l in

velaratel intheridls. Ttpu, theonsng t

ave drnd hid highformeof ou

d currrters a

and

By auxiliary circuits, the transferred current or power can be con-trolled accurately, but the additional feedback control strategiesare also needed, so the control of these converters becomes morecomplicated, and converters are less reliable.

2 so that trter is V = V

By opening and closing the switches of H1 and H2 appropthe output voltage V1 and V2 can be made equal to +Vdc, 0, oTherefore, the output voltage of the inverter can have the+Vdc, +Vdc/2, 0, Vdc/2, Vdc. The ring instants can be obtainedby using phase disposition multicarrier PWM technique.

Voltage balancing using resonant switched capacitor converter

Fig. 2 shows a system conguration of a three-phase diodeclamped ve-level inverter equipped with RSCC voltage balancing

Corresponding author. Tel.: +91 9495686300.E-mail addresses: gayathrideviks@yahoo.co.in (K.S. Gayathri Devi), aruns@

amaljyothi.ac.in (S. Arun), csreeja@amaljyothi.ac.in (C. Sreeja).

Electrical Power and Energy Systems 63 (2014) 363372

Contents lists availab

Electrical Power an

journal homepage: www.eladding some auxiliary balancing circuits [8,9,16] and (3) improvingthe control method by selecting redundant switching states, [4,10].

the output of the second H-bridge is denoted by Vput of this two DC source cascade multilevel invehttp://dx.doi.org/10.1016/j.ijepes.2014.05.0530142-0615/ 2014 Elsevier Ltd. All rights reserved.he out-1 + V2.riately,r Vdc.values,Among the basic multilevel inverters the problem of voltageunbalance of dc link capacitors exists inherently in the diode-clamped converter topology [13,15], which limits the further appli-cation [5] of it, especially at the level above three. To balance thevoltage of DC link series capacitors, three main approaches havebeen proposed. They are: (1) using separate DC sources, [3,7], (2)

where n is the number of levels of output voltage. To operate a cas-cademultilevel inverter [3,7] using a single DC source, it is proposedto use capacitors as the DC sources for all but the rst source. Con-sider a simple inverter with two H-bridges as shown in Fig. 1.

Each H-bridge has a DC power source with an output voltage ofVdc. The output voltage of the rst H-bridge is denoted by V1 andIntroduction

Recently, multilevel inverters [6] hest in the eld of high-voltage abecause it can realize high voltage anlow-voltage switches without transbalance circuits, with the numberharmonics of the output voltage anEM1 are decreasing Multilevel invediode-clamped, capacitor-clampedinverters.awn tremendous inter-gh-power applicationspower output through

r and dynamic voltagetput level increasing,ent are decreasing andre mainly classied as

cascaded H-bridge

Voltage balancing techniques

Cascaded H-bridge inverter

AmultilevelCHBconsistsof anumberofH-bridgecells connectedin seriesperphase, andeachmodule requires a separateDCsource togenerate voltage levels at the output of inverter. The output of eachbridge is +Vdc, 0 andVdc and the combinations of the output volt-ages of series connected bridges give the total output voltage.According to the number of bridges the number of levels of the out-put voltage changes. The number of bridges is equal to (n 1)/2Comparative study on different ve leve

K.S. Gayathri Devi , S. Arun, C. SreejaDept. of Electrical & Electronics, Amal Jyothi College of Engineering, Kottayam, India

a r t i c l e i n f o

Article history:Received 30 July 2013Received in revised form 17 May 2014Accepted 20 May 2014Available online 3 July 2014

Keywords:Diode clamped inverterMultilevel inverterPulse width modulationSwitched-capacitor converter

a b s t r a c t

The diode-clamped multilecan be solved by using seplation control in three-levsources is not viable andimplement. Hence the Hybcan be used for higher levevoltage. The boosting of ouformer or even eliminate itiumhigh voltage applicatiwith other voltage balanciverter topologies

topology has the problem of voltage unbalance of dc link capacitors. Thise DC sources, or adding auxiliary circuits or adopting space vector modu-verter applications. For higher level applications, the use of separate DCcontrol techniques for other two methods will be more complicated toMultilevel Inverter Based on Switched-Capacitor and Diode-Clamped unitshis topology can balance the capacitor voltage and also step up the outputt voltage contributes to lessen the transformation ratio of the input trans-reby reducing the cost, which will benet the design of converters in med-. The topology is simulated using MAT lab simulink software and comparedechniques.

2014 Elsevier Ltd. All rights reserved.

le at ScienceDirect

d Energy Systems

sevier .com/locate / i jepes

er an364 K.S. Gayathri Devi et al. / Electrical Powcircuits [8]. The dc link consists of four dc capacitors C1C4, and it isconnected to a three-phase diode rectier. The rectier keeps thetotal voltage across the four dc capacitors as 4Vdc, but each capac-itor voltage may deviate from Vdc. Therefore, two voltage-balanc-ing circuits are employed to balance their voltages. One isconnected to C1 and C2, and the other is connected to C3 and C4Symmetrical operation of the ve-level inverter makes it possibleto maintain the voltage of node M at the center of the dc-linkvoltage.

While considering the section between P2 and M, the circuitconsists of two half-bridge inverters with four switching devicesS1S4 and a series resonant circuit Lr and Cr. The dc terminals ofthe half-bridge inverters are connected in parallel with the dccapacitor, while an ac terminal is connected to the other onethrough the series resonant circuit. Dc voltage source 2Vdc keepsthe total capacitor voltage vC1 + vC2 as 2Vdc. When the vC2 decreasesby some cause, the dc voltage source supplies an amount of powerto both C1 and C2, and then vC1 increases. The voltage balancing cir-cuit ows a positive current in iPB to discharge C1 and to charge C2.The four switching devices enables the bidirectional power ow

Fig. 1. Five level H-

Fig. 2. Five level Dd Energy Systems 63 (2014) 363372from C1 to C2 or opposite direction. The circuit essentially acts asa switched-capacitor converter or a charge pump circuit, whichstores transferred energy in the resonant capacitor Cr, instead ofan inductor. The resonant inductor Lr makes it possible to suppressthe spike currents, power losses, and electromagnetic noises.

All switching devices are operated with a 50% duty ratio, buttheir switching frequency is higher than the resonant frequencyof the resonant circuit. Therefore, the resonant circuit acts asinductive impedance. The amplitude of the resonant current bymeans of phase shift between the two half bridge inverters. Theproposed method delays S3 and S4 from S1 and S2 for a phase-shifttime T1. Power Prscc will be maximum when the value of T1 liesbetween Tsw/4 and +Tsw/4 where Tsw is the time correspondingto the switching frequency of RSCC.

Hybrid Multilevel Inverter Based on Switched Capacitor and DiodeClamped units

The circuit diagram of three-phase topology [1,2] is deduced bycombination of three HMI-BSD single-leg circuits sharing common

bridge inverter.

CI with RSCC.

dc link capacitors, shown in Fig. 3. All the elements sustain thesame voltage stresses, which equal 1/(n 1)of the dc link voltage.

Principle of operationA single-leg ve-level inverter topology based on HMI-BSD is

shown in Fig. 4. The topology can be divided into two parts, whichare, respectively, indicated by a dash dotted frame and a dashedframe. Part 1 is called switched-capacitor part and part 2 is calleddiode-clamped part. The switched-capacitor part is composed ofthe dc link capacitors (C1, C2), the ying capacitors (C3, C4, C5),and the clamping switching devices (Sc1, Sc6). And the diode-clamped part is composed of the ying capacitors (C3, C4, C5), theclamping diodes (Dc1, Dc6), and the main switching devices(S1, S6). It can be seen clearly that both parts include the yingcapacitors C3, C4, and C5.

The diode-clamped part is a classical four-level diode-clamped

c1 c6

divided into two groups: Group A and Group B.

When the HMI-BSD operates under the condition of a lowermodulation ratio (M < 0.5), the output voltages will change from

K.S. Gayathri Devi et al. / Electrical Power and Energy Systems 63 (2014) 363372 365Group A includes Sc1, Sc3, and Sc5, and Group B includes Sc2, Sc4,and Sc6. The control signals for the switching devices of the samegroup are identical. When the switching devices of Group A areswitched ON, those of Group B are switched OFF, and vice versa.The switching devices of the two groups are switched ON or OFFalternately. If the switching devices of Group A are ON and thoseof Group B are OFF C1 is paralleled with C3, and C2 is paralleled withC4.

Table 1 shows the relationships between the output voltage Voand the working states, where Udc is the voltage of one dc linkcapacitor. The switched-capacitor part not only contributes to bal-ance the DC link capacitors, but also participates in the synthesis ofthe output voltage levels by the common connection of the yingcapacitors with the diode-clamped part. In this case, theHMI-BSD can output ve kinds of voltage levels with only aconventional four-level diode-clamped topology.

Voltage balance of capacitorsIf only the diode-clamped part of the HMI-BSD works without

the switched-capacitor part [12], the voltages of capacitors wouldbe unbalanced due to the asymmetric charge current and dischargecurrent through capacitors. The switched-capacitor part plays atopology with four kinds of working states and four kinds of outputvoltage levels. Among the main switching devices S1, S6, only threeswitching devices in succession are switched on in each workingstate. And (S1, S4), (S2, S5), and (S3, S6) are complementary switch-ing pairs. The switched-capacitor part operates with two kinds ofworking states. The clamping switching devices S , S can beFig. 3. Three phase vrole in balancing the voltages of capacitors by alternativeconduction of the clamping switches Sc1Sc6.

When the switching devices of Group A are ON and those ofGroup B are OFF, C1 and C3, C2 and C4 are in parallel, respectively,so VC1 = VC3 and VC2 = VC4. When the switching devices of Group Bare ON and those of Group A are OFF, C1 and C4, C2 and C5 are inparallel, respectively, so VC1 = VC4 and VC2 = VC5. If the switchingdevices of Group A are turned from ON to OFF, and from OFF toON over and over again, then VC1 = VC2 = VC3 = VC4 = VC5. That isto say, each capacitor can keep voltage balance through the yingcapacitor C4. Actually, the capacitor C4 acts as the second spiker,which is in parallel with different dc link capacitors in differentswitching states. So long as Group A or Group B can switch oncein one period, the voltages of the capacitors can keep balance.

As seen from Table 1, the switching devices of Group B or GroupA are ON for one working state and OFF for the other working state.Accordingly, there exist two kinds of working states combination,shown in Tables 2 and 3.

It can be seen from Table 2 that for the rst working states com-bination, the switching devices of group B are turned ON onlywhen the highest output level (+2Vdc) appears and turned OFFwhen other four levels appear. The pulse width modulation(PWM) [11,14] carriers arrangement for this working states combi-nation is shown in Fig. 5(a). As seen from Fig. 5(a), every carrierband from top down corresponds to Group B, S1, S2 and S3 in turn.

Fig. 4. Single-leg of ve-level HMI-BSD.e-level HMI-BSD.

Table 1Relationships between the output voltage V0 and the working states.

No. of working states Output level Vo S1 S2 S3 S4 S5 S6 Group A Group B

1 +2Vdc 1 1 1 0 0 0 0 1

2 +1Vdc 1 1 1 0 0 0 1 03 0 1 1 1 0 0 0 1

4 0Vdc 0 1 1 1 0 0 1 05 0 0 1 1 1 0 0 1

6 1Vdc 0 0 1 1 1 0 1 07 0 0 0 1 1 1 0 1

366 K.S. Gayathri Devi et al. / Electrical Power and Energy Systems 63 (2014) 3633728 2Vdc 0 0

Table 2First kind of working states combination for HMI-BSD.

No. of working states Output level Vo S1 S2

1 +2Vdc 1 12 +1Vdc 1 14 0Vdc 0 16 1Vdc 0 08 2Vdc 0 0

Table 3Second kind of working states combination for HMI-BSD.

No. of working states Output level Vo S1 S2

1 +2Vdc 1 13 +1Vdc 0 15 0Vdc 0 07 1Vdc 0 0ve levels to three levels, i.e., the highest output level cannot berealized for the lower modulation index, and then Group B will losethe chance of being turned ON and being turned OFF all the time. Inthe like manner, for the second working states combination listedin Table 3, the switching devices of Group B are turned OFF onlywhen the lowest output level (2Vdc) appears and turned ON forthe other four output levels appear. The carriers arrangement forthis working states combination is shown in Fig. 5(b). As seen fromFig. 5(b), every carrier band from top down corresponds to theswitching devices S1, S2, S3, and Group B in turn. When the modu-lation ratio is less than 0.5, the lowest level cannot be realized,andthen Group B will have no chance to be turned OFF and beturned ON all the time.

The capacitor voltage balancing can be realized only when theswitching devices of Group B are turned on and off alternately;otherwise, the voltage balancing in capacitors will be broken underthe condition of a lower modulation ratio. So these two kinds ofworking states combination must be considered together to ensurethe normal operation of the HMI-BSD within the wider range ofmodulation degree (even less than 0.5). As a result, the switching

8 2Vdc 0 0

Fig. 5. PWM carriers for rst and s0 1 1 1 1 0

S3 S4 S5 S6 Group A Group B

1 0 0 0 0 11 0 0 0 1 01 1 0 0 1 01 1 1 0 1 00 1 1 1 1 0

S3 S4 S5 S6 Group A Group B

1 0 0 0 0 11 1 0 0 0 11 1 1 0 0 10 1 1 1 0 1devices of Group B are turned ON over one switching period andturned off over the next switching period so that, the ying capac-itors are, respectively, connected in parallel with the differentcapacitors alternately to ensure the realization of self-voltage bal-ancing mechanism of capacitors.

According to the aforesaid analysis, by rearranging the carrierwaveforms, the special carrier waveforms of the switching devicesfor the HMI-BSD are shown in Fig. 7. The carrier waves for the threemain switches are shown in Fig. 6(a)(c) and Fig. 6(d)shows thecarrier wave for switching devices of group B. By applying the spe-cial carriers in Fig. 6 to the SPWMmethod, the switching devices ofGroup A and Group B will be alternately turned ON or OFF by peri-ods, thus balancing the voltages in capacitors. This balance schemedoes not need complex control strategy, and is adaptive to all kindsof loads.

Realization of stepping up the output voltageIn view of the symmetry of the positive and negative output

levels of the HMI-BSD, the two input terminals of the DC sourceshould be on the output terminals symmetry, so for a single-leg

0 1 1 1 1 0

econd kind of working states.

er anK.S. Gayathri Devi et al. / Electrical Powve-level HMI-BSD, there are three modes of connection of theinput DC source, as shown in Fig. 7(a)(c) The positive and negativeterminals of the DC source are, respectively, connected with (1) thepositive terminal of C1 and the negative terminal of C2; (2) thepositive terminal of C3 and the negative terminal of C5; (3) thepositive and the negative terminals of C4.

If the HMI-BSD is in three-phase operation, the input DC sourceterminals should be only connected to the DC link capacitorsinstead of the ying capacitors because of the symmetry of the

Fig. 6. Carrier waveforms of t

Fig. 7. Three modes of connectio

Fig. 8. Output voltage of FIVE led Energy Systems 63 (2014) 363372 367three-phase topology. And for a ve-level topology, only the con-nection mode 1 can be used. The function of boosting the outputvoltage of the HMI-BSD contributes to lessening the transforma-tion ratio of the input transformer, thereby reducing the cost, evento eliminate the input transformer, which will benet the design ofconverters in mediumhigh voltage application. On the other hand,compared to the conventional switched-capacitor topology, lessswitching devices and dc link capacitors are needed in the HMI-BSD, which can also reduce the production cost in applications.

he switching devices [2].

n of the input DC Source [2].

vel diode-clamped inverter.

er an368 K.S. Gayathri Devi et al. / Electrical PowSimulations and results

The simulation of diode-clamped ve level inverter had donewith 180 V DC input voltage and sinusoidal pulse width modula-tion at 0.5 modulation index. The output obtained has ve stepsand each step differ by a voltage of 50 V. That is the ve levels

Fig. 9. Capacitor voltage of ve le

Fig. 10. FFT analysis

Fig. 11. Output voltage of casd Energy Systems 63 (2014) 363372obtained are 0 V, 45 V, 90 V, 135 V and 180 V. The switching fre-quency is 5 kHz. Figs. 8 and 9 shows the output voltage and capac-itor voltage of ve level diode clamped inverter.

From Fig. 10 it can be observed that the voltage of capacitors 1,and 4 are increasing to 90 while the voltage across capacitors 2 and3 are decreasing to 20 V. From Fig. 10 it can be observed that the

vel diode-clamped inverter.

of ve level DCI.

caded H-bridge inverter.

er anK.S. Gayathri Devi et al. / Electrical Powvoltage across the capacitors are unbalanced due to the chargingand discharging of the capacitors. Hence the capacitor voltageshould be balanced.

Fig. 10 shows the FFT analysis of ve level DCI. The THD is25.18%. The harmonics other than fundamental are below 5%.

The rst method of capacitor voltage balancing is using separateDC sources. If the capacitors in the DCI are replaced with DC

Fig. 12. DC link voltage of cas

Fig. 13. FFT analysis of ve

Fig. 14. Output voltage of d Energy Systems 63 (2014) 363372 369sources the voltage will be balanced but for this each level requiresa separate DC source and is expensive. Another method that usesseparate DC sources are called cascaded H-bridge inverter. Thisneeds separate DC sources for each bridge. The simulation of cas-caded H-bridge inverter is donewith 110 V DC input to each bridge.Phase disposition SPWM is used for generating gate pulses and thecarrier frequency is 5 kHz. The output voltage is shown in Fig. 11.

caded H-bridge inverter.

level H-bridge Inverter.

ve level DCI with RSCC.

Fig. 15. Capacitor voltage with RSCC.

Fig. 16. FFT analysis of ve level DCI with RSCC.

Fig. 17. Output voltage of HMI-BSD at5 kHz.

370 K.S. Gayathri Devi et al. / Electrical Power and Energy Systems 63 (2014) 363372

Fig. 18. Output voltage of HMI-BSD at 1 kHz Switching Frequency.

Fig. 19. Capacitor voltage of ve level HMI-BSD.

Fig. 20. FFT analysis of ve level HMI-BSD.

K.S. Gayathri Devi et al. / Electrical Power and Energy Systems 63 (2014) 363372 371

transformer and balance the capacitor voltages. The THD is veryless compared to other methods.

Table 4Comparison of different ve level topologies.

HMI- DCI with CHB DCI

372 K.S. Gayathri Devi et al. / Electrical Power and Energy Systems 63 (2014) 363372The output voltage of a phase is the sum of output voltages ofindividual bridges belong to the corresponding phase. The voltageacross the DC link is shown in Fig. 12.

Fig. 13 shows the FFT analysis of ve level cascaded H-bridgeinverter. The THD is 18.4%. The harmonic components except fun-damental are below 5%.

The voltage balancing technique using resonant switchedcapacitor [8] had simulated with 180 V dc input and phase shiftcontrol. The output voltage has ve steps each differ by 45 V. Thevalue of resonant capacitor Cr and inductor Lr are 10 lF and10 lH respectively. The ying capacitors are of 6600 lF and theswitching frequency of RSCC and carrier frequency of inverter are30 kHz and 5 kHz respectively.

Figs. 14 and 15 shows the output voltage and ying capacitorvoltage of ve level diode-clamped inverter with RSCC voltage bal-ancing technique. From Fig. 16 it can be observed that the capacitorvoltage is balanced and maintained at 45 V.

The FFT analysis of ve level DCI with RSCC is shown in Fig. 16.The THD is 18.19%. The harmonic components except fundamentalare less than 5%.

The simulation parameters of HMI-BSD are set as follows: Thedc input voltage is 180 V; the capacitances of all capacitors are2200 lF; the load resistance is 20X; the working frequency is1 kHz; the modulation ratio is 0.95. The input DC source terminalsare connected to the positive terminal of C1 and the negative ter-minal of C2 The simulation is done for 5 kHz switching frequencyand as well as 1 kHz switching frequency. If 1 kHz switching fre-quency is considered, the switching losses can be reduced in prac-tical applications. Figs. 17 and 18 shows the output voltage at5 kHz and 1 kHz frequency respectively.

It can be observed from Figs. 17 and 18 that the output voltageis stepped up to 360 V with the help of switched capacitor part. Theoutput has ve levels in which each level differs by 90 V. Also it canbe observed from Fig. 19 that the voltage across all the capacitors

BSD RSCC

DC Link Capacitors 2 0 0 4Flying Capacitors 3 4 0 0Resonant capacitors 0 2 0 0Inductors 0 2 0 0Diodes 4 6 0 6No. of main switching devices 6 8 8 8No. of clamping switching

devices6 8 0 0

No. of DC sources 1 1 2 1THD in % 17.42 18.19 18.42 25.18(both DC link capacitors and ying capacitors) are balanced andmaintained at 90 V.

Fig. 20 shows the FFT analysis of ve level HMI-BSD. The THD is17.42% and the harmonic components other than fundamental areless than 5%.

The components required per phase is given in Table 4. Fromthe results and circuit congurations it can be understand thatthe voltage balancing using auxiliary circuit need additional circuitand control complexity because the current and voltage has to besensed for control purpose. But HMI-BSD has only simple controltechnique as well as it can reduce the transformation ratio of inputConclusion

The structure and the principle of operation of Hybrid Multi-level Inverter Based on Switched Capacitor and Diode Clampedunits are introduced. The switched-capacitor circuits are appliedto balance the voltage of dc link capacitors and ying capacitors,as well as participate in synthesizing the output voltage levels.Both advantages of the switched-capacitor and the diode-clampedcircuits are included in the HMI-BSD by the combination of the twocircuits. Not only this new topology balances dc link capacitors, butalso it can step up the output voltage with kinds of boostingmodes, which will contribute to lessen the turns ratio of the inputtransformer, even to eliminate it. In conclusion, the application ofthe switched-capacitor circuit in the HMI-BSD can reduce the costto a certain extent. The HMI-BSD can operate effectively under thethree-phase condition. Finally, the validity of the HMI-BSD is veri-ed by the simulation of ve level HMI-BSD and compared withother ve level topologies used for voltage balancing.

References

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Comparative study on different five level inverter topologiesIntroductionVoltage balancing techniquesCascaded H-bridge inverterVoltage balancing using resonant switched capacitor converterHybrid Multilevel Inverter Based on Switched Capacitor and Diode Clamped unitsPrinciple of operationVoltage balance of capacitorsRealization of stepping up the output voltage

Simulations and resultsConclusionReferences