Low temperature breakdown behavior analysis of natural esters from the perspective of molecular structure

Abstract

To better promote and apply natural esters safely in cold area transformers, it is very important to master its low temperature insulation performance. Low temperature breakdown properties of two kinds of the most widely used natural esters named soybean oil and palm oil were tested and analyzed from the perspective of molecular structure. Results show that the AC breakdown voltage of the two natural esters first decrease and then increase, showing the shape of U from −20 °C to 20 °C. Based on the molecular dynamic simulation and first principle of density functional theory, it was found that the physical and chemical properties, like free volume, interaction energy, water form and hydrogen bond, are mainly related to the low temperature breakdown behavior of natural esters. The electrical properties, such as molecule orbital, band structure and density of states, are secondary factors. From −20 °C to −5 °C, the viscosity, the interaction energy between natural ester and water are the main influencing factors where the breakdown voltage declines with the increase of temperature. From 5 °C to 20 °C, the ability of natural esters to dissolve water becomes stronger and hydrogen bonds formed by moisture and natural esters decline, which play a major role in process of breakdown voltage rising with temperature. When the temperature is between −5 °C and 5 °C, all the factors work together to breakdown performance. Compared with soybean oil, palm oil has a greater free volume, a weaker ability of gain and lose electrons, and a wider band gap and forbidden band. The difference of breakdown voltage between soybean oil and palm oil at low temperature is mainly caused by these diversities of electrical characteristics. Therefore, the breakdown behavior of natural esters at low temperature is a complex phenomenon, which is closely related to the molecular structure.