A 7-Level Single DC Source Cascaded H-Bridge

Multilevel Inverter with a Modified DTC Scheme for

Induction Motor-Based Electric Vehicle Propulsion

Farid Khoucha, Khoudir Marouani, Mohamed Benbouzid, Abdelaziz Kheloui,

Abdeslam Mamoune

To cite this version:

Farid Khoucha, Khoudir Marouani, Mohamed Benbouzid, Abdelaziz Kheloui, AbdeslamMamoune. A 7-Level Single DC Source Cascaded H-Bridge Multilevel Inverter with a Modi-fied DTC Scheme for Induction Motor-Based Electric Vehicle Propulsion. International Jour-nal of Vehicular Technology, Hindawi Publishing Corporation, 2013, 2013, pp.ID 718920.<10.1155/2013/718920>. <hal-00790313>

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Hindawi Publishing CorporationInternational Journal of Vehicular TechnologyVolume 2013, Article ID 718920, 9 pageshttp://dx.doi.org/10.1155/2013/718920

Research Article

A 7-Level Single DC Source Cascaded H-BridgeMultilevel Inverter with a Modified DTC Scheme forInduction Motor-Based Electric Vehicle Propulsion

Farid Khoucha,1,2 Khoudir Marouani,2 Mohamed Benbouzid,1

Abdelaziz Kheloui,2 and Abdeslam Mamoune1

1 University of Brest, EA 4325 LBMS, Kergoat Street, CS 93837, 29238 Brest Cedex 03, France2Ecole Militaire Polytechnique, UER ELT, 16111 Algiers, Algeria

Correspondence should be addressed to Mohamed Benbouzid; mohamed.benbouzid@univ-brest.fr

Received 1 September 2012; Revised 15 January 2013; Accepted 16 January 2013

Academic Editor: Lingyang Song

Copyright © 2013 Farid Khoucha et al.his is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

his paper presents a new hybrid cascaded H-bridge multilevel inverter motor drive DTC scheme for electric vehicles where eachphase of the inverter can be implemented using a single DC source. Traditionally, each phase of the inverter requires � DC sourcefor 2� + 1 output voltage levels. In this paper, a scheme is proposed that allows the use of a single DC source as the irst DC sourcewhich would be available from batteries or fuel cells, with the remaining (� − 1) DC sources being capacitors. his scheme cansimultaneously maintain the capacitors of DC voltage level and produce a nearly sinusoidal output voltage due to its high numberof output levels. In this context, high performances and eicient torque and lux control are obtained, enabling a DTC solution forhybrid multilevel inverter powered induction motor drives intended for electric vehicle propulsion. Simulations and experimentsshow that the proposed multilevel inverter and control scheme are efective and very attractive for embedded systems such asautomotive applications.

1. Introduction

Currently, automotive applications such as EV’s seem toconstitute an increasingly efective alternative to conven-tional vehicles, allowing vehicle manufacturers to fulill usersrequirements (dynamic performances and fuel consumption)and environmental constraints (pollutant emissions reduc-tion) [1].

he electric propulsion system is the heart of EV. Itconsists of the motor drive, transmission device, and wheels.In fact, the motor drive, comprising the electric motor, thepower converter, and the electronic controller, is the core ofthe EV propulsion system. he motor drive is conigured torespond to a torque demand set by the driver [2].

he induction motor seems to be a very interestingsolution for EV’s propulsion. FOC and DTC have emergedas the standard industrial solutions to achieve high dynamicperformance [3–5]. However some drawbacks of both meth-ods have motivated important research eforts in the last

decades. Particularly for DTC, the high torque ripple andthe variable switching frequency introduced by the hys-teresis comparators have been extensively addressed [6, 7].In addition, several contributions that combine DTC prin-ciples together with PWM and SVM have been reportedto correct these problems. his approach is based on theload angle control, from which a voltage reference vectoris computed which is inally modulated by the inverter [8].Although one major feature of classic DTC is the absence ofmodulators and linear controllers, this approach has shownsigniicant improvements and achieves similar dynamicperformance.

On the other hand, power converter technology iscontinuously developing, and cascaded multilevel invertershave become a very attractive solution for EV applications,due to its modular structure, higher voltage capability,reduced common mode voltages, near sinusoidal outputs,and smaller or even no output ilter [9–12]. In general,cascaded multilevel inverter may be classiied in two groups.

2 International Journal of Vehicular Technology

he irst one refers to the amplitude of isolated DC sourcesdevoted to supply each H-bridge cell. If the amplitude ofall sources is equal, then the inverter is called symmetrical;otherwise, if at least one of the sources presents diferentamplitude, then it will be called asymmetrical. he secondclassiication labels the multilevel inverter whether hybridor not. If the converter is implemented with diferentsemiconductor device technologies, diferent nature of DCsources (fuel cells, batteries, and supercapacitors) and/or if itpresents a hybrid modulation strategy, then it is classiied ashybrid [13–15]. his structure greatly simpliies the convertercomplexity.

he proposed control algorithm eliminates the need ofadditional isolated DC sources.he control strategy regulatesthe DC link voltages of capacitors connected to the smallestvoltages of a two-cell 7-level cascaded H-bridge inverter [16].Speciically and in comparison to previous works [17, 18], theproposed control does not use an angle for capacitor voltageregulation but a comparison voltage level. his will facilitatea DSP implementation.

he carried out simulations and experiments validatethe voltage control strategy and conirm the high dynamicperformance of the proposed method, presenting very lowtorque ripple.

2. Multilevel Inverter Topology

he power circuit of the cascaded H-bridge multilevelinverter is illustrated in Figure 1.he inverter is composed bythe series connection of power cells, each one containing anH-bridge inverter and an isolatedDC source. In the particularcase of asymmetric inverters these sources are not equal(�1 > �2).he asymmetry of the input voltages can reduce or,when properly designed, eliminate redundant output levels,maximizing the number of diferent levels generated by theinverter.herefore, this topology can achieve the same outputvoltage quality with less number of semiconductors, space,costs, and internal fault probability than the symmetric fedtopology.

A particular cell � can generate three voltage levels(+��, 0, −��). he total inverter output voltage for a particularphase � is then deined by

��� =�∑�=1

��� =�∑�=1

�� (��1 − ��2) , � ∈ {�, �, �} , (1)

where ��� is the total output voltage of phase � (resp., theneutral of the inverter�), the output voltage of cell of phase,and the switching state associated to cell.

It should be noticed how the output voltage of one cellis deined by one of the four binary combinations of theswitching state, with “1” and “0” representing the “On” and“Of” states of the corresponding switch, respectively.

he inverter generates diferent voltage levels (e.g., an

inverter with � = 4 cells can generate (2�+1 − 1 = 31)diferent voltage levels).When using three-phase systems, thenumber of diferent voltage vectors is given by 3��(�� − 1) + 1,where �� is the number of levels. For example, for the � = 4case with 31 levels there are 2791 diferent voltage vectors.

IMCell A1: main inverter

DC linkRectiier H inverter

�12

�12

�11

�11Cell B1

Cell B2

Cell C1

Cell C2

NCell B1: auxiliary inverter

a b c

��� ��� ���

Figure 1: Hybrid cascaded multilevel converter topology.

Table 1: 7-Level asymmetric cascaded inverter switching states.

��Cell 1 Cell 2 Total

�11 �12 ��1 �21 �22 ��2 ���1 1 0 3�DC 0 0 0 3�DC

2 1 0 3�DC 0 1 −�DC 2�DC

3 0 0 0 1 0 �DC �DC

4 0 0 0 0 0 0 0

5 0 0 0 0 1 −�DC −�DC

6 0 1 −3�DC 1 0 �DC −2�DC

7 0 1 −3�DC 0 0 0 −3�DC

Table 1 summarizes the output levels for an asymmetric 7-level inverter using only � = 2 cells per phase (only phasevoltage is given). An example of the voltage waveform for anasymmetric 7-level inverter is illustrated by Figure 2.

3. DTC-PWM Control

3.1. DTC Basic Principles. he stator voltage space vector (��)of an induction motor is related to the stator lux vector �� ina stator ixed coordinate system by

�� = ���� +����� . (2)

Neglecting the voltage drop in the stator resistance ��,the stator lux vector is the time integral of the stator voltagevector. Hence, for a small sampling period ��, (2) can beexpressed by

Δ�� ≈ ����. (3)

On the other hand, the motor torque is related to thestator and rotor luxes by

�� =32� �

�� �������

����������

���� sin (�) , (4)

where � = ��/� �. If both lux magnitudes are kept constantin (4), the torque can be controlled directly by changing

International Journal of Vehicular Technology 3

100

0

0.02 0.025 0.03 0.35 0.04 0.045 0.05 0.055 0.06

� �1

(V)

Time (s)

−100

500

−500.02 0.025 0.03 0.35 0.04 0.045 0.05 0.055 0.06

� �2

(V)

Time (s)

200

0

0.02 0.025 0.03 0.35 0.04 0.0450.05 0.055 0.06

� ��

(V)

Time (s)

−200

Figure 2:Asymmetricmultilevel inverterwith 7-level output voltagesynthesis.

the load angle. his can be easily achieved, since variationsin the stator lux vector change the load angle due to slowerrotor lux dynamics.

Considering (3) and (4) it follows that the stator voltagevector can be used to manipulate the load angle and conse-quently to control the torque.

3.2. Flux and Torque Control by Load Angle Tracking. Intraditional DTC, the inluence over the load angle of eachvoltage vector generated by the inverter is determined andstored in a lookup table, according to the stator lux positionin the complex plane. his is diicult to extend for multilevelinverter fed drives, where the complexity increases in hugeproportions in relation to the levels generated by the inverter.herefore, it is easier to look at the problem in a diferentway: the torque error can be used to generate a reference loadangle �∗ necessary to correct the torque behavior. hen thedesired load angle can be used to compute the exact voltagevector that will produce the necessary lux variationΔ��.hisprinciple is illustrated in Figure 3. Note that once providedthe reference load angle �∗, the reference stator lux vector�∗� can be computed by

�∗� = �����∗����� cos (�

∗ + ��) + � �����∗����� sin (�∗ + ��) , (5)

where |�∗� | is the ixed stator lux amplitude reference, and ��is the rotor lux vector angle. Note that (�∗ + ��) correspondsto the reference stator lux vector angle �∗� .

hen the desired stator voltage vector �∗� can be obtainedby

�� ≈Δ����

= �∗� − ����

. (6)

Finally �∗� has to be generated by the inverter. hisis commonly performed with PWM or SVM for 2-level

���� �

�

�∗

�

�∗� �∗�

�∗�

��

��

Δ�� ≈ �∗� ��

Figure 3: Stator voltage vector inluence over the stator lux vector.

inverters [19]. he use of a multilevel inverter will reducethe torque ripple amplitude due to the reduction of therate of change of the common-mode voltage (number ofstep variations in a ixed time span), and, in this case,using the hybrid modulation method [20], a ixed-frequencyoutput voltage will be obtained concentrating the voltagespectrum around the carrier frequency of the small powercell. Hence themost important drawbacks of traditional DTCare corrected. A simpliied control diagram of the proposedstrategy is shown in Figure 4. he outer speed control loop isomitted.

4. Hybrid Modulation Strategy

he hybrid modulation is specially conceived for the asym-metric multilevel H-bridge inverter structure [14].

he basic idea is to take advantage of the diferent powerrates among the cells to reduce switching losses and improvethe converter eiciency. From Figure 5 it can be seen thatthe inverter generates a rectangular waveform, which is atthe same frequency of the reference (turn-on and -of onlyone time during a half reference cycle). he remaining ofthe output voltage, second trace in Figure 5, is synthesized bythe auxiliary inverter at high switching frequency (with sinu-soidal PWM). his completes the generation of a multilevelsteppedwaveformwith a high-frequency component (similarto a multicarrier-based PWM), but with the diference thatfewer switching losses are produced to achieve it. Typicaloutput waveform of the inverter using this modulation isshown in the third trace of Figure 5. Note that the outputhas 7 diferent voltage levels given by all the possible com-binations of the series connection of (+�DC, 0, −�DC) and(+2�DC, 0, −2�DC).

5. Capacitor Voltage Control

Capacitor voltage control in the auxiliary inverter is achallenging task [14, 21]. he proposed control method isbased on a hybrid modulation that consists of adjustingthe main inverter turn-on. his indeed corresponds toadjusting the active and reactive powers that the maininverter injects to the load (if � is chosen to be exactly18∘, the main inverter injects only active power). By shitingthe voltage waveform synthesized by the main inverter(Figure 6), one could also inject some reactive power, which

4 International Journal of Vehicular Technology

IM

Regulation

block

Hybridmultilevel

inverter

Hybrid

modulationFlux

PI

��

�∗�

�∗

��

��Flux and torque estimator

1��

+

+ ++ +

−

− −

��

PI

��

∣�∗� ∣ ���∗� Δ��

Figure 4: Hybrid multilevel inverter-fed DTC motor drive with a single DC source.

1

0

0−10.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

Time (s)

Vo

ltag

e (p

u)

0.5

0

0−0.5

0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02Time (s)

Vo

ltag

e (p

u)

1

0

0−10.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

Time (s)

Vo

ltag

e (p

u)

Figure 5: Desired output, auxiliary and main inverter voltage waveforms.

300

200

100

0

−100

−200

−3000 0.002 0.004 0.06 0.008 0.01 0.012 0.014 0.016 0.018 0.02

Time (s)

���

Δ�

Figure 6: Variation of the main inverter conduction angle by �cm variation.

can be used to charge or discharge the auxiliary invertercapacitor.

herefore, the proposed algorithm, illustrated byFigure 7, regulates the capacitor voltage by an appropriate

adjustment of Δ� which is given by the PI regulator and actsdirectly on the variation of the comparison level �cm thatcontrols the main inverter turn-on and -of. he closed loopregulation for each phase is done by measuring the capacitor

International Journal of Vehicular Technology 5

SHPWM

+

+

+

++

+− −

−

−

PI

c��

��2

��1 ref

�� ref

�� ��

��

�cm

Δ�cm

��� ref

Secondary

H-bridge

inverter

MainH-bridge

inverter

Figure 7: Closed-loop control of the capacitor voltages.

Figure 8: 7-level single DC source cascaded H-bridge multilevelinverter.

voltage. From the capacitors voltage error, the PI regulatorgives directly the necessary Δ�cm to be added to the initialcomparison level which leads to changing the conductionangle � of the main inverter that allows the secondaryinverter capacitor charging.

6. Experimental and Simulations Results

6.1. Hybrid Modulation Strategy Implementation. he above-discussed hybrid modulation strategy has been experimen-tally implemented (Figure 8). he proposed hybrid mod-ulation strategy main results are illustrated by Figure 9.As abovementioned, the inverter generates a rectangularwaveform, which is at the same frequency of the reference(Figure 9(a)). he remaining of the output voltage is synthe-sized by the auxiliary inverter at high switching frequencywith sinusoidal PWM (Figure 9(b)). Figure 9(c) shows thetypical output waveform of the 7-level inverter under hybridmodulation.

6.2. Simulation Tests on an Induction Motor-Based EVPropulsion. To evaluate the dynamic performances of theproposed control strategy, numerical simulations have beencarried out on an EV propelled by an induction motorsupplied with 7-level single DC source cascaded multilevelinverter (Figure 4).

Output voltage

Reference voltage

(a) Main H-bridge inverter voltages

Output voltageReference voltage

Auxiliary source current

(b) Secondary H-bridge inverter voltages and auxiliary source (feeding theH-bridge) current

Auxiliarysourcecurrent

Voltage supplycurrent

Load current

7-level inverter

phase voltage

(c) he main outputs

Figure 9: Desired output, main, auxiliary, and inverter voltage andcurrent waveforms of the realized 7-level inverter under hybridmodulation.

Simulations were done using the standardized ECE-15and EUDCdriving cycle [22].he achieved performances areillustrated by Figures 10 to 15.

he simulation results show that the speed referencerequired by the driver is well-tracked as shown byFigure 10. One can see that the induction motor developedtorque presents fewer ripples compared to a traditional DTC

6 International Journal of Vehicular Technology

180

160

140

120

100

80

60

40

20

0

−200 2 4 6 8 10 12

Speed

Reference speed

Time (s)

Spee

d (

rad

/s)

Figure 10: Speed with the ECE-15 + EUDC driving cycle.

4

3.5

3

2.5

2

1

1.5

0.5

0

−0.50 2 4 6 8 10 12

Torque

Reference torque

Time (s)

To

rqu

e (N

m)

Figure 11: Reference and developed torques.

(Figure 11). his is due to the quality of the voltage supplyingthe inverter (Figure 12). Consequently, the stator currentsare almost sinusoidal (Figure 13) and better dynamicsare obtained; operating practically with constant lux(Figure 14).

Figure 15 shows the capacitor voltages waveforms whichsupply each auxiliary H-bridge of the three phase hybridmultilevel inverter. One can notice the oscillations aroundthe reference. Moreover, these voltages are quiet identical.However, their diference becomes very large when there isa signiicant acceleration or deceleration requested by thedriver. Indeed, in these cases, diferent amounts of energyare required due to the induction motor operating point fastvariation.

Time (s)

0 2 4 6 8 10 12

400

300

200

100

0

−200

−100

−300

−400

Ou

tpu

t vo

ltag

e (V

)

Figure 12: Output voltage waveform.

Time (s)

20

15

10

5

0

−5

−10

−150 2 4 6 8 10 12

Cu

rren

t (A

)

ib

ic

ia

Figure 13: Stator currents waveforms.

he above-presented simulation results conirm that,independently of the requested speed and torque, the capac-itor voltages are controlled and the validity of the proposedcontrol approach is proven.

7. Conclusion

In this paper a control strategy has been proposed for a hybridcascaded multilevel inverter with a modiied DTC schemeintended for induction motor-based EV propulsion. In thiscase, the main inverter uses a single DC voltage source, andthe auxiliary inverter capacitor voltages are controlled in eachphase. In addition, DTC-PWM allows the operation with aixed switching frequency, the reduction in the torque ripplewithout output ilter, and high-performance torque and luxcontrol.

he proposed control method simpliies the DTC appli-cation in embedded systems by reducing the inverter cost

International Journal of Vehicular Technology 7

0.9

0.8

0.6

0.7

0.5

0.4

0.3

0.2

0.1

00 2 4 6 8 10 12

Flu

x m

agn

itu

de

(Wb

)

Time (s)

0.802

0.801

0.8

0.799

0.798

0.797

11 11.0211.004 11.008 11.012 11.016

Time (s)

Flu

x m

agn

itu

de

(Wb

)

(a) Flux magnitude

10.80.60.40.20−0.2−0.4−0.6−0.8−1−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1

Ψ �

��

(b) Flux locus

Figure 14: Magnitude and locus of the stator lux in the DTC-PWM.

102

100

98

96

94

92

900 2 4 6 8 10 12Time (s)

Vo

ltag

e� �1(

V)

(a) Phase a

102

100

98

96

94

92

900 2 4 6 8 10 12

Time (s)

Vo

ltag

e� �2(

V)

(b) Phase b

102

100

98

96

94

92

900 2 4 6 8 10 12

Time (s)

Vo

ltag

e� �3(

V)

(c) Phase c

100.5

100

99.5

99

98.5

98

97.5

97

96.5

960 2 4 6 8 10 12

Cap

acit

or

volt

age

(V)

Time (s)

��2��1 ��3

�ref

(d) Capacitor voltage comparison zoom

Figure 15: Capacitor voltage waveforms.

8 International Journal of Vehicular Technology

Table 2: Rated data of the simulated induction motor.

1 kW, 50Hz, 400/230V, 3.4/5.9 A, 1420 rpm

�� = 4.67Ω, �� = 8Ω, � � = � � = 0.347H, �� = 0.366H� = 0.06 kg⋅m2, � = 0.042Nm⋅sec

and complexity (by decreasing the semiconductor and thesource numbers). Moreover, there is no need for hysteresiscomparators and lookup tables. All these key features makethe proposed control approach very attractive for embeddedsystems such as automotive applications.

Appendix

See Table 2.

Nomenclature

EV: Electric vehicleDTC: Direct torque controlFOC: Field-oriented controlPWM: Pulse width modulationSVM: Space vector modulation�, (�): Stator (rotor) index� (�): Voltage (current)�: Flux��: Electromagnetic torque�: Resistance� (��): Inductance (mutual inductance)

�: Leakage coeicient, � = 1 − �2�/� ����: Load angle�: Pole pair number.

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International Journal of Vehicular Technology 9

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