IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 04 Issue: 10 | Oct-2015, Available @ http://www.ijret.org 379
MULTILEVEL INVERTER WITH LEVEL SHIFTING SPWM TECHNIQUE USING
FEWER NUMBER OF SWITCHES FOR SOLAR APPLICATIONS
Mahajan Sagar Bhaskar Ranjana
1
, Pandav Kiran Maroti
2
, Ruchita Maheshwari
3
, Pachagade Ruchi M.
4
1,2,3,4
Department of Electrical and Electronics Engineering Marathwada Institute of Technology (MIT), Aurangabad,
INDIA- 431028
sagar25.mahajan@gmail.com
1
, kiranpandav88@yahoo.co.in
2
, rchitadahad@gmail.com
3
, ruchi.pachagade@gmail.com
4
Abstract
A multilevel inverter (MLI) is a popular inverter for solar based high power applications. The drawback of conventional H-bridge
inverter is non-sinusoidal output voltage, which reduces the output quality of inverter. Later, the drawback of conventional H-
bridge was overcome by conventional MLI. But, conventional MLI needs maximum number of diodes and switches. In order to
overcome this drawback proposed MLI topology with level shifting sinusoidal pulse width modulation (SPWM) technique can be
employed. Proposed MLI contain fewer number of switches and diodes, which helps in optimizing the circuit layout, reducing gate
driver circuit for those switches. SPWM technique uses multicarrier waveforms with level shifting ensuring the reduction in total
harmonics distortion (THD). In this Paper level shifting SPWM technique has been incorporated in which 5 kHz carrier wave is
compare with 50Hz of sinusoidal wave with a modulation index of 0.8. THD of proposed 9-level inverter is 17.27% without filter
and 4.29% with LC filter. Simulation of proposed inverter is carried out in MATLAB/SIMULINK.
Key Words: Solar, 9-level inverter, Level shifting SPWM, Power diodes and switches, Total Harmonics Distortion (THD)
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1. INTRODUCTION
Renewable energy sources have gained wide importance due
to the depletion of fossil fuels. Also the problem of pollution
caused by fossil fuels can be solved by using clean and
freely available renewable energy. Solar energy is one of the
renewable energy in which most of the researchers are
showing interests as it can be responsible for green energy
concept.
In case of solar energy system, voltage generated from solar
array is needed to be converted into ac signal for high power
AC application. Conventional H-bridge inverter is not a
practical solution for DC-AC conversion because of large
harmonics distortion and switching losses. Later, the
drawbacks of conventional inverter are overcome by
multilevel inverter (MLI) [1]-[2]. The increased number of
level reduces the harmonic content and brings the output
voltage waveform closer to sinusoidal. Conventional MLI
include diode clamped, flying capacitors and cascaded H-
bridge. In diode clamped MLI, diodes are used in majority
and number of diode increases with increase in levels. In
flying capacitor MLI, number of capacitor is increases with
increase in levels. In cascaded MLI, as the cascade stage
increases, certainly the number of switches and sources also
increases [3]-[6]. In Fig.1 (a), cascaded MLI for N-stages is
shown. But, these conventional MLI‟s requires large number
of power devices to generate maximum number of levels.
Further, this drawback is overcome by modifying the
conventional MLI‟s circuits. Diode clamped MLI has been
discussed with lesser number of diodes in [7]. Modified
flying capacitor topology is discussed in [8]. In [9], cascade
MLI topology with reduced number of switches is
discussed. In [10]-[12], 7-level inverter using 7 switches and
9 switches has been discussed. In Fig.1 (b) and Fig.1 (c), 7-
level with 9-switches and 7-level with 7-switches are shown
respectively. In [13], 7-level with 6-switches MLI is
explained. In Fig.1 (d), the schematic of 7-level with 6-
switches MLI is shown. In [13], though the number of
switches is less; but the number of diodes increases with
number of levels. The above discussed MLI‟s gives more
number of levels as compared to conventional MLI using
minimum number of switches. In the above topology, the
emphasis is on minimizing the number of switches but it
still requires large number of voltage sources. This
drawback is overcome by advanced MLI [14] and
asymmetrical MLI. Single phase MLI with single PV source
and less number of switches is proposed in [15]. The
drawback of advance MLI is that it requires inductor and a
switch operating at high frequency. The cost and design
complexity of advance MLI is increased due to the use of
inductor.
Generally, asymmetrical multilevel inverters are used to
increase the number of levels. In [16], 25-level
asymmetrical inverter with 12 switches for renewable power
grid applications is proposed. In [17], 25-level asymmetrical
inverter with 10 switches for solar application is proposed.
In [18], modified cascaded H-bridge inverter is proposed
with three different algorithms to generate 9, 13 and 17
levels. It is observed that THD for 9-level, 13-level and 17-
level inverter without filter is 17.22%, 10.74% and 7.76%
respectively. Also the THD for 9-level, 13-level and 17-
level inverter with filter is 5.86%, 5.18% and 4.71%
respectively.
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 04 Issue: 10 | Oct-2015, Available @ http://www.ijret.org 380
Asymmetric multilevel inverter is more advantageous than
symmetric multilevel inverter in obtaining more number of
output levels using same number of voltage sources. But the
main drawback of asymmetrical inverter is that it increases
the complexity of gate triggering pulses. The other
drawback is that the required rating of the power devices is
not same. Due to complexity of gate triggering pulses
asymmetrical MLI‟s are not beneficial to use. The proposed
MLI requires fewer number of power devices and nearly
removes all major above discussed drawbacks. In this paper,
a novel 9-level MLI with 7-switches is proposed. The
SPWM technique is used to reduce the THD and improve
the quality of output voltage.
2. PROPOSED TOPOLOGY
The proposed MLI is designed for 9-level using 7 switches
as shown below in Fig. 2. This topology is modification of
conventional H-bridge inverter where number of levels can
be increased by stacking specific combination of diode,
switch and voltage source. Switches S
1
, S
5
, S
6
and S
7
represent H-bridge in which S
1
, S
5
are used for generating
negative voltage levels whereas S
6
, S
7
are used for positive
voltage levels. In addition switches S
2
, S
3
and S
4
are used to
increase the number of levels. The proposed scheme 9-level
inverter requires 7-switches, 3-diodes and 4-voltage sources.
Fig. 3 shows circuit diagram of N-level proposed MLI.
Fig.2 Circuit diagram of proposed 9- level inverter.
Fig.3 N-level proposed MLI.
The relationship between number of output levels, switches,
diodes and voltage sources is stated in TABLE-I. It can be
depicted that this topology is suitable only for odd number
of output levels. (N+5)/2 switches with anti-parallel diodes,
(N-3)/2 clamping diodes and (N-1)/2 sources are needed to
design N-Level proposed inverter.
Table-I Relationship between number of output levels,
switches, diodes and voltage sources
No. of
levels
No. of
switches
No. of
clamping diodes
No. of
sources
3
4
0
1
5
5
1
2
7
6
2
3
9
7
3
4
N
(N+5)/2
(N-3)/2
(N-1)/2
3. OPERATION OF PROPOSED TOPOLOGY
The operation for 9-level MLI with 7-switches as shown in
Fig. 2 is discussed below. Switches S
1
and S
5
, S
6
and S
7
are
complementary to each other. S
1
, S
5
are turned „ON‟ to
generate negative output levels and S
6
, S
7
are turned „ON‟ to
generate positive output levels. The switches S
2
, S
3
and S
4
(a) Conventional Cascaded MLI (b) 7-level with 7-switches (c) 7-level with 9-switches (d) 7-level with 6-switches
Fig.1 (a)-(d) Existing multilevel Inverter
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 04 Issue: 10 | Oct-2015, Available @ http://www.ijret.org 381
are turned „ON‟ and turned „OFF‟ accordingly to increase
the number of output levels.
Table-II Switching states with corresponding voltage levels
Switches S2, S3, and S4 are turned „ON‟ when maximum
positive and negative output level is required and are turned
„OFF‟ with corresponding levels as shown in TABLE-II.
The current flow direction through the circuit for positive
and negative voltage levels is shown in Fig.4 (a)-(d) and
Fig.5 (a)-(d) respectively.
3. SPWM TECHNIQUE FOR PROPOSED TOPOLOGY
Sinusoidal pulse width modulation (SPWM) technique is
used for pulse generation where reference wave is sinusoidal
and carrier wave is high frequency triangular wave. The
comparison of both these waves gives rise to the pulses to
trigger the switches. Level shifting SPWM technique is
employed in order to reduce THD. Multiple carrier waves
are compared with single reference wave with 0.8
modulation index. Three schemes of level shifting SPWM
involve phase disposition, phase opposition disposition and
alternate phase opposition disposition. In this paper phase
disposition has been used for the pulse generation for the
proposed MLI.
Fig.6 (a) shows the internal circuitry of SPWM pulse logic
block for 9-level proposed MLI. The internal circuitry of
SPWM block contains bias, relational operator, followed by
logic operators. The inputs for SPWM logic block are
sinusoidal waveform and triangular waveform. Pulses are
generated according to switching logic and amplitude
modulation index. The methodology of generating pulses for
9-level MLI is shown in Fig.6 (b). Phase opposition
disposition and alternate phase opposition disposition
SPWM techniques are also suitable for proposed converter.
The modulation index for SPWM Technique is the ratio of
Amplitude of reference wave (A
R
) to the product of number
of positive or negative level and amplitude of carrier wave
(A
C
).
R
C
A
Modulation Index =
N-1
A
2
(1)
(a) V
out
= Vdc
4
+Vdc
3
+Vdc
2
+Vdc
1
(b) V
out
= Vdc
3
+Vdc
2
+Vdc
1
(c) V
out
= Vdc
2
+Vdc
1
(d) V
out =
Vdc1
Fig.4 (a)-(d) positive levels current direction
Voltage Levels
S
1
S
2
S
3
S
4
S
5
S
6
S
7
Vdc
4
+Vdc
3
+Vdc
2
+Vdc
1
0
1
1
1
0
1
1
Vdc
3
+Vdc
2
+Vdc
1
0
1
1
0
0
1
1
Vdc
2
+Vdc
1
0
1
0
0
0
1
1
Vdc
1
0
0
0
0
0
1
1
0
0
0
0
0
0
0
0
-Vdc
1
1
0
0
0
1
0
0
-Vdc
2
-Vdc
1
1
1
0
0
1
0
0
-Vdc
3
-Vdc
2
-Vdc
1
1
1
1
0
1
0
0
-Vdc
4
-Vdc
3
-Vdc
2
-Vdc
1
1
1
1
1
1
0
0
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 04 Issue: 10 | Oct-2015, Available @ http://www.ijret.org 382
(a) V
out
= -Vdc
1
(b) V
out
= -Vdc
2
-Vdc
1
(c) V
out
= -Vdc
3
-Vdc
2
-Vdc
1
(d) V
out
= -Vdc
4
-Vdc
3
-Vdc
2
-Vdc
1
Fig.5 (a)-(d) Negative levels current direction
(a) SPWM pulse logic block
(b) methodology
Fig.6 (a)-(b) methodology for generation of pulses.
4. SIMULATION RESULTS
The proposed MLI has been designed for 2kW, 9-level
output voltage using 7-switches for solar application. Four
input voltage sources are used and each voltage source is
equal to 60V. Phase disposition level shifting SPWM
technique is used with 50 Hz reference sinusoidal wave and
5 KHz carrier wave with modulation index 0.8. The
proposed 9-level inverter is designed with or without filter
LC Filter. LC filter has been used with inductance of 0.5mH
and capacitance of 15µF to reduce higher order harmonics.
The simulation has been performed in
MATLAB/SIMULINK. The switching pulse generation for
switches S
1
and S
5
, S
2
, S
3
, S
4
, S
6
and S
7
has been shown in
Fig.7 to Fig.11 respectively. It is observed that switches S
2
,
S
3
and S
4
required two carrier waveforms to obtain
switching pulses whereas S
1
, S
5
, S
6
and S
7
required single
carrier waveform. 9-level output voltage and output current
waveforms without LC filter are shown in Fig.12. It is
observed that 4 positive, 4 negative and 1 zero level is
obtained with amplitude of each level is 60V. The highest
positive level is 240V which is the positive addition of all
input voltage sources. The highest negative level is -240V
which is the negative addition of all input voltage sources.
Fig.13 shows the output voltage and output current
sinusoidal.
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 04 Issue: 10 | Oct-2015, Available @ http://www.ijret.org 383
Fig.7 Pulse generation for switch S
1
and S
5
Fig.8 Pulse generation for switch S
2
Fig.9 Pulse generation for switch S
3
The voltage stress across each switch is shown in Fig.14. It
is observed that the voltage stress across switches S1, S5, S6
and S7 is 240V which is equal to the addition of all input
voltage source. The voltage stress across switches S2, S3
and S4 is 60V which is equal to single input voltage source.
The THD of output voltage without filter is 17.27% which is
calculated by using Fast Fourier Transform (FFT) analysis
window shown in Fig.15. Afterwards, higher order
harmonics are eliminated by using LC filter. Fig.16 shows
the THD of filtered output is 4.29%.
Fig.10 Pulse generation for switch S
4
Fig.11 Pulse generation for switch S
6
and S
7
Fig.12 Output Voltage and Output Current Waveform
without filter
Fig.13 Output Voltage and Output Current Waveform with
filter