The Influence Of TiO₂Nanoparticles On Insulation Properties And Charge Transport In Transformer Oil

Transmission and distribution transformers form a critical, highly loaded and expensive part of the electricity generation and distribution network. Electric transformers rely on the high dielectric strength properties of insulating oil to achieve normal operation. Developing new transformer oil with excellent insulation properties means a lot to improve the safe operation level of transformer, and it is also very helpful to decrease the size and weight of ultra-high voltage transformer. Recently, many researches have been focused on the nano-modified transformer oil.In the investigation of nano-modified transformer oil, there are some problems to be solved, among which are the long time stability of nanofluids and the effect of nanoparticle on the charge transport in nano-modified transformer oil.In this dissertation, we aim to develop a new type of nanoparticle modified transformer oil-based nanofluids by suspending semiconductive nanoparticles TiO2into transformer oil, investigate their stability and insulating properties. It reveals that the nanofluid presented high colloidal stability, exhibiting no sedimentation after18months at room temperature (average particle size20nm). Semiconductive nanoparticles can increase the ac and lightning impulse breakdown voltages of nanofluids up to more than1.2times compared with pure oils. Especially, the velocity of streamer in nanofluids is slowed down by65%of that in pure oil. Meanwhile, the partial discharge initiate voltage (PDIV) and resistance against PD of the modified oil was also dramatically improved.In order to promote the practical application of nanofluids, the effect of nanoparticles on the insulation properties of transformer oil in the different humidity and aged conditions was investigated. It is shown that breakdown voltages of samples decrease significantly with deterioration increase. It is interesting to notice that the discrepancy of breakdown voltage between nanofluids and pure oil become more and more obvious with deterioration increase, indicating less reduction of electrical strength in nanofluids.To understand breakdown phenomenon in the insulating materials, the distribution of space charge and the internal electric field has been considered as a key component, which can be assessed by the pulse electroacoustic technique (PEA). The effect of nanoparticles on charge accumulation, decay and transport processes in oil and nanofluid was investigated. The charges in oil are increasing with time and result in an increasing internal field, approximately174%of the applied field. However, the field in the nanofluids appears to be’evening out’ and approximately the same as the applied field. It is found that the charge decay rate for nanofluids is much higher than that for pure oil after the voltage is removed. This rapid charge decay rate is mainly associated with TiO2nanoparticles in nanofluids and effectively prevents the distortion of electric field in the oil. The mode transition and the development of streamer were studied with an ICCD camera.In order to investigate the relationship between charge transport and the trap characteristics, thermally stimulated current method (TSC) measurements were performed. It is believed that TSC measurements can be used to investigate the nature and origin of charge carrier traps in dielectrics, including the change of the number and energy of the trap sites. It is noted that shallower trap centers with higher trap density were formed in nanofluids. The trap density in nanofluid is almost2times compared with that in pure oil. According to the Electrophoresis measurements, it is verified that the internal interfaces between nanoparticles and oil induces alter local structure in the vicinity of the nanoparticles and give rise to altered trap characteristic.Based on our measurement results, it is believed that the surface area in contact with the oil is dramatically increased and creates large interaction zones and the corresponding higher shallow trap density. The charge created at high field in the oil can be captured by the shallow traps and decay fast by the trapping and de-trapping process, which can reduce the the velocity and amplitude of electric field wave at the point of streamer. Meanwhile, facile charge movement induced by the trapping and de-trapping process can impair the ionization process caused by fast electrons and offer more uniform internal electric field and high dielectric strength of nanofluid.