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Even for gas-insulated metal-enclosed switchgear (GIS) equipment that has passed an on-site dielectric test, some dielectric failure can occasionally occur without warning. It is necessary to evaluate the effectiveness of on-site dielectric tests of GIS equipment. In this study, two standard test procedures, power-frequency partial discharge (PD) and lighting impulse (LI) withstand test, were carried out in a real model test platform of 550 kV GIS to evaluate the effectiveness of on-site dielectric tests. Through experiments, the minimum detectable defect sizes were determined for different curvature radius values. The results show the limitation of the power-frequency PD test and the need to use of the LI withstand test as supplementary assessment on site. Two calculable constraints of defect detectability were proposed to extend the results to GIS equipment at other voltage levels. The results clarify the cause of dielectric failure without any warning and provide technological guidance for manufacturing and installation. [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]

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]