Dissolved Gas Analysis of Transformer Oil

1. Dissolved Gas Analysis
By:
Rahul Buswala,
Chemical Engineer
Email: rahulbuswala1988@gmail.com

2. INTRODUCTION
N2
C2H6
H2
CH4
C2H4
C2H2
C3H8
C3H6
CO2CO
O2
Transformer oil sample analysis for dissolved gases is a useful,
predictive, maintenance tool for determining transformer health.
The breakdown of electrical insulating materials and related
components inside a transformer generate gases within the
transformer. The amount and relative distribution of these gases
depend upon the type & severity of the fault.
When gassing occurs in transformers enough useful information
can be derived from following 09 gases so the additional gases
(propane & propene) are usually not examined:
1. Hydrogen
2. Oxides of carbon: Carbon dioxide & Carbon monoxide
3. Hydrocarbons: Methane, Ethane, Ethylene & Acetylene
4. Atmospheric gases: Oxygen & Nitrogen.
Dissolved Gas Analysis

3. INTRODUCTION
Principle causes of gas formation:
1. Electrical disturbances
Partial or Disruptive discharge through the insulation
2. Thermal decomposition
Excessive temperature rise in insulation due to
insufficient cooling, excessive current circulating
through the metal parts or insulation, overheating of
internal winding or bushing connection lead, or
overloading.
These two fault may occur in combination.
Dissolved Gas Analysis

4. INTRODUCTION
The DGA procedure consists of essentially four steps:
• Sampling of oil from the transformer.
SAMPLING
• Extraction of the gases from the oil.
EXTRACTION
• Analysis of the extracted gas mixture
in a gas chromatography (GC).ANALYSIS
• Interpretation of the analysis
according to an evaluation schemeINTERPRETATION
Dissolved Gas Analysis

5. INTRODUCTION
REFERENCE STANDARDS
Dissolved Gas Analysis
SAMPLING
METHODS:
• IEC 60475
• ASTM D 3613
• IS 6855
EXTRACTION OF
GASES & ANALYSIS
METHODS:
• IEC 60567
• ASTM D 3612
• IS 9434 (included
sampling methods)
INTERPRETATION:
• IEC 60599
• IEEE C57.104
• IS 10593
• IEC 61181

6. SAMPLING OF
TRANSFORMER OIL
Dissolved Gas Analysis
Precaution & Place of sampling
Make sure that the oil in energized transformer is not under a negative pressure
when taking an oil sample. Since this could introduce air bubbles in the oil.
Precautions should be taken during sampling of oil to deal with any sudden release
of oil and avoid oil spillage.
Much care is required during sampling normally oil sample should be taken from a
point where it is representative of the bulk of the oil in the transformer, like from
the bottom oil drain valve or the oil sample valve.
However, sometime sampling shall be done where it is not representative of bulk to
locate the site of fault like tap changer, selector switch or gas relay.

7. Dissolved Gas Analysis
Cleaning of sampling point
To prevent from contamination of oil samples, clean the sampling equipment
carefully and flushing of the sampling point shall be done.
The drain valve is flush with a sufficient quantity of oil under turbulent flow to
remove any contaminants like water and particles, that might have deposited /
accumulated in the drain valve and at its orifice.
Sample Container
Syringes --------------------------> Glass Hypodermic
Flexible bottle --------------------> Metal (tin) can
Bottle ------------------------------> Glass ( Amber colour)
Bottle--------------------------------> Stainless Steel
Plastic bottles should not be used for DGA as the dissolved may diffuse through the
wall of plastic bottle.
SAMPLING OF
TRANSFORMER OIL

8. analysis FOR
DISSOLVED GASES
There are three methods for gas extraction:
Method A (Extraction by vacuum)
Toepler and Partial Degassing
Method B (Stripping)
Displacement of dissolved gases by bubbling the carrier gas through the oil
Method C (Headspace)
Partition of gases between the oil sample & a small volume of carrier gas
The dissolve gases extracted then will be analysed in Gas Chromatograph
for composition.
Dissolved Gas Analysis

9. Dissolved Gas Analysis
Method A is usually require mercury to compress the extracted gas under
vacuum and mercury affects health and environment. This method is more
laborious and require continuous involvement of operator. However, it is having
better sensitivity & accuracy.
Method B is not widely in use for DGA. This method having minimum detection
limit for hydrogen is 20 ppm which is higher than method A & C. This could
affect interpretation of results when low levels of gases are present.
Method C is having advantage of mercury free extraction & automatic sampling.
Require very sensitive equipment to analyse at lower concentration. Care shall
be taken to select the headspace temperature and use the respective partition
coefficients which is a function of temperature.
analysis FOR
DISSOLVED GASES

10. GAS CHROMATOGRAPHY
Dissolved Gas Analysis
After extraction of gases they shall be analysed over an Gas Chromatograph
which shall have following facilities:
1. TCD Detector to detect atmospheric gases & hydrogen
2. FID Detector to detect hydrocarbon gases
3. Methanizer to convert CO & CO2 to CH4 so that they can detect over FID.
4. Columns to separate the constituent gases from mixture.
Argon: Use as a carrier gas in
some cases helium can also be
used but care shall be taken
for hydrogen determination.
Air & Hydrogen: These gases
are required for flame of FID
detector

11. GAS CHROMATOGRAPHY
Dissolved Gas Analysis
Thermal Conductivity Detector (TCD) is used to
determine the concentration of Hydrogen &
atmospheric gases.
Basically it is based on wheatstone bridge circuit.
This detector senses changes in the thermal
conductivity of the column effluent and compares
it to a reference flow of carrier gas.
Temperature changes leads to the change in the
resistance.
Thermal conductivity of some carrier gases is
given in this table.

12. GAS CHROMATOGRAPHY
Dissolved Gas Analysis
Flame Ionization Detector (FID)
The effluent from the column is mixed with
hydrogen and air, and ignited.
Organic compounds burning in the flame
produce ions and electrons which can
conduct electricity through the flame.
A large electrical potential is applied at the
burner tip, and a collector electrode is
located above the flame.
The current resulting from the pyrolysis of
any organic compounds is measured.

13. Mechanism of
gas formation
OIL DECOMPOSITION
DECOMPOSITION OF CELLULOSIC INSULATION
STRAY GASSING OF OIL
OTHER SOURCES
Dissolved Gas Analysis

14. OIL DECOMPOSITION
Oil decomposition products are Hydrogen & Hydrocarbon gases.
Some solid particles of carbon & hydrocarbon polymer (X wax) are other
possible product of oil decomposition. However oil may oxidize with
formation of small quantities of CO & CO2 which may accumulate over long
period of time.
Low Energy faults like corona discharge favours the formation of Hydrogen
gas while other hydrocarbon gases can be formed if the energy of discharge
& temperature is high enough to break the C-C, C=C or C≡C bond.
Ethylene is favoured above temperature 500°C
while Acetylene favoured above temperature 800°C.
Dissolved Gas Analysis
Mechanism of
gas formation

15. CELLULOSE DECOMPOSITION
Cellulosic insulation are made up of large
no. of anhydrous glucose rings having weak
C-O bonds glycosidic bonds. This
decomposes at lower temperature than oil.
Carbon monoxide and dioxides, water
vapour forms with minor quantity of
hydrocarbon gases & furan compounds.
Decomposition occurs at 105°C or above
with complete decomposition or
carbonization occurs at 300°C or above.
Dissolved Gas Analysis
Mechanism of
gas formation

16. STRAY GASSSING OF OIL
It is formation of gases in oil heated to temperature <200°C.
In general Hydrogen , methane, & ethane forms at this temperature or due to
oxidation of oil. It is a non-damage fault.
Dissolved Gas Analysis
Mechanism of
gas formation

17. OTHER SOURCES OF GAS
Gases can be formed in the equipment not as a result of fault but through rusting,
chemical reaction involving steel, uncoated surface or protective paints.
Hydrogen can be formed due to reaction of moisture with steel/galvanised steel
surface in presence of oxygen from oil, or due to moisture with special coatings on
metal surface.
Hydrogen & acetylene can be formed in new steel surface which absorbed during
manufacturing process or welding and releases slowly into the oil.
Internal transformer paints may also form gases.
High value of hydrogen with absence of hydrocarbon gases indicates this source &
even sometime gases form in a transformer which is never been energized due to this
reason.
Dissolved Gas Analysis
Mechanism of
gas formation

18. interpretation
Dissolved Gas Analysis
The major (minor) fault gases can be categorized as follows by the type of
material that is involved and the type of fault present:
• Oil: H2
• Cellulose: CO , CO2
Partial
Discharge
• Oil
LT: CH4 , C2 H6 HT: C2H4 , H2 ( CH4 , C2H6 )
• Cellulose
LT: CO2 ( CO ) HT: CO ( CO2 )
Thermal
• H2, C2H2
(CH4, CH6, C2H4)
Arcing
LT: Low Temperature, HT: High Temperature

19. interpretation
Dissolved Gas Analysis
The solubilities of the fault gases in mineral oil as well as their temperature
dependence are also important factors for consideration in fault gas analyses.
Table below lists the saturation solubilities for the fault gases.
Solubility of gases in transformer oil
Static Equilibrium at 760 mm Hg and 25°C (by Volume).
Hydrogen
Nitrogen
Carbon monoxide
Oxygen
Methane
Carbon dioxide
Ethane
Ethylene
Acetylene
7.00 %
8.60%
9.00 %
16.0 %
30.0 %
120 %
280 %
280 %
400 %

20. interpretation
Dissolved Gas Analysis
When rates of gas generation are
being followed it is important to
take into account the solubilities of
these gases as a function of the oil
temperature. Over a temperature
range of 0 to 80°C some gases
increase in solubility up to 79%
while others decrease their
solubility up to 66%.

21. interpretation
Dissolved Gas Analysis

22. Case study-1
Dissolved Gas Analysis
Sl.No Components Unit Concentration of gas
1 CH4 ppm(V) 98.64
2 C2H6 “ 15.15
3 C2H4 “ 109.5
4 C2H2 “ 1.10
5 H2 “ 80.66
6 CO “ 2.92
7 CO2 “ 16.34
8 O2 “ 1962.18
9 N2 “ 6484.01
10 TGC % 1.0
Type of Sample:
Transformer Insulating Oil (after
over excitation)
Details:
80 MVA, 765 KV, Reactor
Date of Analysis:
20-07-2017

23. Case study-1
Dissolved Gas Analysis
Rogers
Ratio
C2H2 / C2H4 =
0.01
CH4 / H2 =
1.22
C2H4 / C2H6=
7.23
Thermal >
700°C
DIAGNOSIS

24. Case study-1
Dissolved Gas Analysis
Doernenburg
C2H2 /C2H4 =0.01
CH4 / H2 =1.22
C2H2 / CH4=0.01
C2H6/ C2H2=13.77
Thermal
Decomposition
DIAGNOSIS

25. Case study-1
Dissolved Gas Analysis
Duvals
Triangle
% C2H2 = 0.5
% C2H4 = 52.0
% CH4 = 47.0
Thermal
Fault
DIAGNOSIS

26. Case study-2
Dissolved Gas Analysis
Sl.No Components Unit Concentration of gas
1 CH4 ppm(V) 41.0
2 C2H6 “ 1.80
3 C2H4 “ 32.0
4 C2H2 “ 44.0
5 H2 “ 161.0
6 CO “ 9.40
7 CO2 “ 11.0
8 O2 “ 2262.87
9 N2 “ 6196.70
10 TGC % 1.0
Type of Sample:
Transformer Insulating Oil (after
short circuit)
Details:
315 MVA, 400 KV, GT
Date of Analysis:
09-01-2018

27. Case study-2
Dissolved Gas Analysis
Rogers
Ratio
C2H2 /C2H4 =1.38
CH4 / H2 =0.25
C2H4 / C2H6= 17.28
Arching
High
Energy
Discharge
DIAGNOSIS

28. Case study-2
Dissolved Gas Analysis
Doernenburg
C2H2 / C2H4 =1.38
CH4 / H2 =0.25
C2H2 / CH4=1.07
C2H6 / C2H4=0.04
Arcing ( High
density PD )
DIAGNOSIS

29. Case study-2
Dissolved Gas Analysis
Duvals
Triangle
% C2H2 =38.0
% C2H4 =27.0
% CH4 =35.0
D2 :
Discharge
of high
energy.
DIAGNOSIS

30. Case study-2
Dissolved Gas Analysis
CO2 / CO
Ratio
Analysis
according to
IEC 60599
CO2 / CO =
1.17
Recommended to
perform a Furan-
Analysis or
measurement of
polymerization
with paper
samples if possible
DIAGNOSIS

31. THANK YOU