Gassing Tendency of Fresh and Oxidative Aged Mixed Insulating Oils on Electric Sparking

Gases are generated in the liquid insulation during its decomposition caused by thermal and/or electrical faults. The generated gases dissolve in the oil and their measure play an important role in predicting the nature of the fault. With the advent of mixed insulating oils (MIOs) as potential alternative to the mineral oil (MO), there is a need to study the gassing tendency of these oils to predict their in-service performance. Despite the mixing ratio of oils, there is uncertainty about the oil mixture present around conductor. Further, gassing in such oils is not widely investigated. This article addresses the issue by considering a wide range of mixtures and investigating their gassing tendency by subjecting to multiple electric sparks. The long-term gassing performance of these oils is also investigated by oxidative aging the oils and sparking them. Further, this work explores the applicability of various existing gas ratios and graphical methods available for the mineral and ester oils (EOs) to MIOs. The graphical methods are found to be superior in establishing the electric fault in comparison to ratio methods. Duval triangles are recommended for predicting electric faults. In addition to providing an insight into the gassing in MIOs and applicability of the existing gas analysis methods, this work will serve the scientific community in defining new limits for gas ratios and gas relays.