Post on 01-Jun-2018
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MULTI-STAGE IMPULSEGENERATORS
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INTRODUCTION
The single-stage impulse generator circuit is
inconvenient and impractical for generation ofvery high voltages because of various reasons
like requirement of a very high dc voltagesource ,difficulties encountered in switching ofvery high voltages,physical size of the circuitelements etc.
To overcome these difficulties , in 1!" #ar$
suggested a multistage impulse generator circuit.
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A typical four-stage ipulse
ge!erator circuit
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PRINCIPLE O" OPERATION
The dc voltage %o charges the stage capacitors &1
through the high value charging resistors '1 as well as
through resistors '!,which are much smaller than the
resistors '1. (fter a long time period, the points (,).&,* will acquire
the potential of the dc source %o,w.r.t the earth point +.
The points ,,,/ will remain at earth potential .
The load capacitance &b remains at earth potential
during charging of state capacitors.
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0et the charging voltage attain a value of say ,%o.
2hen +1 breaks down there will be a momentaryredistribution of charge potential on capacitor &1connected across (.
ts top plate falls to zero potential while its lower platesuddenly changes to 3%o.
4uddenly there appears a potential of !%o across the gap+! and this gap instantly breaks down. The process isrepeated .
5or an n-stage impulse circuit the output impulse voltageacross &b will be 36n%o. -is voltage efciency
It is essential that Switching gaps are adjustable tomake breakdown almost simultaneously
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ANALYSIS OF THE MULTI-
STAGE CIRCUIT
When all the gaps G1G!G" and G# havebroken down the spark resistance in eachgap may be regarded as negligiblecompared to the resistances $1 and $!%
& and G' and () and I* and + are at e,ualpotentials so that the links '()I and *+may all be omitted%
or an n-stage impulse circuit )s . )1/n 0 $e.n$!
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COMPONENTS
1. *& &harging 4et
!. &harging 'esistors
". +enerator &apacitors and 4park +aps
7. 2ave 3shaping 'esistors and &apacitors
8. Triggering 4ystem
9. %oltage *ividers
:. +as insulated impulse generators
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1.a dc (variable) supply for charging hesage capaciors!for his use co"binaion
of sep up r"r and reci#er. Charging i"eno less han $!1% second. Charging
should be suiably conrolled (hyrisorconrolled)
&.'se non inducive ire ound resisors.They should be placed such ha hey canbe easily echanged * replaced
+.Paper i"pregnaed capacior of loinducance and high capaciy for fasdischarge..
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RATING O" IMPULSE GENERATORS
mpulse generators are rated by
;1
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A Marx Generator is a clever way ofcharging a number of capacitors inparallel, then discharging them in
series. Here R1and R2are the wave shapingresistors.
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Mar ,eneraor (&$%-)
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Need for Modi#caion
Rating of the control resistors requiredis high.
Hence cost is high.Also the efficiency is low.
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Modi#ed Mar Circui
Here the resistances R1and R2are incorporated in the unit.
R1is divided in to n equal parts of R1/n and put in series
with the gap G. R2is divided in the n equal parts and arranged across eachcapacitor after gap G.
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/dvanages * 0i"iaions
This arrangement saves space.
Rating of the resistors(R1& R2)required is reduced.
Cost is reduced.
Efficiency is increased.
Wide variation of the wave shape isnot possible.
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GENERATION OF
SWITCHING SURGES
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Switching surges are overvoltages in power systems that occurdue to sudden opening or closing of a circuit breaker or due toarcing at a fault point in the system
Switching surges are short-duration transient voltages
Their magnitude is dependent on the system voltage
Switching surges are slow rising impulse having a wave-fronttime of 0.1 to 10millisecond and a tail time of 1 to severalmilliseconds
Standard switching impulse voltage is defined as2502500micro sec wave with the same tolerances for time tofront of !250 " 50# micro sec and time to half value of!2500 " 500# microseconds
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Several circuits have been adopted for producing
switching surges.
They are grouped as
1.$mpulse generator circuit modified to give longer
duration waveshapes
2. %ower transformers or testing transformers e&cited
by dc voltages
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D.C
G R1
C1 R2 C2 CX
V0
t
The values of R1 and R2 for producing waveshapes of long duration
such as 100!1000"s or #00!#000"s will range fro$ 1 to %&'oh$s and% to 20 &'oh$s respectivel(
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GENERATION OF
IMPULSE CURRENTS
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3 Mai"u" value of i"pulse curren isnor"ally independen of and C forgiven energy :15&C&.and
e;ecive inducance 0.3 / lo value of inducance 0 is
needed o ge high curren
"agniudes for given chargingvolage .
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GENERATION OF HIGH
IMPULSE CURRENTS
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IMPULSE CURRENT
GENERATOR / dc charging uni giving variable volage ocapacior ban-.
Capaciors each ih lo self inducance
capable of giving high shor circui currens. /n addiional air cored inducor for highcurren value.
Proper shuns and oscillograph for
"easure"en purposes=and / riggering uni and spar- gap for iniiaion
of curren generaion.