A comparative study of cascaded h bridge and reversing voltage multilevel inverter topologies

1. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
131
A COMPARATIVE STUDY OF CASCADED H BRIDGE
AND REVERSING VOLTAGE MULTILEVEL INVERTER
TOPOLOGIES
ROSHNY MARY B 1
, NEETHU S 2
1
P G Scholar, Department of EEE Sree Narayana Gurukulam College of Engineering Kolenchery, India
2
Assisstant Professor, Department of EEE Sree Narayana Gurukulam College of Engineering, Kolenchery, India
ABSTRACT
Multilevel inverters are mainly using for high power high voltage applications. It can overcome the limitations
of a conventional two level inverter such as higher total harmonic distortion and higher electromagnetic interference. The
main peculiarity of multilevel inverter is that it can generate nearly sinusoidal output voltage. Conventional multilevel
inverter topologies include diode clamped multilevel inverter, flying capacitor multilevel inverter and cascaded H bridge
multilevel inverter. The main limitations of the conventional topologies are large number of components, complex PWM
control methods and voltage balancing problem. As the number of level increases, these limitations will become more
pronounced. So in this paper a new multilevel inverter topology called reversing voltage is proposed. The proposed
topology needs less number of components and carrier signals and THD of the proposed topology is less compared to
conventional topologies. In this paper proposed seven level topology is compared with the conventional cascaded H
bridge seven level inverter in terms of THD and number of components required. Finally an experimental set up of the
proposed seven level topology is built and evaluated.
Keywords: Phase Disposition (PD) SPWM, Reversing Voltage (RV) Topology, Total Harmonic Distortion.
1. INTRODUCTION
Advanced power electronic converters are required to meet high power demand. As a result, different multilevel
converter topologies are developed. Multilevel converter topology was first introduced by Nabe in 1981. A conventional
voltage source inverter (VSI) can produce an output voltage with two levels ±Vdc/2, commonly known as two level
inverter. The main problem associated with the conventional two level inverter is the high ripple content present in the
output voltage or current [1]. So to improve the performance of the inverter, different PWM control methods have to be
adopted along with high switching frequency. But switching frequency cannot be increased beyond a particular limit due
to switching losses [2].
Multilevel inverter consists of series of semiconductors and capacitors, whose output will be a stepped
waveform. Figure.1 shows one leg of a multilevel inverter with different number of levels. Renewable energy sources
such as wind, fuel cell and solar panels can be directly interfaced with the multilevel inverter for high power applications
[3].
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 12, December (2014), pp. 131-139
© IAEME: www.iaeme.com/IJEET.asp
Journal Impact Factor (2014): 6.8310 (Calculated by GISI)
www.jifactor.com
IJEET
© I A E M E

2. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
132
Fig.1.One leg of an inverter with (a) two levels (b) three levels (c) N levels
The various multilevel inverter topologies include diode clamped multilevel inverter, capacitor clamped
multilevel inverter and cascaded H bridge multilevel inverter. In diode clamped multilevel inverter a bank of series
connected capacitors will divide the dc link voltage into small steps. Inverter poles can be connected to any one of these
voltage steps to generate the complete multilevel output. Flying capacitor multilevel inverter consists of pre charged
capacitors and this capacitor voltage is added or subtracted from the dc voltage to obtain the required levels. Cascaded H
bridge multilevel inverter includes a number of single phase H bridges connected in series, the output of which is equal
to the sum of voltage produced by each bridge. Table 1 shows the total number of components required for different
conventional multilevel inverter topologies [4].
Table.1- Number of Components for Three Phase Inverters.
The reliability of a multilevel inverter is inversely proportional to the number of components used in the
inverter. So as the number of components decreases the reliability of the inverter increases. Referring to Table.1,
cascaded H bridge multilevel topology is found to be the best in terms of reliability and modularization.
The objective of this paper is to introduce a new multilevel inverter topology called Reversing Voltage (RV).
The proposed topology requires less number of components compared to conventional inverters and has lower control
complexity.
2. CASCADED H BRIDGE AND PROPOSED INVERTER BASED ON REVERSING VOLTAGE
Cascaded multilevel inverter consists of a number of single phase H bridges connected in series. The output
voltage of the multilevel inverter is the sum of voltage produced by each bridge. Figure.2 shows a conventional cascaded
seven level inverter. It consists of three single phase bridges connected in series. Each bridge can generate three different
levels, ie +V, 0 or –V. For N level output it requires 2(N-1) high frequency switches, and N-1 carrier signals. When the
number of level increases, the control of the inverter become more complex [5]

3. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
133
Fig.2.Cascaded H bridge seven level inverter
In conventional topologies, all switches are combined to produce a complete multilevel output. But there is no
need to use all these switches for generating multilevel output. This idea is implemented in the new topology.
This is a hybrid multilevel inverter topology. It actually separates the output voltage into two sections. First section is
called level generation part and is responsible for generating the required positive level voltage. The peculiarity of level
generation part is that it will generate only positive level voltage. The switches used in this section should have high
switching frequency capability. The other section is called polarity generation part which decides about the polarity of
the output voltage. This section requires low frequency switches. So there is no need for all switches to be of high
frequency type. The block diagram representation of the inverter is shown in figure.3.
Fig.3.Block diagram of multilevel inverter using reversing voltage
Circuit diagram for the single phase seven level inverter based on reversing voltage is shown in figure.4. Left portion
of the circuit represents level generation part and right portion represents polarity generation part. Six high frequency
switches are used in the level generation part. The positive level generated by the level generation part is transferred to
polarity generation part. Polarity generation part consists of a single phase H bridge inverter which can work in two
modes forward and reverse mode. Switches 1 and 2 are operated in the forward mode, whereas switches 3 and 4 are
operated in the reverse mode. In forward mode, positive level generated by the level generation part is transferred to the
output in the same polarity. In reverse mode, positive level is transferred to the output in the reverse polarity, so that
complete cycle can be obtained.

4. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
134
Fig.4.Single phase seven level inverter based on reversing voltage
It can also be used for three phase applications. Three phase seven level model is shown in figure.5. Here output
of each leg is given to a single phase full bridge inverter, output of which drives the primary of a transformer. The
secondary of the transformer is delta connected which can be connected to the three phase system. Fig.5. shows the
corresponding three phase RV model.
Fig.5.Three phase RV model

5. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
135
3. SWITCHING SEQUENCE
Switching sequence in this inverter is very easy. Table.2. shows the switching modes for the proposed seven level
inverter. While selecting a particular switching pattern, make sure that switching transitions are minimum. Referring to
table.2, there are six switching patterns to control the inverter. The sequence of switches (2-3-4), (2-3-5), (2-6-5), and (1-
5) are used for levels 0 up to 3, respectively. The output voltage will be the sum of voltage sources which are included in
the current path. Current path for each level is shown in figure.6 according to the above switching pattern.
Table.2.Switching modes for seven level RV model
Fig.6.Switching sequence
4. CONTROL STRATEGY
LEVEL
0 Vdc 2Vdc 3Vdc
MODE
1 2,3,4 2,3,5 1,4 1,5
2 2,4,6 2,6,5

6. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
136
PD (Phase Disposition) SPWM is adopted to control the inverter. Here all carriers are in phase but have definite
offset from each other. Conventional multilevel inverter requires N-1 carriers for N level output. Since only positive
levels are generated in the proposed topology, we can reduce the number of carriers required to (N-1)/2 [6, 7]. All high
frequency switches are operated under PDSPWM control and the low frequency switches are operated at line frequency.
The three carriers and sinusoidal modulating signal for inverter control are shown in figure.7. [8, 9]
Fig.7.Carrier and modulating signal
5. COMPARISON
In order to validate the superiority of RV topology over cascaded inverter, first of all the number of components
required are compared. Referring to table.3, the number of components required for the RV topology is smaller.
Table.3. Number of three phase components required
Fig.8.Number of components required
6. SIMULATION RESULTS
Inverter Type RV Cascade
Main Switches 3((N-1)+4) 6(N-1)
Main diodes 3((N-1)+4) 6(N-1)
DC Bus Capacitors/
Isolated Supplies
(ܰ − 1)
2
3(ܰ − 1)
2
Total Numbers
(13ܰ + 35)
2
27(ܰ − 1)
2

7. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
137
Simulation results for both the inverters are shown below. Here each DC power supply is adjusted to 50V and the
load used is RL load. Output voltage is 300Vp-p.
Fig.9. output of cascaded inverter
Fig.10. THD of cascaded inverter
Fig.11.Output of RV topology

8. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
138
Fig.12. THD of RV topology
THD of the cascaded inverter is 17.72% and THD of RV topology is 1.25%. So the THD of the RV topology is
very small in comparison with the THD of cascaded inverter. There for the proposed topology is very advantageous since
it requires less number of components and carrier signals for the same output.
7. EXPERIMENTAL RESULTS
A prototype of seven level inverter using RV topology is built. Each DC voltage source is set to 5V. Control
signals are generated by PIC16F876.Output voltage is 30Vp-p.
Fig.13. Hardware set up
Fig.14.Output Voltage
8. CONCLUSION

9. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
139
This paper has provided a brief summary of different conventional multilevel inverters. The cascaded multilevel
inverter is compared with RV topology to show the superiority of the proposed inverter. In the proposed topology
switching operations are divided into high and low frequency parts. Proposed topology is better for all applications
because it has less control complexities; cost is also less and gives less % THD. Hence proposed topology is preferred
than conventional cascaded inverter. The carrier based PWM scheme using the PD strategy is used here. PD-SPWM
control method for this topology has fewer complexities since it use only positive carriers. The results show that this
proposed topology can work as a multilevel inverter with reduced number of carriers for SPWM. The hardware
implementation of power and control circuit has been implemented.
REFERENCES
[1] EhsanNajafi and Abdul Halim Mohamed Yatim” Design and implementation of a new multilevel inverter
topology” IEEE Trans. Ind.Electron., Vol.59, N0.11, November 2012.
[2] L. M. Tolbert, F. Z. Peng, and T. G. Habetler, “Multi level converters for large electric drives,” IEEE Trans. Ind.
Appl., vol. 35, no. 1, pp. 36–44,Jan./Feb. 1999.
[3] J. S. Lai and F. Z. Peng, “Multilevel converters – A new breed of power converters,” IEEE Trans. Ind.
Applicant., vol. 32, pp. 1098–1107, May/June 1996.
[4] S. Srikanthan and M. K. Mishra, “DC capacitor voltage equalization in neutral clamped inverters for
DSTATCOM application,” IEEE Trans. Ind. Electron., vol. 5.7, no. 8, pp. 2768–2775, Aug. 2010.
[5] M. Malinowski, K. Gopakumar, J. Rodriguez, and M. A. Perez, “A survey on cascaded multilevel inverters,”
IEEE Trans. Ind. Electron., vol. 57,no. 7, pp. 2197–2206, Jul. 2010.
[6] A. Radan, A. H. Shahirinia, and M. Falahi, “Evaluation of carrier-based PWM methods for multi-level
inverters,” in Proc. IEEE ISIE, 2007, pp. 389–394
[7] B.Urmila and D.Subbarayudu” Multilevle inverters: A comparative study of pulse width modulation techniques”
ISSN 2229-5518, Volume 1, Issue 3, December-2010.
[8] Jose Rodríguez,Jih-Sheng Lai and FangZhengPeng,” Multilevel inverters: A survey of topologies, controls and
applications” IEEE Trans. Ind. Electron, Vol.49, No.4, August 2002.
[9] HemantJoshi,P.N, Tekwaniand Amar Hinduja,” Implementation of a five-level inverter using reversing voltage
topology: A competitive solution for high-power IMdrive applications”, National power electronics conference
2010.
[10] Muhammad H. Rashid, Power Electronics circuits, devices, and applications. 2004 by Pearson education Inc.
[11] Anuradha Tomar, Dr. Yog Raj Sood, “All About Harmonics In Non-Linear PWM Ac Drives” International
Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 187 - 19 138 - 144, ISSN
Print : 0976-6545, ISSN Online: 0976-6553.