Investigation of the thermal aging of the natural monoester/paper mixed insulation.

Palm kernel and castor oils are two vegetable oils available in tropical climates which have been proposed as an alternative to mineral oils, and their methyl esters, also called monoesters, have been previously elaborated. As the insulation system of many power transformers is made up of oil and paper, it is essential to study the interaction between these methyl esters and insulating paper. In this paper, we present the results of thermal aging of insulating paper immersed in methyl esters of palm kernel (MEPKO) and castor (MECO) oils. A comparison is made with insulating paper immersed in mineral oil (MO) based on the total acid number, the kinematic viscosity, the quantity of dissolved decay products through Ultraviolet–Visible (UV–visible) spectroscopy, the paper breakdown voltage, and the physical appearance of the insulating oils. UV–visible spectroscopy and total acid number show that the quantity of decay dissolved products of methyl esters is greater than the quantity of decay dissolved products generated by MO during initial thermal aging. However, the viscosity of MEPKO is smaller than the viscosity of MO. In addition, the paper breakdown voltage when dipped in methyl esters is greater than when dipped in MO. Therefore, methyl esters better preserve the insulating paper than mineral oils during initial thermal aging. [ABSTRACT FROM AUTHOR]