Study On Dynamic Characteristics And Inhibition Approaches Of Streamers In Natural Ester Insulating Oil

Natural ester(NE)insulating oil is a type of environment-friendly liquid dielectric with distinctively high fire point.It has been widely used in distribution transformers and is towarding to applications in large power transformers.Since the molecular structures of NE insulating oil and mineral insulating oil are different,the molecules ionization and propagation in the NE insulating oil are different from those in mineral insulating oil.Upon the request for promotion and application of NE insulating oil in large power transformers,it is critically important to clarify the generation and dissipation of charge carriers(electrons and ions)in NE insulating oil and oil-pressboard insulation under high field strength.The research could provide experimental data and theoretical support for the insulation design and operation maintenance of NE insulating oil power transformer.This dissertation studies the dynamics of streamers in NE insulating oil and on oil-pressboard insulation.A route to streamer suppression in NE insulating oil is proposed resultantly.The main research contents are described as follows:(1)An observation platform for the dynamic characteristics of the streamer in the insulating oil under an extremely uneven electric field was built for measuring the lightning impulse(LI)breakdown voltage of the insulating oil,the discharge current,and the streamer gray-scale evolution images during the streamer development process.The discharge current and the shadow images of the streamer in NE insulating oil during the propagation process under the LI voltage were analyzed.It was the first time found that the low charge density "precursory branch" of the streamer in NE insulating oil under the positive LI voltage.This branch could effectively increase the ionization rate of molecules at the streamer head,and then make it evolve into the main branch of the streamer.Besides,a multi-framing ultra-high-speed camera was used to obtain luminous images of the streamer in NE insulating oil during the initiation and propagation process.The light radiation intensity of the streamer is proportional to the gray value of the streamer branch.This light radiation of the precursory branch could also be conducive to the ionization of molecules,which promotes the streamer propagation in NE insulating oil.The luminescent intensity of the streamer near plate-electrode is significantly higher than that of the main branch,and its propagation velocity is much higher than the average velocity.(2)The molecular ionization potential(IP)and electron affinity(EA)of NE insulating oil were calculated by the quantum chemistry method based on density functional theory.The influence of the molecular structure of fatty acid esters such as carbon chain number,double bond,and chain length on molecular IP and EA were analyzed.The molecular IP and EA could be strongly affected by the number of the double bond and carbon chain.The integrated streamer observation platform was used to investigate the influence of ester molecular structure on streamer propagation and dissipation process in NE insulating oil.Under the positive LI voltage,the streamer propagation speed in the single-chain fatty acid ester with lower EA and higher IP is faster than that in the triglyceride ester with higher EA and lower IP.While the streamer propagation speed of the former is slower than that of the latter under the negative LI voltage.The streamer simulation model considering electrons collision with molecules was established for the simulation of the electric field and space charge distribution of the streamer in NE insulating oil.The results show that the collision process between electrons and molecules in a positive streamer could improve the space charge density at the streamer head and then promote the streamer propagation in NE insulating oil.The more easily electrons are absorbed by molecules in the negative streamer,the higher the electric field intensity of the streamer head is,which promotes the negative streamer propagation in NE insulating oil.(3)The observation system for dynamic characteristics of streamer along the oil-pressboard surface was constructed to simultaneously obtain the voltage,streamer current,and streamer shadow images during the streamer propagation and dissipation process.Under the positive LI voltage,the charge density of the surface streamer in the single-chain fatty acid ester is lower than that of the corresponding branch in triglyceride during the initial stage.The resistance during the space charge migration process in the single-chain fatty acid ester is smaller than that in triglyceride ester,which makes the space charge at the surface streamer head in the single-chain fatty acid ester easier to diffuse into the oil,and then the main branches of the streamer could spread in the oil.The propagation distance of main branches in single-chain fatty acid ester could be increased,which makes the positive LI surface breakdown voltage of the oil-pressboard system slightly higher than that of pure oil.Under the negative LI voltage,the existence of the pressboard promotes the development of the surface streamer in single-chain fatty acid ester,thus reducing the negative LI surface breakdown voltage of the oil-pressboard system.Also,the surface breakdown process under negative LI voltage causes more damage to the pressboard,and the surface streamer is easier to spread along the damaged carbon traces under negative LI voltage,thus reducing the surface breakdown voltage of the damaged oil-pressboard system.(4)Aiming at the problem that the fast streamer in NE insulating oils is easy to form under positive LI voltage,a method of doping ultraviolet absorption molecules in NE insulating oil was proposed to suppress streamer propagation in NE insulating oil.The Density functional theory was used to simulate the IP,EA,and excitation energy of the molecules of based oil and UV absorption molecules.The molecules with different UV absorption capacities were selected,and the IP of chosen molecules was similar to that of the molecules in the based oil.Based on the streamer discharge observation platform,the influence mechanism of UV absorption molecules on streamer initiation and propagation process in NE insulating oil was studied.The molecules with maximum absorption wavelength corresponding to the first excitation energy of base liquid molecules were able to inhibit the development of streamer in NE insulating oil under the positive LI voltage,thereby significantly increasing the LI breakdown and acceleration voltage of NE insulating oil.The above research work provides not only theoretical guidance for the selection of NE insulating oil but also insulation design and operation maintenance of NE insulating oil power transformers,which promotes the application of NE insulating oil in power transformers.