the loop are then introduced into a gas chromatograph.

STANDARDS AND GUIDELINES GOVERNINGDISSOLVED GAS ANALYSIS

REFERENCE DESCRIPTION

IEEE Std. C57.104.1991 IEEE Guide for the Interpretation of Gases Generated in Oil Immersed Transformers

IEEE PC57.104 Draft 11d Draft Guide for the Interpretation of Gases in Oil Immersed Transformers

IEEE Std. C57.12.80-2002 Terminology for Power and Distribution Transformers

IEC 60599-1999 Mineral Oil Impregnated Electrical Equipment in Service: Guide to the Interpretation of Dissolved and Free Gas Analysis

IEC 60599-1999-03 Reference to Duval Triangle Diagnostic Model and C

2H

2/H

2 Ratio Interpretation

STANDARDS AND GUIDELINES GOVERNINGGAS EXTRACTION FROM OIL

REFERENCE DESCRIPTION

ASTM D2945-90 (2003) Standard Test Method for Gas Content of Insulating Oils

ASTM D3305-95 (1999) 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 D2759-2000 Standard Practice for Sampling Gas from a Transformer under Positive Pressure

IEC 60567-1992 Guide for the sampling of gases and of oil from oil-fi lled electrical equipment and for the analysis of free and dissolved gases www.serveron.com

DGA

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Lightning

Through Faults

Insulation Deterioration

Inadequate Maintenance

Moisture

Loose Connections

Workmanship

Overloading

All Others

ANALYSIS

ASTM method D3612 and IEC 60567, specifi es gas chromatography (GC) as the analysis method.

The GC results are calibrated to known gas standards and normalized to standard temperature and

pressure levels so that data obtained under different conditions may be compared meaningfully.

Gas chromatography separates each gas from the others and directly measures their concentrations

individually. When recorded over time, the resulting detector signal is called a chromatogram.

Gas CARBON MONOXIDE

Formula CO

Structure

Molecular Weight 28.010

Solubility in Oil @ 25˚C 7.52:1


Temperature at which Gas forms signifi cant amount

105˚ - 300˚C(complete decomposi-tion & carbonization

occurs > 300˚C)

Source of Gas

Thermal fault involving cellulose (paper, press-board, wood blocks);

slowly from oil oxidation

Gas METHANE

Formula CH4

Structure





<150˚ - 300˚ C

Source of Gas

Corona partial-discharge; low &

medium temperature thermal faults

Gas OXYGEN

Formula O2

Structure





Following drop in oiltemperature (vacuum)

Source of Gas

Exposure to atmosphere(air); leaky gasket (under vacuum); air-breathing

conservator; leaky bladder

Gas HYDROGEN

Formula H2

Structure




Temperature atwhich Gas formssignifi cant amount

<150˚C for “cold plasma” ionization;

(corona in oil) >250˚C for thermal& electrical faults

Source of Gas

Partial-discharge;thermal faults; power

discharges; rust, galva-nized parts; stainless

steel; sunlight

Gas ETHYLENE

Formula C2H4

Structure


Solubility in Oil @ 25˚C 1:1.76



300˚ - 700˚C

Source of Gas High-temperaturethermal fault

Gas ACETYLENE

Formula C2H2

Structure





>700˚C

Source of Gas

Very hot spot;low- energy discharge

(spitting fromfl oating part); high-

energy discharge (arc)

Gas ETHANE

Formula C2H6

Structure




Temperature atwhich Gas formssignifi cant amount

200˚ - 400˚C

Source of Gas Low & medium

temperaturethermal faults

Gas CARBON DIOXIDE

Formula CO2

Structure





105˚ - 300˚C

Source of Gas

Normal aging (accelerated by amount of O

2-in-oil

& H2O-in-paper); thermal

fault involving cellulose (paper, pressboard, wood

blocks); accumulationfrom oil oxidation

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INDICATION / FAULT GAS CO CO2 CH4 C2H2 C2H4 C2H6 O2 H2 H2O

Cellulose aging

Mineral oil decomposition

Leaks in oil expansion systems,gaskets, welds, etc.

Thermal faults – Cellulose

Thermal faults in Oil@ 150°C - 300°C

TRACE

Thermal faults in Oil@ 300°C - 700°C

TRACE

Thermal faults in Oil@ >700°C

Partial Discharge TRACE

Arcing

Guidelines for surveillance range1 for Type 1 transformers(IEEE PC57.104 D11d)

N <350C 350 - 570

W >570

N <120C 120 - 400

W >400

N <2C 2 - 5W >5

N <50C 50 - 100

W >100

N <65C 65 - 100

W >100

N <100C 100 - 700

W >700

1ppm for Normal (N), Caution (C), Warning (W) – alarm thresholds

KEY GAS METHOD (IEEE PC57.104 D11d)

KEY GAS FAULT TYPE TYPICAL PROPORTIONS OF GENERATEDCOMBUSTIBLE GASES

C2H4 Thermal oilMainly C

2H

4Smaller proportions of C

2H

6, CH

4, and H

2

Traces of C2H

2 at very high fault temperatures

CO Thermal oiland cellulose

Mainly COMuch smaller quantities of hydrocarbon

gases in same proportions as thermal faultsin oil alone.

H2

Electrical Low Energy Partial

Discarge

Mainly H2

Small quantities of CH4

Traces of C2H

4 and C

2H

6

H2 & C2H2

Electrical High Energy (arcing)

Mainly H2 and C

2H

2Minor traces of CH

4, C

2H

4, and C

2H

6Also CO if cellulose is involved

TDCG METHOD (IEEE PC57.104 D11d)

SURVEILLANCERANGE

TDCGLEVEL IN PPM

DAILY RATEOF CHANGE1

SUGGESTED OPERATORGUIDELINES

SAMPLINGINTERVAL

OPERATINGPROCEDURE

Normal <700

<0.3% Normal Continue normaloperation

≥0.3%, ≤0.5% Monthly Caution: Checkload dependence

Caution 700 to1,900

>0.5%, ≤3% Monthly Caution: Checkload dependence;

advise manufacturer or insurer

≥3%, <7% Weekly

>7% Daily

Warning >1,900

<7% Weekly Extreme caution:Plan outage; advise

manufacturer or insurer>7% Daily

12% of change from initial sample, per day

CIGRE SC15

New Guidelines for Interpretation of Dissoved Gas Analysis in Oil-Filled Transformers, (ELECTRA No. 186 October 1999)

NAME RATIO VALUESIGNIFICANCE INDICATION

KEY RATIO #1 C2H

2/C

2H

6>1 Discharge

KEY RATIO #2 H2/CH

4>10 Partial Discharge

KEY RATIO #3 C2H

4/C

2H

6>1 Thermal Fault in Oil

KEY RATIO #4 CO2/CO

>10 indicates overheating of cellulose <3 indicates degradation of cellulose

by electrical fault

CellulosicDegradation

KEY RATIO #5 C2H

2/H

2

>2 (>30 ppm) indicates diffusion from OLTC

or through a common conservator

In Tank LoadTap Changer

BASIC GAS RATIOS (IEC 60599-1999)

C2H2/C2H4 CH4/H2 C2H4/C2H6

SUGGESTEDFAULT TYPE

NS1 <0.1 <0.2Partial

Discharge(PD)

>1.0 0.1 - 0.5 >1.0Discharge of low energy

(D1)

0.6 - 2.5 0.1 - 1.0 >2.0Discharge of high energy

(D2)

NS1 >1.0 <1.0 Thermal fault, <300ºC (T1)

<0.1 >1.0 1.0 - 4.0Thermal fault,

<300ºC – <700ºC (T2)

<0.2 >1.0 >4.0 Thermal fault,>700ºC (T3)

1Non-signifi cant regardless of value

PARTITIONING

Each gas has a temperature-dependent affi nity (solubility) for the

oil; the hydrocarbon gases such as methane and ethane are more

strongly dissolved in oil while fi xed gases such as hydrogen or

nitrogen are less strongly dissolved. As temperatures increase, the

fi xed gases are more strongly dissolved while the hydrocarbon gases

are less strongly dissolved.

The process of reaching equilibrium is called partitioning, and the

fi nal gas-to-oil concentration ratio is called the solubility coeffi cient.

This ratio must be known accurately at the temperature of the oil

sample undergoing analysis. Once the gases are analyzed by gas

chromatography the original gas-in-oil concentrations are calculated

from the gas-in-oil solubility coeffi cients in the table to the left.

The Key to Transformer Management

IEEE PC57.104 D11d

NAME RATIO VALUESIGNIFICANCE INDICATION

CO2 vs. CO Ratio CO

2/CO

<3 Excessive>7 - <10 Normal

>10 Excessive

Thermal CellulosicDegradation

Note: Ratio valid when levels exceed minimums: CO >500 ppm; CO2 >5,000 ppm

Graphical Representation Applicable to IEEE PC57.104 D11d Rogers Ratios

IEC 60599 (1999-03 Annex B.2 Basic Gas Ratios)

ROGERS RATIOS (IEEE PC57.104 D11d)

Ratio 1 Ratio 2 Ratio 3 SUGGESTEDFAULT TYPECH4/H2 C2H2/C2H4 C2H4/C2H6

<0.1 <0.01 <1.0 Case 0: Normal

≥0.1, <0.5 ≥1.0 ≥1.0Case 1: Discharge

of low energy

≥0.1, <1.0 ≥.0.6, <3.0 ≥2.0Case 2: Discharge

of high energy

≥1.0 <0.01 <1.0 Case 3: Thermal fault,low temp <300ºC

≥1.0 <0.1 ≥1.0, <4.0Case 4: Thermal fault,

<700ºC

≥1.0 <0.2 ≥4.0Case 5: Thermal fault,

>700ºC

C2H2/H2 RATIO (IEC 60599-1999)

OLTC’s (On-Load Tap Changers) produce gases corresponding to dischargesof low energy. The pattern of oil decomposition in the OLTC differs from the pattern of oil decomposition in the main tank resulting from low energydischarges. If oil or gas contamination (communication) exists between theOLTC and the main tank, an incorrect diagnosis of the main tank may result.

A C2H

2/H

2 ratio ≥3.0 in the main tank indicates possible OLTC contamination.

DUVAL TRIANGLE (IEC 60599-1999-03 Annex B.3)

This method uses three ratios to locate the point within the triangle.

%CH4 = CH4/(CH4 +C2H4+C2H2) x 100

%C2H4 = C2H4/(CH4 +C2H4+C2H2) x 100

%C2H2 = C2H2/(CH4+C2H4+C2H2) x 100

Sections within the triangle designate:

Zone INDICATION

T1 Thermal fault ≤300˚C

T2 Thermal fault >300˚C, ≤700˚C

T3 Thermal fault >700˚C

D1 Discharges of low-energy

D2 Discharges of high-energy

DT Combination of thermal faults and discharges

PD Partial discharge

Note: Ratio based diagnostic tools should be calculated only if at least one of the gas values is above typical concentration values and typical rates of change for the type

of equipment. Indications obtained should be viewed only as guidance and any resulting action should be undertaken only with proper engineering judgment.

PRACTICING DGA

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