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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]

The standard lightning impulse (LI), defined as a smooth full lightning impulse voltage having a front time of 1.2 ?s and a time to half-value of 50 ?s, obtained from field observation, has remained unchanged from IEC Ed. 1 in 1962 through to Ed. 3 in 2010. LI test plays an important role in the equipment design and insulation defects detection. In order to evaluate the reasonability of the standard LI waveform from the point of insulation characteristics, breakdown characteristics of GIS under LI with different wavefront times Tt and wavetail times Tt were studied. Those experimental results show that the LI waveform parameters have little influence on the breakdown characteristics of a sound GIS equipment, but have great influence on the GIS equipment with defects. For a highly inhomogeneous electric field, with the wavefront time increasing, the 50% breakdown voltages U50% have remarkable change and the U50%?Tt curve tends to be U-shaped. The 50% breakdown voltages decrease significantly with the increase of impulse wavetail time, especially when wavetail time is in the range of 1.5 to 15 ?s, but the difference of the values decreases after 15 ?s. Ignoring the generating possibility in the field for the large series inductance and load capacitance, a LI with wavefront time in the range of 0.2 to 0.7 ?s is proposed in the field for it may increase the probability of detecting insulation defects. Finally, the polarity reversion phenomenon and critical radius phenomenon, observed from the breakdown characteristics, can be explained by the migration and diffusion of space charges. [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]

Discharges in liquid obey time effect, pressure effect and scale effect. Traditional formulas are usually applied in a narrow range to keep the formula form consistent with Martin?s formula. In this paper, we have developed a new empirical formula for the electric strength of transformer oil depending on pulse duration, hydrostatic pressure and gap geometry in form Εbr = κe at-beff Ρ/,sub>spl gamma/ A>sub>-Β d-ϵ to be applied in a wider range. In this formula, the time dependent pressure effect is represented by constant γ, which can be expressed as γ=ape -bpt-c>peff . Time effect, area effect and distance effect are represented by e at-beff, Β and ϵ, respectively. For transformer oil, κ=0.42, a=0.53, b=0.23, ap=0.14, bp=0.93, cp=0.41, Β=0.1 and ?=0.25?0.06, with Ebr, teff, P, A and d in MV/cm, μs, MPa, cm2 and cm, respectively. [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]

The nanosecond single-pulse voltage was employed here to generate a large-volume discharge between the wire-plane electrodes in nitrogen. In order to find out the effect of pulse polarity on the propagation of a large-volume discharge, both the positive and negative pulses were applied to the wire-plane electrodes, respectively. Images presented here reveal that the discharge channels are more homogeneous under the negative pulse, and the propagation speed of the discharge channels is faster under the positive pulse. [ABSTRACT FROM AUTHOR]

The wavefront time T f of lightning impulse plays an important role in the detection of insulation defects in metal enclosed gas insulated switchgear (GIS). Usually, a rod-plane electrode system with different geometric parameters is adopted to simulate local field enhancements in GIS. But, for GIS, the defects may occur on the high-voltage electrode which can not be properly simulated by a rod-plane electrode. To research the influence of waveform parameters on actual defects in GIS, cylinder-plane, coaxial cylinder, and basin-type insulator with a conductive protrusion, were used in this study. A generating system of double-exponential impulses with wavefront times in the range of 0.08 to 15 μs and wave tail times of approximately 50 μs was used to simulate different impulse test waveforms based on a fully enclosed oil-immersed Marx generator. The results show that for the cylinder-plane gap with a protrusion, when the gap distance of the needle tip to the plane is constant, increasing the length of the needle decreases the 50% breakdown voltage. Comparing with the insulation characteristics of rod-plane gap, the U 50 %-T f curves of the cylinder-plane electrode with needle defects thereafter no longer show a U shape as the rod-plane electrode, but increase with an increase in the wavefront time. The difference can be explained by the shielding effect of the cylinder electrode on the defect, which changes the electric field distribution of the gap. The U 50 %-T f curves of GIS bus with different voltage levels and basin-type insulator with surface conductive defect also show the similar increase trend. For V-t characteristics of GIS bus with different voltage levels, we find that the higher the voltage level, the more obvious the trend of the V-t characteristic bathtub curve. [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]