Examining acceptable Dissolved Gas Analysis level of in-service transformers

Abstract

Dissolved Gas Analysis (DGA) is widely accepted as the most reliable tool for the detection of incipient faults in oil-immersed transformers. Currently there are several standards/guidelines such as IEC 60599, IEEE C57.104 and CIGRE 296 which are available to aid the interpretation of DGA test results. According to these standards, there are several methods that can be used to diagnose types of fault in transformers such as Rogers ratio and Duval Triangle. In particular, IEC 60599 and CIGRE 296 have suggested the usage of 90 % typical values for diagnosis. Since 90 % typical values vary with the load, type of transformer and climate, utilities are encouraged to calculate their own 90 % typical values. In this paper, the 90 % typical values of distribution transformers for different age intervals and manufacturer groups are calculated and the results shown that in general 90 % typical values increase with age, different manufacturers' transformers have given different 90 % typical values. Besides, a comparison between different gas extraction methods is carried out and its impact on 90 % typical values is identified. There is a difference of 90% typical values for different gas extraction methods.