line inductance effect
TRANSCRIPT
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Efect o Source Inductance on the
Perormance o AC to DC Converters
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The presence o source inductance have signicant
efect on the perormance o the converter. With
source inductance present the output voltage o aconverter does not remain constant or a given ring
angle. Instead it drops graduall !ith load current.
The converter output voltage and input current
!aveorms also change signicantl."or simplicit it is assumed that the converter
operates in the continuous conduction mode.
"urther# it is assumed that the load current ripple is
negligi$le and the load can $e replaced $ a dc
current source the magnitude o !hich e%uals the
average load current.
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Diode&'ectier (ridge Analsis !ithAC&Side Inductance
Single Phase rectier !ith )s
*utput current is assumed to $e pure dc
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+nderstanding Current Commutation
Assuming inductance in this circuit to $e
,ero
Current Commutation ckt and waveform assumes Ls =0
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+nderstanding Current Commutation-cont.
Source Inductance in this circuit is included
a) Ckt during commutation, b) Circuit after the current
commutation is completed.
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Current Commutation Waveorms
Sho!s the volt&seconds needed to commutate current
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Current Commutation in "ull&(ridge'ectier
Sho!s the necessar volt&seconds
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Three&Phase# "ull&(ridge 'ectier
Including the ac&side inductance
/ ph rectier !ith nite source inductance )s and aconstant load current
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/&Phase 'ectier0 CurrentCommutation
output current
is assumed to$e pure dc
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'ectier !ith a )arge "ilter Capacitor
a /ph rectier !ith nite )s and constant dc voltage$ E%uivalent circuit# c Waveorms
*utput voltage is assumed to $e pure ripple ree dc
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The presence o source inductance have signicant
efect on the perormance o the converter. With
source inductance present the output voltage o aconverter does not remain constant or a given ring
angle. Instead it drops graduall !ith load current.
The converter output voltage and input current
!aveorms also change signicantl."or simplicit it is assumed that the converter
operates in the continuous conduction mode.
"urther# it is assumed that the load current ripple is
negligi$le and the load can $e replaced $ a dccurrent source the magnitude o !hich e%uals the
average load current.
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ig. 1-$ sho!s the corresponding !aveorms. It
s assumed that the thristors T/ and T2 !ere
onducting at t 3 4. T1 and T5 are ired at 6t 3 7. there !ere no source inductance T/ and T2
ould have commutated as soon as T1 and T5 are
urned *8. The input current polarit !ould have
hanged instantaneousl. 9o!ever# i a source
nductance is present the commutation and
hange o input current polarit can not $enstantaneous. Thereore# !hen T1 and T5 are
urned *8 T/ T2 does not commutate immediatel.
nstead# or some interval all our thristors
ontinue to conduct as sho!n in "ig.
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A$ove e%uation o :o can $e represented $ the ollo!ing
e%uivalent circuit
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Three phase ull controlled converter !ith source inductance
• In the three phase ull controlled converter !hen the
source inductance is ta=en into account# the%ualitative efects on the perormance o theconverter is similar to that in the case o a singlephase converter. "ig. 2-a sho!s such a converter. Asin the case o a single phase converter the load is
assumed to $e highl inductive such that the load can$e replaced $ a current source.
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• As in the case o asingle phase converter#commutations are notinstantaneous due tothe presence o sourceinductances.
• It ta=es place over anoverlap period o ;>1<
instead. During theoverlap period threethristors instead ot!o conducts. Currentin the outgoingthristor graduall
decreases to ,ero !hilethe incoming thristorcurrent increases ande%uals the total loadcurrent at the end o
the overlap period.
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• I the duration o the overlap period is greater than ?44 our thristors ma also conduct clamping the outputvoltage to ,ero or sometime.
• 9o!ever# this situation is not ver common and !illnot $e discussed an urther. Due to the conduction ot!o devices during commutation either rom the topgroup or the $ottom group the instantaneous outputvoltage during the overlap period drops -sho!n $ the
hatched portion o "ig. 2 -$ resulting in reducedaverage voltage. The e@act amount o this reductioncan $e calculated as ollo!s
• In the time interval 7 6t B 7 ># T? and T5 rom the
$ottom group and T1
rom the top group conducts. Thee%uivalent circuit o the converter during this period isgiven $ the circuit diagram o "ig. .
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A$ove e%uation suggests the same dc e%uivalent circuit orthe three phase converter !ith source inductance as sho!n in"ig./ !ith
It should $e noted that 'C is a ;loss less< resistance# since the
overlap process does not involve an active po!er loss.