Ester based dielectric fluids have gained wide popularity for applications in high voltage apparatus. Synthetic and natural esters have been subjected to research for decades vis-à-vis mineral insulating oils across the globe. Despite many researchers are affirmative towards the application of ester fluids, utilities are still under a dilemma and application of these alternatives is still a contemporary challenge. The intent of this survey is to present the recent research progress and highlights the state of the art for the key aspects that need to be emphasized in future research. The contemporary research scenario pertaining to performance of ester fluids, miscibility, and retro filling of insulating fluids vis-à-vis mineral oils are discussed. Furthermore, the topic of pre-breakdown phenomena of ester fluids, usage of esters on load tap changers, environmental and fire properties, use of the esters in cold climatic regions are also discussed. Importantly, challenges and future aspects that are to be investigated to improve the existing knowledge of ester dielectric fluids for applications in transformer technology are emphasized.

The present work is focused on the oxidation aspects (focused on gelling) of natural esters and its influence on the performance of transformer insulation systems. Even though natural esters are not suitable for breathing units, the scenario considered simulates a defect in the sealing system and thus bringing the liquid in direct contact with atmospheric air. Commercially available natural ester liquids are thermally degraded under open beaker conditions in the presence of cellulose with a controlled aging history. The accelerated aging conditions are maintained until the formation of sol and gel is evident in the bulk of the liquid. Later, the insulating liquids and insulating papers are analyzed to understand the behavioral impact of sol and gel on the insulation. The discussions in this article are supported by the physicochemical and electrical characterizations of the degraded liquids and papers. The characterizations include Fourier transform infrared spectroscopy (FTIR), ac breakdown voltage (BDV) failure probability rate, X-ray diffraction (XRD), degree of polymerization (DP), and viscosity. It is found that before the formation of gelling, sol is witnessed in the bulk of the liquid. The presence of the sol highly influences the viscosity of the liquid and aids in rapid acceleration of the oxidation process. It is understood that that the gelling phenomena in natural esters do not have a noticeable impact on the liquid BDV failure rate and paper degradation.