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Tuesday, May 24, 2011

Substation maintenance: oil sampling for dissolved gas analysis

Oil sampling is being done to our 50 MVA Power Transformer quarterly. The maintenance group draws out power transformer oil sample for analysis. Most of the time, they draw the oil from gate valve located near the base of the transformer main tank. But for power transformer with Dissolve Gas Analyzer installed like in our substation, they just connect an adaptor to a port dedicated for oil sampling. The drawn-out oil must not contain bubbles inside the syringe as these bubbles may give false results on the test data.
The analysis to be done on the oil sample will be a comprehensive analysis of gas content of the oil. The most  important gases to be determined are: hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO), and carbon dioxide (CO2). These gases will be expressed in part per millions (ppm)/day. The different limits for the different gases are as follows as per our company's standard:
1. Hydrogen (H2)     <100 ppm/day
2. Methane (CH4)    <120 ppm/day
3. Ethane (C2H6)      <65 ppm/day
4. Ethylene (C2H4)   <50 ppm/day
5. Acetylene (C2H2) <35 ppm/day
6. Carbon Monoxide (CO) <350 ppm/day
7. Carbon Dioxide (CO2)   <2500 ppm/day
If any of the gases are abnormally high, further investigation must be done. This will include trending of the test results to determine if the gases with high values will continue to increase. If this happens, comprehensive testing will be done on the power transformer to abate transformer failure. The next post will discuss the different interpretations of high values of certain gases and corresponding implications to power transformer.
Oil sampling was drawn-out at the port of the Dissolve Gas Analyzer

DGA port for oil sampling

Calisto Dissolve Hydrogen & water monitor

reading the parameters of the Calisto DGA


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