THESIS
2018
xx, 113, that is, xxii, 114 pages : illustrations ; 30 cm
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...[
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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.
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