Study On Electric Field And Space Charge Distribution Characteristics In Liquid Dielectric Under Switching Impulse Voltage

Space charge is one of the most important parameters to electric-field andinsulation breakdown performance. With the development of EHV and UHVtransmission lines, power transformer makes progress towards the direction ofhigh-insulation, large capacity and miniaturization, which makes traditional oil-papercombination insulation no longer meeting the requirement of high dielectric strengthand protection. It bears great theoretical and engineering significance to explore themechanism of the space charge to the electric field, improve insulating properties oftransformer oil, and enhance the level of overvoltage defense. With the combination ofexperimental observation and theoretical analysis, this paper mainly focuses on thedistribution characteristics of space charge in liquid dielectric and impulse insulationperformance in nanofluids on the basis of Kerr electro-optic measurement technique.Firstly, a high-speed measuring system of electric field and space charge in liquiddielectric was studied. The electric field intensity and space charge density betweenparallel plate and blade-plane electrodes in HV pulsed propylene carbonate with highKerr constant were observed by using a CCD high speed camera. Secondly, thetwo-dimensional adaptive moving average filtering technique and image processingmethod of electro-optic field mapping were proposed, the electric field intensitydistribution in propylene carbonate using brass, stainless steel and aluminumparallel-plane electrodes and stainless steel blade-plane electrode has been studied, so asto provide a reference for a comprehensive study of the impact of space charge onbreakdown process. Further, the photoelectric detection measurement in transformer oilwith low Kerr constant was carried out, which has great significance for theestablishment and improvement of two-dimensional space charge measuring system inoil.In this paper, pure propylene carbonate and Karamay25#mineral transformer oilwere used as dispersion medium of nanofluids, and Fe₃O₄, TiO₂and Al₂O₃nanoparticleswere added to modify the liquids on the basis of depth analysis of the nanoparticles’dispersion, agglomeration properties and surface modification methods. In order toimprove physicochemical property and insulation performance of liquid dielectrics,polyethylene glycol, oleic acid, stearicacid and span80were chosen as surfactant,according to the different surface properties of nanoparticles. Switching lightning impulse voltage tests were applied to the pure liquids and the nanofluids to check theirinsulation breakdown performance. It was found that nanoparticles can enhance bothpositive and negative breakdown voltage compared to that of pure liquid. Themeasurement results were analyzed by using Kerr electro-optic high-speed system,which verified the electron scavenger theory. The nanoparticles act as electronscavengers in electrically stressed nanofluids converting fast electrons into slownegative charged particles to form a net space charge region near bottom electrode,which makes a more symmetrical electric field distribution that can improve theimpulse insulation performance of liquid dielectric.