The key gas chart in Figure 4 shows that the proportion of carbon monoxide and hydrogen has
decreased over time period. In contrast, the proportion of methane and ethane was constantly
increased. This indicates that during early operation of the transformer, there is a possibility
of occurrence of low energy discharges that produces hydrogen, and slightly methane, ethane
and ethylene. This approach has also indicated an occurrence of cellulose decomposition for the
following years. However; after five years in service the key gas interpretation suggested no
fault and the transformer has been classified into normal operation see Table 1.
20 40
60 80
100
CO H2
CH4 C2H6
C2H4 C2H2
G a
s C
o m
p o
s it
io n
2008 2009 2010 2011 2012
Figure 4:
Trend of hydrocarbon gas composition of Transformer 1
DGA interpretation using Doernenburg’s ratio has shown different results from year to year. There
was an indication of low energy discharge in 2008 and thermal fault in 2011. In contrast, no fault
indication was given by IEC ratio for five years in service. Similarly, Duval’s triangle approach
provided no results. This is due to a low monthly increase rate of the hydrocarbon gases. Details of
the DGA interpretations are shown in Table 1.
Table 1:
DGA interpretations of Transformer 1
Year Technique
2008 2009 2010 2011 2012
Key Gas
Low energy
Discharge Cellulose
decom- position
Cellulose decom-
position Cellulose
decom- position
Normal
Doernenburg Ratio
Low energy
Discharge High
energy discharge
NA Thermal
fault NA
IEC Ratio Normal Normal Normal Normal
Normal
Duval’s Triangle
NA NA NA NA NA
The formation of comparably high carbon oxides and hydrogen during first year of transformer
operation may relate to an early stage oxidation of the transformer oil. These gases may be generated
not only as a result of faults within the equipment but also because of rusting process or other
chemical reactions involving steel, uncoated surfaces or protective paints [7]. The amount of the
produced gasses is very dependent on the characteristics of the oil, cellulose materials and
other metallic components within the transformer. This condition is signified by the reduction in
hydrocarbon gas concentrations for the following years, which may indicate that the chemical
reaction is reaching a new equilibrium point. The increase of methane and ethane proportion
between year 2011 and 2012 however, has been a concern for further investigation. The transformer is
currently continued in normal service.
3.2 Transformer 2 10 years in service
The second transformer has a similar type of Transformer 1. This apparatus has been in service
for about ten years. The maximum transformer loading is about 14 MW. No failure was previously
occurred during operation of the transformer. Oil degassing process or reclamation has also never
been carried out. Individual gas concentration in oil for the last five
years of Transformer 2 is presented in Figure 5. It can be seen that there was a consistence increase
in concentration of carbon dioxide, methane, ethane and ethylene for the last four years. In
2008, a high concentration of ethane was recorded. The gas concentration suddenly
decreased on the following year, before beginning to gradually increase again. In 2009, there was an
abrupt increase in methane concentration and occurred only in single year.
200 400
600 800
1,000 1,200
2008 2009
2010 2011
2012
Year G
as C o
n cen
trat io
n p
p m
CO2 CO H2 CH4 C2H6 C2H4 C2H2
Figure 5:
Trend of individual gas concentration of Transformer 2
The key gas analysis shows that the proportion of hydrogen was gradually decreased over the five
year period, as illustrated in Figure 6. In contrary, ethane was present at a very high proportion in
year 2008. A high increase in methane was also noticeable in year 2009. A relatively small increase
2138
in acetylene proportion was noticed in 2010. For the last three years, the proportion of the key
gases was dominated by carbon monoxide. Note also that in year 2012, the proportion of ethylene
was increased significantly from the previous year. The summary of DGA interpretation using key gas
and other techniques over the last five years is presented in Table 2.
20 40
60 80
100
CO H2
CH4 C2H6
C2H4 C2H2
G a
s C
om pos
it ion
2008 2009 2010 2011 2012
Figure 6:
Trend of hydrocarbon gas composition of Transformer 2
Interpretation using Doernenburg’s ratio for the last four years indicated fault occurrence involving high
energy discharge and thermal fault. Particularly in year 2012, similar diagnosis was also indicated by
IEC ratio and Duval’s triangle approaches. Quite similar interpretation results were also observed in
year 2009, however with a PD indication by Duval’s triangle technique. In other years of
observation, results of the DGA interpretation techniques varied, thus it is difficult to provide a
conclusive interpretation.
Table 2:
DGA interpretations of Transformer 2
Year Approach
2008 2009 2010 2011 2012 Key Gas
Normal Normal Normal Cellulose
decom- position
Normal
Doernenburg Ratio
NA Thermal
fault High
energy discharge
Thermal fault
Thermal fault
IEC Ratio Normal
Thermal Fault
Normal Normal Thermal
fault
Duval’s Triangle
NA PD NA NA T3
Regular increase in carbon monoxide and other hydrocarbon gasses for the last four years have
been a concern for the continuity of the transformer service. DGA interpretations results indicated a
possibility of fault occurrence involving high temperature. It has not been decided yet however,
whether the transformer will be put into maintenance. Currently, the transformer is still in
service with a low loading scheme and further monitoring has been planned accordingly.
3.3 Transformer 3 17 years in service