Thyristor_Converters

8/8/2019 Thyristor_Converters

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6-1

Thyristor Converters

Chapter 6

In some applications (battery charger, some ac/dc drives),

the dc voltage has to be controllable Thyristor converters provide controlled conversion of ac

into dc

Primarily used in three-phase, high power application

Being replaced by better controllable switches


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6-2

Thyristors (Review Class)

Semi-controlled device

Latches ON by a gate-current pulse

if forward biased

Turns-off if current tries to reverse


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6-3

Thyristor in a Simple Circuit (Review Class)

F

or successful turn-off, reverse voltage required


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6-4

Thyristor Converters

Fully controlled converter shown in Fig. 6-1a

Average dc voltage Vd

can be controlled from a positive maximum to a

negative minimum on a continuous basis

The converter dc currentIdcan not change direction

Two-quadrant operation

Rectification mode (power flow is from the ac to the dc side): +Vd

& +Id

Inverter mode (power flow is from the dc to the ac side): : -Vd

& +Id

Inverter mode of operation on a sustained basis is only possible if a source of

power, such as batteries, is present on the dc side.


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6-5

Basic thyristor circuits: Line-frequency voltage source connected to a load

resistance

In the positive half cycle ofvs, the current is zero until [t=E, at which a

gate pulse of a short duration is applied

With the thyristor conducting, vd=vs

vdbecomes zero at [t=T

By adjusting the firing angle E, the average dc voltage Vd

and currentId

can be controlled


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6-6

o Basic thyristor circuits: Line-frequency voltage source connected to a RL

load

o Initially, the current is zero until [t=E, at which the thyristor is fired

during the positive half cycle ofvs

o With the thyristor conducting, current begins to flow, vd=vs

o Voltage across the inductor: vL=vs-vRo During E to U1, vL is positive, and the current increases

o Beyond U1, vL is negative, and the current begins to decline

S U2

is the instant at which current becomes zero and stays at zero until 2T+E

at which the thyristor is fired again


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6-7

o Basic thyristor circuits: The load consists ofL and a dc voltage Ed

o The thyristor is reverse biased until U1

o The thyristor conduction is further delayed until U2

at which the thyristor is

fired

o With the thyristor conducting, vd=vs

o Between U2

to U3, vL is positive, and the current increases

o Beyond U3, vL is negative, and the current begins to decline

o WhenA1 is equal to A2, current goes to zero at U4


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6-8

Thyristor Gate Triggering

Generation of the firing signal

The sawtooth waveform

(synchronized to the ac input) is

compared with the control signal

vcontrol, and the delay angle E with

respect to the positive zero crossingof the ac line voltage is obtained in

terms ofvcontroland the peak of the

sawtooth waveform Vst.

! st

controloo

V

v

180E


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6-9

Full-Bridge (Single- and Three-Phase) Thyristor Converters


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6-10

Single-Phase Thyristor Converters

One thyristor of the top group and one of the bottom group will conduct

If a continuous gate pulse is applied then this circuit will act like a full

bridge diode rectifier and the web forms are as shown below

E=0 for 1 and 2 and E=T for thyristors 3 and 4


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6-11

1-Phase Thyristor Converter Waveforms

Assumptions: Ls=0 and purely dc current

Id

Edelay angle or firing angle

Prior to [t=0, current is flowing through 3and 4, and v

d= -vs

Beyond [t=0, thyristors 1 and 2 become

forward biased, but cannot conduct until

E.

vdbecomes negative between 0 and E as aconsequence of the delay angle

At [t=E, gate pulse applied and current

commutation from thyristors 3 and 4 to 1

and 2 is instantaneous (Ls= 0), and vd=vs

Thyristors 1 and2

will keep conductinguntil 3 and 4 are fired


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6-12

The expression for the average voltage Vd:

Average dc Voltage as a Function of the DelayAngle

E[[T

TE

E

E cos9.0sin21

ssd VtdtVV !!

Let Vd0

be the average dc voltage with E=0,

ssd VtdtVV 9.0sin21

0

0 !! [[T

T

Then, drop in average voltage due to E,

EEE

cos19.00 !!( sddd VVVV

The average power through the converter,

dtivT

dttpT

PT

dd

T

!!00

11

With a constant dc current (id=I

d),

Ecos9.01

0dsdd

T

dd IVVIdtv

T

IP !!

-

!


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6-13

Average dc Output Voltage

The variation ofVd

as a function ofE

Average dc voltage is positive until E=90o: this region is called

the rectifier mode of operation

Average dc voltage becomes negative beyond E=90o: this

region is called the inverter mode of operation

EEE cos

9.0cos9.0

0

!!

s

s

d

d

V

V

V

V


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6-14

1-Phase Thyristor Converter

o AC side inductance is included, which generally cannot be ignored

in practical thyristor converters.

o For a given delay angle, there will be a finite commutation interval

o Commutation process is similar to that in diode bridge rectifiers

o During the commutation interval, all four thyristors conduct, and

therefore, vd=0, and the voltage vLs=vs.


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6-15

1-Phase Thyristor Converter

o During the commutation interval, all four thyristors conduct, andtherefore, vd=0, and the voltage vLs=vs.

? A

[

EQ

[QEE[

[[[

QE

E

Q

QE

EQ

-

!@

!!!@

!

!

!@

!!

s

ds

dsss

ds

dI

dIsss

sssLs

V

IL

ILVdttVA

IL

diL

dtt

VA

dt

diLvv

2

2

coscos

2coscos2sin2

2sin

2

1


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6-16

1-Phase Thyristor Converter: with and without Ls

o Voltage drop due to the inclusion ofLs.

? A

? AT

[E

ET

[

T

(

Q

Q

Q

Q

dssd

sd

dsd

ILVV

VV

ILAV

2cos9.0

cos9.0

2

0

0

!

!

!!

{

!

withoutLs with Ls


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6-17

Example

In the converter circuit, Ls is 5% with the rated voltage of

230 V at 60 Hz and the rated volt-ampere of 5 kVA.

Calculate the commutation angle Q and Vd/Vd0 with the

rated input voltage, power of 3 kW, and E=

30o

.


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6-18

Solution

VILVV

V

IL

AI

IILVIVP

mHZ

L

I

VZ

AI

dssd

s

ds

d

ddssddd

bases

rated

ratedbase

rated

5.1732

cos9.0

9.52

2coscos

3.17

30002

cos9.0

30

4.1377

05.0

58.10

74

.2

1230

5000

01

0

!!

!

-

!

!

!

-

!!

!

!!

!!

!!

[

T

E

E[

EQ

[T

E

E

;


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6-20

3-Phase Thyristor Converters

CurrentIdflows through the one thyristor of the top group and one of the

bottom group

If a continuous gate pulse is applied then this circuit will act like a three-

phase full bridge diode rectifier and, as a result,

LLdVV

35.10!


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3-Phase Thyristor Converter Waveforms


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6-22

Average Output DC Voltage

E

E

T

E

T

Q

E[[

E

[

T

EE

E

E

Q

Q

E

cos35.1

35.1cos35.1

3

cos12

35.13

cos12sin2

sin2

3

0

0

0

0

dLLddd

dLL

LL

LLdd

LLLL

LLac

dd

IVIVP

PowerAverage

VV

V

V

A

VV

VtdtVA

angledelaythetoduevoltagedcaveragetheinreductionThe

tVV

AVV

!!

!!

!

!@

!!

!

!


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dc-side voltage waveforms

as a function ofE

Vd

repeats at six times the

line frequency


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Conclusions

Thyristor converters provides controlled transfer of power

between the line frequency ac and adjustable-magnitude dc

By controlling E, transition from rectifier to inverter modeof operation can be made and vice versa

Thyristor converters are mostly used at high-power levels

Thyristor converters inject large harmonics into the utilitysystem

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