Comparison of online and lab DGA methods for condition assessment of mineral and vegetable transformer oils

Transformers are one of the most critical and cost-intensive components in the electrical power system. Dissolved gas analysis (DGA) has been proven to be a reliable technique for diagnosis of incipient faults in oil-filled transformers. Traditionally, DGA is usually performed in laboratories using standardized methods. In the past decade, an increasing number of multi-gas online monitors have gradually become commercially available. However, it is still not clear whether the monitor readings are accurate and reliable and whether they agree with laboratory results. This raises additional concerns for the application of vegetable oils in power transformers in recent years. This paper comparatively studied the lab and online DGA results of a mineral oil and a vegetable oil under various faults, including thermal faults, partial discharge faults and sparking faults. The results show that online DGA results can be correlated with laboratory DGA results for hydrocarbon gases within an error of 30%. However, gas leakage or air ingression might occur during the transport of oil samples for laboratory analysis. Therefore, oil samples should be analyzed as soon as sampled for laboratory analysis.