Study on the nanosecond pulse vacuum flashover property of polystyrene based insulation materials

Abstract

The fast development of high-performance pulse power equipment in military and civil industry places great demands on the performance of polymeric insulation materials. Among the high voltage insulation issues, flashover is the most serious one since it happens on the insulator surface and is triggered by lower voltage comparing to that of bulk breakdown and gap discharge with similar size. This makes it significant to explore the mechanism of flashover on the insulator surface and to improve their flashover resisting ability through material design. In this thesis, two modification methods on polystyrene, crosslinking and side group modification, are delivered to improve its flashover resisting ability. And the mechanism of the effect of different chemical modification methods on the flashover resisting performance of polystyrene materials is explored. Firstly, in order to more accurately characterize the nanosecond pulse vacuum flashover resistivity of the modified polystyrene materials, the testing platform and procedure is thoroughly studied and optimized. By comparing the pulse waveforms generated by different experimental loop parameters, it is found that the nanosecond pulse front steepness is affected by the surface impedance of the samples. And this instability of the pulse waveform causes a considerable error in the measured flashover voltage values. Therefore, the influence of the front steepness of the nanosecond pulse on the flashover voltage is studied. A linear relationship between nanosecond pulse vacuum flashover voltage and pulse front steepness is found and verified in both simulation and experiments. Based on this linear relationship, a set of test methods that can eliminate the influence of the front steepness of the pulse waveform is proposed for better evaluating nanosecond pulse vacuum flashover of different materials. Based on the optimized flashover test method, the influence of crosslinking density of styrene-divinylbenzene (St-DVB) copolymers on nanosecond pulse vacuum flashover resisting performance is studied. It is found that as the crosslinking density increases in St-DVB copolymers, the nanosecond pulse vacuum flashover voltage is obviously enhanced. To further study the influence of crosslinked network on the flashover resisting performance of St-DVB copolymers, the trapping-detrapping behavior of samples with different crosslinking densities are characterized by both thermally stimulated current (TSC) and broadband dielectric spectroscopy (BDS). The results show that the formation of network structure in St-DVB copolymers not only hinders the movement of polymer segments, but also suppresses trapping-detrapping behavior for main chain polarized structure. This leads to the suppression of secondary electron emission (SEE) process, and finally results in the enhancement of vacuum flashover voltage increase. Finally, the influence of side group chemical modification of polystyrene on vacuum flashover resisting performance is studied by comparing the performance of polystyrene and polystyrene modified with different side groups, including chlorophenyl, fluorophenyl and naphthalene group. The nanosecond pulse vacuum flashover characteristics of polyfluorostyrene, polychlorostyrene, styrene-vinyl naphthalene copolymers and poly (vinyl naphthalene) are characterized and compared. The trapping behavior of the above materials are also studied with the help of density functional theory (DFT) calculation. The results show that the introduction of halogen in polystyrene materials can effectively improve the vacuum flashover resistance of insulating materials; after the introduction of naphthalene side groups in polystyrene materials, it significantly inhibits the secondary electron emission of materials and improve nanosecond pulse vacuum flashover resisting performance.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology