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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]

A new method was introduced to evaluate and analyze synergistic effect including negative synergistic effect of SF₆/N₂ gas mixtures breakdown voltage. This method provides an intuitional comparison among different degree of synergistic effect through a simple equation of the normalization coefficient h. The disadvantages of traditional method, which fails to evaluate negative synergistic effect and would be inaccurate when breakdown voltages of the two gas compositions are close, are overcome via the method. In this paper, a typical example of the above is shown to analyze synergistic effect and abnormal negative synergistic effect of SF₆/N₂ gas mixtures in non-uniform electric field under lightning impulse. Meanwhile the validity of the normalization coefficient h with the synergistic effect of SF₆/N₂ gas mixtures under various voltage conditions and electrodes in references was verified. [ABSTRACT FROM PUBLISHER]

In this paper, the influence of surface charges is quantitatively analyzed based on the study of surface charge distribution under DC voltage and flashover characteristics of insulators under superimposed voltage of DC and lightning impulse. It is found that when DC and impulse voltage are in the same polarity, surface charges weaken the electric field along the insulator surface and flashover voltages under superimposed voltage increases. When DC and impulse are in the opposite polarity, surface charges enhance the electric field along the insulator surface, and flashover voltages will decrease. The calculation results of the electric field along the flashover path show that the changing rate of the maximum field strength is approximately equal to the changing rate of flashover voltage, which proves that the influence of surface charges on flashover characteristics of insulators mainly depends on their distortion of the electric field. [ABSTRACT FROM AUTHOR]

The gas-solid interface is generally considered to be the weakest part in gas-insulated systems. The flashover voltage of a gas-solid interface is difficult to predict because it involves an extremely complex physical process affected by multiple factors. In this paper, we employ the support vector regression (SVR) method based on mass data on the features of the electric field that are indicators of the flashover voltage. We cast 48 kinds of epoxy resin post insulators as samples, and 61 features are extracted to characterize the surface electric field distribution and the flashover voltage under standard lightning impulse voltage. Four indicators that may be useful in predicting the flashover voltage were obtained; the maximum electric field along the surface of the insulator (E max), the normal electric field components of E max (E n_Emax), the maximum gradient of the surface tangential electric field component (E t_gm), and the integral of the area in which the E value exceeds x% of E max along the insulator surface (V t60). We establish an SVR flashover voltage prediction model with these four features, whose average deviation is 7% in 10,000 predictions. It is expected that this study may provide a reference for the continued study of the gas-solid surface flashover mechanism. [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]

Alumina filled epoxy resin is important insulating material, which discharge characteristics are greatly determined by its material properties. In this paper, the surface flashover characteristics of the epoxy resin with various alumina contents (250- 400 phr) in 0.4 MPa SF6 are studied. The short-term and long-term withstand voltages and the application times are measured, and then the V-T characteristics are compared accordingly. The withstand voltage of the epoxy resin with certain alumina content decreases exponentially with time and the long-term withstand voltage is about 20-30% lower than the short-term withstand voltage. The discharge damages to alumina filled epoxy resin material are also studied through the carbonization traces and the microcosmic damage patterns. It is found that the epoxy resin with 350 phr alumina has the lowest short-term and long-term withstand voltages and most serious carbonizations according to the experimental results. The ?wormholes? formation in the discharge and its effect in the flashover process are analyzed and discussed based on the carbonation traces and the microcosmic patterns. It is found that the flashover paths shift from creeping-discharge-based to bulk-breakdown-based due to wormholes effect with the increase of alumina content and the critical alumina content is about 300-350 phr. [ABSTRACT FROM PUBLISHER]