Surface flashover fault is one of the most challenge issues in oil-cellulose insulation pressboard system used in power transformer. In this study, a comparative study of the AC surface flashover properties of the novel 3-element mixed oil-cellulose insulation pressboard (3EMO-IP) and mineral oil-cellulose insulation pressboard (MO-IP) was performed under needle-plate and finger-finger electrode, respectively, measurement including dielectric property, surface flashover voltage, damage of cellulose pressboard surface and the gas generation behaviors after multiple surface flashover. Results show that the cellulose insulation pressboard immersed in the novel 3-element mixed insulation oil (3EMO) has higher relative permittivity and dielectric loss factor at 50 Hz, and also has slightly lower surface resistivity. The AC surface flashover voltage of the 3EMO-IP is higher than that of MO-IP under needle-plate and finger-finger electrode (electrode distance 5 mm, 10 mm, 15 mm and 20 mm). Compared to MO-IP, the lower electric field intensity at the oil-pressboard interface, as well as more difficult for surface charge accumulation of 3EMO-IP and the higher breakdown voltage of 3EMO lead to the higher AC surface flashover voltage of 3EMO-IP. Moreover, the carbonization of fibers in 3EMO-IP is slightly less. After multiple surface flashover, C2H2 and total hydrocarbon gases are the main differences between 3EMO-IP and MO-IP, which is more marked with the increase of flashover times. This study offers a reference for improving the surface flashover property of oil-pressboard insulation system by using 3EMO.

Introduction

Polymer insulation materials, such as cellulose, are widely used in high-voltage equipment as electrical insulation [1]. The cellulose products (pressboard, kraft paper, etc.) and insulating oil constitute the compound insulation systems in oil-immersed transformer and its property directly affects the safe operation of transformer [2], [3]. Insulating oil has a significant impact on the performance of oil-paper insulation system, which is regarded as the ‘‘blood’’ of transformer. Among all types of oils, mineral oil (MO) has a long history of application in transformer because of its low price and good insulation properties. However, the poor fire performance and biodegradability of mineral oil makes it inconsistent with the development of electric equipment [4], [5]. As a promising alternative for mineral oil, natural esters are not only renewable and biodegradable, but also can delay the aging of cellulose [6]–[۹]. However, some defects, such as poor oxidation stability and high kinematic viscosity, are needed to be improved [10], [11]. Mixed oil developed from mineral oil and natural ester retains the good physical and electrical properties of mineral oil, and also absorbs the advantages of natural ester on fire resistance and anti-aging properties.