Influence Mechnism Of Nanoparticles On The Insulating Properties Of Nanofluid/Pressboard Under Lightning Impulse Voltage

With the fast development of ultra-high voltage grid in China, the demands on the insulating reliability of power transformer get more pressing. As the major insulation components used in oil-immersed transformer, the insulating properties of transformer oil and oil impregnated pressboard plays a vital role in the safe and stable operation of power transformers. To intensify insulation level of ultra-high voltage power transformer and reduce its size and weight of, it is necessary to improve the insulating performance of transformer oil and oil impregnated cellulose. Recently, suspending nanoparticles into transformer oil has been found to signifivantly improve the insulating properties of transformer oil. Serveral modification models has also been carried out to explain thier modification mechanisms. Both electron capturing model for conductive nanoparticles and electron transportation by shallower trap for semi-conductive nanoparticles have been proposed. But none of them are universality. Thus, in this paper, we aimed to propose the modification mechanism of nano-size particles on insulating properties of transformer oil/pressboard by study the characteristics and principles of streamer propagation in nanofluids.Three types of nanoparticles, which are Fe3O4(conductive)、TiO2(semi-conductive) and Al2O3 (insulating) nanoparticles with same size and oleic acid surface modification, were used to modify the transformer oil.The effects of nanoparticle types, concentrations and gap distances on insulating properties of nanofluid under lightning impulse voltage were examed. It was found that, with Fe3O4 and TiO2 nanoparticle modification, the positive impulse breakdown voltage and chop time of transformer oil were increased but the negative impulse breakdown voltage and chop time of transformer oil were decreased. Al2O3 nanoparticles, on the other hand, had little effects on insulating properties of transformer oil. In addition, with the increase of nanoparticles concentration of and gap distance, the modification effects of nanoparticles on positive impulse breakdown properties of transformer oil were also increased.To study the initating and propagating characteristics of streamers in transformer oil, a streamer shadowgraph measurement system was establised. The pre-breakdown streamer in tranformer oil and nanofluids were measured.It was foun that nanoparticles can increase the inception voltage of positive streamer and change the propagating spatial structure of streamers in transformer oil. The positive streamers in nanofluids propagate slower with thicker body and more branches while the negative streamers propagate faster and more filamentary. The shadograph pictures also indicated that Fe3O4 and TiO2 nanoparticles showed remarkable changes on the streamer shape and propergation speed, whereas Al2O3 nanoparticles have little obvious impacts on the streamer characteristics of transformer oil.In this paper, the trap characteristics of nanofluids were also studied by using Thermal Stimulated Current method. It was found that the nanoparticles can increase the shallow trap density in transformer oil. The relationships between traps characteristics, breakdown voltage and streamer structure properties of oil samples were built and a possible influence mechanism of nanoparticles on insulating properties of nanofluid was proposed. The produced electrons would lose their kinetic energy through repeatly trapping and de-trapping process, resulting in the decrease of electron mobility. The higher trap density of nanofluids, the lower average energy and mobility of electrons are obtained. The electrons with lower energy are easier to be captured by those molecules with high electron affinity, and be converted into negative space charges. Those negative spache charges will decrease the electrical field strength at the front of positive streamer and therefore reduce the propagating velocity of positive streamer. The positive streamer stimulation, which based on field dependent molecular ionization model, further verified such influence mechanism of nanoparticles on propagating properties of positive streamer in nanofluids.By using TiO2 nanofluid, a new kind of nanoparticle modificated oil impregnated pressboards obtained. The effect of nanoparticle on the creeping flashover properties of oil impregnated pressboard was measured. It is found that TiO2 nanoparticle enhanced both positive and negative creeping flashover voltages of oil impregnated pressboard. The flashover chop times was also prolonged. The effect was also found to increase with gap distances. Such changes were believed to be caused by the matching of permitivities between nanofluid and nanofluid impregnated pressboard which can reduce the interface charge production on the surface of pressboard. The charges density reduction on the surface of pressboards further lead to less attractive force applied to the streamer. The reduced attractive forece can weaken the squeeze effect of streamers along the surface of pressboard and decrease the electrical strength in front of streamer, resulting in improvement of creeping flashover voltage of nanofluid impregnated pressboards.