Practicing DGA - Diagnóstico DGA

1. DIAGNOSISGASES
DISSOLVED GAS ANALYSIS
(DGA) is the single most comprehensive asset condition assessment
and management tool for oil-filled 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.
METHODS
OIL COLLECTION
Manual Collection - A small volume of oil is collected for laboratory
analysis and transferred into a gas-tight container from a dedicated
fitting 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 specifies 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 flushed
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 filled 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 GOVERNING
DISSOLVED 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
C2
H2
/H2
Ratio Interpretation
STANDARDS AND GUIDELINES GOVERNING
GAS 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-filled
electrical equipment and for the analysis of free and
dissolved gases
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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, specifies 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
Solubility in Oil @ 100˚C 8.33:1
Temperature at which
Gas forms significant
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
Molecular Weight 16.043
Solubility in Oil @ 25˚C 2.28:1
Solubility in Oil @ 100˚C 2.27:1
Temperature at which
Gas forms significant
amount
<150˚ - 300˚ C
Source of Gas
Corona partial-
discharge; low &
medium temperature
thermal faults
Gas OXYGEN
Formula O2
Structure
Molecular Weight 31.999
Solubility in Oil @ 25˚C 5.59:1
Solubility in Oil @ 100˚C 5.88:1
Temperature at which
Gas forms significant
amount
Following drop in oil
temperature (vacuum)
Source of Gas
Exposure to atmosphere
(air); leaky gasket (under
vacuum); air-breathing
conservator; leaky bladder
Gas HYDROGEN
Formula H2
Structure
Molecular Weight 2.016
Solubility in Oil @ 25˚C 17.92:1
Solubility in Oil @ 100˚C 13.51:1
Temperature at
which Gas forms
significant 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 C2
H4
Structure
Molecular Weight 28.054
Solubility in Oil @ 25˚C 1:1.76
Solubility in Oil @ 100˚C 1:1.47
Temperature at which
Gas forms significant
amount
300˚ - 700˚C
Source of Gas
High-temperature
thermal fault
Gas ACETYLENE
Formula C2
H2
Structure
Molecular Weight 26.038
Solubility in Oil @ 25˚C 1:1.22
Solubility in Oil @ 100˚C 1.08:1
Temperature at which
Gas forms significant
amount
>700˚C
Source of Gas
Very hot spot;
low- energy discharge
(spitting from
floating part); high-
energy discharge (arc)
Gas ETHANE
Formula C2
H6
Structure
Molecular Weight 30.069
Solubility in Oil @ 25˚C 1:2.59
Solubility in Oil @ 100˚C 1:2.09
Temperature at
which Gas forms
significant amount
200˚ - 400˚C
Source of Gas
Low & medium
temperature
thermal faults
Gas CARBON DIOXIDE
Formula CO2
Structure
Molecular Weight 44.010
Solubility in Oil @ 25˚C 1:1.17
Solubility in Oil @ 100˚C 1:1.02
Temperature at which
Gas forms significant
amount
105˚ - 300˚C
Source of Gas
Normal aging (accelerated
by amount of O2
-in-oil
& H2
O-in-paper); thermal
fault involving cellulose
(paper, pressboard, wood
blocks); accumulation
from 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 <350
C 350 - 570
W >570
N <120
C 120 - 400
W >400
N <2
C 2 - 5
W >5
N <50
C 50 - 100
W >100
N <65
C 65 - 100
W >100
N <100
C 100 - 700
W >700
1
ppm for Normal (N), Caution (C), Warning (W) – alarm thresholds
KEY GAS METHOD (IEEE PC57.104 D11d)
KEY GAS FAULT TYPE
TYPICAL PROPORTIONS OF GENERATED
COMBUSTIBLE GASES
C2
H4
Thermal oil
Mainly C2
H4
Smaller proportions of C2
H6
, CH4
, and H2
Traces of C2
H2
at very high fault temperatures
CO
Thermal oil
and cellulose
Mainly CO
Much smaller quantities of hydrocarbon
gases in same proportions as thermal faults
in oil alone.
H2
Electrical Low
Energy Partial
Discarge
Mainly H2
Small quantities of CH4
Traces of C2
H4
and C2
H6
H2
& C2
H2
Electrical High
Energy (arcing)
Mainly H2
and C2
H2
Minor traces of CH4
, C2
H4
, and C2
H6
Also CO if cellulose is involved
TDCG METHOD (IEEE PC57.104 D11d)
SURVEILLANCE
RANGE
TDCG
LEVEL IN PPM
DAILY RATE
OF CHANGE1
SUGGESTED OPERATOR
GUIDELINES
SAMPLING
INTERVAL
OPERATING
PROCEDURE
Normal <700
<0.3% Normal
Continue normal
operation
≥0.3%, ≤0.5% Monthly
Caution: Check
load dependence
Caution
700 to
1,900
>0.5%, ≤3% Monthly
Caution: Check
load 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
1
2% 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
VALUE
SIGNIFICANCE
INDICATION
KEY RATIO #1 C2
H2
/C2
H6
>1 Discharge
KEY RATIO #2 H2
/CH4
>10 Partial Discharge
KEY RATIO #3 C2
H4
/C2
H6
>1 Thermal Fault in Oil
KEY RATIO #4 CO2/CO
>10 indicates overheating
of cellulose <3 indicates
degradation of cellulose
by electrical fault
Cellulosic
Degradation
KEY RATIO #5 C2
H2
/H2
>2 (>30 ppm) indicates
diffusion from OLTC
or through a common
conservator
In Tank Load
Tap Changer
BASIC GAS RATIOS (IEC 60599-1999)
C2
H2
/C2
H4
CH4
/H2
C2
H4
/C2
H6
SUGGESTED
FAULT TYPE
NS1 <0.1 <0.2
Partial
Discharge
(PD)
>1.0 0.1 - 0.5 >1.0
Discharge of
low energy
(D1)
0.6 - 2.5 0.1 - 1.0 >2.0
Discharge of
high energy
(D2)
NS1 >1.0 <1.0
Thermal fault,
<300ºC (T1)
<0.1 >1.0 1.0 - 4.0
Thermal fault,
<300ºC –
<700ºC (T2)
<0.2 >1.0 >4.0
Thermal fault,
>700ºC (T3)
1
Non-significant regardless of value
PARTITIONING
Each gas has a temperature-dependent affinity (solubility) for the
oil; the hydrocarbon gases such as methane and ethane are more
strongly dissolved in oil while fixed gases such as hydrogen or
nitrogen are less strongly dissolved. As temperatures increase, the
fixed gases are more strongly dissolved while the hydrocarbon gases
are less strongly dissolved.
The process of reaching equilibrium is called partitioning, and the
final gas-to-oil concentration ratio is called the solubility coefficient.
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 coefficients in the table to the left.
The Key to Transformer Management
IEEE PC57.104 D11d
NAME RATIO
VALUE
SIGNIFICANCE
INDICATION
CO2 vs. CO
Ratio
CO2
/CO
<3 Excessive
>7 - <10 Normal
>10 Excessive
Thermal Cellulosic
Degradation
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 SUGGESTED
FAULT TYPECH4
/H2
C2
H2
/C2
H4
C2
H4
/C2
H6
<0.1 <0.01 <1.0 Case 0: Normal
≥0.1, <0.5 ≥1.0 ≥1.0
Case 1: Discharge
of low energy
≥0.1, <1.0 ≥.0.6, <3.0 ≥2.0
Case 2: Discharge
of high energy
≥1.0 <0.01 <1.0
Case 3: Thermal fault,
low temp <300ºC
≥1.0 <0.1 ≥1.0, <4.0
Case 4: Thermal fault,
<700ºC
≥1.0 <0.2 ≥4.0
Case 5: Thermal fault,
>700ºC
C2H2/H2 RATIO (IEC 60599-1999)
OLTC’s (On-Load Tap Changers) produce gases corresponding to discharges
of low energy. The pattern of oil decomposition in the OLTC differs from the
pattern of oil decomposition in the main tank resulting from low energy
discharges. If oil or gas contamination (communication) exists between the
OLTC and the main tank, an incorrect diagnosis of the main tank may result.
A C2
H2
/H2
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
+C2
H4
+C2
H2
) x 100
%C2
H4
= C2
H4
/(CH4
+C2
H4
+C2
H2
) x 100
%C2
H2
= C2
H2
/(CH4
+C2
H4
+C2
H2
) 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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