Influence of chemical composition on the gassing behavior of biodegradable dielectric liquids: A review

The increasing adoption of biodegradable dielectric liquids in power transformers introduces new challenges for condition assessment, as Dissolved Gas Analysis (DGA) methods developed for mineral oils often yield inconsistent results when applied to alternative insulating fluids. Understanding how chemical composition influences gas generation is therefore essential for improving diagnostic reliability and supporting the transition toward sustainable transformer technologies. This review provides a comprehensive bibliographic synthesis of the influence of molecular structure, oxygen and moisture content, additives, and oil–paper interactions on the gassing behavior of bio-based hydrocarbon oils (Bio-MO), natural esters (NE), and synthetic esters (SE). The principal experimental and analytical methodologies used to investigate gas generation are critically compared, together with their advantages, limitations, and implications for DGA interpretation. A comparative assessment of insulating liquid performance under thermal, electrical, moisture, contamination, and cellulose interaction conditions is also presented, highlighting the relative strengths and limitations of each fluid family. The literature indicates that Bio-MO exhibit degradation mechanisms similar to mineral oils, whereas natural esters generally display positive gassing and superior moisture management, while synthetic esters frequently exhibit negative gassing associated with hydrogen consumption. The review further identifies the major knowledge gaps and research priorities required to develop fluid-specific diagnostic criteria and reliable condition assessment methodologies for next-generation biodegradable transformer insulation systems.