6-1

Chapter 6

Thyristor Converters

• Thyristor circuits and their control• Line-frequency phase-controlled rectifiers and inverters• line-frequency ac <-> controlled dc

6-2

Thyristor Converters

• Two-quadrant conversion

• positive Vd implies rectification

• negative Vd implies inverter mode

6-3

Primitive circuits with thyristors (1)

R load

• The current is zero until t=• The current and voltage vd are in phase• Average voltage of load R controlled by adjusting

)cos1(2

sin2

1

V

dxxVvd

6-4

Primitive circuits with thyristors (2)

• The current is zero until t=• The current lags voltage vd

• At Instant 2 current becomes zero

R L load

0)()(1

)()(1

)(

)()(1

)(

2

1

1

2

dvvL

dvvL

i

dvvL

ti

vvdt

diLv

RdRd

t

Rd

RdL

Rv

6-5

Primitive circuits with thyristors (3)

L Load with dc voltage

dEvti

Evdt

diLv

t

dd

ddL

2

)()(

6-6

Thyristor Gate Triggering

st

controlo

V

vˆ

180

Delay angle is

6-7

Full-Bridge Thyristor Converters (1)

6-8

• constant dc current load

Full-Bridge Thyristor Converters (2)

6-9

Thyristor Converter Waveforms (1)

• Assuming zero ac-side inductance

• at t=0 thyristors 1,2 are triggered

•At t= thyristors 3,4 are triggered

RMSRMSd VVV

tdtVv 9.0222

)()sin(1

0

0

6-10

Thyristor Converter Waveforms (2)

cos9.0cos

22cos

2)()sin(

1RMSRMSd VV

VtdtVv

0

6-11

Thyristor Converter Waveforms (3)

• The drop in the average value due to is

• The average power in the converter is

• Average power becomes negative when > 90º ; this is inverter mode of operation

)cos1(9.0 RMSddd Vvvv

cos9.01

1

0

0

RMSddd

T

dd

T

dd

VIvIdtvT

I

dtivT

P

6-12

Inverter mode

• 90º < < 180º• average vd is a negative value

6-13

Average DC Output Voltage

• Assuming zero ac-side inductance

6-14

Input Line-Current Waveform

• input line current is harmonics

h

IIIII

tItItIti

sshdds

ssss

11

531

9.022

with

)5(sin)3(sin)(sin)(

6-15

THD, Power Factor, Power

• THD

• Power

• Power Factor PF

%31.481001

21

2

s

ss

I

IITHD

cos9.0cos11

__

s

s

s

s

RMSsRMSs I

IDPF

I

I

IV

PPF

cos9.0cos __1_ dRMSsRMSsRMSs IVIVP

6-16

Effect of Ls

• Finite ac-side inductance; constant dc output current

6-17

Finite ac-side inductance (1)

• finite comutation interval u• during u all thyristors conduct (vd=0)

6-18

Finite ac-side inductance (2)

dRMSdRMS

d

u

d

d

d

du

d

u

u

d

I

I

s

u

sLsd

ILVILV

ILV

tdtVtdtVv

vV

ILu

ILuVA

ILtdtVA

ILdiLtdtV

dt

diLvvv

d

d

2cos9.0

2cos22

2cos

2

)()sin()()(sin1

by obtained becan

2)cos()cos(

2)cos()cos(

2)()sin(

2)()sin(

:interval comutation over the integrate u, find to

0

6-19

Input line current is

• Is has a trapezoidal waveform• DPF is

• The RMS value of fundamental frequency is

)2/cos(cos 1 uDPF

)2/cos(

/2cos9.0 2

1

1

uV

ILIVI

IvDPFIV

RMSs

ddRMSsRMSs

ddRMSsRMSs

6-20

• waveform for =45º

• average value of vd

Practical Thyristor Converter

min,

2cos9.0 dRMSsd ILVv

6-21

Thyristor Converter Waveforms: Discontinuous Conduction Mode

• PSpice-based simulation

6-22

DC Voltage versus Load Current

• at constant , if Id falls below a certain value, Vd increases sharply

6-23

Inverter mode

• 90º < < 180º• average vd is a negative value• on dc side average power <vd>Id is negative• on ac side average power VS RMS Is1 RMS cos 1 is negative