A Review of the Functional Properties of Ester Liquids for Power Transformers

Ester liquids have become viable alternatives to conventional mineral oils for power transformer applications, mainly driven by increasing demands for enhanced fire safety and environmental sustainability. This paper reviews the historical development and functional properties of synthetic and natural ester liquids, with particular emphasis on their implications for transformer design and operation. Key physical and chemical characteristics of ester liquids are introduced, followed by a comprehensive review of their dielectric performances under alternating current and lightning impulse stresses, including pre‐breakdown, breakdown and partial discharge behaviours under different electric field conditions. The thermal performance of ester liquids is examined through their intrinsic thermal properties, results from laboratory‐scale experiments, computational fluid dynamics studies and full‐scale transformer temperature rise tests under forced and natural cooling modes. Ageing processes of ester liquids and ester‐impregnated cellulose insulation are critically reviewed, highlighting the different degradation mechanisms compared with mineral oil insulation systems and their impact on insulation lifetime and thermal class. Practical application aspects such as moisture characteristics, dissolved gas analysis, compatibility, retrofilling and low‐temperature applications are also discussed. The paper consolidates recent advances in understanding ester liquids, identifies remaining technical challenges to support the wider adoption of ester liquids in high‐voltage power transformers.