| The vacuum performs well as insulation. It can be used as a filling dielectric, along with isolating and supporting solid insulators. But it is found that the dielectric strength of the vacuum and solid insulation system is much smaller than the vacuum and solid respectively. And the insulation breaks down in the form of flashover. There are complex processes in the vacuum and solid interface different from the traditional insulation breakdown, which is theoretically important. At the same time, in general vacuum insulation is applied in components and equipment with complex structure, and also extremely expensive, quite a part are as the core equipment of high-tech areas. Therefore, the flashover will bring huge economic and attached losses, seriously affect scientific research and society.Vacuum insulator is always subjected to nanosecond pulses in engineering. The flashover mechanism of vacuum insulator under nanosecond pulse was accepted as secondary electron emission by lots of scholars, yet lack of experiment support and mechanism study. There are few studies on surface charge accumulation under nanosecond pulse. The influence mechanism of surface charge accumulation on flashover voltage on vacuum insulator is not clear yet under nanosecond pulse.Therefore, for research on vacuum surface flashover under nanosecond pulse which is general in engineering, a vacuum flashover test platform under nanosecond pulsed was built in this paper. Based on nanosecond pulsed power technology, nanosecond pulse sensing technology and surface charge measurement technology, a nanosecond pulse voltage generator, nanosecond pulse voltage measuring sensor and high precision surface charge distribution on solid surface 3D measurement system in high vacuum were designed and developed. With this test platform, pulse wave with the rise time of 10 ns, pulse duration of 100 ns was able to be applied, and nanosecond pulse voltage was able to be measured. Solid surface charge density measurement was able to be carried on, with 3D control mechanism the static capacitance probe was able to scan along the insulator surface side surface with radial accuracy of 0.1mm, and circumferential accuracy of 1 degree, to cover the whole radial length and circumferential angle on the insulator surface. The charge resolution of the static capacitance probe was up to 0.17094pC/(mm2·MV), and the spatial resolution was 1.053mm2.Through experiment and theoretical analysis, three-dimensional distribution characteristics and the generation and distribution mechanism of surface charge 45 degree cone insulator was obtained, and the effect of surface charge accumulation on the surface electric field of insulator in vacuum was also obtained. Under the positive pulse voltage the surface charge is positive, while under negative pulse voltage the surface charge is negative. The surface charge density lager close to the conical tapered end of the insulator, and decreased towards the other end of the insulator along the radial, and surface charge along the circumference of a circle is evenly distributed. There are corresponding relations between the surface charge distribution and the voltage and electric field distribution. The increase of the amplitude of the pulse voltage and the increase of strike times will lead to more surface charge accumulation. Under the positive pulse voltage, the surface charge may come from the anode hole emission, and the surface charge of the negative pulse voltage may be mainly derived from the electron emission.The influence of surface charge accumulation is manifested as the distortion of the surface electric field and the pushing of high field area to the weak side. In this system, high field area can be pushed the distance of 5% of flashover length due to surface charge accumulation. Surface charge measurement and flashover test ware carried on with defect samples, and the results showed that the accumulation of surface charge directly affect the electrical strength of vacuum insulator. Under the test condition, there was a flashover voltage decrease of 16% caused by surface charge accumulation compared to the sample without surface charge accumulation.Through the experimental study, the influence of secondary electron emission coefficient on surface charge accumulation and flashover voltage was obtained, which provides experimental support for the secondary electron emission mechanism under nanosecond pulse. Effect of secondary electron emission coefficient of different kinds of materials on the surface charge accumulation was studied comparatively. The secondary electron emission coefficient of cross-linked polystyrene, PMMA, PTFE and alumina ceramic materials were measured, the results showed that the cross-linked polystyrene material of the secondary electron emission coefficient was the smallest. The secondary electron emission coefficient of PMMA and PTFE is slightly larger, the secondary electron emission coefficient of alumina ceramic is much larger than that of three kinds of organic materials, up to 1.9 times in the integral form. The surface charge accumulation and material secondary electron emission coefficient of vacuum insulator is positive correlation under the same conditions, that is the surface charge accumulation of the material with larger secondary electron emission coefficient tend to be more serious, the surface charge density differences can be up to 5 times, which can lead to 20% flashover voltage decline.Through the experimental study and theoretical analysis, mechanism of electron multiplication and charge transfer for 45 degree cone vacuum insulators under nanosecond pulse was put forward. It was suggested that the initial electrons impact the insulator surface, leads to secondary electron emission. And the secondary electrons are trapped when moving forward to the anode, forming surface charge accumulation. Surface charge accumulation affected the surface electric field intensity and distribution range, leads to more and even more serious secondary electron emission. The process repeats on and on, then electron multiplication and transfer take place due to secondary electron emission and surface charge accumulation. Therefore, high field strength area keeps moving forward, and flashover will occur on the vacuum insulator eventually. |
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