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Transcript
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013)
Improvement of Input Power Factor and Reduction in Harmonic
Content of a Single Phase AC-DC Converter Via sPWM
Technique
Dr. S.K.Gupta1, Rachit Goyal2, Ashutosh Tripathi3, Akash Deep4, Arvind Garg5
1
2,3,4,5
Chairman Electrical Engineering Department, DCR University of Science & Technology, Murthal, Sonepat
UG scholar Department of Electrical Engineering, DCR University of Science & Technolgy, Murthal, Sonepat, Haryana
In the conventional firing scheme used for single phase
full bridge converter the source current contains higher
order harmonics and also a low power factor is exhibited
[3].
Figure1 shows the basic circuit configuration for an ACDC bridge converter. Now these switches can be fired by
different switching schemes. First the switches will be fired
by the conventional firing and later by the sPWM switching
scheme and a comparison of input power factor and THD is
done.
Abstract— Electrical machines utilise power electronic
devices to start, stop and run the machines smoothly and
efficiently. The harmonics produced, distort the source
current to be a non- sinusoidal current. Input power factor
also suffers due to low displacement factor. The classical
control of switching devices results into a poor power factor.
The harmonic content of the input current is very high and
thus there is a requirement of a large filter. Sinusoidal PWM
control attempts to correct all the above shortcomings of an
AC/DC converter. In this paper, an improvement in the input
power factor and the total harmonic distortion is
demonstrated by comparing conventional two pulse converter
with a Sinusoidal PWM converter with appropriate switching
scheme. The analysis is done using MATLAB
SIMULATIONS and different conclusions are drawn.
Keywords-Sinusoidal PWM technique, AC-DC conversion,
Input power factor improvement, THD improvement.
I. INTRODUCTION
It is known that the power control of a DC load fed by
the grid is achieved by the use of an AC-DC converter
structure operating through a sPWM technique [1]. The
sPWM operation is performed by comparison of a
sinusoidal voltage waveform in phase to the source voltage
with a high frequency triangular waveform in order to
obtain a switching pulse waveform.. It is observed that
pulse of the maximum width is located exact at the middle
of the half period, while the pulse of the minimum width
appears at the beginning of the waveform [1].
Figure 1 BASIC CIRCUIT CONFIGURATION
76
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013)
On the input side the source inductance is being
modelled by an inductor of value 0.5 mH [2] and on the
output side a capacitor of value 400 µF is used to regulate
the DC output.
II. FIRING SCHEME
Figure 2 Gate pulses for (S1, S2) and (S3, S4) respectively for
conventional converter.
Figure1 shows four switches connected to form a bridge
rectifier. The bridge is connected to a RL load with a
capacitor filter at load end.
Figure 2 shows the conventional firing scheme for the
switches. For 0-π, switch S1 and S2 are fired and for π-2π
switch S3 and S4 are fired.
Figure 3 shows the sPWM firing scheme that can be
used to obtain a very good input power factor in AC-DC
conversion. The upper switches are controlled by the PWM
generated pulses and lower half is controlled by the pulse
generator. The PWM pulses can be generated by comparing
a rectified sine wave with a high frequency triangular
wave. The gate pulses are given according to the following
scheme [4]:
1. 0-π: Switch S-4 is closed and S-2 is open while in the
upper half of the bridge switches S-1 and S-3 are
switching complementary with sPWM pulses.
2. π-2π: Switch S-2 is closed and S-4 is open while in
the upper half of the bridge switches S-1 and S-3 are
switching complementary with sPWM pulses.
Figure 3 Gate pulses for S1, S3, S2, and S4 respectively for sPWM
converter
III. INVESTIGATING POWER FACTOR AND THD USING
SIMULINK/MATLAB
The input power factor for an AC-DC converter depends
on the harmonic distortion of the source current.
77
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013)
The circuit in figure 2 is modelled in the MATLAB for
conventional firing and sPWM firing. The results are
compared for different loads and for different firing angles.
Different firing angles used were 0, 30, 45, 60 and 90
degrees. The analysis was done using the FFT analysis
available in MATLAB.
Harmonic distortion is given by: [2]
THD=√
∑
Various loads have been taken and THD and input
power factors were calculated for different firing angles in
case of conventional converter and for sPWM firing
technique the results have been calculated using the FFT
analysis using the above formula. The frequency of
switching in PWM scheme has been taken as 5 kHz (50
pulses per half a cycle) and the graphical representation is
done in figure 5 and figure 6. The graphs are plotted for
fixed value of resistance and varying inductive loads. The
fix value of resistance is taken to be 10Ω and the different
values of inductive load are plotted on X axis. While for
the conventional firing the power factor can be reached up
to a maximum of .95 and that too for the case of a (firing
angle) =0, for the sPWM scheme the power factor remains
majorly equal to 1.
The THD analysis is also studied for fixed resistance and
varying inductive load. The harmonic distortion is going
very high, it’s only for the PWM technique that the THD is
almost equal to zero. Figure 4 shows the MATLAB model
for the study of sPWM technique.
(i)
Where Ik represents the magnitude of the kth harmonic
component and I1 represents the magnitude of the
fundamental component.
Input Power factor is given by: [2]
Pf= ⁄
√
*
)
(ii)
Where
is called the Displacement Factor
(DF) where (α1) is the angle between the fundamental
components of the input current and the input voltage.
Figure 4 MATLAB MODEL
When the bridge circuit is controlled by the conventional
firing scheme the power factor can’t be achieved equal to 1.
Even if the simple diode bridge is used for rectification
of AC voltage and a proper filter is used the best THD and
power factor achieved is 5.591 % and .931 respectively [6].
But as is clear from the figure 5 and 6 obtained by the
readings taken from the MATLAB simulations the THD
obtained for sPWM firing scheme is almost 0 and the
power factor obtained is found to be unity.
Table 1 shows a comparative analysis of different cases
considered also including the input power taken.
78
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013)
For different cases as the input power factor is falling the
Reactive Power intake is increasing which is uneconomical
for the supply system [5]. In the sPWM technique where
the input power factor is unity the Reactive Power intake is
minimum.
Table 2 shows the observation taken in the MATLAB
simulations for the sPWM technique for different loads.
The THD can be seen to have an improved value as
compared to the conventional case. The input power factor
is seen to be unity for all the loads considered.
Figure 7 and Figure 8 shows the amplitude of different
frequency components of the input source current for a load
of resistance of 10 Ω and an inductance of 30 mH for
conventional firing and sPWM firing. It can be clearly seen
that the different frequency components of source current
for conventional firing results in a net distortion of 42.18%
whereas it’s only equal to 0.17 % in the case of the used
firing scheme. Also the magnitude of the fundamental
component for conventional firing was only 5.413 amperes
whereas it was 15.42 amperes in the case of the used firing
scheme. The FFT analysis of all the loads was calculated
similarly and was found to be larger than the sPWM firing
scheme.
Once the THD is calculated and the FFT analysis done,
the input power factor can be calculated using (ii) and it
can be compared in the similar manner and the results
represented graphically in figure 6.
55
50
45
THD(%)
40
a=0
a=30
a=45
a=60
a=90
1.05
1
0.95
POWER FACTOR
0.9
0.85
0.8
0.75
0.7
0.65
0.6
0.55
0.5
10 15 20 25 30 35 40 45 50 55 60
LOAD(R=10Ω and varying L in mH)
Figure 6 Graph showing variation of input power factor with
different loads computed for conventional and sPWM converter
TABLE I
COMPARISON BETWEEN P, Q, THD, POWER FACTOR FOR
VARIOUS FIRING CONFIGURATIONS
sPWM
a=0
a=30
a=45
a=60
a=90
60
sPWM
35
Configu
ration
Active
Power(W)
Reactive
Power(VAR
S)
THD(
%)
a=0
1501
322.95
a=30
1382
540
5.73
.93
a=45
1254
624
14.55
.88
a=60
1074
686
23.8
.82
sPWM
2398
18.15
0.17
1
12.24
PF
.96
30
25
20
15
10
5
0
5
10 15 20 25 30 35 40 45 50 55 60
LOAD(R=10Ω and varying L in mH)
Figure 5 Graph showing variation of THD for various loads computed
in case of conventional and sPWM converter
79
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013)
TABLE II
SIMULATION RESULTS FOR sPWM CONTROL SCHEME
Active
Power(W)
Reactive
Power(Vars)
THD(
%)
PF
10,10
2398
18
.19
1
10,15
2397
18.16
.17
1
10,20
2397
18.1
.17
1
10,25
2397
18.15
.17
1
10,30
2398
18.15
.16
1
10,35
2397
18.1
.17
1
10,40
2397
18.1
.16
1
10,45
2398
18.1
.16
1
10,50
2397
18.08
.16
1
10,60
2398
18.07
.17
1
Load
Figure 8 FREQUENCY SPECTRUM OF sPWM CONVERTER
IV. CONCLUSIONS
From the analysis done using the MATLAB simulations
some conclusions are reached.
It's clear from the graphs (figure 5, figure 6) that the
Total Harmonic Distortion and the input power factor is
much better in the sPWM firing than in the conventional
firing scheme. The highest power factor for conventional
controller is obtained for zero firing angle but if we wish to
change/control the output voltage and also want good
power factor then we have to go for sPWM firing as it does
not change the displacement angle for changing the output
voltage but changes the modulating index [2]. The second
factor that controls the power factor is the harmonic
content. In the sPWM circuit the source inductance is
enough for the removal of harmonic distortion for small
loads but as the load will increase we will require a filter to
remove these harmonics but the size of that filter will be
comparatively smaller.
REFERENCES
[1 ] K. Georgakas and A. Safacas, July 27-29, 2007. Power Factor
Improvement of an AC-DC Converter via Appropriate sPWM
Technique Mediterranean Conference on Automation, AthensGreece.
[2 ] Muhammad H. Rashid. Power Electronics,Circuits, Devices, And
Applications.
Figure 7 FREQUENCY SPECTRUM OF TWO PULSE
CONVERTER
80
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013)
[3 ] Ali I. Maswood and M. H. Rashid, 1991. Input current harmonic
reduction in high power ac/dc rectifiers. Industrial Electronics,
Control and Instrumentation, Proceedings. IECON '91.
[4 ] http://nptel.iitm.ac.in/video.php?subjectId=108101038.
[5 ] IEEE SM 519-1992 IEEE Recommended Practices and
Requirements for Harmonic Control in Electrical Power Systems.
[6 ] Rohit Gupta,Ruchika,JUNE 2012“ A Study of AC/DC Converter
with Improved Power Factor and Low Harmonic Distortion”
International Journalon Computer Science and Engineering (IJCSE).
81