Research On The Charge Transport Mechanism In Transformer Oil-based Nanofluid Under Impulse Voltage

As the scale of power industry continues to expand,higher requirements are placed on the safety and stability of power transformers and other power equipment.Transformer oil as the main insulating medium of the transformer,its insulation performance will directly affect the operational reliability of the transformer.Nanoparticle modification provides a new idea for improving the insulation performance of transformer oil.It is helpful to ensure the safe and stable operation of the ultra-large-scale transmission system effectively and reduce the insulation size of the oil-immersed electrical equipment and the equipment cost.Therefore,the systematic research on transformer oil-based nanofluid is of great significance for the development of new high-performance insulating media and high-reliability UHV power equipment.This paper focuses on three aspect problems.The influences of nanoparticle microscopic properties(e.g.size,morphology and surface modification)on transformer oil insulation performance are lacking.The mechanism of nanoparticle polarization trapping electron or shallow trap modification lacks direct experimental verification.Morover,the mechanism of microscopic charge transport in nanofluid is not clear.Therefore,the distribution state of nano-particles in transformer oil,the breakdown performance of nano?oil,the development characteristics of flow injection and the micro-charge transport characteristics were studied,taking the titanium dioxide nano-particle modified transformer oil as the research objectThis paper prepared titanium dioxide nanoparticles with different microscopic properties,including different morphologies(spherical,rod-like),different sizes and different surface modifications(acetic acid,caproic acid and oleic acid),and the corresponding nano-modified transformers were obtained by two-step method,oil.Using liquid microchips,the distribution of nanoparticles in transformer oil was observed by TEM for the first time.It is found that nanoparticles have good dispersion in transformer oil.The breakdown properties of pure oil and transformer oil-based nanofluid under the action of positive lightning impulse voltage were measured.The results show that the morphology,size and surface modification of nanoparticles have a significant impact on the lightning breakdown performance of transformer oil-based nanofluid.Compared with pure oil the breakdown voltage of rod-shaped transformer oil-based nanofluid is increased by 55.5%.and the spherical transformer oil-based nanofluid is increased by 25.9%.The modification effect of rod-shaped nanoparticles on breakdown voltage of transformer oil is 2.1 times that of spherical nanoparticles;The positive lightning impulse breakdown performance of transformer oil increases with the decrease of the size of nanoparticles and the decrease of the modifier carboxylic acid molecule..A high-definition photograph platform for transformer oil based on schlieren method was established.The measurement of streamer propagation characteristics of transformer oil-based nanofluid containing nanoparticles with different characteristics were realized for the first time.The results show that the influence of nanoparticle moiphology,size and surface modification on the streamer propagation iin transformer oil-based nanofluid is consistent with the impact on the lightning breakdown performance.Compared with spherical nanoparticles,rod-shaped nanoparticles make the initial voltage of the streamer in the transformer oil lower.Streamers of nanofluid with rod-shaped nanoparticles have denser branches,and the development speed is slower.The inhibitory effect of rod-shaped nanoparticles on the streamer propagation is more obvious.Therefore,the breakdown voltage of the rod-shaped nanofluid is higher.As the size or the modifier carboxylic acid lolecule of the nanoparticles decreases,the branches of the nanofluid is denser and the development speed is slower.Therefore,the nanoparticles inhibits the development of the streamer propagation,thereby increasing the breakdown voltage.Based on the surface potential decay method,the potential attenuation decay under vacuum environment was built.The influence of the microscopic characteristics of TiO2 nanoparticles on the trap characteristics of transformer oil was studied.The relationship between surface characteristics and trap characteristics of TiO2 nanoparticles was established.It is found that there are more Ti3+defect states on the surface of the rod-shaped nanoparticles than the spherical nanoparticles,introducing more shallow traps in the naofluid.As the average particle size of the nanoparticles decreases,the specific surface area of the nanoparticles increases,and the surface of the particles increases.The Ti3+defect state increases,and more shallow traps are introduced into the transformer oil.As the molecular length of the surface modifier increases,the trap density increases.Based on the Kerr photoelectric method,the measurement method for electron mobility in transformer oil was designed.For the first time,the electron mobility measurement of pure oil and transformer oil-based nanofluid under high electric field was realized.The results show that the nanoparticles significantly increase the electron mobility in the transformer oil.The electron mobility of nanofluid wirh spherical nanoparticles is 2.6 times that of the pure oil.Moreover,the electron migration in the nanofluid with rod-shaped nanoparticles is faster.The lodification mechanism of nanoparlicle polarization trapping electrons was firstly studied by test.It is found that the electrons captured by nanoparticles and the transition between traps existed simultaneously.The synergistic effect mechanism of polarization and trap characteristics on electron transport is proposed.Then,combined with the characteristics of microscopic charge transport and the macroscopic streamer propagation characteristics,the charge transport model and the mechanism of streamer propagation in nanofluid are proposed.Therefore,the modification mechanism of nanoparticles on the breakdown of transformer oil by shallow trap is proposed.A large number of shallow traps are introduced by nanoparticles,promoting electron migration and inhibiting gas discharge in the low-density region of the streamer head.In that case,streamer propagation in transformer oil is inhibitted,which significantly increases the breakdown voltage of transformer oil.In addition,the change of the microscopic characteristics of the nanoparticles will cause the change of the trap state on the surface of the particles,which will cause the trap characteristics to change,and affect the electron migration process and the development of the streamer propagation.In tahat case,it affects the breakdown voltage of the transformer oil,which provides a theoretical basis for the functional design of nanoparticles.