Jawaban Soal Gas Insulation Santos Edun

g as media is 1. Explain properties of the gas when in the gas applied low and high voltage Various phenomena occur in gaseous dielectrics diele ctrics when a voltage is applied. -When low voltage is applied, small current fow between the electrodes and the insulation retains its electrical properties. -I the applied voltage is large, the current fowing through the insulation increases very sharply and an electrical breakdown occur. A strongly conducting spark ormed during breakdown, practically produces a short circuit between the electrodes. he ma!imum voltage applied to the insulation at the moment o breakdown is called the breakdown voltage.

2. Why does Towsend’s Towsend’s theory theory fail to explain explain about gas gas phenomena?  ownsend"s  ownsend"s mechanism ailed to e!plain e!plain the ollowing# According to the ownsend theory, current growth occurs as a result o ioni$ation processes only. only. %ut in practice, breakdown voltages were ound to depend on the gas pressure and the geometry o the gap. &echanism predicts time lags o the order o '(-)*, while in actual practice breakdown was observed to occur at very short times o the order o '(-+*.  ownsend  ownsend mechanism predicts a very diused orm o discharge, discharge, in actual practice, discharges were ound  ound to be lamentary and irregular. irregular. •

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3. What are are advantages advantages of the the streamer streamer theory theory in the gas insulation?  

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 ownsend"s  ownsend"s &echanism is not not consistent with non-uniorm eld gaps. It was dicult to envisage how the ownsend mechanism would apply or long gaps where the sparks are observed to branch and to have an irregular character o growth. *treamer theory was proposed by,  /oeb 0'1))2 and &eek or the positive streamer, and  3aether or the negative streamer In both versions, spark discharge develops directly rom a single avalanche which the space charge transorms into a plasma streamer. streamer. 4rincipal eatures o both versions are the photo-ioni$ation photo-i oni$ation o gas molecules in the space ahead o the streamer and the local enhancement o the electric eld by the space charge at its i ts tip. Version by /oeb and &eek#

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5igure 6.'#7athode-directed streamer as envisaged by &eek and /oeb# 0a2 5irst Avalanche 7rossing the 8ap9 0b2 *treamer :!tending rom the Anode9 0c2 *treamer 7rossing the 8ap. When the avalanche has crossed the gap, the electrons are swept into the anode, the positive ions remaining in a cone shaped volume e!tending across the gap. A highly locali$ed space-charge eld is produced near the anode, but elsewhere in the gap the ion density is low. 4hotoelectrons are produced by photons emitted rom the densely ioni$ed gas constituting the avalanche stem  hese photoelectrons initiate au!iliary avalanches, which are directed by both the space-charge eld and the e!ternally applied eld  he greatest multiplication in these au!iliary avalanches will occur along the a!is o the main avalanche. 4ositive ions let behind by these avalanches eectively branch, lengthen, and intensiy the space charge o the main avalanche toward the cathode.  he process thus develops a sel propagating streamer, which eectively e!tends the anode toward the cathode. ;ltimately, a conducting lament o highly ioni$ed gas bridges the whole gap between the electrodes  E r 

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=

κ   E 

where κ  ≈ 1

An electron avalanche changes into a streamer when the radial eld Er  produced by the positive ions at the head o the avalanche is o the order o the e!ternally applied eld E, that is, 7

5.27 × 10 αe

α x

 E  = V  / m r  (13,500 x /  p ) 1/ 2

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3aether