Transformer 2 10 years in service

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