Firing Pulse Generation

8/10/2019 Firing Pulse Generation

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F IRING P ULSE GENERATION S CHEMES FOR T WO AND S IX P ULSE CONVERTERS

AIMi. To Design the firing pulse generator(s) for thyristor based converter systems

using.a) Ramp Firing Schemeb) Cosine Firing Scheme

ii. To Validate the relation between the control voltage and output voltage ofconverter system.

iii. To Design and validate through simulation using MATLAB .

THEORY

A generalized block diagram of Phase Controlled Rectifier with typical firing scheme isgiven in Fig.1.1. The converter is operated from ac power. Since synchronization isneeded for all converters with ac input, the firing pulses must be synchronized withthe ac supply. Isolation is essential as the control circuit uses very low power devicessuch as various chips, logic gates etc. The strength of the pulse obtained from logicgates may not be sufficient to drive the gate of thyristors, so amplification of thepulse along with isolation is used at the final stage.

The output voltage control of a phase controlled rectifier is achieved by varying thedelay angle of its firing circuit. The firing circuit consists of a reference signal and acontrol signal. The firing pulses are generated based on the comparison between thetwo signals. The generation of firing pulses can be classified into two schemes basedon the reference signal employed. The two schemes are:

(i) Ramp firing scheme(ii) Cosine Firing Scheme

1.1 RAMP FIRING SCHEME:

The ramp firing scheme with unipolar positive slope ramp signal as reference is

shown in Fig.1.2. In this scheme, the reference signal is a ramp waveform andsynchronized to the input source. Its magnitude is chosen to vary from 0 to +4.5V as

t varies from 0 to 180 (i.e., one half-cycle of the input supply) as shown in Fig.1.2& Fig 1.3.

The control signal is a dc with the variable magnitude but limited to rampmagnitudes. The firing pulses are generated based on the comparison of two signals.Pulses are produced when the magnitude of the reference signal is greater than themagnitude of control signal.


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Fig.1.1 Schematic for Converter with firing control circuit

Fig.1.2 Schematics of firing pulse generator using Ramp and constant voltage scheme.

The instant at which the pulse is produced is called the delay angle or firing angle. The firing angle can be varied by adjusting the magnitude of control signal. Thegeneration of pulses for a firing angle of 60 is shown in fig.1.3. Thyristors T1 and T2conduct in the positive half cycle. The gate pulses are logically same for T1 and T2.

Thyristors T3 and T4 conduct in the negative half cycle of input supply. Pulses for T3and T4 are produced with ramp reference whose magnitude varies from 0V to +4.5Vwhen t varies from 180 to 360(negative half cycle of the input supply).

Firing angle () can be varied from 0 to 180 by varying Control voltage fromVc(min)[0V] to V c(max)[+4.5V] respectively . The relationship between and V c is givenby


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Fig1.2(b) Bipolar positive ramp referene signal

Fig.1. 3 Ramp Firing scheme for firing angle,=60


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1.2 Cosine Firing Scheme:

In order to achieve improved closed loop response of converters, the control voltageof the firing circuit must be linear with respect to the output voltage of the converter.

To linearize the output voltage with respect to the control voltage, the inverse cosine-firing scheme is adopted. In this scheme the reference signal is cosine waveforminstead of ramp signal and such a scheme is shown in Fig.4. Pulses are produced forthe duration in which V c is greater than V ref . To limit the maximum pulse duration to180

, a sine reference with zero crossing detector is used.The firing angle is theinstant when V c exceeds V ref .i.e.,

V p cos =V c

(1.3 )

Where,

Vp is the cosine reference peak magnitudeVc is control voltage magnitude.

cos =V c /V p

= cos -1 (V c /V p ) (1.4)

Fig1.4 Cosine firing pulse generation scheme

The above equation shows that the relation between and V c is an inverse cosinefunction. Substituting equation 4 in equations 6&8,

cos -1 (Vc /V p)} (1.5)

From the above equation it is seen that since V c is an inverse cosine function of , theoutput voltage is cosine of inverse cosine of V c which means that the output voltage islinear with respect to V c. Thus in this scheme, the output voltage is linearized withrespect to the control voltage. The firing pulse generation is shown in Fig.1.5.


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1.3 THREE PHASE CONVERTER

1.31 Ramp Firing Scheme

Firing pulse generation using ramp method for three phase converter is similar tothat of single phase except that the reference signals are shifted mutually by 60 foreach successive thyristor. This is shown in fig.1.6.

1.32 Cosine Firing Scheme

The schematic of cosine firing scheme (for triggering one thyristor (T1)) is shown inFig.1.7. The principle of pulse generation is same as that for single phase cosinescheme. Here, the sine and cosine references for each phase are derived respectivelyfrom the line and phase voltages of the three phase supply. Each line voltage

waveform is taken as sinusoidal reference for one thyristor (-V BR for T1). Among thesix available phase voltage waveforms, the waveform which leads the chosen sinereference by 90 is chosen as the cosine reference (-V Y) for T1). This can be identifiedusing the vector diagram shown in Fig.1.10. The similar procedure is followed toobtain the gate pulses for other thyristors. The pairs of sine and cosine referencessignals used to generate the firing pulses for each thyristor of the six-pulse converteris shown in Fig.1.11 and summarized in Table-1.The pulse generation(for thyristor

T1) for firing angle of 60 is shown in fig.1.12.

Fig.1.5Cosine Firing scheme for single phase converter with firing angle,=60


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Fig.1.6 - Pulse generation for Three phase converter with Ramp firing scheme

Fig.1. 7 Cosine firing pulse generation scheme for 3-phase converter


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Fig.1.8Vectorial representation of phase and line voltages

Fig.1.9 Sine reference (blue w/f) and Cosine reference(green w/f) for eachthyristor


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Fig.1.9(a) Sine reference (blue w/f) and Cosine reference(green w/f) for eachthyristor

Table-1.1: Sine and cosine reference pairs taken from the vector diagram

S.No. ThyristorSine

ReferenceCosine

reference

Phase Shiftof sine

reference()

Phase shift of cosinereference()

1 T 1 -VBR -VY -30 60

2 T 2 VYB VR -90 03 T 3 -VRY -VB -150 -60

4 T 4 VBR VY -210 -120

5 T 5 -VYB -VR -270 -180

6 T 6 VRY VB -330 -240


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Fig.1.10 Pulse generation for 6 pulse converter with Cosine firing scheme

Table 1.2 Relation between control voltage and output voltage for bothschemes

Firing angle (deg) Control voltage (Vc) forRAMP scheme Control voltage (Vc) forCOSINE scheme0 -4.5 4.5

30 -3 3.89760 -1.5 2.2590 0 0

120 1.5 -2.25150 3 -3.897180 4.5 -4.5


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Table 1.3 Single phase converter with r load

Vm=325.26V, CONSTANT CURRENT LOAD=15A

FIRINGANGLE

()

DC OUTPUTVOLTAGE

I s (A) I s1 (A) PF HF DF RF

THEO(V) SIMU(V)0 207.1 205.5 11.15 11.15 1 0 1 0.486

30 193.2 191.9 11.02 10.84 0.852 0.1829 0.866 0.61460 155.3 154.3 10.1 9.266 0.458 0.4337 0.5 0.87590 103.5 102.8 8.038 6.431 0 0.7498 0 1.213

120 51.77 51.32 5.108 3.8 -0.311 1.257 -0.5 1.692150 13.87 13.7 2.056 0.633 -0.266 3.086 -0,866 2.627

176 0.252 0.296 0.366 0.319 -0.896 0.4852-

0.99675.862

Table-1.4- FOR Single phase converter with R load Vs=240V Observation fromthe above experiment:

AL PHA

DC OUTPUTVOLTAGE Vrms

For R=10ohms DF PF IS1 HF RF

THEO(V)

SIMU(v

Idc Irms

0 216.1 214.4 238.5 21.44 23.85 1 1 23.85 0.002 .48730 201.6 197.5 234.1 19.75 23.41 .866 .8534 23.07 .1723 .635560 162.1 156.8 211.1 15.68 21.11 .5 .464 19.59 .4015 .901590 108 107.8 168.7 10.72 16.87 0 0 14.11 .6542 1.214120 54.02 49.08 99.18 4.908 9.918 -0.5 -0.339 6.73 1.083 1.756150 14.48 14.09 39.94 1.109 3.994 -.866 -0.425 1.961 1.774 2.651

Fig.1.11 Single phase converter with R- load for =60


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PERFORMANCE OF SINGLE PHASE FULL CONVERTER

Schematic representation of single phase full bridge converter with load

Performance Parameters: (Single Phase)

For R Load1.Average Output Voltage (Ramp)

1 cosm

oavg V

V *2 (1.6)2.Average Output Voltage (cosine)

cos -1 (1.7)

For Highly Inductive Load1.Average Output Voltage (Ramp)

2

cosm

oavg V

V

2 (1.8)

2.Average Output Voltage (cosine)

Vo,avg = ( ) (1.9)

3.RMS Value of Fundamental Harmonic Current

01

2 2 s

I I

(1.10)

4.RMS Value of the Input Current0 s I I (1.11)

5.Power Factor


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1

cos s

s

I PF

I

(1.12)

6.Harmonic Factor

2

11

Is HF Is

(1.13)

7.Ripple Factor

2

1rms

oavg

V RF

V

(1.14)

Fig.1.12 Single phase converter with RL- load for =60

Table 1.5 - Single phase converter with highly inductive load


FIRINGANGLE

()

DC OUTPUTVOLTAGE

I s(A) I s1 (A)

PF

HF DF RFTHEO

(V)SIMU

(V)THEO SIMU

0 207.1 205.5 14.99 13.52 0.899 0.901 0.4791 1 0.48730 179.3 177.7 14.95 13.2 0.778 0.764 0.5318 0.866 0.81160 103.5 101.9 15 12.95 0.449 0.431 0.5857 0.5 2.017

90 0 1.661 15.04 12.84 0 0 0.6098 0 138.6120 -103 -105.2 14.99 12.92 -0.44 -0.431 0.5882 -0.5 1.953150 -179.3 -181 14.93 13.17 -0.77 -0.76 0.5348 -0.866 0.795176 206.6 -208.2 14.98 13.47 -0.89 -0.896 0.4875 -0.951 0.485


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Where,Vm- Input/Supply Peak Voltage (V)v c(max)- Cosine Peak Voltage (V)Vc - Control Voltage (V)Io - Average Load Current (A)

- Fundamental Harmonic Current (A) The simulation results for single phase converter with resistive and inductive loadsfor =60 are shown in the figures and tables:

Model Calculation:

(for R-load)1.Average Output Voltage (Ramp)

325.26

1 cos 60oavg V =155.3 V


1325.26 4.51 cos cos9

oavg V

=155.3 V

(for RL-load) 1.Average Output Voltage (Ramp)

2 325.26

cos60oavg V

=103.5 V


12 325.26 4.5cos cos9

oavg V

=103.5 V

3.RMS Value of Fundamental Harmonic Current

1

2 2 15 s I

=13.50 A

4.RMS Value of the Input Current

0 s I I =15 A

5.Power Factor

13.50

cos 6015.01

PF

=0.45

6.Harmonic Factor

215.01

113.50 HF =0.486

7.Ripple Factor


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2233

1103.5

RF

=2.017

Fig.1.13 Control Characteristics for R-load:

Fig.1.14 Control Characteristics for RL-load:


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PERFORMANCE OF SIX PULSE CONVERTER

Schematic representation of 3-phase full bridge converter with load

Performance Parameters: (Three Phase)

For R Load

Continuous Conduction mode ( 60)

1. Average Output Voltage (Ramp)

3 3cos

moavg

V V

(1.15)

2.Average Output Voltage (Cosine )

3 3 m coavg

m

V V V

E

(1.16)

Discontinuous Conduction mode ( > 60)

1. .Average Output Voltage (Ramp)

3 31 cos

3

moavg

V V

(1.17)

2. .Average Output Voltage (Cosine)


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2 23 3

1 0.5 0.866 /m c

oavg m c mm

V V V E V E

E

(1.18)

Fig.1.15 Three phase converter with R- load for =60

Fig.1.16 Three phase converter with RL- load for =60


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For Highly Inductive Load


3 3cos

moavg

V V

(1.19)

2. Average Output Voltage (Cosine)3 3 m c

oavg m

V V V

E

(1.20)

3. RMS Value of Fundamental Harmonic Current

01

2 2sin

3 s

I I

(1.21)

4..RMS Value of the Input Current

0

2

3 s I I (1.22)

5. Power Factor

1

cos s

s

I PF

I

(1.23)

6. Harmonic Factor

2

11

Is HF

Is

(1.24)

7. Ripple Factor

2

1rms

oavg

V RF

V

(1.25)

Model Calculation:

Continous Conduction mode ( 60)


3 3cos

moavg

V V

3 3 325.26cos 45oavg V

=380.2 V



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3 3 m coavg

m

V V V

E

3 3 325.26 6.36

9oavg V

=380.2 V

Table 1.6 - Three phase converter with r-load

Vm=325.26V

FIRINGANGLE

()

DC OUTPUTVOLTAGE

I s (A) I s1 (A) PF HF DF RFTHEO

(V)SIMU

(V)0 538 537.6 8.173 7.812 0.955 0.3075 0.9877 0.042

10 529.8 529.4 8.056 7.68 0.938 0.3166 0.984 0.06230 465.9 465.4 7.245 6.827 0.816 0.3551 0.866 0.17345 380.2 379.4 6.139 5.64 0.65 0.4277 0.707 0.29660 269 268.5 4.743 4.089 0.431 0.5877 0.5 0.50790 72.08 71.83 1.878 1.165 0 1.264 0 1.235

120 0 0.3 0.049 0.003 -0.034 14.61 -0.5 7.282

Table 1.7 - Three phase converter with highly inductive load


FIRINGANGLE

()

DC OUTPUTVOLTAGE

I s(A) I s1 (A)PF

HF DF RFTHEO

(V)SIMU

(V)THEO SIMU

0 538 537.6 8.164 7.797 0.954 0.955 0.3105 1 0.04210 529.8 529.4 8.164 7.796 0.94 0.9389 0.3104 0.984 0.04230 465.9 465.4 8.164 7.795 0.826 0.8161 0.3112 0.866 0.16145 380.2 379.4 8.163 7.794 0.675 0.6501 0.3115 0.707 0.29260 269 268.5 8.163 7.793 0.477 0.4311 0.3116 0.5 0.51490 0 13.09 8.163 7.792 0 0 0.3118 0 328.7

120 -269-

257.68.163 7.793 -0.47 -0.034 0.3116 -0.5 0.5126

135-

380.4-

371.18.163 7.794 -0.67 -0.674 0.3115

-0.707

0.292


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Fig.1.17: Control Characteristics for Three Phase Full Converter with Ramp firing

Discontinous Conduction mode ( > 60)

1. .Average Output Voltage (Ramp)

3 3 1 cos3

moavg V V

3 3 325.261 cos 90

3oavg V

=72.08 V


2 23 3

1 0.5 0.866 /m c

oavg m c mm

V V V E V E

E

2 23 3 325.26 0

1 0.5 0.866 9 0 / 99

oavg V

=72.08

Model Calculation:

1. Average Output Voltage (Ramp) 3 3

cosm

oavg V

V

3 3 325.26cos 45

=380.4 V

2. Average Output Voltage (Cosine) 3 3 m c

oavg m

V V V

E


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3 3 325.26 6.36

9

=380.6 V

3. RMS Value of Fundamental Harmonic Current

01

2 2 sin3

s I I

2 2 10sin

3

=7.796 A

4.RMS Value of the Input Current

0

2

3 s I I =

210

3=8.164 A

Fig.1.18: Control Characteristics for Three Phase Full Converter with Cosine firing


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Fig.1.19 Normalized control voltage Vs firing angle

5. Power Factor

1

cos s

s

I PF I

8.163

cos 457.794

=0.675

6. Harmonic Factor

2

11

Is HF

Is

28.163

17.794

= 0.3108

7. Ripple Factor

2

1rms

oavg

V RF

V

2396.2

1

380.2

RF

=0.292


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CONCLUSION:

The Control Characteristics shows that the variation of output voltage with respect tocontrol voltage is non-linear when ramp firing scheme is used and linear whencosine-firing scheme is used.

Also, theoretical average output voltage of the converters is verified throughsimulation and other performance parameters such as harmonic factor, power factor,ripple factor and input current are determined for single and three phase convertersfor different firing angles.

It is found that the performance of three phase converter are better than that ofsingle phase converter due to more number of pulses in the output and lessharmonic distortion. The performance can be increased further by using higher pulseconverters.

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Firing Pulse Generation