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$ation processes only. only. %ut in practice, breakdown voltages were ound to depend on the gas pressure and the geometry o the gap. &echanism predicts time lags o the order o '(-)*, while in actual practice breakdown was observed to occur at very short times o the order o '(-+*. ownsend ownsend mechanism predicts a very diused orm o discharge, discharge, in actual practice, discharges were ound ound to be lamentary and irregular. irregular. •
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3. What are are advantages advantages of the the streamer streamer theory theory in the gas insulation?
ownsend"s ownsend"s &echanism is not not consistent with non-uniorm eld gaps. It was dicult to envisage how the ownsend mechanism would apply or long gaps where the sparks are observed to branch and to have an irregular character o growth. *treamer theory was proposed by, /oeb 0'1))2 and &eek or the positive streamer, and 3aether or the negative streamer In both versions, spark discharge develops directly rom a single avalanche which the space charge transorms into a plasma streamer. streamer. 4rincipal eatures o both versions are the photo-ioni$ation photo-i oni$ation o gas molecules in the space ahead o the streamer and the local enhancement o the electric eld by the space charge at its i ts tip. Version by /oeb and &eek#
5igure 6.'#7athode-directed streamer as envisaged by &eek and /oeb# 0a2 5irst Avalanche 7rossing the 8ap9 0b2 *treamer :!tending rom the Anode9 0c2 *treamer 7rossing the 8ap. When the avalanche has crossed the gap, the electrons are swept into the anode, the positive ions remaining in a cone shaped volume e!tending across the gap. A highly locali$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$ed gas constituting the avalanche stem hese photoelectrons initiate au!iliary avalanches, which are directed by both the space-charge eld and the e!ternally applied eld he greatest multiplication in these au!iliary avalanches will occur along the a!is o the main avalanche. 4ositive ions let behind by these avalanches eectively branch, lengthen, and intensiy the space charge o the main avalanche toward the cathode. he process thus develops a sel propagating streamer, which eectively e!tends the anode toward the cathode. ;ltimately, a conducting lament o highly ioni$ed gas bridges the whole gap between the electrodes E r
=
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
3aether
he total eld, thus enhanced, would promote secondary anode-directed avalanches ahead o the initial one, orming a negative streamer. hese secondary avalanches are initiated by photoelectrons in the space ahead o the streamer. At higher pd values, the streamer mechanism plays the dominant role in e!plaining breakdown phenomena. *treamer mechanism has been widely used to e!plain breakdown in non-uniorm elds, such as in point-plane or point-point gaps. In long gaps, i the voltage gradient at the stressed electrode e!ceeds the corona onset level, the ioni$ation activity in the gap increases. As a result, a highly ioni$ed and luminous lamentary channel, called the leader channel, develops at the electrode and propagates toward the other electrode.
Figure 2.2:Anode-directed streamer as envisagebdy Raether
Figure 2.3: eader channe! and !eader streamers.
. What is that mean with !ollision?
7ollision is an accident that happens when two ion-electron hit each other with orce.
". Why do the ele!trons move fast to !ompare the ions?
