Research on Creeping Flashover Characteristics of Nanofluid-Impregnated Pressboard Modified Based on Fe₃O₄ Nanoparticles under Lightning Impulse Voltages.

Bingliang Shan,Yuzhen Lv,Chengrong Li,Meng Huang,Mingkang Niu,Zhaoliang Xing,Qian Sun,Yupeng Ying,Bo Qi

doi:10.3390/nano9040524

Abstract

Creeping flashover of mineral-oil-impregnated pressboard under impulse stress is a common insulating failure in oil-immersed transformers, arousing increasing attention. Recent studies have shown that the breakdown strength of transformer oil under positive lightning impulse voltage can be significantly improved through nanoparticles-based modification, and Fe3O4 has shown the best improvement in breakdown strength compared to other nanoparticles that have been used. This paper presents the creeping flashover characteristics of pure oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP) based on Fe3O4 nanoparticles under positive and negative lightning impulse voltages, respectively. It was found that NIP possessed higher resistance to creeping flashover than OIP. The relative permittivities of oil and oil-impregnated pressboard before and after nanoparticles-based modification were measured, and the results revealed that the addition of nanoparticles led to a better match in relative permittivity between oil and oil-impregnated pressboard, and a more uniform electric field distribution. Furthermore, the shallow trap density in NIP was obviously increased compared to that of OIP through the thermally stimulated depolarization current (TSDC), which promoted the dissipation of surface charges and weakened the distortion of the electric field. Therefore, the creeping flashover characteristics of oil-impregnated pressboard were greatly improved with Fe3O4 nanoparticles.

Highlights

Oil/pressboard composite insulation systems have been widely applied in large power transformers due to their excellent dielectric properties and relatively high mechanical strength [1]
One conclusion that can be reasonably drawn is that, compared to that of oil-impregnated pressboard (OIP), the rapid surface charge decay rate in nanofluid-impregnated pressboard (NIP) is mainly attributable to its increased shallow traps density resulting from the addition of Fe3O4 nanoparticles, which is consistent with our previous studies [7,22]
With the addition of Fe3O4 nanoparticles, plenty of shallow traps were produced in the NIP, which can promote the dissipation of surface charges, leading to the suppression of local electric field distortion caused by charge accumulation

Summary

Introduction

Oil/pressboard composite insulation systems have been widely applied in large power transformers due to their excellent dielectric properties and relatively high mechanical strength [1]. The lightning impulse breakdown strengths of pure oil and various nanofluids have been studied by Zhou et al and their results showed that the insulating property of pure oil could be improved with the addition of nanoparticles [9]. Since the pioneering work of Segal et al, it was reported for the first time that Fe3O4 nanoparticles could move freely in and out of nanofluid-impregnated pressboard (NIP) without binding to cellulose [19] This phenomenon has provided a new idea for improving the dielectric properties of OIP by immersing the pressboard in the transformer-oil-based nanofluid modified using Fe3O4 nanoparticles, since these nanoparticles would not be blocked out of the pressboard. Combined with the results of thermally stimulated depolarization current (TSDC), the mechanism of creeping flashover characteristics modification is proposed

Samples Preparation

Effect of Change in Relative Permittivities

Effect of Shallow Trap Characteristics

Findings

Conclusions