Study On Electrical Properties And Streamer Discharge Characteristics Of Nano C₆₀ Modified Insulating Oil

As the main insulating dielectric of transformer,the insulation performance of transformer insulating oil will directly affect the operation reliability of transformer.The research shows that the intrinsic performance（such as electrical performance,thermal conductivity,etc.）can be improved by adding nanoparticles into insulating oil.However,Despite being discovered more than 20 years ago,nanofluids still cannot be used in the power industry.The fundamental reason is that nano-insulating oil has poor stability,significant increase in dielectric loss and its electrical performance decreases under negative impulse voltage.In view of the above problems,nano-C₆₀ particles were selected to modify the insulating oil,and the stability,electrical properties and physical properties of the modified insulating oil were studied.In this paper,the mechanism of the modification of C₆₀ particles is analyzed based on trap characteristics and streamer discharge characteristics which were obtained by the experiments of the thermally stimulated current trap test and the morphological character and velocity observation,respectively.The main research contents and important achievements are as follows:（1）The morphology of C₆₀ nanoparticles was tested by TEM,and the size distribution of C₆₀ nanoparticles in insulating oil and zeta potential of modified oil were studied.The dispersion stability of the modified nano insulating oil was studied.The solubility difference between C₆₀ nanoparticles and other conventional nanoparticles in insulating oil and stability of modified oil were analyzed.The results show that the absolute zeta potential of nano-C₆₀ modified insulating oil is greater than 30 m V,and the particle size distribution of the fresh modified oil had no obvious change compared with modified oil kept for 12 months.Nano-C₆₀ modified insulating oil has excellent dispersion stability and good stability.（2）The effects of concentration and temperature on the viscosity and thermal conductivity of nano-C₆₀ modified insulating oil were obtained by experiments.The influence of C₆₀ nanoparticles on the breakdown voltage of insulating oil under positive and negative lightning impulse and power frequency voltage were studied.The dielectric constant and dielectric loss factor of different concentration of nano-C₆₀modified insulating oil in wide frequency domain were measured and analyzed.The trap characteristics of nano-C₆₀ modified insulating oil were studied.The test results show that the addition of C₆₀ nanoparticles is helpful for the improvement of thermal conductivity and the decrease of viscosity of insulating oil,C₆₀ nanoparticles can improve both the positive and negative lightning impulse and power frequency breakdown voltage of insulating oil under the concentration range of 50-150mg/L,and almost do not increase the dielectric loss.The addition of C₆₀ nanoparticles deepen the trap level and increase the trap density.（3）The characteristics of the streamer development morphological and velocity in the modified oil were studied.The cut-off length,projection area and fractal dimension of the streamer were selected to quantitatively analyze the characteristics of the streamer morphological.The streamer development velocity was tested by using the photomultiplier array.The experimental results show that the cut-off length and projection area of the streamer in the modified oil are smaller than that in the fresh oil under the same applied voltage,and the fractal dimension of the streamer in the modified oil is smaller than that in fresh oil under the condition of prebreakdown.The addition of nano-C₆₀ particles is helpful to slow down the steamer velocity in the modified oil.There was no significant difference in the streamer velocity with different C₆₀ concentration.（4）Based on the experimental results of streamer morphological characteristics,TSC tests and velocity observation and the intrinsic characteristics of C₆₀ nanoparticles,the modification mechanism of C₆₀ nanoparticles was proposed as follows:The addition of C₆₀ nanoparticles deepens the electron trap energy level and increases the trap density of insulating oil.C₆₀ can capture the electrons during the development of discharge in insulating oil,thus reducing the number of electrons that can participate in the formation of electron avalanche under the electric field.C₆₀ molecules can absorb photons generated during streamer development through photon absorption polymerization（most are dimers）,which weakens the process of photoionization.Thus,the nanoparticles weaken the development of streamer.We believe that the absorption of photons by C₆₀ is the reason for the improvement in the negative impulse breakdown performance of C₆₀ modified insulating oil.