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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]

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]