Influence of Gelling in Natural Esters Under Open Beaker Accelerated Thermal Aging

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.