Study On Spatial And Temporal Distribution Of Space Charge In Nano-modified Liquid Dielectric Under Impulse Voltage

The breakdown of liquid dielectric is based on streamer propagation, and the presence of space charge can change the electric field distribution inside the liquid dielectric, which has a significant influence on the streamer development process. Studies have shown that the addition of nanoparticles to the liquid dielectric can influence the insulation strength the liquid dielectric. Thus, it is of great importance to master the measurement technique for the space charge in the new liquid dielectric, and to analyze the mechanism of space charge to the insulation performance and that of nanofluids to space charge distribution, in order to guide the large electric power equipment insulation optimization of grid design and the improvement of voltage level. Based on the improved Kerr electro-optic field mapping measurement, this paper led an in-depth and meticulous research on: the measurement methods for electric field and space charge distribution in liquid dielectric(as well as nano- modified liquid dielectric), the influence of nanoparticles on the space charge dynamics and the space charge effect in the liquid dielectric discharge process.By studying the basic principles of Kerr electro-optic field mapping measurement, the electric field calculation formulas under different combinations of polarized elements and wave plates were deduced and classified by using optical vector and Jones matrix theory. The array photodetector was introduced into Kerr electro-optic field mapping measurement system, which solved the problem that the CCD camera cannot be triggered continuously. The CCD camera can record the light intensity with spatial variation, which is continuous in space but discrete in time; while the array photodetector can record light intensity variation over time, which is continuous in time but discrete in space. According to the respective characteristics of the CCD and the array photodetector, both measurement systems were set up by choosing the appropriate optical components, and the data processing methods for both systems were described in detail. By using the measurement method developed in this paper, three-dimensional distributions of the electric field intensity and the space charge density over time and gap between the parallel plates can be obtained by the array photodetector system. The accuracy and reliability of the novel system was verified based on error analysis of the measurement results and the comparison with the CCD measurement results.Ferromagnetic nanoparticles(Fe3O4 nanoparticles) with size 20 nm was added into propylene carbonate liquid for modification. The nanofluid was prepared by using ultrasonic dispersion method, and its stability was verified by using spectrophotometric method. Switching impulse breakdown test results for propylene carbonate before and after modification showed that, the addition of ferromagnetic nanoparticles could increase the positive switching impulse breakdown voltage between plate electrodes by 11.65%, and the average time to breakdown declined meanwhile. Time-continuous Kerr electro-optic field mapping measurement was conducted to investigate the electric field and the space charge distributions in the pure and modified propylene carbonate between parallel plates under impulse voltage, and thus the spatial and temporal distributions of the electric field and the space charge were obtained. From the distribution of the electric field, it can be seen that the electric field distribution formed an upward convex shape in pure propylene carbonate, while in nanofluid the curve shape was much more flat. This is because the nanoparticles reduced the distortion effect of the space charge on the electric field distribution. From the dynamics of the space charge, it can be seen that the addition of the Fe3O4 nanoparticles could not only reduce the peak value of the space charge density, but also apparently make the space charge dissipation much faster. Analysis shows that after modification, the Fe3O4 nanoparticles can capture fast electrons and convert them into slow negatively charged particles, inhibiting the generation and transportation of the space charge, especially the negative space charge.