Study of water molecules state in transformer oil via FT-IR spectrum and molecular dynamic simulation

Mineral transformer oil has been widely used in the power transformers due to its good insulating property and thermal conductivity. It is well-known that water molecules in the oil have detrimental effect on its conduction and breakdown strength and tend to cause accelerated aging of oil and paper insulation system. The relation of the total amount of water to the electrical properties of oil has been widely investigated. A turning point of moisture content has been found that beyond this point the abrupt decrease of AC breakdown strength of oil is alleviated and keep almost constant with the further increasing of moisture content. Water state in saturated and unsaturated hydrocarbons and aromatic hydrocarbons have been studied and indicated that the state of water molecules is closely related to the electrical conductivity of hydrocarbons. Therefore, it is of great importance to investigate the water state in the mineral transformer oil with different moisture contents. In this paper, mineral transformer oil was treated at different humidity and Fourier transform infrared spectrometer (FT-IR) was used to characterize the water state of oil samples with different moisture contents. Moreover, the water state in the oil samples was investigated using a molecular dynamics simulation. By analyzing the formation of hydrogen bonds and the diffusion behavior of water molecules, it is found that the hydrogen bond energy is greatly changed by varying the moisture content of mineral oil, which lead to the change of the state of water clusters.

I. Introduction

Mineral transformer oil has been widely used in electrical power equipment Due to its great insulation strength and thermal conductivity. Water is one of the main factor affecting the performance of transformer oil. With the water content increasing, dielectric strength and breakdown voltage of transformer oil will decrease significantly. The relationship between water content and electrical properties of transformer oil has been extensively studied [1]–[5]. Under AC condition, the breakdown strength of transformer oil decreases rapidly and then slowly decreases with the increase of water content, and there is an obvious turning point on the curve [6]. To explain this phenomenon, the state of water in saturated and unsaturated hydrocarbons was studied, and finding that compared with the saturated hydrocarbons, the conductivity of unsaturated hydrocarbons is more sensitive to the change of water content. The results show that the turning point of AC breakdown strength curve of transformer oil is caused by the change of water state in oil [7]. This is very crucial to explain the effect of water on the dielectric strength of transformer oil and its degradation. Therefore, it is of great importance to investigate the water state in the mineral transformer oil with different water contents.