2 DGA MONITORING
As part of asset management strategy, the state electricity company of the Republic of Indonesia,
has implemented DGA as a standard method for condition monitoring of HV transformers. The time
interval of the DGA monitoring is determined based on the condition of the equipment. The DGA has
been applied as a standard method, since it is efficient to determine and classify the thermal and
electrical faults. In addition to the DGA monitoring, condition
monitoring of transformer using electrical, mechanical and thermal methods have also been
applied. The later include Partial Discharge PD, Frequency Response Analysis FRA, Acoustic
Emission AE, vibration analysis and infrared thermography techniques.
DGA test results of three different transformers of similar type and capacity are discussed in this
paper. The first transformer has been operated for five years with high vibration and noise. The
second has operated for about ten years with thermal fault indication, while the third represents a
seventeen years in service transformer. The DGA tests have been carried out for five years
in series. The time interval between measurements is approximately a year. Interpretation techniques
of the DGA test results presented here are of key gas method, Doernenburg’s ratio, IEC ratio and
Duval’s triangle. In addition to the diagnosis using these interpretation methods, the trends of
individual concentration and composition of the hydrocarbon gases are also presented. The trend
of gas quantity may provide a useful fault indication within the transformer.
The sample for DGA tests was of bottom oil of the main tank. The oil was taken from bottom drain
valve of the transformer see yellow arrow sign in Figure 1. The oil sample was obtained during
normal operation of the transformers, with an approximate temperature of 60°C.
3 RESULTS AND DISCUSSION
3.1 Transformer 1 5 years in service
The power transformer was manufactured in 2007, and commenced to operate in early 2008. It is an
ONANONAF type with a capacity of 60MVA. The equipment is installed within a network substation,
with an operating voltage of 15020kV. The cooling fan is set to operate at an oil temperature of 60°C
and above. Previous measurement showed that the transformer produced a vibration acceleration
of 311.27ms
2
, the highest among other tested power transformers in the region [8]. As a result of
the high vibration, the transformer also produces a high audible noise.
The maximum transformer loading during first year of operation was slightly above 15MW. It has been
100 increased of load since the time. Figure 2 shows typical daily load pattern of the transformer.
The transformer is currently operated with a low loading scheme, mostly below 50 of its maximum
capacity. During day time, the load is comparatively steady of about 21MW. Beginning
about 5pm, the load increases to meet night peak demand. The maximum transformer loading is
about 32MW and lasts for a few hours before decreases because of less power demand later at
night. The recorded maximum oil temperature is about 65°C.
5 10
15 20
25 30
35
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00
Time Lo
a d
M W
Figure 2:
Typical load pattern of Transformer 1 Individual gas concentration in oil for Transformer
1 over the time period is illustrated in Figure 3. There was an increase in concentration of carbon
dioxide, carbon monoxide, hydrogen, methane, and ethane during first year operation of the
transformer. The concentrations of these gases were comparably high, with carbon dioxide of
2714.29 ppm, carbon monoxide of 755.93 ppm, hydrogen of 281.45 ppm and the ethane of 192.48
ppm. Over the following three years however, the concentrations of the hydrocarbon gases have
decreased gradually.
500 1,000
1,500 2,000
2,500 3,000
2008 2009
2010 2011
2012
Year G
as C o
n cen
tr a
ti o
n p
p m
CO2 CO H2 CH4 C2H6 C2H4 C2H2
Figure 3:
Trend of individual gas concentration of Transformer 1
2137
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
