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CHAPTER TWOLine or Naturally Commutated

Converters

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Introduction ContdFully controlled converter

This circuit uses SCRs to provide an

adjustable output voltage by controlling

what we call phase or firing angle of the

SCR.

The load voltage can also reverse thusallowing power inversion.

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Fully controlled converter ContdTurn-off of switching devices is brought

about by ac line voltage reverse-biasingthe SCRs, a process called natural or line

commutation.

They are used in applications such as high-

voltage dc power transmissions and dcmotor and ac motor drives where

regeneration is required.

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Introduction Contd

Half or Semi-controlled converterIt uses a combination of diodes and

SCRs.

Its output voltage is adjustable but it only

allows power flow from the supply to the

load.

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Half or Semi-controlled converter contd

They are used in applications whereadjustable dc voltage is required but

regeneration is not.

Examples are dc power supplies with

adjustable dc output voltage, battery

chargers and dc motor drives whereregeneration is not required.

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Introduction Contd In this chapter, we analyze practical

single- and three-phase thyristorconverters.

The diode converter is considered to be a

special case of the fully controlled

thyristor converter where the firing angleis zero.

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Introduction ContdAfter going through this chapter, the

student will know

the converter configurations,

understand the operation of these circuits

and also

know the analytical expressions required to

determine their performances.

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1-Phase Fully Controlled Bridge Converteror Rectifier

It is a 2-pulse bridge converter

It uses 4 SCRs to control the average load

voltage.

The circuit is shown on the next slide

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Fig. 2.1 Single-phase fully controlled bridge

converter circuit

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Analysis of Single-Phase ControlledConverter

Case I: Converter Feeding Highly Inductive

Load Contd

In this analysis, it is assumed thatLsis zeroandL

dis infinite. With the assumption that

, is constant.

The effects ofLsnot being zero andL

dnot

being infinite will be considered later

dd Iti )(

dL

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The converter uses two groups of SCRs:

one group consists of T1 and T3

which have common cathodeconnection and

the other consists of T2and T4which

have a common anode connection.

The two groups operate independently of

each other.

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If the gate currents to the SCRs were applied

continuously, then they would behave like

diodes and for the common cathode connection, the

thyristor with its anode at the highest

potential would conduct and

for the common anode connection, the

SCRs with its cathode at the lowest

potential would conduct.

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It follows that if the SCRs were gated

continuously,

then T1and T2would start conducting as

soon as the voltage vsbecame positive

and T3and T4would start conducting as

soon as the voltagevsbecame negative.

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Using gate current pulses,the start of conduction can be delayed by

time

with respect to where as diodes they wouldnaturally start conducting.

The angle is called the firing angle

or delay angle.

dt

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SCRs T1and T2are fired simultaneouslyduring thepositive half wave of the source

voltage and

SCRs T3and T4fired simultaneously during

the negative half wave of the sourcevoltage.

To ensure simultaneous firing, each pair of

SCRs uses the same firing signal.

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When T1and T2are conducting When T3and T4are conducting

Sd vv

Sd vv

dtts

tt

dtt

Iiii

ii

Iii

41

43

21

0 dtts

tt

dtt

Iiii

ii

Iii

41

21

43

0

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Fig. 2.2 Waveforms of the fully controlled bridge rectifier with dL

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The rms value of the output voltage:

2

12

max

21

22

max

2cos12

1

sin1

)(

dV

dVRMSVd

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Input Line CurrentThe input current is given by

It is a square waveform with amplitude

The waveform is phase shifted from the

source voltage by the delay angle .

41 tts iii

dI

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Input Line Current ContdIt can be expressed in terms of its Fourier

components as

From the basic definition of rms, the rms

value of the input current can be shown to

be

Note that

,...5,3,1

)(sin4)(

k

d

s

k

tkIti

dS II

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Voltages and Currents of ThyristorsThe peak inverse or reverse voltage

The peak value of the thyristor current

From KCL

Hence and

sRM VVV 2max

dT II

max dTT

Iii 31

dTT IAVIAVI )()( 31

2)(dT

IAVI

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Voltages and Currents of Thyristors ContdBecause T1and T3do not conduct

simultaneously, we can also write

Hence

or

or

or

22

3

2

1 dTT Iii

)()()( 22

3

2

1 dTT IAviAviAv 22

3

2

1 )()( dTT IRMSIRMSI

2)(dT

IRMSI 22 )(2 dT IRMSI

P d S P f P t

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Power and Some Performance Parameters

The total harmonic distortion (THD) of the

input current is given by

Since the semiconductor switches areconsidered to be ideal,

the input power = the output power.

%43.48100

9.0

9.0

100

22

22

2222

1

2

1

2

d

dd

I

I

d

s

ss

I

II

I

I

IITHD

d

d

P d S P f P t

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Contd

The input power is given by

And the output power by

cos22cos

1

11

0

sd

ss

T

ss

VIIVdtiv

TP

cos221

00

sd

ddo

T

d

d

T

dd

VIIVdtv

T

IdtIv

TP

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Contd

The displacement factor defined as the

cosine of the phase angle between the

fundamental component of the input current

and voltage is given by

Input power factor

coscos 1 DPF

cos9.0DPFpowerapparentinput

powerinput 1 s

s

I

IPF

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Contd

The output dc power

And the output ac power

d

s

ddodcdcdc I

VIVIVP

cos22

dsddac IVRMSIRMSVP )()(

P d S P f P t

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Contd

Therefore the rectification efficiency is

Transformer utilization factor

cos9.0cos22

ac

dc

P

P

cos9.0powerapparentinput

ds

dcdc

IV

PP

TUF

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