Dissolved gas analysis of power transformer oil

Dissolved Gas Analysis
(DGA) of Power Transformer Oil

Shivaji choudhury

1.Introduction
 Transformer is one of the most
important but complex component of
electricity generation and transmission
system.
 Much attention is needed on
maintenance of transformers in order
to have fault free electric supply and to
maximize the lifetime and efficacy of a
transformer.

2. Gases in oil filled transformers

 The detection of certain gases
generated in an oil-filled transformer
in service is frequently the first
available indication of a malfunction
that may eventually lead to failure if
not corrected.

2.1.Benefits of DGA
 Assesses the internal condition of the
transformer
 Helps calculate probability of failure and
end of life
 Identifies degradation before it leads to
failure
 Essential for effective maintenance and
replacement strategies
 Low cost test process

3.Internal View of a Large Power Transformer

4.Cause of gas formation

 The two principal causes of gas
formation within an operating
transformer are
 4.1.Cellulosic Decomposition
 4.2.Oil Decomposition

4.1.Cellulosic Decomposition

 The thermal decomposition of oil-
impregnated cellulose insulation
produces carbon oxides (CO, CO2)
and some hydrogen or methane.

4.2.Oil Decomposition

 Mineral transformer oils are mixtures
of many different hydrocarbon
molecules, and the decomposition
processes for these hydrocarbons in
thermal or electrical faults are
complex.

4.3.Oil Decomposition
 -some of these gases
will be formed in larger
or smaller quantities
depending on the
energy content of the
fault.
 -for example, low
energy faults such as
corona partial
discharges in gas
bubbles, or low
temperature hot spots,
will form mainly H2
and CH4.

5.Interpretation of Gas Analysis

 Thermal Faults
 Electrical Faults—Low Intensity
Discharges
 Electrical Faults—High Intensity Arcing

5.0.Halstead's Thermal Equilibrium Partial
Pressures as a Function of Temperature

5.1.1.Thermal faults

 The decomposition of mineral oil from 150 °C
to 500 °C produces relatively large quantities
of the low molecular weight gases, such as
hydrogen (H2) and methane (CH4), and trace
quantities of the higher molecular weight gases
ethylene (C2H4) and ethane (C2H6).


 At the upper end of the thermal fault range,
increasing quantities of hydrogen and
ethylene and traces of acetylene (C2H2) may
be produced.
 In contrast with the thermal decomposition
of oil, the thermal decomposition of cellulose
and other solid insulation produces carbon
monoxide (CO), carbon dioxide (CO2), and
water vapor at temperatures much lower
than that for decomposition of oil.


 The ratio of CO2/CO is sometimes used
as an indicator of the thermal
decomposition of cellulose.
 As the magnitude of CO increases, the
ratio of CO2/CO decreases. This may
indicate an abnormality that is
degrading cellulosic insulation.

5.2.Electrical Faults—
Low Intensity Discharges

 Low intensity discharges such as partial
discharges and very low level intermittent
arcing produce mainly hydrogen, with
decreasing quantities of methane and trace
quantities of acetylene.
 As the intensity of the discharge increases,
the acetylene and ethylene concentrations
rise significantly .

5.3.Electrical Faults—
High Intensity Arcing
 As the intensity of the electrical
discharge reaches arcing or
continuing discharge proportions that
produce temperatures from 700 °C to
1800 °C, the quantity of acetylene
becomes pronounced.

6.Interpretation of Dissolved Gas
Analysis (DGA)

 Key gas Method- IEEE
 Type of faults –IEC 60599
 IEC Gas ratio method
 Duval Triangle
 Rogers ratio method flow chart
 Deornenburg method flow chart

6.1. Key Gas Method
 Thermal –oil
 Thermal –cellulose
 Electrical –corona
 Electrical -arcing

6.1.1.Thermal -oil
 Decomposition products include ethylene
and methane ,together with smaller
quantities of hydrogen and ethane
.traces of acetylene may be formed if
the fault is severe or involves electrical
contacts.
 Principal gas - ethylene

6.1.2.Thermal -cellulose
 Large quantities of carbon dioxide
and carbon monoxide are evolved
from overheated cellulose
.hydrocarbon gases ,such as methane
and ethylene ,will be formed if fault
involves an oil impregnated structure
 Principal gas—carbon monoxide

6.1.3.Electrical - relative corona
 Low energy electrical discharges
produce hydrogen and methane ,with
small quantities of ethane and
ethylene .
 Principal gas –hydrogen

6.1.4.Electrical –arcing
 Large amounts of hydrogen and
acetylene are produced ,with minor
quantities of methane and ethylene
.carbon dioxide and carbon monoxide
may also formed if fault involves
cellulose. Oil may be carbonized.
 Principal gas- acetylene

6.2.Type of faults –IEC 60599

 1. PD- Partial Discharges (corona)
 2. D1- Discharges of low energy Electrical

 3. D2- Discharges of high energy
 4. T1 - Thermal faults < 300°
 5. T2 - Thermal faults > 300°< 700
Thermal
6. T3 - Thermal faults > 700°

6.2.1-Partial discharges of the
corona-type (PD).-

 Typical examples are discharges in
gas bubbles or voids trapped in
paper, as a result of poor drying or
poor oil-impregnation.

6.2.2.Discharges of low energy
(D1)

 -Typical examples are partial
discharges of the sparking-type,
inducing pinholes or carbonized
punctures in paper.-or low-energy
arcing, inducing carbonized
perforations or surface tracking of
paper, or carbon particles in oil.

6.2.3.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,
metal fusion, tripping of the
equipment or gas alarms .

6.2.4.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 oil ducts, stray flux in beams.

between 300 and 700°C (T2)

 Faults T2 are evidenced by :
 -carbonization of paper.
 -formation of carbon particles in oil.
 Typical examples are defective
contacts or welds, circulating
currents.

> 700°C (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.

6.5.Rogers ratio method flow chart

6.6.Deornenburg ratio method flow chart

7.Evaluation of Transformer Condition Using
Individual and TDCG (total dissolved combustible
gas) Concentration

 A four-level criterion has been developed to classify
risks to transformers.
 Condition 1 TDCG below this level indicates the
transformer is operating satisfactorily .
 Condition 2 TDCG within this range indicates greater
than normal combustible gas level.
 Condition 3 TDCG within this range indicates a high
level of decomposition.
 Condition 4 TDCG within this range indicates
excessive decomposition. Continued operation could
result in failure of the transformer.

7.1.Action based TCG (total combustible gas )

7.2.Dissolved Gas Concentration

8.Sampling

 ASTM D3613 requires that transformer oil
sampling be taken via a syringe and stopcock
system from a mineral-oil insulated
transformer's drain point to ensure no oil
contact with air.
 To minimise air ingress, it is important that the
syringe not be pulled forcefully, i.e. the
transformer oil's natural gravity flow should be
allowed to work the oil into the syringe .

Dissolved gas analysis of power transformer oil

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