6-1

Thyristor ConvertersChapter 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

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

6-3

Thyristor in a Simple Circuit (Review Class)

• For successful turn-off, reverse voltage required

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 current Id can 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.

6-5

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

• In the positive half cycle of vs, the current is zero until t=, at which a gate pulse of a short duration is applied

• With the thyristor conducting, vd = vs

• vd becomes zero at t =

• By adjusting the firing angle , the average dc voltage Vd and current Id can be controlled

6-6

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

o Initially, the current is zero until t=, at which the thyristor is fired during the positive half cycle of vs

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

o Voltage across the inductor: vL=vs-vR

o During to 1, vL is positive, and the current increases

o Beyond 1, vL is negative, and the current begins to decline

2 is the instant at which current becomes zero and stays at zero until 2+ at which the thyristor is fired again

6-7

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

o The thyristor is reverse biased until 1

o The thyristor conduction is further delayed until 2 at which the thyristor is fired

o With the thyristor conducting, vd = vs

o Between 2 to 3, vL is positive, and the current increases

o Beyond 3, vL is negative, and the current begins to decline

o When A1 is equal to A2, current goes to zero at 4

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 with respect to the positive zero crossing of the ac line voltage is obtained in terms of vcontrol and the peak of the sawtooth waveform Vst.

st

controloo

V

v180

6-9

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

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

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

6-11

1-Phase Thyristor Converter Waveforms

• Assumptions: Ls=0 and purely dc current Id

delay angle or firing angle

• Prior to t=0, current is flowing through 3 and 4, and vd = -vs

• Beyond t=0, thyristors 1 and 2 become forward biased, but cannot conduct until .

• vd becomes negative between 0 and as a consequence of the delay angle

• At t=, gate pulse applied and current commutation from thyristors 3 and 4 to 1 and 2 is instantaneous (Ls = 0), and vd = vs

• Thyristors 1 and 2 will keep conducting until 3 and 4 are fired

6-12

The expression for the average voltage Vd:

Average dc Voltage as a Function of the Delay Angle

cos9.0sin2

1ssd VtdtVV

Let Vd0 be the average dc voltage with =0,

ssd VtdtVV 9.0sin21

0

0

Then, drop in average voltage due to ,

cos19.00 sddd VVVV

The average power through the converter,

dtivT

dttpT

PT

dd

T

00

11

With a constant dc current (id=Id),

cos9.01

0dsdd

T

dd IVVIdtvT

IP

6-13

Average dc Output Voltage

The variation of Vd as a function of

Average dc voltage is positive until =90o: this region is called the rectifier mode of operation

Average dc voltage becomes negative beyond =90o: this region is called the inverter mode of operation

cos

9.0

cos9.0

0

s

s

d

d

V

V

V

V

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.

6-15

1-Phase Thyristor Converter

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

s

ds

dsss

ds

dI

dIsss

sssLs

V

IL

ILVdttVA

ILdiLdttVA

dt

diLvv

2

2coscos

2coscos2sin2

2sin2

1

6-16

1-Phase Thyristor Converter: with and without Ls

o Voltage drop due to the inclusion of Ls.

dssd

sd

dsd

ILVV

VV

ILAV

2cos9.0

cos9.0

2

0

0

without Ls with Ls

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 and Vd/Vd0 with the rated input voltage, power of 3 kW, and =30o.

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

74.21230

5000

01

0

6-19

Thyristor Converters: Inverter Mode (Vd is negative)

• Average value of vd is negative for 90o<<180o. Average power Pd is negative (Pd=VdId) and thus power flows from the dc to the ac side

• On the ac side, Pac=VsIs1cos is also negative because 1>90o

• Inverter mode of operation is possible because there is a source of energy on the dc side

• ac side voltage source provides commutation of current from one pair of thyristors to the others

6-20

3-Phase Thyristor Converters

• Current Id flows 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,

LLd VV 35.10

6-21

3-Phase Thyristor Converter Waveforms

6-22

Average Output DC Voltage

cos35.1

35.1cos35.1

3

cos1235.1

3

cos12sin2

sin2

3

0

0

0

0

dLLddd

dLL

LLLLdd

LLLL

LLac

dd

IVIVP

PowerAverage

VV

VV

AVV

VtdtVA

angledelaythetoduevoltagedcaveragetheinreductionThe

tVV

AVV

6-23

dc-side voltage waveforms as a function of

Vd repeats at six times the line frequency

6-24

Conclusions

• Thyristor converters provides controlled transfer of power between the line frequency ac and adjustable-magnitude dc

• By controlling , transition from rectifier to inverter mode of operation can be made and vice versa

• Thyristor converters are mostly used at high-power levels

• Thyristor converters inject large harmonics into the utility system