Multilevel Converter Topology Using Two Types of Current-Source Inverters

1558 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 6, NOVEMBER/DECEMBER 2006

Multilevel Converter Topology Using Two Typesof Current-Source Inverters

Sangshin Kwak, Member, IEEE, and Hamid A. Toliyat, Senior Member, IEEE

Abstract—A multilevel power-converter topology based on twotypes of current–source inverters (CSIs) has been proposed forlarge induction motor drives. The topology utilizes the combina-tion of the load-commutated inverter (LCI) using thyristors andthe CSI using gate turn-off thyristors (GTOs). As a result, themultilevel-inverter operation takes advantage of a soft switchingof the LCI and hard switching of the CSI. The output capacitor isrequired to generate the leading power factor for load commuta-tion of the LCI. It is shown that the proposed multilevel operationcontributes to the leading-power-factor generation by providingthe effective phase shift. Thus, the leading power factor requiredfor the load commutation of the LCI is obtained with significantlysmaller capacitors at the inverter’s output terminals. As a result,the proposed approach can employ the LCI and utilize its soft-switching operation, yielding a cost-effective solution comparedwith the conventional multilevel CSI using two GTO CSIs. Theswitching losses are curtailed due to the natural commutation ofthe LCI and the six-step operation of the GTO CSI. Therefore,the proposed multilevel inverter can both increase the outputpower level and decrease the output capacitor size for LCI. Thesimulation and experimental results have been shown to verify thefeasibility of the proposed multilevel topology and its operation.

Index Terms—Gate turn-off thyristor (GTO) inverter, load-commutated inverter (LCI), multilevel converter.

I. INTRODUCTION

THE pulsewidth-modulated (PWM) voltage-source inverter(VSI) is the most common power-converter topology for

adjustable speed induction motor drives. The VSI ensures asimple and effective motor control since the power circuit canbe operated over wide ranges of load frequency and voltages.Furthermore, the popular space vector PWM techniques forthe VSI have been well established to obtain a direct controlof the magnitude and phase of the output voltage. However,the VSI, based on fast-switching insulated gate bipolar tran-sistors (IGBTs), has shown intrinsic weakness for high powerapplications due to substantial switching losses and high dv/dtof the PWM operation, leading to hazardous over voltages

Paper PID-06-04, presented at the 2004 Industry Application Society AnnualMeeting, Seattle, WA, October 3–7, and approved for publication in theIEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Mining IndustryCommittee of the IEEE Industry Applications Society. Manuscript submittedfor review October 15, 2004 and released for publication July 6, 2006. Thiswork was supported in part by the ERC Program of the National ScienceFoundation under Award Number EEC-9731677.

S. Kwak is with the Research and Development Center, Samsung SDICorporation, Youngin, Korea (e-mail: [email protected]).

H. A. Toliyat is with the Department of Electrical Engineering, TexasA&M University, College Station, TX 77843-3128 USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/TIA.2006.882645

and electromagnetic interference problems [6]. Moreover, theVSI fed from a diode rectifier prevents regeneration of theoutput power back to supply utility, which is often an importantrequirement for high power applications. In high power areas,current-source inverters (CSIs) using gate turn-off thyristors(GTOs) are of special interest. Most of all, the power circuitof the CSI is simpler and more robust than the VSI due to nofreewheeling diodes with unidirectional current flow. The CSIcan provide a higher reliability related with a dc-link inductorthan a capacitor for the VSI and inherent overcurrent protectionby current regulation of the controlled rectifier. The CSI permitsfour quadrant operations transferring the electric power in bothdirections using the controlled rectifier with closed-loop currentcontrol. In addition, the CSI is more efficient because of thequasi-square-wave mode operation, which turns ON and OFF

only once per cycle of the output current. However, the quasi-square-wave output-current waveforms are, in turn, the mostobvious disadvantages of the CSI. The current waveforms,rich in low-order harmonics, can produce considerable currentharmonics, which can cause losses and heating inside themachine. Furthermore, they can lead to voltage spikes in thestator leakage inductance of the motor.

Two approaches have been considered to reconcile the har-monic current problems, because the harmonic concerns gainimportance in high power applications. One solution is to usethe PWM for GTO inverters, equipped with output capacitors.However, the PWM GTO inverter has shown their weakness,such as the increased size and cost, and greater complexityof the control algorithm. The susceptible resonance betweenthe output capacitors and the motor inductance has seriouslyrestricted the system performance [1]. Among all, the PWMoperation of the GTO thyristors greatly reduces the systemefficiency and increases the GTO and snubber power losses.Keeping in mind that one of the most important issues of theGTO CSI is the low-switching frequency, another approach isto use multilevel CSIs. The multilevel CSIs have been proposedto improve the output-current quality as well as to increasethe output power level, while keeping the switching frequencyof the GTO as low as possible [7]–[11]. The multilevel CSIconsists of two GTO CSI modules based on a dual config-uration, and each GTO CSI operates in the six-step mode,switching once ON and OFF per one output-current cycle. Theoutput capacitors are required to absorb the inductive energy byproviding commutation paths during the forced commutation ofthe GTO thyristors. The capacitive filters also help to avoid theovervoltages in the motor. This paper presents the investigationof the multilevel CSI for large power induction motor drives.The proposed CSI topology is a combination of a GTO CSI

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KWAK AND TOLIYAT: MULTILEVEL CONVERTER TOPOLOGY USING TWO TYPES OF CURRENT-SOURCE INVERTERS 1559

using the GTO thyristors and a load-commutated inverter (LCI)with the thyristors. The LCI based on the thyristors has shown abetter performance than the GTO-based CSI, because the GTOthyristor compromised its performance to achieve the turn-off capability [2]. As a result, the LCI has several advantagesover the GTO CSI, such as higher voltage blocking ability,smaller ON-state voltage drop, better cost effectiveness, andbetter reliability [2]. The serious deficiency of the LCI is itsinability to turn off the switching devices by applying gatesignals. The only way to turn off the thyristors in the LCI is forthe external circuit to force the reverse-biased voltage across thedevice for a minimum specific time period. This requirementfor the natural turn-off process is epitomized to generate a lead-ing power factor for the three-phase LCI [3]–[5]. The leadingpower factor required for natural commutation can be providedby additional output capacitors connected in parallel with theinduction motor, since the induction motor cannot provide theleading power factor through excitation control employed forthe synchronous motor. Based on the proposed topology, itis possible to realize a synergism effect with the GTO CSIand the LCI. Under the appropriate modulation strategy, themultilevel CSI can contribute to generate 30◦ leading angle forthe natural commutation of the LCI. With the aid of 30◦ leadingangle provided by the CSI operation itself, the proposed systemcan perform the load commutation successfully with greatlyreduced capacitor size.

The multilevel CSI is designed to provide four output-currentlevels. Both the GTO CSI and the LCI are modulated to switchonly at fundamental frequency of the motor output cycle. As aresult, the switching losses can be minimized because it utilizesthe soft switching of the LCI and the six-step mode of the GTOCSI. Furthermore, the proposed multilevel CSI can be a cost-effective topology compared with the conventional multilevelCSI consisting of two GTO CSIs, replacing one GTO CSI withthe LCI. With the increased output power level and robust CSIstructures, the proposed multilevel converter can be beneficialfor high power areas such as milling operations for the miningapplications. Simulation and experimental results are shown todemonstrate the feasibility of the proposed system and controlstructure.

II. CAPACITOR COMMUTATION OF LCI-BASED

INDUCTION MOTOR DRIVE

A typical schematic diagram of an LCI-based inductionmotor drive is shown in Fig. 1. It consists of a three-phasecontrolled rectifier at the input side and an LCI at the outputside with a large dc-link inductor. The amplitude of the currentssupplied to the motor is controlled by the phase-controlled rec-tifier through a dc-link inductor. The dc-link inductor reducesthe current harmonics and ensures that the input of the LCIand hence to the motor appears as a current source. The dc-current magnitude as well as the motor-current magnitude canbe controlled by adjusting the firing angle of the controlledrectifier. The LCI can control only the fundamental frequencyof the motor currents by selecting the gating instances ofthyristors. Since the system has thyristor-based topologies, itmust guarantee a safe commutation for thyristors, requiring that

Fig. 1. LCI-based induction motor drive.

Fig. 2. Vector diagram of LCI-based induction motor drive.

the LCI be faced with a leading power factor in all the operatingregions. The leading power factor required for natural commu-tation is generated by additional output capacitors connected inparallel with the induction motor. As the power rating of theinduction motor is increased, a larger capacitance is requiredto create higher leading var requirement taken by the capacitor,which could become unreasonably high. The requirement of alarge capacitor has limited the use of this drive for high powerapplications. A vector diagram of Fig. 2 explicitly explainshow the output capacitor provides a phase shift of the current,resulting in a leading-power-factor angle φ. This leading powerfactor allows thyristors of the LCI to commutate at speedsabove critical frequency of induction motors.

The output capacitor also smoothes out the output-currentwaveform coming from the LCI to nearly sinusoidal in thehigh-frequency operation by providing a low-impedance pathfor current harmonics. However, for speeds lower than thelower limit of the operating range, such as the startup and thelow-speed region, these output capacitors cannot make enoughleading angle, because the capacitor currents are too small dueto the high impedance of the capacitors. Thus, additional forceddc-commutation circuits are required so as to facilitate thecommutation from one phase to another phase, by effectivelybypassing the flow of the dc-link current around the load. Withthe operation of this circuit, the induction motor can start upand bring the operation to above the critical speed, which willensure a natural commutation by output capacitors.

III. PROPOSED MULTILEVEL CSI SYSTEM

A complete power circuit diagram of the proposed multilevelCSI system is illustrated in Fig. 3. The proposed multilevelsystem combines the LCI using the thyristors and a GTOinverter. In the proposed multilevel inverter, both the LCI andthe GTO CSI are operated in the six-step mode to minimize the


Fig. 3. Circuit diagram of proposed multilevel CSI system.

resultant switching losses. Thus, the inverters have no controlof the magnitude of the output current. The amplitude of thecurrents supplied to the motor is controlled by adjusting thefiring angle of the controlled rectifier. The LCI and the GTOinverters are fed from dedicated controlled rectifiers. The inputtransformer with delta and Y connections for its secondarywinding is employed for each controlled rectifier to reduce theinput harmonic components. The ac capacitors are connected atthe output terminals of the multilevel CSI to avoid overvoltagesin the motor as well as provide a leading power factor for theLCI. The dc-link currents for both the LCI and GTO inverter areequally controlled with the controlled rectifiers. The thyristorsin the LCI naturally turn ON and OFF only once per cycleof the output current; consequently, their switching loss isnegligible. The GTO inverter is also modulated to switch onlyat the same fundamental frequency of the LCI output waveform.Furthermore, the GTO inverter is controlled to switch with60◦ phase delay from the LCI output current. As a result, theLCI and the GTO inverters generate the same current waveformwith only 60◦ phase difference.

The difference between the thyristor and the GTO is the self-turn-off capability. The LCI is naturally commutated based onthe leading power factor established by the external condition,while the GTO inverter is turned off through the gating signal.It is observed that the leading power factor for the naturalcommutation of the LCI is generated by the GTO CSI operationin conjunction with the output capacitor. Under the proposedtopology and modulation strategy, the multilevel inverter canprovide 30◦ phase shift for the LCI, and, accordingly, the outputcapacitor size required for the phase shift of the output currentcan be greatly reduced in the system. Therefore, the naturalcommutation for the LCI can be obtained with the smalleroutput capacitor. Fig. 4 illustrates the idealized output-currentwaveforms of the LCI output current ILCI, the GTO inverteroutput current IGTO, and the multilevel-inverter output currentIO to explain the proposed control method for the multilevel

Fig. 4. Idealized output current and fundamental waveforms of LCI, GTOCSI, and multilevel inverter.

CSI. The fundamental current waveforms of the currents arealso depicted. The gating signal of the GTO inverter is con-trolled with 60◦ phase delay from the gating signal of the LCI.Thus, the two output currents of the LCI and the GTO inverterhave 60◦ phase-angle difference with respect to each other. Itis possible to synthesize a stepped current waveform with fourlevels, such as −2Idc, −Idc, Idc, and 2Idc at the output phasewith this topology. From Fig. 4, it is shown that the multilevel-inverter output current IO and the LCI output current ILCI have30◦ phase shift by the GTO inverter operation. In other words,the proposed multilevel operation can effectively contribute togenerate a 30◦ leading power factor for the load commutationof the LCI. Based on the leading power factor by the multilevelinverter, the phase-shift amount required by the output capacitoris greatly reduced and, thus, smaller output capacitor can beused to perform the natural commutation of the LCI in theproposed multilevel inverter.

Fig. 5 shows the per-phase equivalent circuit of the pro-posed multilevel inverter. The proposed inverter has a parallel


Fig. 5. Per-phase equivalent circuit of proposed multilevel inverter.

Fig. 6. Vector diagram of proposed multilevel inverter.

connection of two CSIs, the LCI represented by the currentsource ILCI and the GTO inverter by the current source IGTO.The output current of the multilevel inverter IO is producedby the sum of two output currents from the LCI and theGTO inverter. The proposed multilevel-inverter operation notonly increases output power level but also contributes to thegeneration of the leading power factor for the LCI naturalcommutation. The small output capacitor can be installed toprovide an additional leading power factor for the LCI naturalcommutation and filtering function for the output current drawnfrom the multilevel inverter. With the small capacitor, the nearlysinusoidal motor currents and voltages are achieved due tothe superior harmonic characteristics of the multilevel currentwaveforms.

The output vector diagram of the proposed multilevel CSIsystem is shown in Fig. 6. The phase angles φ and θ, inthe same way as Fig. 3, represent the leading-power-factorangle for natural commutation of the LCI and the power-factorangle of the induction motor, respectively. The induction motorcurrent IM is lagging the motor phase voltage VM accordingto the power-factor angle θ. For natural commutation of thethyristors in the LCI, the LCI output current ILCI must leadthe corresponding phase voltage VM. The phase shift from theinduction motor current IM to the LCI output current ILCI

should be provided by the external circuit condition. Under theproposed control strategy of the multilevel CSI system, it isseen that the resultant output current from the multilevel CSIIO is lagging the LCI output current ILCI by 30◦. As a result,the output capacitors need to generate a phase shift from themotor current IM to the multilevel CSI output current IO. Thephase-angle amount shifted by the output capacitor is shown tobe (φ + θ) − 30◦. The lagging power factor of induction motorsis normally 0.75 to 0.8, which means that the angle θ, in turn,is about 35◦ to 40◦. In addition, the leading-power-factor anglefor load commutation φ can be set to about 5◦. Considering theamount for phase shift, the 30◦ reduction of the phase shift is

Fig. 7. LCI output current ILCI, GTO CSI output current IGTO, and multi-level CSI output current IO.

quite big. The additional phase shift is provided by the outputcapacitor.

IV. SIMULATION RESULTS

In order to investigate the performance of the proposedmultilevel CSI system, a detailed computer simulation wasperformed using a 3-hp induction motor. Fig. 7 shows the LCIoutput current, the GTO CSI output current, and the multilevelCSI output current. As shown in Fig. 7, the GTO CSI outputcurrent has 60◦ phase delay with the LCI output current. Theresultant multilevel CSI output current obtained by two cur-rents provides output-current levels with four step values and30◦ phase delay with the LCI output current.

Fig. 8 illustrates the LCI output current ILCI, motor phasevoltage VM, multilevel CSI output current IO, and motor cur-rent IM with phase-angle information. It is seen that the LCIoutput current has a leading angle φ with respect to the motorphase voltage. Based on this leading power factor, the naturalcommutation is assured for the LCI. The induction motorvoltage and current show a lagging power-factor angle θ inFig. 8. The phase-shift amount of φ + θ is achieved by theproposed multilevel CSI operation and the output capacitor.It is also shown that the motor voltage and current are nearlysinusoidal due to the multistep output-current waveform and theoutput capacitor. Spectral performance of the motor current andthe voltage is enhanced through the synthesized output currentIO with four-level conversion.

Fig. 9 shows the multilevel CSI output current IO, motorcurrent IM, capacitor current IC, and motor phase voltage VM,respectively. Fig. 10 shows the dc-link current, the rectifierinput voltage, and the rectifier input current, respectively. Ascan be observed in Fig. 10, the input current has the typ-ical six-step waveform. The dc-link inductor current showssome harmonic ripple components because of the finite dc-linkinductor.


Fig. 8. LCI output current ILCI, motor phase voltage VM, multilevel CSIoutput current IO, and motor current IM.

Fig. 9. Multilevel CSI output current IO, motor current IM, capacitorcurrent IC, and motor phase voltage VM.

V. EXPERIMENTAL RESULTS

To validate the proposed multilevel CSI topology and controlalgorithm, the prototype experimental setup was fabricatedusing the LCI, the GTO CSI, and the two dedicated controlledrectifiers. The setup was composed of a digital-signal-processor(DSP) board using TMS320LF2407, pulse transformer boardsfor gate drivers, analog interface board such as a zero-crossingdetector (ZCD), a dc-link inductor, and the power board for thecontrolled rectifier, the LCI, and the GTO CSI. Prototypes of theLCI and controlled rectifiers were fabricated in the laboratoryusing thyristors (Motorola MCR16N). On the other hand, IGBTswitches connected in series with diodes were used for theGTO CSI. The dc-link inductance used in the experiment was

Fig. 10. DC-link current Idc, rectifier voltage VR, and controlled rectifiercurrent IR.

Fig. 11. ZCD signal and gating signals of the controlled rectifier (10 V/div,5 ms/div).

200 mH. The proposed control structure was implemented onthe single TMS320LF2407 DSP to control the LCI, the GTOCSI, and the two controlled rectifiers. The digital I/O ports ofthe DSP board are assigned to provide the gating signals dueto the limitation of the PWM ports in the DSP board. Therectangular signals from the digital I/O ports were convertedto 20-kHz pulse trains to turn ON the thyristors through anexternal 20-kHz oscillator. The generated pulse train throughthe pulse transformer board (FCOAUX60) was used to turn ON

the thyristors in the LCI and the controlled rectifiers. In theexperiment, a 230-V, 60-Hz, 1-hp general-purpose inductionmotor is employed as the load.

Fig. 11 shows the gating pulses for the controlled rectifieralong with the output of the ZCD. The pulse train has 20-kHzfrequency and 50% duty cycle to avoid the transformer satu-ration. The ZCD signal from the supply voltage was used to


Fig. 12. DC-link current (1 A/div, 10 ms/div).

Fig. 13. LCI output current (upper trace: 1 A/div, 5 ms/div) and GTO CSIoutput current (lower trace: 1 A/div, 5 ms/div).

Fig. 14. Multilevel CSI output current (1 A/div, 5 ms/div).

determine the firing instants for the rectifier. Fig. 12 illustratesthe dc-link current waveforms of the controlled rectifiers.

The output-current waveforms from the LCI and the GTOCSI are shown in Fig. 13. Two current waveforms have the samesix-step waveforms with 60◦ phase shift. Figs. 14 and 15 showthe multilevel CSI output current and motor current, respec-tively. The multilevel output current has four-level values witha better harmonic performance over the six-step waveform. Thesinusoidal motor current was obtained by filtering operation ofthe output capacitor, which also generated the leading powerfactor.

Fig. 15. Motor current (1 A/div, 1 ms/div).

VI. CONCLUSION

A multilevel power-converter topology based on a dual CSIhas been proposed in this paper. The proposed topology utilizesthe combination of the soft switching of the LCI and the hardswitching of the GTO CSI. It is shown that the proposedmultilevel operation contributes to the leading-power-factorgeneration for the LCI natural commutation by providing theeffective phase shift. With the help of the leading angle fromthe multilevel operation, the leading power factor required forthe load commutation of the LCI is obtained with significantlysmaller capacitors at the inverter’s output terminals. As a re-sult, the proposed approach can employ the LCI and utilizeits soft-switching operation, yielding a cost-effective solutioncompared with the conventional multilevel CSI using two GTOCSIs. The switching losses are very small, due to the naturalcommutation of the LCI and the six-step operation of theGTO CSI. Simulation and experimental results have shown thevalidity of the proposed topology.

REFERENCES

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Sangshin Kwak (S’02–M’05) received the B.S. andM.S. degrees in electronics engineering from Kyung-pook National University, Daegu, Korea, in 1997 and1999, respectively, and the Ph.D. degree in electricalengineering from Texas A&M University, CollegeStation, in 2005.

From 1999 to 2000, he worked as a ResearchEngineer with LG Electronics, Changwon, Korea.In 2004, he was also with Whirlpool R&D Center,Benton Harbor, MI. Since 2005, he has been workingas a Senior Engineer with the Research and Develop-

ment Center, Samsung SDI Corporation, Youngin, Korea. His research interestsinclude topology design, modeling, control, and analysis of ac/dc, dc/ac, ac/acpower converters for adjustable speed drives and digital display drivers as wellas DSP-based power electronics control.

Hamid A. Toliyat (S’87–M’91–SM’96) received theB.S. degree from Sharif University of Technology,Tehran, Iran, in 1982, the M.S. degree from WestVirginia University, Morgantown, in 1986, and thePh.D. degree from University of Wisconsin, Madi-son, in 1991, all in electrical engineering.

After receipt of the Ph.D. degree, he joined thefaculty of Ferdowsi University of Mashhad, Mash-had, Iran, as an Assistant Professor of electrical en-gineering. In March 1994, he joined the Departmentof Electrical Engineering, Texas A&M University,

College Station, where he is currently the E. D. Brockett Professor of electricalengineering. His research interests and experience include analysis and designof electrical machines, variable speed drives for traction and propulsion ap-plications, fault diagnosis of electric machinery, and sensorless variable speeddrives. He has supervised more than 35 graduate students, published over265 technical papers, raised over $3.4 M in research funding, presented morethan 35 invited lectures all over the world, and has ten issued and pendingU.S. patents in these fields. He is the author of DSP-Based ElectromechanicalMotion Control (CRC Press, 2003) and the Co-Editor of Handbook of ElectricMotors—2nd Edition, (Marcel Dekker, 2004).

Dr. Toliyat has received the prestigious Cyrill Veinott Award in electro-mechanical energy conversion from the IEEE Power Engineering Societyin 2004, Outstanding Professor Award, in 2005, from Texas A&M, TEESFellow Award in 2004, Distinguished Teaching Award in 2003, E. D. BrockettProfessorship Award, in 2002, Eugene Webb Faculty Fellow Award, in 2000,and Texas A&M Select Young Investigator Award, in 1999, from Texas A&MUniversity. He has also received the Space Act Award from NASA, in 1999,and the Schlumberger Foundation Technical Awards, in 2001 and 2000. He isan Editor of the IEEE TRANSACTIONS ON ENERGY CONVERSION, and wasan Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS.He is also Vice Chair-Papers, IEEE-IAS Industrial Power Conversion SystemsDepartment of IEEE-IAS Electric Machines Committee, and is a member ofSigma Xi. He is the recipient of the 1996 IEEE Power Engineering SocietyPrize Paper Award for his paper entitled “Analysis of Concentrated WindingInduction Machines for Adjustable Speed Drive Applications-ExperimentalResults.” He was the General Chair of the 2005 IEEE International ElectricMachines and Drives Conference, San Antonio, TX.

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Multilevel Converter Topology Using Two Types of Current-Source Inverters