Post on 20-Dec-2015
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