Thyristor commutation Techniques

A. K. Gautam

By: Aniruddha K. GautamAKGEC, Ghaziabad

Methods of Commutations

• Natural Commutations

• Forced commutations

A. K. Gautam

~

T

+

v ov sR

Natural Commutation

•Occurs in AC circuits•Natural Commutation of Thyristors takes place in

•AC voltage controllers.•Phase controlled rectifiers.•Cyclo-converters

A. K. Gautam

A. K. Gautam

t

t

t

t

S u p p ly v o lta g e v s S in u s o id a l

Vo l ta g e a c r o s s S C R

L o a d v o lta g e v o

Tu rn o ffo c c u r s h e re

0

0

2

2

3

3

tc

G a te P u ls e

Forced Commutation

• Applied to • dc circuits

• Choppers• Inverters.

• Commutation achieved by reverse biasing the SCR or by reducing the SCR current below holding current value.

• Commutating elements such as inductance and capacitance are used for commutation purpose.

A. K. Gautam

Methods of Forced Commutation

• Self commutation.

• Resonant pulse commutation.

• Complementary commutation.

• Impulse commutation.

• External pulse commutation.

• Line Commutation.

A. K. Gautam

CLASS A COMMUTATION: LOAD COMMUTATIONOR

(SELF COMM.)

• When R is low : L & C connects in series with R

• When R is high : C connects in Parallel with R

• Useful for dc Circuits.

•System should be under damped.

•When energized from a dc source, current must have a natural tendency to decay to zero.

•Change in direction of current make the thyristor turn- off.

•Oscillating current flows.

•SCR is turned off when current is zero.

A. K. Gautam

Self commutation

•Circuit is under damped by including suitable values of L & C in series with load.

•Oscillating current flows.

•SCR is turned off when current is zero.

A. K. Gautam

V

R L V (0)c

C

T i

Load

+ -

Expression for Current

A. K. Gautam

VS

R SL1

C S

V C (0 )S

C

T I(S) + +- -

Fig. shows a transformed network

2

2

1

0

1

0

1

0C

C

CCS V V

SI SRC

V V

SI SR SL

CS

C

S S LC

V V

RLC S S

L LC

2

2 22

0

1

0

12 2

C

C

V V

LI SR

S SL LC

V V

LI SR R R

S SL LC L L

A. K. Gautam

2 2 22 2

2

0

12 2

Where,

0 1, ,

2 2

C

C

V VALI S

SR RS

L LC L

V V R RA

L L LC L

2 2

is called the natural frequency

Taking inverse Laplace transforms

sint

AI S

S

Ai t e t

A. K. Gautam

2

Expression for current

0sin

Peak value of current

0

RtC L

C

V Vi t e t

L

V V

L

Expression For Voltage Across Capacitor At The Time Of Turn Off

V

R L V (0)c

C

T i

L oad

+ -

A. K. Gautam

Applying KVL to figure

Substituting for i,

sin sin

c R L

c

t tc

v t V v V

div t V iR L

dt

A d Av t V R e t L e t

dt

sin cos sin

sin cos sin

sin cos sin2

sin cos2

t t tc

tc

tc

tc

A Av t V R e t L e t e t

Av t V e R t L t L t

A Rv t V e R t L t L t

L

A Rv t V e t L t

A. K. Gautam

Substituting for A,

0sin cos

2

0sin cos

2

SCR turns off when current goes to zero.

i.e., at

C tc

C tc

V V Rv t V e t L t

L

V V Rv t V e t t

L

t

2

Therefore at turn off

00 cos

0

0

C

c

c C

R

Lc C

V Vv t V e

v t V V V e

v t V V V e

A. K. Gautam

2

For effective commutation

the circuit should be under damped.

1That is

2

With R = 0, and the capacitor initially uncharged

that is 0 0

sin

Note:

C

R

L LC

V

V ti t

L LC

1But

sin sin

and capacitor voltage at turn off is equal to 2V

Fig. shows the waveforms for the above conditions.

Once the SCR turns off voltage across it is

negative voltage.

LC

V t C ti t LC V

L LLC LC

A. K. Gautam

C u r r e n t i

C a p a c i to r v o l t a g e

G a te p u l s e

Vo l ta g e a c r o s s S C R

0 / 2t

t

t

t

V

V

2 V

CLASS B COMMUTATION: Resonant-pulse commutation

T1: main thyristor.

TA: Auxiliary thyristor.

iT1 : Current through thyristor.

iC : Capacitor current.A. K. Gautam

• Series LC circuit connected across thyristor ‘T’.

• Initially ‘C’ is charged to ‘V’ volts with plate ‘a’ as positive.

• Current in LC oscillates when SCR is ON.

• ‘T’ turns off when capacitor discharges through thyristor in a direction opposite to IL

Operations:

Mode 1:• TA= OFF, T1= OFF

• Flow of current through C, current ic starts flowing and C charges up to VS.

Mode 2:• TA = OFF, T1= ON at t=0

• iT1 = iO

• When 0<t>t1

VC = VS

iC = 0

iO = IO

iT1 = iO

Mode 3:TA = ON, T1= ON at t=t1

Current iC path : C→TA →L →C

C S P O

Ci V Sin t I Sin t

L

1C S OV idt V Cos t

C

Mode 4:

T= t2 + TA= OFF , Vc= - Vs

Flow of resonant current: C→L →D →T1

As well as Ic increases which is opp. To

T1, iT1=Io-Ic, begin to decrease.

When Ic- Io, iT1=0, means T1= off

Mode 5:

T1= OFF at t = t3

Io flow through, C→L →D. Vc ↑ 0 → Vab

At t= t4 , Vc ↑0 → Vs

A. K. Gautam

3 2( )o S o

CI V Sin t t

L

1

3 2

1( )

( )o

IoSin

t t Ip

p s

CI V

L

4 3ab

cO

Vt t t C

I

3 2cos ( )ab oV Vs t t

….(1)

….(2)

….(3)

….(4)

….(5)

Circuit turn=off time

Peak resonant current

A. K. Gautam

CLASS C Commutation: Complementary commutation

• One thy. Commutates another and vise-versa

Mode 1:

T1= ON , Load current dirn:

Battery →R1 →T1

Battery →R2 → C→ T1

Vc

Mode 2:

T2 = ON, Capr voltage appears as reverse bias across T1, and turns it off.

Current dirn:

Battery →R1 → C→ T2

Battery →R2 → T2

SV

A. K. Gautam

1 11

SR

VI I

R 2

2

So R

VI I

R

When T1=ON at t = 0

or

1 2

1 11 1 ( )T C S R RI i i V

VC changes from 0→VS

2( )

2

tR CS

C

Vi e

R

2( )

(1 )tR C

C SV V e

When t= t+

So that VT1 =Vc(t)

After transient condition, Vc =VT2= VS , iC=0, 11

ST

Vi

R

When t= t1

T2=ON, Vc (across T1) reverse and T1=OFF, VT2=0

A. K. Gautam

1T SV V1

2 SC

Vi

R

1 2

2 12 ( )T S R RI V , ,

• Applying KVL law:

1

1C C SR i i dt V

C

• Laplace transformation:

1

( )1( ) C S S

C

I S CV VR I S

C S S S

• After taking inverse transformation

1( )

1

2( )

tR CS

C

Vi t e

R

Ci = opposite direction , So that 1( )

1

2( )

tR CS

C

Vi t e

R

• Again, 0

1 t

C S CV V i dtC

= 1( )

1

21 tR CS

C S

VV V e

C R

A. K. Gautam

12 2CR

t

sc eVV

2 11 2

11

( )

2S R R

ST

V

T Vi

R

i

When T=t2 transient condition are over now

VT1=Vs, ic=0, Vc=-Vs, VT2=Vs/R2, iT1=0

When T=t3

T1= ON, T2= commutated

iT2=0, VT2 =-Vs, VT1 =0, ic=2Vs/R2 iT1= 1 2

2 1( )S R RV

11

( )

1 0 1 2tc

R C

T SV V e

1 1 ln(2)Ct R C

2 2 ln(2)Ct R C

Turn of T2 at t1, capacitor voltage Vs suddenly appears as reverse biased across T1 to turn it off.

Turn off time for T1 and T2

A. K. Gautam

A. K. Gautam

CLASS D COMMUTATION: Impulse Commutation

In such type of commutation circuit , a main thyristor, a auxiliary thy.,and an inductor is used.

Mode 1:

T1-ON , io flows through Battery →T1 →load

Capacitor discharge dirn, T1 →D →L, Capacitor charged with opp. Polarity, which is not allowed due to diode.

sin sinC S O P O

Ci V t I t

L

1T C Oi i I (due to initial condition)

1T O P Oi I I Sin t 1

OLC

A. K. Gautam

Ip= Peak capacitor voltage

0

1 offtL off

C L

i tV i dt

C C L off

C

i tC

V

P S

CI V

L

m 1I ( )P axI throughTmIS ax

CV

L

2

2mI ax

V CL , , ,

Mode: 2

,

TA-ON, T1-off, C discharges through TA →T1, when this current= iO →T1-off

At t=t1

TA-ON, VT1= -VS, iT1=0

Now load current will flow through, C →TA

VC charges through –Vs to Vs

This method is called voltage commutation, due to T1 turns off due to reverse voltage application A. K. Gautam

A. K. Gautam

External Pulse Commutation (Class E Commutation)

V S V A U X

L

C

T 1 T 3T 2

R L 2V A U X

+

•

•This Method of commutation used a pulse obtain from a source external to the main circuit or obtain from a pulse forming network fed by an auxiliary voltage source.

• The pulse is used to apply a reverse bias and turn off the thyristor.

• Vaux Auxiliary voltage source

• L, C Oscillatory circuit to general a pulse.

•

Mode -1• T1 T3– ON, VS – Used to supply the current through load.• A current pulse flows having a peak value Vaux √C/L from V1 T3 L

C to charge up to 2VAUX

• When C is fully charged, Charging current tends to 0 , and T3 turns off.

Mode -1• T2 ON ,

• Capacitor voltage appears as reverse bias across T1 and turns it off.• Capacitor C discharge through load.

A. K. Gautam

A. K. Gautam

• T1 is conducting & RL is connected across supply.

• T3 is fired & ‘C’ is charged to 2VAUX with upper plate positive.

• T3 is self commutated.

• To turn off T1, T2 is fired.

• T2 ON results in a reverse voltage VS – 2VAUX appearing across T1