International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
Implementation of Seven Level Cascaded Multilevel Inverter in
Closed Loop For Different Modulation Index
R. Kavitha1, Dr. Rani Thottungal2, S. Agalya3
1
2
3
Asst. Professor, Professor, PG Scholar, Dept. of EEE, Kumaraguru College of Technology, Coimbatore, India-641049
The proposed closed loop control system reduces the
harmonics by varying the optimized switching angles and
DC voltage values provides to the inverter based on the
modulation index. For the lower modulation index the
device will operates in the reduced levels.
Abstract-This paper explains a control strategy SHE-PWM
and a closed loop control of the cascaded multilevel inverter
based on the modulation index. The modulation index is taken
as the feedback to the cascaded multilevel inverter, the
proposed system reduces the total harmonic distortion by
optimizing the angles of the inverter and the DC voltage
sources applied to the cascaded inverter. The specific lower
order harmonics are eliminated by using SHE-PWM method.
The optimized angles for the cascaded inverter are obtained
by using the particle swarm optimization (PSO) algorithm for
the various modulation indexes. The objective function is used
to reduce the lower order harmonics in the output voltage.
The simulation results of various modulation indexes and
equal, unequal DC voltage sources for seven level cascaded
multilevel inverter are carried out.
II. PRINCIPLE OF OPERATION
The seven level cascaded H bridge multilevel inverter
includes[3] three H bridge which are connected in the
cascaded connection.
Index terms –Cascaded H Bridge (CHB), Modulation Index
(MI), Particle Optimization Technique (PSO), Selective
Harmonic Elimination (SHE), Total Harmonic Distortion
(THD).
I. INTRODUCTION
Multilevel inverters are used widely in the industrial
applications because of the high voltage capability and it
produces the multilevel with low harmonics with minimum
stress in the switching devices. Compared [1] to the
conventional single bridge inverter the multilevel inverter
reduces the harmonics due to the multi switching. The
multilevel inverter produces different voltage levels by
varying the switching sequence of the inverter. In
multilevel inverter[1] as the number of voltage levels
increased the harmonics produced in the output waveform
decreases relatively. The simplicity of cascaded H bridge
multilevel inverter tends to used in many applications. The
cascaded multilevel [2] inverter requires the n DC sources
for 2n+1 level and it has the reduced number of switches
compared to the other topologies of the multilevel
inverters. Each H Bridge is connected to the separate DC
sources. Thus this topology provides high voltage at higher
modulation frequencies with low switching losses.
Fig. 1 Circuit diagram of the seven level cascaded MLI
Each H bridges having a separate DC source and it
produces three levels (+Vdc, 0,-Vdc), so the levels
produced in the seven level cascaded H bridge inverter are
(3Vdc, +2Vdc, +Vdc, 0,-Vdc,-2Vdc,-3Vdc). The equal and
unequal DC [4] voltage sources are used in this technique
to reduce the[5] lower order harmonics presents in the
output of the inverter.
567
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
In the control section the modulation index is calculated
and a lookup table is created for the optimized switching
angles and for DC voltage values by using PSO [6]
technique.
In this the number of particles is moving around in the
search space to get the best solution to the problems. The
flow chart of the particle swarm optimization is shown
below.
In the first stage of the flow chart searching points are
generated for each agent and the searching points are
evaluated in the second stage. In the third stage searching
points are modified by using the state equation if it reached
maximum iteration means this cycle is stopped otherwise
this cycle continues for searching the best solution.
START
Generation on the initial searching points
of each agent
Evaluation of searching point of each agent
Fig. 2 Block diagram of the closed loop MLI
The input parameter to the lookup table is modulation
index selects the angles and input voltages. The switching
angles are converted into delay time by using the
MATLAB embedded function and the reference voltages
are entered into the rectifier control section.
Thus the capacitor voltages are compared to the
corresponding reference values and error signal is
generated. These signals are processed by the PI controller
to generate the current signals required for the rectifier. The
current signals are compared by the reference current and
these errors signals are passed through the hysteresis band
to produce the gate signals required for the cascaded MLI.
Modification of each searching point by
state equation
Reach maximum Iteration
YES
NO
STOP
III. PARTICLE SWARM OPTIMIZATION
Fig. 3 Flow chart of PSO algorithm
The particle swarm optimization is a stochastic
optimization [7] which has population based search
procedure.
568
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
IV. SIMULATION RESULTS
Table.1
Optimum switching angles and lower order harmonics for equal DC sources
ᶿ1
ᶿ2
ᶿ3
THD
THDline
h1
1
10.70841
29.52743
59.50533
12.69012
7.869036
3.005054
0.053914
0.015011
0.088724
0.073584
0.95
15.07427
39.2501
60.26497
17.24922
8.156116
2.996764
0.050287
0.054942
0.013766
0.142538
m
h5
h7
h11
h13
0.9
14.79514
40.62699
66.62137
18.5118
11.7529
3.002886
0.070259
0.054074
0.009536
0.303118
0.85
23.83169
51.85433
61.88664
19.94008
9.40847
3.001276
0.011445
0.067173
0.0306
0.164962
0.8
5.729578
36.67221
84.928
19.63669
8.5147
3.00083
0.097914
0.01043
0.051109
0.089453
0.75
38.26712
48.58377
71.44602
42.60728
10.5341
2.99611
0.032892
0.015631
0.207054
0.7
38.67567
55.60725
72.28277
47.19903
12.41043
3.000985
0.060618
0.014652
0.059263
0.079375
0.65
38.81435
56.16402
78.73415
46.9267
12.01148
2.999674
0.019577
0.031965
0.115681
0.108789
0.6
38.58575
58.7127
83.30663
46.6284
11.67529
3.008158
0.009944
0.029827
0.049212
0.153928
0.55
40.63859
62.31352
85.94367
49.75228
16.26492
2.996056
0.042112
0.004287
0.036436
0.032897
0.014392
Table.2
Optimum switching angles, and lower order harmonics for unequal DC sources
Thd
v1
0.85
v2
1
v3
1.15
ᶿ1
11.15
ᶿ2
31.03
h1
h5
h7
h11
h13
h17
%
Thd
line
%
12.67
8.61
2.99
0.04330
0.09659
0.01569
0.06521
0.06549
ᶿ3
58.58
Table 1 show that the optimized angles for different
modulation index with lower order harmonics for equal DC
voltage sources. For m=1 the obtained THD is minimum
compared to the lower modulation indexes. The total
harmonic distortion is increased when the modulation index
is decreased.
Table 2 shows the voltage values and angles for unequal
Dc voltage sources. Here the degree of freedom is
increased as six so the harmonics eliminated is up to h17.
Figure 4a shows the output voltage of cascaded MLI in
closed loop control and the figure 4b shows the harmonic
profile 12.62 of the output voltage waveform.
Fig. 4a Output voltage waveform of seven level cascaded MLI in
closed loop
569
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
AUTHORS DETAILS
R. Kavitha completed her B.E from
Bharathiar University and M.E Degree from
Anna University in
2001 and 2004
respectively. She has 9 years of teaching
Experience and pursuing Ph.D. in Anna
Univerity. She is now working as senior
Grade Assistant professor in Kumaraguru
college of Technology, India. She is a life
member of ISTE. Her research interests are Multilevel inverters
and Optimisation Techniques.
Dr. Rani Thottungal obtained her B.E and
M.E degree fromAndhra University,India
and her Ph.D degree from Bharathiar
university,India. She has around 20 years of
teaching Experience. She is currently
working as Professor in Kumaraguru college
of Technology,India.She is member of ISTE
and IE. Her research interests includes power system and FACTS.
Fig. 4b Harmonic analysis for seven level output voltage
V. CONCLUSION
This paper is intended for closed loop control of the
cascaded multilevel inverter in order to reduce the lower
order harmonics. The PSO algorithm is recommended to
determine the optimized switching angles for the seven
level cascaded multilevel inverter for various modulation
index. Flexibility in the DC source values which are
obtained from the rectifier increases the degree of freedom
to eradicate the harmonic contents. Simulation and the
experimental results illustrates that the SHE values of
seven level for different modulation indexes and also equal
and unequal DC voltage sources.
S. Agalya completed her B.E Degree from
Tamilnadu
College
of
Engineering,
Coimbatore and currently persuing her M.E
Degree
in
Kumaraguru
College
of
Technology.
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