line inductance effect

8/17/2019 Line Inductance Effect

1/21

Efect o Source Inductance on the

Perormance o AC to DC Converters


2/21

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.


3/21

Diode&'ectier (ridge Analsis !ithAC&Side Inductance

Single Phase rectier !ith )s

*utput current is assumed to $e pure dc


4/21

+nderstanding Current Commutation

Assuming inductance in this circuit to $e

,ero

Current Commutation ckt and waveform assumes Ls =0


5/21

+nderstanding Current Commutation-cont.

Source Inductance in this circuit is included

a) Ckt during commutation, b) Circuit after the current

commutation is completed.


6/21

Current Commutation Waveorms

Sho!s the volt&seconds needed to commutate current


7/21

Current Commutation in "ull&(ridge'ectier

Sho!s the necessar volt&seconds


8/21

Three&Phase# "ull&(ridge 'ectier

Including the ac&side inductance

/ ph rectier !ith nite source inductance )s and aconstant load current


9/21

/&Phase 'ectier0 CurrentCommutation

output current

is assumed to$e pure dc


10/21

'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


11/21

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.


12/21

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.


13/21


14/21

A$ove e%uation o :o can $e represented $ the ollo!ing

e%uivalent circuit


15/21


16/21

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.


17/21

• 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.


18/21

• 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. .


19/21


20/21


21/21

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.

line inductance effect

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