Researches On The Creeping Discharge Properties Of Pressboards Impregnated By Transformer-Oil Based Nanofluid

As the important electrical equipment in the power system, transformer is essential to the reliable operation of the entire grid. Oil-pressboard composite insulation is widely used in large power transformer. Under long term operation, with increasing moisture level and aging, the surface creeping discharge at the oil-pressboard interfacewould become more serious, leading to insulation failure in the transformer. Therefore, to ensure the safety operation of power transformer, it is important to improve the creepage properties of pressboard with high water content and aging time.In this paper, the nanofluid impregnated pressboard was prepared by impregnatingthe pressboard with titanium dioxide nanoparticle modified transformer oil.Transformer oil impregnated pressboards (OP) and nanofluid impregnated pressboards (NP) with different water content and thermal aging time were prepared by using humidity and thermal aging treatment. In order to study the effect of nanoparticle on the creepage properties at the oil-pressboard interfacial of oil impregnated pressboards, AC and lightning impulse breakdown measurements with different electrode gap distances under werecarried out. The results indicated that the higher water content of pressboard could significantly reduce the creepage discharge inception voltage and flashover voltage of the oil-pressboard interface, accelerate the development of the creepage. The presence ofnanoparticle could improve the interfacial insulating characteristics of NP significantly. For pressboard witha moisture content of 3.5%, the creepage discharge inception voltage and flashover voltage of NP is 1.25-1.27 and .11～1.15times higher respectively, as compare to OP.Under positive lightning impulse voltage, the flashover voltage of NP is 20%-28%higher than OPs, and the chop time of NPs is 50%-72% higher than OP. Thermal aging could also reduce the interfacial insulating strength of oil-pressboard. Compared with OPs, the creepage discharge inception voltage, flashover voltage and chop time of NPs are increased. In conclusion, the presenceof nanoparticlecould reduce the effects of thermal aging deterioration on oil-pressboard interface creepage characteristics.Based on the above test results, the surface charge transport characteristic and trap characteristic of both OPs and NPs is studied by measuring and analyzing thesurface charge density and thermally stimulated depolarization current (TSDC). The results indicated that the charge dissipation rate of NPs was nearly 25 times higher than OPs. Such a huge difference is related on the difference in dielectric constant matching rate between oil and pressboard. High matching rate is not only beneficial to the uniformly distribution of the electrical field at the oil-pressboard interface, but also reduce the attractiveness of surface charge. Therefore, resulting in an inhibition of surface charge accumulation near the interface region. In addition, the presence of nanoparticles are not only reduces the trap level of the nanofluid immersed pressboard, but also improve the trap density, leading to faster charge trapping and desorption rate and therefore accelerating the transmission speed of the surfacecharge, further reducing the electric field distortion at oil-pressboard interface, improving theanti-creepageproperties at the oil-pressboard interface of oil impregnated pressboard.