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In recent years, SF6/N2 binary mixtures have been used as insulation in second generation gas-insulated metal-enclosed transmission lines mainly because of the greenhouse effect and diseconomy of SF6. Though the synergistic effect of SF6 is known in a uniform electric field, there is little understanding of the synergistic effect phenomenon with parameters in a non-uniform electric field under standard lightning impulse. A series of experiments were designed to increase our knowledge of this and the breakdown characteristics of SF6/N2 under multiple parameters show a negative synergistic effect. Our experimental results with negative lightning impulse demonstrated that the breakdown voltage of N2 increased greater than with both SF6 and its mixtures with increasing gas pressure. When the gas pressure exceeded 0.3MPa, the breakdown voltage of pure N2 was higher than SF6/N2 gas mixtures and even exceeded the value of SF6 at 0.5MPa. A normalization coefficient h was introduced in this research to evaluate the negative synergistic effect. Under negative lightning impulse the analytical results show that the negative synergistic effect increased with the augment of gas pressure. On the other hand, the synergistic effect was reduced with decreasing gas pressure under positive polarity impulse and it appeared to be a negative synergistic effect with the enhancement of field non-uniformity. The analysis of streamer discharge propagation indicates three reasons lead to the negative synergistic effect: similar streamer corona onset voltage, different space charge effect, and dissimilar discharge form of N2 and SF6/N2 gas mixtures. [ABSTRACT FROM AUTHOR]

For the application of SF6/N2 in power equipment such as GIS and GIL, the breakdown characteristics of SF6/N2 in varying electric field non-uniformity under standard lightning impulse were studied in this paper based on a lightning impulse experimental platform. The research indicates that the reversal of polarity effect appears in SF6/N2 gas mixtures. That is, when the pressure is relatively low, the 50% breakdown voltage under positive polarity is lower than which under negative polarity. When gas pressure reaches the critical pressure, the situation would be opposite. The augment of SF6 mixing ratio or decrease of electric field non-uniformity would both result in the decline of the critical pressure. Meanwhile the change of polarity effect with SF6/N2 is later than that with pure gas with the increase of electric field non-uniformity. In addition, the phenomenon of critical radius of SF6/N2 gas mixtures was found. The critical radius reduces with the increase of gas pressure and is smaller under positive lightning impulse than negative. And the phenomenon of critical radius with pure gas is more obvious than that with gas mixtures. [ABSTRACT FROM AUTHOR]

Follow Phenomenon, defined as a series of continuous breakdowns or withstands without sufficient time lag of successive applied voltages, is observed during the research of SF6 discharge characteristics under lightning impulse. Follow Phenomenon of SF6 gas plays a decisive role for the insulation recovery time between continuous discharges, but little relevant research has been reported before. In this work, Follow Phenomenon as a function of gas pressure, time lag of applied voltages in quasiuniform and non-uniform field, under positive and negative lightning impulse is studied. It is found that, when the time lag t=1 min, Follow Phenomenon is more obvious under positive lightning than that under negative lightning, and it becomes more pronounced under larger pressure; as time lag t increases, the increasing speed of Follow Phenomenon with pressure becomes lower. When time lag t=5 min, the measure uncertainty influenced by Following Phenomenon could be avoided when P⩽0.2 MPa in quasi-uniform field and P⩽0.3 MPa in non-uniform field. As pressure rises, a longer time lag is needed to eliminate the influence of Follow Phenomenon. At the end, analysis is made which shows that Follow Phenomenon is caused by the detachment of negative ion formed at previous discharge processes and by ?the Sweeping effect? of effective initial electrons. [ABSTRACT FROM AUTHOR]

By using 1 MV Marx generator and a 220 kV GIS bus, an experiment platform has been established, and a stainless steel needle is applied to simulate spike defect, so as to study the influence of pressure, wave front time and electric field on 50 % breakdown voltage (U50) and voltage-time (V-t) characteristics of SF6 gas gap with existence of a spike defect on the bus of GIS under double exponential impulse voltage. When wave front time is short, 50% breakdown voltage of the SF6 gap hardly changes with various pressures, and U50%-P curve turns to be a hump shape when wave front time increases. However, pressure has no significant effect on V-t characteristics. The slope of V-t characteristics curve is negative when wave front time of applied voltage is short (0.08~0.9μs), and it will turn to be positive when wave front time becomes longer (several ~15μs). A new method to fit these V-t characteristics curves under impulse voltage with longer wave front times is proposed. It is found that 50 % breakdown voltage decreases almost linearly with the increase of electric field non-uniformity factors (f), and V-t characteristics cluster with valley-like profile have been drawn at different f. Dispersion of V-t characteristics cluster reduces and the flat region expands with the increase of f, and minimum value of breakdown voltage of SF6 gap with spike defect under impulse voltage, which is of guiding significance for engineering, can be obtained. [ABSTRACT FROM AUTHOR]