Stability of mineral oil and oil-ester mixtures under thermal ageing and electrical discharges

This study summarises the results of an experimental investigation on the stability of mineral-based oil mixtures when they are confronted with different ratios of synthetic ester under conditions of electrical discharge, electrical breakdown and a combination of both. The condition of the oil samples was assessed using diagnostic techniques such as gassing tendency, turbidity, dissolved decay products (DDP) and dielectric dissipation factor (DDF), according to ASTM standards. A comparison is made between the performances of fresh and aged samples. The results provide experimental evidence that the chemical composition of hydrocarbon blend is contributing factors to oil gassing. It was observed that aged oils release more gases than new one. It was also observed that the gassing tendency increased with increasing amount of ester for the mixed fluids. The pressure and the absorbance of gases vary proportionally with ester content. Under thermal stress, an increase in pressure is observed especially for the mineral oil sample. The turbidity, DDP and DDF measurements revealed higher values for mineral oil. Importantly, the stability improved with increasing ester content in the blends. Mixed mineral oil/ester therefore offers many advantages with concomitant cost reductions compared with pure synthetic esters.