Treść książki
Przejdź do opcji czytnikaPrzejdź do nawigacjiPrzejdź do informacjiPrzejdź do stopki
Inasimplifiedform–assumingthatUext=0,tpcanbeexpressedas
t
p
=−τ
DC
⋅
ln1
⎛
⎜
⎝
−
U
U
inc
c
0
⎞
⎟
⎠
(3.18)
DenotingtheratioofthecavityvoltagetoPDinceptionvoltage
nU
=
c
0/
U
inc
,
the
aboveequationtakesthefollowingform:
t
p
=−τ
DC
⋅
ln1
⎛
⎜
⎝
−
1
n
⎞
⎟
⎠
Thetimeconstantintheaboveconfigurationsisequalto
τ
DC
=
R
RR
0
0
+
R
A
A
C
c
(3.19)
(3.20)
Thus,forUinc=0.9Uc0(forexample),theapproximatetimebetweenthePDpulsesis
t
p
=
2.3
τ
DC
=
2.3
R
RR
0
0
+
R
A
A
C
c
(3.21)
Asmentionedabove,thesimplifiedcalculationdoesnottakestatisticaleffectssuch
astimelagormemoryeffectsintoaccount;however,itdoesprovidegoodengineering
proximity.
ThePD,selectricfieldstrengthinagaseousvoid(denotedasEp)shouldbesimilar
atagivenpressureforACandDCaccordingtoPaschen,sformula.Theelectricfield
inceptionlevelintheairvoiddependsstronglyonthecavitythickness.Forsmallcavities
(a–cavitythicknessincm),theapproximateelectricbreakdownstrengthinairunder
normalpressurecanbeexpressedbythefollowingempiricalformula[FLOR03b]:
E
p
=
23
+
7
a
[
kV/cm
]
(3.22)
ThecommonvalueofanelectricfieldwithstandingEpforafew-millimeter
interspacedcavityundernormalpressureis3kV/mm,whereasitrisesto5kV/mm
forsubmillimeterdistancesandevenfurtherfortinyvoidsofa0.01mmthickness(up
to9kV/mm).
Theinvestigationswereperformedonthemodelflatandroundcavity(ofdia-
meterDandthicknessd2)embeddedinthehomogenousinsulatingmaterialthatis
asourceofthedischarges.Twoglassplatesontheupperandbottomsidesappliedpres-
suretothevoidinordertoachievemechanicalstability.Suchaspecimenconsistsoffive
layers;i.e.,twoglassplates(eachwithathicknessofd0),asinglelayercontainingthe
void(thickness–d2),andtwoflakesformingthetopandbottomsidesofthecavity
50
