![International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 4, No. 3, September 2014, pp. 299~307
ISSN: 2088-8694 299
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Multilevel DC Link Inverter with Reduced Switches and
Batteries
Y. R. Manjunatha*, B.A. Anand**
* Department of Electrical Engineering, U.V.C.E, K.R. Circle, Bengaluru-1
** Electrical Engineering, Department of Agricultural Engineering, UAS-B, GKVK Campus, Bangalore-65
Article Info ABSTRACT
Article history:
Received Oct 3, 2013
Revised Mar 1, 2014
Accepted Mar 18, 2014
Multilevel inverters are the best solution for medium and high voltage power
electronic drives. Because of its unique characteristic of synthesizing
sinusoidal voltage with less harmonic contents using several DC sources. In
a three phase multilevel inverter, each phase of a cascaded H-bridge inverter
requires ‘n’ DC sources to obtain 2n + 1 output voltage levels. One particular
disadvantage is that, it increases number of power semiconductor switches.
To overcome this disadvantage a multilevel DC link inverter (MLDCLI)
with reduced number of switches and batteries is proposed.Keyword:
Cascaded H-bridge inverter
DC link
MLDCLI
Multilevel inverter
Power electronic drives Copyright © 2014 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
B.A. Anand
Assistant Professor,
Department of Agricultural Engineering,
UAS-B, G.K.V.K, Bangalore-560065
Email: lovanand1586@gmail.com
1. INTRODUCTION
Multilevel converters have received more and more attention because of their capability of high
voltage operation, high efficiency, and low electromagnetic interference (EMI). The desired output of a
multilevel converter is synthesized by several sources of dc voltages. With an increasing number of dc
voltage sources, the converter voltage output waveform approaches a nearly sinusoidal waveform while using
a fundamental frequency switching scheme. These results in low switching losses, and because of several dc
sources, the switches experience a lower voltage stress. As a result, multilevel converter technology is
promising for high power electric devices such as utility applications [1]. There are three major multilevel
topologies: cascaded, diode clamped, and capacitor clamped. For the number of levels (M) greater than three
or some applications such as reactive and harmonic compensation in power systems, these multilevel
converters do not require a separate dc power source to maintain each voltage level. Instead, each voltage
level can be supported by a capacitor with proper control. However, for M > 3 and applications involved in
active power transfer, such as motor drives, these multilevel converters all require either isolated dc power
sources or a complicated voltage balancing circuit and control scheme to support and maintain each voltage
level. In this aspect, the three existing multilevel converters are neither operable nor complete for real
(active) power conversion because they all depend on outside circuits for voltage balancing [2]. Multilevel
inverters produce a stepped output phase voltage with a refined harmonic profile when compared to a two-
level inverter-fed drive system. However, these configurations are also complex for higher number of levels.
The three-level inverter is realized by connecting two 2-level inverters in cascade. This three-level inverter
structure does not show the voltage fluctuations of the neutral point, as isolated power supplies are employed
to power the individual inverters. [3].](https://image.slidesharecdn.com/031mar14id4618edit-171213034253/85/Multilevel-DC-Link-Inverter-with-Reduced-Switches-and-Batteries-1-320.jpg)
![ ISSN: 2088-8694
IJPEDS Vol. 4, No. 3, September 2014 : 299 – 307
300
For many applications, to get many separate DC sources is difficult, and too many DC sources will
require many long cables and could lead to voltage unbalance among the DC sources [4]. To reduce the
number of DC sources and semiconductor switches required, when the cascaded multilevel converter is
applied to a motor drive, a scheme is proposed in this paper, that allows the use of two unequal voltage DC
sources and eight switches to generate 7 level equal step multilevel inverter output. Whereas the conventional
cascaded multilevel inverter requires three equal voltage DC sources and twelve switches to generate 7
levels. This scheme provides the capability to produce higher voltages at higher speeds (where they are
needed) with a low switching frequency, which has inherent low switching losses and high conversion
efficiency. For electric/hybrid electric vehicle motor drive applications, one H-bridges and four switches for
cascading the batteries for each phase is a good tradeoff between performance, cost and reliability.
2. MULTILEVEL CASCADED H-BRIDGES INVERTER STRUCTURE
To operate a cascaded multilevel inverter using two unequal DC source, [5] proposed to use the
first DC sources (i.e., the battery connected to first H-bridge, H1) as Vdc and the magnitude of voltage of
second battery as 2Vdc.To understand the concept, consider a cascaded multilevel inverter with two H-bridges
as shown in Figure 1. The DC source for the first H-bridge (H1) is a battery or fuel cell V1 with an output
voltage of Vdc, and the DC source for the second H-bridge (H2) is V2 with an output voltage of Vdc/2. The
output voltage of the cascaded multilevel inverter is:
tVtVtV 21 (1)
Figure 1. Single phase structure of a Multilevel
cascaded H-bridges inverter
Figure 2. Multilevel converter output with two
unequal batteries
By giving the triggering pulses to the switches of H1 appropriately, the output voltage V1 can be
made equal to VdC, 0, or -VdC. while the output voltage of H2 i.e., V2 can be made equal to 2Vdc , 0, or –
2Vdc by giving the triggering pulses to the switches of H2 appropriately. Therefore, the output voltage of the
converter can have the values, (2Vdc+Vdc), 2Vdc, Vdc, 0, -Vdc, -2Vdc and -3Vdc(-Vdc-2Vdc) which are 7
possible output levels. Figure 2 shows the 7 level equal step output voltage waveform [5].
The significant advantages of multilevel configuration are, voltage sharing both statically and
dynamically and it produces better voltage waveforms with less harmonic contents. One particular
disadvantage of cascaded H-bridges multilevel inverter is that, it increases greater number of power
semiconductor switches. The work proposed in [6] reduces the number of switches, their gate drivers,
compared with the existing multilevel inverter counterparts with harmonic profile improvement.
3. CASCADED HALF-BRIDGE–BASED MULTI LEVEL DC LINK INVERTER STRUCTURE
WITH UNEQUAL VOLTAGE SOURCES
The work proposed in [6] utilizes three batteries and ten switches to generate seven voltage levels
per phase. This paper proposes a scheme in which only two batteries and eight switches are utilized to
generate same number of voltage levels per phase and it is shown in Figure 3.](https://image.slidesharecdn.com/031mar14id4618edit-171213034253/85/Multilevel-DC-Link-Inverter-with-Reduced-Switches-and-Batteries-2-320.jpg)
![IJPEDS ISSN: 2088-8694
Multilevel DC Link Inverter with Reduced Switches and Batteries (Y. R. Manjunatha)
303
Figure 5. Comparison of required number of Batteries
5. MODULATION AND SWITCHING CONTROL
Generally, traditional PWM control methods and space vector PWM methods are applied to
multilevel inverter modulation control. The disadvantage of the traditional PWM methods is the power loss
in the switches due to the high switching frequency [7]-[9]. For these reasons, low switching frequency
control methods, such as a fundamental frequency method [10], and the active harmonic elimination method
[11] has been proposed for motor drive applications. This is the simplest switching control method for the
proposed multilevel motor drive. It also is an effective modulation control method for the proposed
Multilevel DC link inverter with unequal voltage sources motor drive.
The Fourier series expansion of the 7-level equal step output voltage waveform is:
tnnnn
n
V
tV
n
dc
sincoscoscos
4
321
...5,3,1
(2)
Where ‘n’ is the order of harmonic in the output voltage of multilevel inverter. Given a desired
fundamental voltage V1, one wants to determine the switching angles θ1, θ2, θ3 so that tVtV sin1 ,
and specific higher harmonics of tnV are eliminated [6]-[8]. For three-phase motor drive applications,
the triplen harmonics in each phase need not be considered as they automatically cancel in the line-to-line
voltages. The lower order harmonics can be eliminated by choosing proper values for θ1, θ2 and θ3 in the
following equations:
07cos7cos7cos
05cos5cos5cos
coscoscos
321
321
321
m
(3)
This is a system of three transcendental equations in the three unknowns θ1, θ2, and θ3. There are
many ways one can solve for the angles [10], [12]-[14] Here the resultant method [11], [15] was used to find
the switching angles. The modulation index m is defined as:
dcV
V
m
2
1
(4)
And the total harmonic distortion (THD) up to the 50th
harmonic (odd, non-triplen) is computed as:
1
2
49
2
7
2
5 ......
V
VVV
THD
(5)](https://image.slidesharecdn.com/031mar14id4618edit-171213034253/85/Multilevel-DC-Link-Inverter-with-Reduced-Switches-and-Batteries-5-320.jpg)
![ ISSN: 2088-8694
IJPEDS Vol. 4, No. 3, September 2014 : 299 – 307
306
(i.e. CT60AM-18F of Mitsubishi make) as switching devices. These devices have a maximum turn ON time
of 0.75 μs and turn off time of 0.7 μs.
8. CONCLUSION
This paper developed a Multilevel DC Link Inverter with unequal voltage sources motor drive
control scheme that required only two-battery source for each phase. A 7-level equal step output voltage
switching control method has been applied to the motor drive. Multilevel DC Link Inverter with unequal
voltage sources is simulated using MATLAB Simulink. The hardware prototype is implemented using
Microcontroller. A seven level-cascaded multilevel DC-link inverter with unequal voltage sources is
successfully fabricated and tested. The new MLDCL inverter with unequal voltage sources needs least
number of components than the other counterpart existing multilevel inverters for the same level of output
waveform. By increasing the number of levels of the Multilevel DC Link Inverter with unequal voltage
sources switches, gate driver and battery are reduced with better output waveform.
REFERENCES
[1] Z Du, L Tolbert, J Chiasson. Active Harmonic Elimination on Multilevel power converters. IEEE Trans. Power
Electron., 2006; 21(2): 459–469.
[2] Fang Zheng Peng, Senior Member, IEEE. A Generalized Multilevel Inverter Topology with Self Voltage Balancing.
IEEE Transactions on industry applications. 2001; 37(2): 611-618
[3] VT Somasekhar, K Gopakumar, Senior Member, IEEE, M. R. Baiju, Krishna K. Mohapatra, Student Member,
IEEE, L Umanand. A Multilevel Inverter System for an Induction Motor with Open-End Windings. IEEE
Transactions on industrial electronics. 2005; 52(3).
[4] Zhong Du, Leon M Tolbert, John N Chiasson, Burak Ozpineci Hui Li, Alex Q Huang. Hybrid Cascaded H-bridges
Multilevel Motor Drive Control for Electric Vehicles.
[5] Manjunatha YR, Dr MY Sanavullah. Generation of Equal Step Multilevel Inverter Output Using Two Unequal
Batteries. International Journal of Electrical and power engineering. 2007; 1(2): 206-209.
[6] R Kavitha, P Dhanalakshmi Rani Thottungal. Implementation of Novel Low Cost Multilevel DC-Link Inverter with
Harmonic Profile Improvement. Asian Power Electronics Journal. 2008; 2(3): 158-162.
[7] HS Patel, RG Hoft. Generalized harmonic elimination and voltage control in thyristor inverters: Part I -harmonic
elimination. IEEE Trans. Industry Applications. 1973; 9: 310-317.
[8] HS Patel, RG Hoft. Generalized harmonic elimination and voltage control in thyristor inverters: Part II -voltage
control technique. IEEE Trans. Ind. Applications. 1974; 10: 666-673.
[9] PN Enjeti, PD Ziogas, JF Lindsay. Programmed PWM techniques to eliminate harmonics: A critical evaluation.
IEEE Transactions on Industry Applications. 1990; 26(2): 302-316.
[10] Z Du, LM Tolbert, JN Chiasson. Harmonic elimination for multilevel converter with programmed PWM method.
IEEE Industry Applications Society Annual Meeting. Seattle, Washington. 2004: 2210-2215.
[11] PN Enjeti, PD Ziogas, JF Lindsay. Programmed PWM techniques to eliminate harmonics: A critical evaluation.
IEEE Transactions on Industry Applications. 1990; 26(2): 302 - 316.
[12] T Kato. Sequential homotopy-based computation of multiple solutions for selected harmonic elimination in PWM
inverters. IEEE Trans. Circuits and Systems I. 1999; 46(5): 586-593.
[13] JN Chiasson, LM Tolbert, KJ McKenzie, Z Du. A new approach to solving the harmonic elimination equations for a
multilevel converter. IEEE Industry Applications Society Annual Meeting. Salt Lake City, Utah. 2003: 640-645.
[14] Z Du, LM Tolbert, JN Chiasson. Modulation extension control for multilevel converters using triplen harmonic
injection with low switching frequency. IEEE Applied Power Electronics Conference. Austin, Texas. 2005: 419-423.
[15] S Sirisukprasert, JS Lai, TH Liu. Optimum harmonic reduction with a wide range of modulation indexes for
multilevel converters. IEEE Trans. Ind. Electronics. 2002; 49(4): 875-881.
[16] Zhong Du, Leon M Tolbert, John N Chiasson2, Burak Özpineci. A Cascade Multilevel Inverter Using a Single DC
Source. 426-430.](https://image.slidesharecdn.com/031mar14id4618edit-171213034253/85/Multilevel-DC-Link-Inverter-with-Reduced-Switches-and-Batteries-8-320.jpg)
![IJPEDS ISSN: 2088-8694
Multilevel DC Link Inverter with Reduced Switches and Batteries (Y. R. Manjunatha)
307
BIOGRAPHIES OF AUTHORS
Y.R. Manjunatha did his BE from Mysore University in Electrical And Electronics
Engineering from J.N.N College of Engineering, Shimoga. He has completed his Master of
Technology in Power Electronics Engineering at B.M.S College of Engineering under
Visvesvaraya Technological University Belgaum. Dr. Manjuntha has obtained his Doctoral
Degree from V.M.U, Salem in Electrical Engineering. His area of interests in power electronics
modeling, power engineering in the field of energy conversion, Distribution System Automation
and Designing of Power Electronics Converters. He is currently Chairman and Head of the
Department in Department of Electrical Engineering at U.V.C.E, Bangalore University.
B.A. Anand did his BE from Visvesvaraya Technological University during 2004-08 in
Electrical and Electronics Engineering from Sri Krishna Institute of Technology,
Chikkabanavara, Bangalore. He has completed his Master of Engineering in Power and Energy
Systems at University Visvesvaraya College of Engineering [U.V..C.E] during 2010-2011. Mr.
B.A. Anand is obtaining his Doctoral Degree from Visveswaraya Technological University in
Department of Electrical Engineering. He is currently working as Assistant Professor of
Electrical Engineering in Department of Agricultural Engineering at University of Agricultural
Sciences-Bangalore, G.K.V.K Campus, Bangalore-560065. His interests are in the area of
power system operation and control, Substation automation and Power distributions system
studies.](https://image.slidesharecdn.com/031mar14id4618edit-171213034253/85/Multilevel-DC-Link-Inverter-with-Reduced-Switches-and-Batteries-9-320.jpg)
Multilevel DC Link Inverter with Reduced Switches and Batteries
- 1. International Journal of Power Electronics and Drive System (IJPEDS) Vol. 4, No. 3, September 2014, pp. 299~307 ISSN: 2088-8694 299 Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS Multilevel DC Link Inverter with Reduced Switches and Batteries Y. R. Manjunatha*, B.A. Anand** * Department of Electrical Engineering, U.V.C.E, K.R. Circle, Bengaluru-1 ** Electrical Engineering, Department of Agricultural Engineering, UAS-B, GKVK Campus, Bangalore-65 Article Info ABSTRACT Article history: Received Oct 3, 2013 Revised Mar 1, 2014 Accepted Mar 18, 2014 Multilevel inverters are the best solution for medium and high voltage power electronic drives. Because of its unique characteristic of synthesizing sinusoidal voltage with less harmonic contents using several DC sources. In a three phase multilevel inverter, each phase of a cascaded H-bridge inverter requires ‘n’ DC sources to obtain 2n + 1 output voltage levels. One particular disadvantage is that, it increases number of power semiconductor switches. To overcome this disadvantage a multilevel DC link inverter (MLDCLI) with reduced number of switches and batteries is proposed.Keyword: Cascaded H-bridge inverter DC link MLDCLI Multilevel inverter Power electronic drives Copyright © 2014 Institute of Advanced Engineering and Science. All rights reserved. Corresponding Author: B.A. Anand Assistant Professor, Department of Agricultural Engineering, UAS-B, G.K.V.K, Bangalore-560065 Email: [email protected] 1. INTRODUCTION Multilevel converters have received more and more attention because of their capability of high voltage operation, high efficiency, and low electromagnetic interference (EMI). The desired output of a multilevel converter is synthesized by several sources of dc voltages. With an increasing number of dc voltage sources, the converter voltage output waveform approaches a nearly sinusoidal waveform while using a fundamental frequency switching scheme. These results in low switching losses, and because of several dc sources, the switches experience a lower voltage stress. As a result, multilevel converter technology is promising for high power electric devices such as utility applications [1]. There are three major multilevel topologies: cascaded, diode clamped, and capacitor clamped. For the number of levels (M) greater than three or some applications such as reactive and harmonic compensation in power systems, these multilevel converters do not require a separate dc power source to maintain each voltage level. Instead, each voltage level can be supported by a capacitor with proper control. However, for M > 3 and applications involved in active power transfer, such as motor drives, these multilevel converters all require either isolated dc power sources or a complicated voltage balancing circuit and control scheme to support and maintain each voltage level. In this aspect, the three existing multilevel converters are neither operable nor complete for real (active) power conversion because they all depend on outside circuits for voltage balancing [2]. Multilevel inverters produce a stepped output phase voltage with a refined harmonic profile when compared to a two- level inverter-fed drive system. However, these configurations are also complex for higher number of levels. The three-level inverter is realized by connecting two 2-level inverters in cascade. This three-level inverter structure does not show the voltage fluctuations of the neutral point, as isolated power supplies are employed to power the individual inverters. [3].
- 2. ISSN: 2088-8694 IJPEDS Vol. 4, No. 3, September 2014 : 299 – 307 300 For many applications, to get many separate DC sources is difficult, and too many DC sources will require many long cables and could lead to voltage unbalance among the DC sources [4]. To reduce the number of DC sources and semiconductor switches required, when the cascaded multilevel converter is applied to a motor drive, a scheme is proposed in this paper, that allows the use of two unequal voltage DC sources and eight switches to generate 7 level equal step multilevel inverter output. Whereas the conventional cascaded multilevel inverter requires three equal voltage DC sources and twelve switches to generate 7 levels. This scheme provides the capability to produce higher voltages at higher speeds (where they are needed) with a low switching frequency, which has inherent low switching losses and high conversion efficiency. For electric/hybrid electric vehicle motor drive applications, one H-bridges and four switches for cascading the batteries for each phase is a good tradeoff between performance, cost and reliability. 2. MULTILEVEL CASCADED H-BRIDGES INVERTER STRUCTURE To operate a cascaded multilevel inverter using two unequal DC source, [5] proposed to use the first DC sources (i.e., the battery connected to first H-bridge, H1) as Vdc and the magnitude of voltage of second battery as 2Vdc.To understand the concept, consider a cascaded multilevel inverter with two H-bridges as shown in Figure 1. The DC source for the first H-bridge (H1) is a battery or fuel cell V1 with an output voltage of Vdc, and the DC source for the second H-bridge (H2) is V2 with an output voltage of Vdc/2. The output voltage of the cascaded multilevel inverter is: tVtVtV 21 (1) Figure 1. Single phase structure of a Multilevel cascaded H-bridges inverter Figure 2. Multilevel converter output with two unequal batteries By giving the triggering pulses to the switches of H1 appropriately, the output voltage V1 can be made equal to VdC, 0, or -VdC. while the output voltage of H2 i.e., V2 can be made equal to 2Vdc , 0, or – 2Vdc by giving the triggering pulses to the switches of H2 appropriately. Therefore, the output voltage of the converter can have the values, (2Vdc+Vdc), 2Vdc, Vdc, 0, -Vdc, -2Vdc and -3Vdc(-Vdc-2Vdc) which are 7 possible output levels. Figure 2 shows the 7 level equal step output voltage waveform [5]. The significant advantages of multilevel configuration are, voltage sharing both statically and dynamically and it produces better voltage waveforms with less harmonic contents. One particular disadvantage of cascaded H-bridges multilevel inverter is that, it increases greater number of power semiconductor switches. The work proposed in [6] reduces the number of switches, their gate drivers, compared with the existing multilevel inverter counterparts with harmonic profile improvement. 3. CASCADED HALF-BRIDGE–BASED MULTI LEVEL DC LINK INVERTER STRUCTURE WITH UNEQUAL VOLTAGE SOURCES The work proposed in [6] utilizes three batteries and ten switches to generate seven voltage levels per phase. This paper proposes a scheme in which only two batteries and eight switches are utilized to generate same number of voltage levels per phase and it is shown in Figure 3.
- 3. IJPEDS ISSN: 2088-8694 Multilevel DC Link Inverter with Reduced Switches and Batteries (Y. R. Manjunatha) 301 Figure 3. 7-level Cascaded multilevel DC link inverter with unequal source The cascaded Multi Level DC Link (MLDCL) inverter with unequal voltage batteries consists of half bridge cells and one full bridge cell. Each half-bridge cell has two switches S1 and S2. They operate in a toggle fashion. The cell source is bypassed when S1 is on and S2 is off. The cell source adds to the DC link voltage when S1 is off and S2 is on. The half bridge cell produces DC bus voltage waveform in the shape of staircase and the full bridge inverter cell alternates the voltage polarity to produce an AC voltage of staircase waveform. Single-phase bridge inverter contains four switches from Sa, Sb, Sc and Sd. Table 1. Switching Pattern For Cascaded H-Bridge Mldci With Unequal Voltage Sources—Seven Level Levels S1 S2 S3 S4 Sa Sb Sc Sd VRL 1 on off off on on off off on Vdc 2 off on on off on off off on 2Vdc 3 off on off on on off off on 3Vdc 4 off off off off off off off off 0V 5 on off off on off on on off -Vdc 6 off on on off off on on off -2Vdc 7 off on off on off on on off -3Vdc Table 1 shows switching pattern for cascaded H-bridge MLDC inverter with unequal voltage sources. Specifically, the MLDCL formed by the 2 half-bridge cells provides a staircase-waveform in the dc-bus voltage of 3 steps to the full bridge inverter, which in turn alternates the voltage polarity to produce an ac voltage Van of a staircase shape with 7 levels. 4. COMPARISION OF MLDCL INVERTER WITH UNEQUAL SOURCES AND EXISTING COUNTERPARTS The multilevel dc-link inverter effectively reduces the number of switches and their gate drivers. Cascaded multilevel inverter requires 2* (m-1) number of switches and the cascaded MLDCL require only (m+3) number of switches. Whereas the proposed MLDCL inverter with unequal voltage sources requires just (m+8) switches, Where m takes the values 0, 2, 4, 6, 8,.., for 7, 15, 31, 63,.., switching levels respectively. In addition, the new multilevel dc-link inverter saves the cost of the inverter circuit by having an additional module of single-phase full bridge inverter. With higher voltage levels, only two switches are enough for fabricating each bridge in multilevel dc-link (MLDCL) with four switches in single phase full bridge inverter.
- 4. ISSN: 2088-8694 IJPEDS Vol. 4, No. 3, September 2014 : 299 – 307 302 Table 2. Component Count Comparison—Seven Levels Types of Inverter No. of Levels No. of Batteries No. of Switches Conventional Multi Level Inverter with equal source voltages 7 3 12 15 7 28 31 15 60 63 31 122 MLDCL Inverter with equal source voltages 7 3 10 15 7 18 31 15 34 63 31 65 MLDCL Inverter with unequal voltage sources 7 2 8 15 3 10 31 4 12 63 5 14 Figure 4. Comparison of required number of Switches Figure 4 shows the reduction in number of switches in the MLDCL inverter with unequal voltage sources. The number of levels as compared to the Conventional Multi Level Inverter with equal source voltages and MLDCL Inverter with equal source voltages the MLDCL inverter with unequal voltage sources scheme considerably reduces the number of switches. Figure 5 shows the reduction in number of batteries in the MLDCL inverter with unequal voltage sources. The number of batteries as compared to the Conventional Multi Level Inverter with equal source voltages and MLDCL Inverter with equal source voltages the MLDCL inverter with unequal voltage sources scheme considerably reduces the number of batteries.
- 5. IJPEDS ISSN: 2088-8694 Multilevel DC Link Inverter with Reduced Switches and Batteries (Y. R. Manjunatha) 303 Figure 5. Comparison of required number of Batteries 5. MODULATION AND SWITCHING CONTROL Generally, traditional PWM control methods and space vector PWM methods are applied to multilevel inverter modulation control. The disadvantage of the traditional PWM methods is the power loss in the switches due to the high switching frequency [7]-[9]. For these reasons, low switching frequency control methods, such as a fundamental frequency method [10], and the active harmonic elimination method [11] has been proposed for motor drive applications. This is the simplest switching control method for the proposed multilevel motor drive. It also is an effective modulation control method for the proposed Multilevel DC link inverter with unequal voltage sources motor drive. The Fourier series expansion of the 7-level equal step output voltage waveform is: tnnnn n V tV n dc sincoscoscos 4 321 ...5,3,1 (2) Where ‘n’ is the order of harmonic in the output voltage of multilevel inverter. Given a desired fundamental voltage V1, one wants to determine the switching angles θ1, θ2, θ3 so that tVtV sin1 , and specific higher harmonics of tnV are eliminated [6]-[8]. For three-phase motor drive applications, the triplen harmonics in each phase need not be considered as they automatically cancel in the line-to-line voltages. The lower order harmonics can be eliminated by choosing proper values for θ1, θ2 and θ3 in the following equations: 07cos7cos7cos 05cos5cos5cos coscoscos 321 321 321 m (3) This is a system of three transcendental equations in the three unknowns θ1, θ2, and θ3. There are many ways one can solve for the angles [10], [12]-[14] Here the resultant method [11], [15] was used to find the switching angles. The modulation index m is defined as: dcV V m 2 1 (4) And the total harmonic distortion (THD) up to the 50th harmonic (odd, non-triplen) is computed as: 1 2 49 2 7 2 5 ...... V VVV THD (5)
- 6. ISSN: 2088-8694 IJPEDS Vol. 4, No. 3, September 2014 : 299 – 307 304 6. SIMULATION AND EXPERIMENTAL RESULTS To validate the proposed MLDCL inverter with unequal voltage sources, a motor drive control scheme has been developed. The switches used for this inverter are the IGBTs GT60M303 (Toshiba Make). The gating pulses are generated by microcontroller board. A 3-phase induction motor is selected with the specifications shown in the Table 3 below. Also motor model is developed using MATLAB Simulink. Table 3. Specification of motor S. No. Type of the motor 3-φ Induction motor 1 Rated Output Power 3700 watts (5 HP) 2 Rated line to line voltage 415 volts 3 Rated Current 8.4 amps 4 Number of Poles 4 5 Frequency 50 Hz 6 Rated Speed 1485 rpm 7 Type of winding Y- connected The parameters of this motor are calculated by conducting No-load test, Blocked Rotor test and Retardation test. This motor model is simulated using MATLAB and the simulated results are compared with that of practical results. It is found that these two results are very close to each other. Then the motor model is simulated with the proposed cascaded multilevel inverter. Time Figure 6. Stator current, motor fed from SPWM Inverter Figure 7. Stator current, motor fed from ML Inverter Figure 8. Output voltage waveform of MLI (3 phase)
- 7. IJPEDS ISSN: 2088-8694 Multilevel DC Link Inverter with Reduced Switches and Batteries (Y. R. Manjunatha) 305 Figure 6 shows the stator currents of three phase induction motor on no-load fed from 2 level inverter. In this figure even though the waveform looks close to sinusoidal, it has distortion. Figure 7 is the stator currents of the same motor fed with 7 level inverter. It is observed that the distortion is almost reduced. Hence the motor runs smoothly and safely. Fig. 8 is the three phase output voltage of multilevel (7 Level) inverter with two unequal voltages. 7. EXPERIMENTAL VALIDATION To experimentally validate the proposed multilevel inverter, a prototype MLDCL inverter with two unequal batteries per phase and 8 IGBTs as switching devices. The fundamental frequency of the inverter output is 50Hz. Figure 9 shows the block diagram of the experimental setup. Figure 9. Block diagram of the experimental setup The microcontroller unit is a PIC 18F8722 microcontroller. It is an 80 pin TQFP package needs +12V DC supply for its working and operates at a frequency of 40MHz. This microcontroller has 128KB of program memory, 3936 bytes of data memory and 1024 bytes of data EPROM memory. It also has 9 I/O ports and 5 timers. This microcontroller is used to implement the control algorithm (i.e. to generate the gating pulses) for switching devices in the MLDCL inverter with unequal voltage sources. The gating pulses generated by microcontroller cannot be connected directly to the power semiconductor switching devices. Hence the driver circuit is introduced between microcontroller and MLDCL inverter with unequal voltage sources. The detailed circuit diagram of this driver unit is shown in Figure 10. Figure 10. Gate drive circuit for IGBT The driver circuit consists of an optocoupler (PC 817) to which the gating pulse generated by microcontroller is given. This optocoupler is used for isolation purpose. Output of optocoupler is passed through a buffer to improve the output drive capacity of the driver circuit so that the switching device can turn on quickly without delay. The driver circuit is for only one switching device. MLDCL inverter with unequal voltage sources was shown in Figure 3. This inverter circuit has been built using 900V, 60A IGBTs
- 8. ISSN: 2088-8694 IJPEDS Vol. 4, No. 3, September 2014 : 299 – 307 306 (i.e. CT60AM-18F of Mitsubishi make) as switching devices. These devices have a maximum turn ON time of 0.75 μs and turn off time of 0.7 μs. 8. CONCLUSION This paper developed a Multilevel DC Link Inverter with unequal voltage sources motor drive control scheme that required only two-battery source for each phase. A 7-level equal step output voltage switching control method has been applied to the motor drive. Multilevel DC Link Inverter with unequal voltage sources is simulated using MATLAB Simulink. The hardware prototype is implemented using Microcontroller. A seven level-cascaded multilevel DC-link inverter with unequal voltage sources is successfully fabricated and tested. The new MLDCL inverter with unequal voltage sources needs least number of components than the other counterpart existing multilevel inverters for the same level of output waveform. By increasing the number of levels of the Multilevel DC Link Inverter with unequal voltage sources switches, gate driver and battery are reduced with better output waveform. REFERENCES [1] Z Du, L Tolbert, J Chiasson. Active Harmonic Elimination on Multilevel power converters. IEEE Trans. Power Electron., 2006; 21(2): 459–469. [2] Fang Zheng Peng, Senior Member, IEEE. A Generalized Multilevel Inverter Topology with Self Voltage Balancing. IEEE Transactions on industry applications. 2001; 37(2): 611-618 [3] VT Somasekhar, K Gopakumar, Senior Member, IEEE, M. R. Baiju, Krishna K. Mohapatra, Student Member, IEEE, L Umanand. A Multilevel Inverter System for an Induction Motor with Open-End Windings. IEEE Transactions on industrial electronics. 2005; 52(3). [4] Zhong Du, Leon M Tolbert, John N Chiasson, Burak Ozpineci Hui Li, Alex Q Huang. Hybrid Cascaded H-bridges Multilevel Motor Drive Control for Electric Vehicles. [5] Manjunatha YR, Dr MY Sanavullah. Generation of Equal Step Multilevel Inverter Output Using Two Unequal Batteries. International Journal of Electrical and power engineering. 2007; 1(2): 206-209. [6] R Kavitha, P Dhanalakshmi Rani Thottungal. Implementation of Novel Low Cost Multilevel DC-Link Inverter with Harmonic Profile Improvement. Asian Power Electronics Journal. 2008; 2(3): 158-162. [7] HS Patel, RG Hoft. Generalized harmonic elimination and voltage control in thyristor inverters: Part I -harmonic elimination. IEEE Trans. Industry Applications. 1973; 9: 310-317. [8] HS Patel, RG Hoft. Generalized harmonic elimination and voltage control in thyristor inverters: Part II -voltage control technique. IEEE Trans. Ind. Applications. 1974; 10: 666-673. [9] PN Enjeti, PD Ziogas, JF Lindsay. Programmed PWM techniques to eliminate harmonics: A critical evaluation. IEEE Transactions on Industry Applications. 1990; 26(2): 302-316. [10] Z Du, LM Tolbert, JN Chiasson. Harmonic elimination for multilevel converter with programmed PWM method. IEEE Industry Applications Society Annual Meeting. Seattle, Washington. 2004: 2210-2215. [11] PN Enjeti, PD Ziogas, JF Lindsay. Programmed PWM techniques to eliminate harmonics: A critical evaluation. IEEE Transactions on Industry Applications. 1990; 26(2): 302 - 316. [12] T Kato. Sequential homotopy-based computation of multiple solutions for selected harmonic elimination in PWM inverters. IEEE Trans. Circuits and Systems I. 1999; 46(5): 586-593. [13] JN Chiasson, LM Tolbert, KJ McKenzie, Z Du. A new approach to solving the harmonic elimination equations for a multilevel converter. IEEE Industry Applications Society Annual Meeting. Salt Lake City, Utah. 2003: 640-645. [14] Z Du, LM Tolbert, JN Chiasson. Modulation extension control for multilevel converters using triplen harmonic injection with low switching frequency. IEEE Applied Power Electronics Conference. Austin, Texas. 2005: 419-423. [15] S Sirisukprasert, JS Lai, TH Liu. Optimum harmonic reduction with a wide range of modulation indexes for multilevel converters. IEEE Trans. Ind. Electronics. 2002; 49(4): 875-881. [16] Zhong Du, Leon M Tolbert, John N Chiasson2, Burak Özpineci. A Cascade Multilevel Inverter Using a Single DC Source. 426-430.
- 9. IJPEDS ISSN: 2088-8694 Multilevel DC Link Inverter with Reduced Switches and Batteries (Y. R. Manjunatha) 307 BIOGRAPHIES OF AUTHORS Y.R. Manjunatha did his BE from Mysore University in Electrical And Electronics Engineering from J.N.N College of Engineering, Shimoga. He has completed his Master of Technology in Power Electronics Engineering at B.M.S College of Engineering under Visvesvaraya Technological University Belgaum. Dr. Manjuntha has obtained his Doctoral Degree from V.M.U, Salem in Electrical Engineering. His area of interests in power electronics modeling, power engineering in the field of energy conversion, Distribution System Automation and Designing of Power Electronics Converters. He is currently Chairman and Head of the Department in Department of Electrical Engineering at U.V.C.E, Bangalore University. B.A. Anand did his BE from Visvesvaraya Technological University during 2004-08 in Electrical and Electronics Engineering from Sri Krishna Institute of Technology, Chikkabanavara, Bangalore. He has completed his Master of Engineering in Power and Energy Systems at University Visvesvaraya College of Engineering [U.V..C.E] during 2010-2011. Mr. B.A. Anand is obtaining his Doctoral Degree from Visveswaraya Technological University in Department of Electrical Engineering. He is currently working as Assistant Professor of Electrical Engineering in Department of Agricultural Engineering at University of Agricultural Sciences-Bangalore, G.K.V.K Campus, Bangalore-560065. His interests are in the area of power system operation and control, Substation automation and Power distributions system studies.