The Influence Mechanism Of Nanoparticle Charging-Discharging Behavior And Microscopic Disturbance On Streamer Evolution In Insulating Oil

Establishing a complete theoretical system for nano-oil breakdown theory is an important foundation for in-depth understanding of the nano-effects and precise control of insulating properties.The existing theories simplifies the nanoparticle to electron traps that capture electrons.By considering the relationship between physical parameters and charge trap characteristics,those theories clarify the trapping effect on charges and its influence mechanism on the streamer propagation,which lays an important foundation for in-depth understanding of the modification mechanism of nano-insulating fluids.However,do the nanoparticles only act as charge traps in insulating oil is still not clear.To clarify the dynamic mechanism of nanoparticle on the charge transfer and streamer propagation is still a important scientific problem.In order to advance the optimization of nanoparticle parameters and the perfection of the breakdown mechanism of nano-oil,this thesis focuses on the electrodynamic behaviors of nanoparticles and the effect of perturbation on the electric field of streamer head.By establishing the response equations of nanoparticles under the action of electric field,a mapping system between physical parameters of nanoparticles and their dynamic electrodynamic behaviors is constructed.And then the mechanism of the electrodynamic behaviors of nanoparticles,such as charge-discharge or spin,on the development of streamer are clarified.Considering the interaction of nanoparticles as a disturbance source on the streamer interface,a dynamic model of streamer morphology evolution is established.Based on the model,the influence of the disturbance mode on streamer morphology is clarified.The main research results are summarized as follows:The conductive Fe₃O₄ nanoparticles and non-conductive Al₂O₃ nanoparticles were selected as modified materials to prepare nano-oil with good dispersibility and stability by surface modification and ultrasonic dispersion method.A platform for observation of streamer development was built up and the morphological evolution characteristics of streamer in nano-oil under the action of lightning impulse voltage and power frequency voltage were obtained.The results show that under the action of lightning impulse voltage,Fe₃O₄ nanoparticles can increase the inception and stopping voltage of the positive streamer,hence hinder the development of positive streamer.But they also reduce the inception and stopping voltage of negative streamer,and accelerate the propagation of negative streamer.Fe₃O₄ nanoparticles significantly increase the branches of positive and negative streamer.Al₂O₃ nanoparticles have no significant effect on the inception voltage,stopping voltage and stopping length of positive and negative streamers,but they increase the streamer branches.Under the action of power frequency voltage,Fe₃O₄ nanoparticles reduce the inception voltage of positive streamer,but still slow down the development of positive streamer.Al₂O₃ nanoparticles can effectively increase the inception voltage of streamer,especially the negative one,but has no obvious effect on the stopping length of streamer.By analyzing the influence of the conductance process of conductive nanoparticles under electric field excitation,the potential well generated by the surface charge was calculated.Based on the trapping effect on electrons,the influence law on charge transfer of charged conductive nanoparticles with different physical parameters were further analyzed,which revealed the influence mechanism of nanoparticle physical parameters on the streamer propagation and the electric field of streamer head.The results indicate that under the action of external electric field,the conductive nanoparticles can generate charge traps and play the role of trapping electrons.However,the trapped electrons can enhance the electric field of the negative charged area,resulting in electron tunneling effect,which enhances electron release process of nanoparticles.The above mechanism reduces the electron capture ability of nanoparticles and promotes the migration of electrons at the streamer head,thereby weakening the electric field of positive streamer head but enhancing the electric field of negative streamer head.This theory makes a supplement of the electron trapping theory and reveals the reason why certain conductive nanoparticles（such as Si C nanoparticle）cannot improve the insulating properties of insulating oil.By establishing the response equations under electric field excitation,the influence of the conductance and polarization process of non-conductive nanoparticles on the surface charge distribution was analyzed,and the charge trapping process of non-conductive nanoparticles was explained.The analysis and calculation of non-conductive nanoparticles various spin behaviors revealed the relationship between spin behavior,nanoparticle physical parameters and electric field parameters,which also clarified the mechanism of spin on charge transfer and streamer initiation.Studies have found that non-conductive nanoparticles can also capture electrons through polarized charges,thereby affecting the development of streamers.On the other hand,non-conductive nanoparticles may spin due to various factors under the action of different voltage forms.The spin behavior of nanoparticles can enhance the flow of insulating oil and reduce its viscosity,thereby increasing the mobility of ions.The time required for spin is longer than the development time of the streamer,so the spin process cannot affect the development of the streamer.However,due to the influence of the spin behavior on the ion mobility,it can increase the inception voltage of streamer under the action of the power frequency voltage.This theory enriches the mechanism of non-conductive nanoparticles and explains the reason why Al₂O₃ nanoparticles increase the inception voltage of the streamer at the power frequency voltage but cannot affect the stopping voltage of the streamer.Considering the contact/non-contact interaction forces between nanoparticles and streamer interface,the mechanism of microscopic disturbance generated by nanoparticles on the gas-liquid interface of the streamer was proposed.And then a hydrodynamic model that takes into account the surface mechanical behavior of the streamer was established.Based on the model,the evolution law of the random disturbance generated by nanoparticles on the streamer surface was analyzed,and the influence mechanism of the disturbance mode on the streamer morphology evolution was analyzed.It indicates that the evolution of the disturbance on the streamer surface is affected by the disturbance mode,applied voltage and other parameters.When the voltage is lower than a critical value,only the symmetrical disturbance with a wavelength close to the circumference of the streamer can cause the instability of the streamer body,leading the streamer body to expand,contract or even pinch off.When the voltage is higher than a critical value,the smaller wavelength disturbance can easier lead the morphology change of the streamer body.But when the disturbance wavelength is less than a critical value,the disturbance has limited influence on the shape of the streamer cylinder.