Comparison between Online and Laboratory Measurements of Fault Gases for Mineral Oil and Natural Ester

Dissolved gas analysis (DGA) is a well-approved technique to diagnose incipient faults in liquid-filled transformers. It is usually performed by periodically sampling liquids from power transformers and analyzing the dissolved gases in laboratories. In the past decade, the emergence of multi-gas online monitors avails a much timelier monitoring for transformers. However, it is still not certain whether the monitor readings are accurate and reliable and whether they agree with laboratory results. This raises additional concerns when applying natural esters in power transformers. This paper comparativel y studied the lab and online fault gas measurement results of a mineral oil and a natural ester under various faults, including thermal faults, partial discharge faults and sparking faults. The results show that the online gas measurement results agree with the laboratory analysis generally within an error of 30%. Gas leakage or air ingression might occur during the transport of liquid samples for laboratory analysis. Therefore, li quid samples should be analyzed as soon as sampled for laboratory analysis.