This paper reports on experimental investigations concerning the behavior of pulsative corona discharges from free conducting wire particles of different sizes in parallel plane and concave electrode systems, insulated with SF6 and its mixtures with nitrogen Nz, triethylamine (CzH5)3N and perfluorocarbon CsF16O gases. Corona inception and particle lift-off voltages as well as corona charge levels are measured for different experimental conditions. The results show that corona and particle lift-off characteristics are affected by particle parameters, electrode shape and gas composition. Generally, addition of triethylamine and perfluorocarbon to SF6 reduces the corona charge level.
Thc new gases or gas mixtures can be used in the ClCCUiC power indusuy. provided they have a high dielccaic strength with a sufficient high pressure at working temperature and they explore an economical advantage. This paper presents an experimental investigation on the corona activity in a coaxial elecnodc geomcay in Nz. SF6 and SF#z mixtures. In this study the low content of N2 in sF6 reveals a reduction in the corona charge indepent of the inner wire diameter and surfaces. Moreover, in these mixtures the prebreakdown corona occurs at a higher voltage. The corona frequency for different N2 content in SF6 records a lower level. The physical mechanisms underlying this phenomenon are discussed to through better understanding for futthcr application. INlRODUCllONThere has been a growing intercst in the application of compressed gases, which is mostly sulfur hexafluoride, sF6. not only in a gas insulated switchgear (CIS) but also in a high voltage underground wnsmission [l]. In practice. the elecmcal breakdown strength of sF6 is very often determined by a local field enhancement due to protrusion, surface roughness and conducting particles left in the system [1,2]. In order to increase the system reliability, SFdgases m i x m s reveal considerable improvement in the dielccuic propetlies of sF6 [3-61. The SFdNz mixtures ataact many researchers for possible adoption in practice [7,8]. The voltagdpressure characteristic and the corona activity has been investigated [9-111 in different nonuniform geometry. However, 50% SFdN2 mixtures are " m e n d e d [12] for compressed gas insulated uansmission system and which arc also involved in the reduction of the gas cost This paper is a continuation of the above intensive research to investigate the corona activity in a nonuniform coaxial elecvode geomehy. In the present work different wire diameters and surfaces arc used. The corona charge and the corona frequency are measured for sF6. N2 and SF#z mixtures. Such mixtures d u c e the corona charge and improve the dielectric strength of sF6 gas. EXPUIIMEI'4TAL ARRANGEMENTSIn the coaxial cylindrical elecaode cage type. wires with diameters (4) ranging from 0.4 mm -1.9 mm ax mounted under tension within a 400 mm long outer electrode of 100 mm internal diameters. This arrangement is fixed inside a plexiglass pressurc vessel. Smooth and stranded wire surfaces are selected. A Greinacher Doubler circuit is uscd in order to supply the D.C. voltage to the inner electrode.For the corona measurement, the discharge free capacitor is connected in series with the measuring impedance and in paraUel with the cell. This arrangement helps to by-pass high frequency cumnt pulses coming from the cell. The m n a measurement meter is also connected in parnllel with the measuring impedance and serves to vaify a high frequency. An electronic digital counter is connected in parallel with a corona measurement meter to record the corona frequency. RESULTSFigure 1 shows the corona level for a smooth wire of 0.4 mm + and 1.9 mm 4 in Nz. sF6 and SFdS% N2 mix...
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