new switching scheme for z-source inverter

7/28/2019 New Switching Scheme for Z-Source Inverter

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International Journal of Automation and Computing 9(2), April 2012, 200-210

DOI: 10.1007/s11633-012-0634-4

A New Switching Scheme for Z-source Inverter to

Minimize Ripples in the Z-source Elements

Sengodan Thangaprakash1 Ammasai Krishnan2

1School of Electrical Systems Engineering, University Malaysia Perlis, Perlis 02000, Malaysia2Dean (Academic), K.S.R. College of Engineering, Tamilnadu 637215, India

Abstract: This paper presents a modification in pulse width modulation (PWM) scheme with unequal shoot-through distributionfor the Z-source inverter (ZSI) which can minimize ripples in the current through the Z-source inductors as well as the voltage acrossthe Z-source capacitors. For the same system parameters, the proposed control technique provides better voltage bo ost across theZ-source capacitor, DC-link, and also the AC output voltage than the traditional PWM. The ripples in the Z-network elements arefound to be reduced by 75 % in the proposed modulation scheme with optimum harmonic profile in the AC output. Since the Z-networkrequirement will be based on the ripple profile of the elements, the Z-network requirements can be greatly reduced. The effectivenessof the proposed modulation scheme has been simulated in Matlab/Simulink software and the results are validated by the experimentin the laboratory.

Keywords: Z-source inverter (ZSI), pulse width modulation (PWM), shoot-through, ripples, total harmonic distortion.

1 Introduction

Z-source inverter (ZSI) has found widespreadapplications[110], and attracted the interest of re-searchers in recent years, since it overcomes the limitationsof the traditional inverters. The schematic of a three phaseZSI is shown in Fig. 1. ZSI based systems advantageouslyutilize the shoot-through states to boost the direct current(DC) bus voltage by gating on both the upper and lowerswitches of the same phase leg. Shoot-through modeallows simultaneous conduction of devices in same phaseleg and is forbidden in traditional inverter topologies.Therefore, a ZSI can buck or boost the voltage whichis equal to a desired output voltage that is less/greaterthan the DC bus voltage based on the shoot-through timeperiod and boost factor. Since the shoot-through state hasno harmful effect on the inverter and is advantageouslyutilized, the reliability of the inverter is greatly improved.It also provides a low cost and highly efficient singlestage power conversion structure for reliable operation.ZSI ensures smooth operation by supplying the desiredvoltage to the load even during the supply voltage sagsand fluctuations[11].

Fig. 1 Three phase ZSI

Manuscript received September 24, 2010; revised March 11, 2011

Inverter bridge with lattice impedance network con-nected after DC power supply with the feature ofbuck-boost capability was first proposed by Peng[11]. Inthis research, simple boost control (SBC) method withconstant boost factor was used to control the shoot-through and DC link voltage of the inverter. The range ofshoot-through duty ratio is very limited in SBC in additionto the high voltage stress across the power insulated gatebipolar junction transistors (IGBT). Maximum boostcontrol (MBC) was proposed by Peng et al.[12] to produce

maximum voltage gain (boost) under a given modula-tion index. MBC turns all traditional zero states intoshoot-through state and voltage stress across the powerIGBTs is greatly reduced by fully utilizing the zero states.Indeed, turning all zero states into shoot-through statecan minimize the voltage stress; however, doing so alsocauses a shoot-through duty ratio varying in a line cycle,which causes inductor current ripple. This will require ahigh inductance for low-frequency or variable-frequencyapplications. Shen et al.[13] proposed a constant boostcontrol (CBC), which can obtain maximum voltage gainat any given modulation index without producing anylow-frequency ripple that is related to the output frequency.

A detailed analysis for how the various conventional pulsewidth modulation (PWM) strategies can be modified toswitch a ZSI either continuously or discontinuously, whileretaining all the unique harmonic performance featuresis discussed in [14]. Modified space vector modulation(MSVM) algorithm for reducing the switching stress bycapacitor voltage control with good transient responsehas been presented in [15]. In this research, the referencecapacitor voltages are derived for minimizing the voltagestress at any desired alternating current (AC) outputvoltage by considering the DC input voltage. In [16],minimization of voltage stress across switching devicesin the ZSI has been presented by modifying the space vector


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S. Thangaprakash and A. Krishnan / A New Switching Scheme for Z-source Inverter to Minimize Ripples in 201

algorithm to generate shoot-through states. An inte-grated control algorithm using modified voltage space vec-tor has been presented by Thangaprakash and Krishnan[17]

to achieve maximum boost in the DC link voltage withwide range of controllability and the same has been im-plemented for induction motor drives[18, 19]. The perfor-

mance of the drive is found to be very smooth when a stepchange is applied to the load or during the supply voltagesags. Development of a ZSI based power conversion sys-tem for photovoltaic applications is proposed in [5] and thesame with quasi-Z-source inverters has been discussed in[20]. Detailed comparison of ZSI with traditional invertersin terms of total switching device power, passive compo-nents requirement and cost of the overall system has beengiven in [8]. An indirect controller for the DC link voltageon the DC side of the ZSI along with AC voltage controlis discussed in [21]. Controller design for specific applica-tions, namely fuel cell and voltage sag compensation arediscussed by Jung and Keyhami[22]. Unified control tech-

nique has been presented

[23]

for ZSI with minimal numberof sensors and PI controllers to achieve good stability of theoverall system. In [24], an improved ZSI has been proposedwith certain modifications in the Z-network connections ofthe traditional ZSI topology. Improved ZSI greatly reducesthe inrush current of the Z-network inductors and facili-tates the soft start capability. Space vector modulation(SVM) of nine switches inverter and nine switches Z-sourceinverter was proposed in [25] where the switching sequenceis composed of the upper active vectors, the lower activevectors and the zero vectors. In this research, the upperand lower active vectors are determined via two space vec-tor diagrams. Comparative evaluation of different PWMschemes that can be used for ZSI control to achieve buck-

boost energy conversion with random and reduced commonmode switching has been presented in [26, 27]. Operation ofpower inverter fed induction motor drive has been analyzedin [28, 29].

The same space vector modulation strategies of tradi-tional inverters with proper insertion of shoot-through pe-riods could be applied to three phase ZSI with each havingthe same characteristic spectrum as its conventional coun-terpart. In these methods, the shoot-through period over ahalf of the sampling period is divided into three equal in-tervals and inserted in the switching waveforms without al-tering the active time periods. Therefore the shoot-throughtime period is diminished from the traditional zero vector

time period. By doing so, one can increase the switch-ing frequency and improve the harmonic profile in the ACoutput. But this will introduce significant ripples in theZ-source elements and causes ripples associated with theoutput frequency.

In this paper, a new PWM scheme is proposed which canminimize the ripples in the current through the Z-source in-ductors and voltage across the Z-source capacitors withoutsacrificing the features achieved by the traditional methods(i.e., DC link voltage boost, voltage gain, switching stress,and AC output voltage). In addition to the above, pro-posed method provides better voltage boost in the DC link.The theoretical and modulation concepts of continuous and

discontinuous switching of ZSI using modified modulation

is presented in this paper, and the same have been verifiedby the simulation and experimental results.

2 Modulation of Z-source inverter(ZSI)

Space vector PWM (SVPWM) techniques have beenwidely used in PWM inverters due to lower current har-monics and a higher modulation index. The unique fea-tures of a ZSI presented in [1] can be accomplished by thesame SVPWM technique with a few modifications to insertthe shoot-through. In addition to the six active and twonull states associated with traditional inverters, the threephase ZSI has seven shoot-through zero states representingthe short-circuiting of a single phase-leg, two phase-legs orall three phase-legs. These shoot-through states again boostthe DC link capacitor voltages and can partially supplementthe null states within a fixed switching cycle without alter-ing the normalized volt-sec average. Shoot-through states

short-circuit the corresponding inverter phase leg and pro-duce zero voltage across the output terminals similar to thetraditional zero states. Shoot-through states can thereforebe inserted into the existing PWM state patterns of tradi-tional inverters to derive different modulation strategies forcontrolling the ZSI.

In a continuous centered SVM state sequence of a tra-ditional inverter, there are three state transitions whichoccur in a half of the sampling period and the tradi-tional zero states are placed at the start and end of theswitching cycle. With these three-state transitions (i.e.,000100110111), three equal-interval (T0/3) shoot-through states are added immediately adjacent to the ac-tive states per switching cycle for modulating a ZSI. In thethree state transitions, the middle shoot-through state issymmetrically placed about the original switching instant.The traditional switching pattern of a voltage source in-verter (VSI) and ZSI for the sector I has been shown inFigs. 2 (a) and (b). In this switching pattern, the activestates {100} and {110} are left/right shifted accordingly byT0/3 with their time intervals kept constant, and the re-maining two shoot-through states are inserted in the endwithin the null intervals, immediately adjacent to the leftof the first state transition and to the right of the secondtransition[2]. At this switching pattern, the zero state pe-riod is reduced from Tz/2 to Tz/22T0/3 and Tz/2 to Tz/2T0/3. As both zero state periods should be greater than 0,

the shoot-through time is less than 0.75 times ofTz at pe-riod Tz/22T0/3, and less than 1.33 times of Tz at periodTz/2 to Tz/2T0/3. In this modulation scheme, the DC linkvoltage cannot be boosted to the maximum level, since theshoot-through state is limited to 0.75Tz, and also it resultsripples in the current/voltage in the Z-source elements.

3 Modified shoot-through distribution

The process of inserting the shoot-through states inswitching waveforms produce ripples in the current throughthe Z-source elements. These ripples cause additional heat-ing of the elements and ultimately degrade their lifespan.

The ripple in shoot-through duty ratio results in ripple in


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202 International Journal of Automation and Computing 9(2), April 2012

Fig. 2 Switching pattern for sector I of (a) traditional inverter, (b) ZSI in continuous mode, and (c) ZSI in 60 0 discontinuous mode

the current through the inductor, as well as in the volt-age across the capacitor. These ripples are influenced bychanges in power factor angle, modulation index, shoot-through time period and amplitude of the load current.Instead of having equal shoot-through states, this sec-

tion presents a modified shoot-through state distributionin SVM scheme for the ZSI which can minimize the ripplesin the current through the Z-source inductors and voltageacross the Z-source capacitors without sacrificing any fea-tures derived in the traditional modulation.

The proposed method also results reduction in the ACoutput voltage/current harmonics of the ZSI over the tra-ditional switching. State sequence and the placement ofshoot-through for both the continuous and discontinuousswitching of the proposed modulation schemes are pre-sented. Fig. 3 shows the continuous and discontinuousswitching pattern for the MSVM with modified shoot-through states at the space vector angle 0


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Fig. 3 Modified switching pattern for sector I using novel SVM with (a) continuous switching, (b) 60 0 discontinuous switching for

+ve DC rail clamping, (c) 60 0 discontinuous switching for -ve DC rail clamping

PWM switching pattern for the MSVM is shown in Fig. 4.

As the symmetrical shoot-through time periods with dis-similar duration are inserted in the switching waveforms,the ripples in the Z-source inductor current and the Z-source capacitor voltage can be reduced having retainedall the features such as, voltage gain, voltage stress limita-tion, AC output controllability, and the optimum harmonicprofile. In addition to the above a better voltage boostcan be achieved in the DC link. During the shoot-throughperiod, both switches of the phase leg are conducted si-multaneously for boosting the DC capacitor voltage. Fig. 4shows the modified switching with unequal shoot-throughdistribution for sector I.

The modified switching can also be realized by carrierbased implementation. For carrier based implementation,

the modified reference signals needed to produce the modi-

fied switching pulses to the ZSI in the continuous switchingcan mathematically be expressed as follows,

Vmax(sp) = Vmax + Voff + T

Vmax(sn) = Vmax + Voff +T

4(1)

Vmid(sp) = Vmid + Voff +T

4

Vmid(sn) = Vmid + Voff T4

(2)

Vmin(sp) = Vmin + Voff +T

4

Vmin(sn) = Vmin + Voff T (3)


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where{sp, sn} = {1, 4}, {3, 6}, {5, 2} andT = T0/3Ts.The modified reference signals of all the six switching de-

vices are generated for the switching frequency fs=10kHz,shoot-through duty ratio D0 = 0.2 and modulation index,ma = 0.79 using modified modulation with unequal shoot-through state and is compared with high frequency trian-

gular carrier signal to generate the switching pulses to thegates of the power IGBTs of the ZSI.

Fig. 4 Switching pattern for the modified shoot-through distri-

bution

In (1)(3), all the waveforms are altered while insertingthe unequal shoot-through states, by switching the ZSI in

this way ensures a better voltage boost in the DC link alongwith added features, however, the voltage stress acrossthe power devices is similar to the traditional modulation.These features can also be realized with reduced switchingstress as discussed in the previous section and the switchingpattern for the same is shown in Fig. 5.

For carrier based implementation, the modified referencesignals needed to produce the modified switching pulseswith symmetrical unequal shoot-through states inserted byonly altering four switching waveforms in the continuousswitching can now mathematically be expressed as follows,

Vmax(sp) = Vmax + Voff +5T

4Vmax(sn) = Vmax + Voff (4)

Vmid(sp) = Vmid + Voff


(5)

Vmin(sp) = Vmin + VoffT2

Vmin(sn) = Vmin + Voff 7T4

(6)

where{sp, sn} = {1, 4}, {3, 6}, {5, 2} andT = T0/3Ts.

In discontinuous PWM, a null state is eliminated eitherat the start or end of the switching cycle. In the 60 dis-continuous switching of ZSI using the proposed SVM two

shoot-through states T0/6 and 5T0/6 are inserted at the

state transitions. There are only two switching transitionsin a half of the sampling period. 16 % of the shoot-throughperiod is inserted in between the active states {100} and{110}, and the remaining 84 % is inserted at the active andzero states either in between {000} and {100} or {110} and

{111

}. The switching pattern of the proposed MSVM with

60

discontinuous switching for positive and negative DCrail clamping is shown in Figs. 3 (b) and (c), respectively.The existing zero state interval is again reduced to Tz/2T0. Carrier based implementation of proposed modifiedmodulation with discontinuous switching with positive DCrail clamping (as shown in Figs. 3 (b)) and negative DC railclamping (as shown in Fig. 3 (c)) is possible with the mod-ified reference signals derived using the same procedure asdescribed in the continuous switching. The reference volt-ages are given in (7)(9) for positive DC rail clamping and(10)(12) for negative DC rail clamping with 60 discontin-uous switching mode.

Fig. 5 Modified switching pattern for sector 1 with unequal

shoot-through distribution

Modified reference voltages for positive DC rail clampingare as follows:

Vmax(sp) = Vmax + Voff

Vmax(sn) = Vmax + Voff (7)

Vmid(sp) = Vmid + Voff


(8)

Vmin(sp) = Vmin + Voff T6

Vmin(sn) = Vmin + Voff T3. (9)

Modified reference voltages for negative DC rail clampingare as follows:

Vmax(sp) = Vmax + Voff +T

3

Vmax(sn) = Vmax + Voff +T

6(10)

Vmid(sp) = Vmid + Voff +T

6


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Vmid(sn) = Vmid + Voff (11)

Vmin(sp) = Vmin + Voff

Vmin(sn) = Vmin + Voff. (12)

The proposed modulation can be adopted for all kindsof applications since it greatly reduces the current/voltage

ripples along with additional boost in the DC link for thesame system parameters and shoot-through duty ratio. TheDC capacitor voltage can be boosted significantly, since themaximum shoot-through time period in the MSVM is in-creased to the traditional zero state time period (Tz/2).

4 Current ripple calculation

Current ripples in the DC link can produce ripples inthe output and it would cause ripples in the electromag-netic torque waveform, and ultimately this will lead totorque pulsation in the rotor during operation. This fa-cilitates an undesirable operation to the ZSI fed induction

motor drive. Expression for the ripple currents throughthe DC inductor produced by different modified state se-quences can conveniently be formulated by assuming thatthe Z-source capacitors and inductors are symmetrical (im-plying L1 = L2 = L and C1 = C2 = C). Current flowingthrough a single Z-source capacitor can then be expressedas: IC2 = IL1 in a shoot-through state during time inter-val T0 and IC2 = IL1 Iin in a non-shoot-through activeor null state during time interval Ts T0. Averaging thecurrent across a Z-source inductor over a switching periodgives

IL =

Ts T0Ts 2T0

IDClink. (13)

Further the Z-source inductor current in shoot-throughand non shoot-through states can be calculated as

ILshootthrough =

Ts T0Ts 2T0

IDClink (14)

ILnonshootthrough =

T0

Ts 2T0

IDClink. (15)

To proceed with the formulation of RMS expression forthe inverter input current IDClink, two assumptions shouldpreferably be made firstly, in order to simplify the deriva-tions without any degradation of accuracy. These assump-tions are respectively the presence of smooth three-phasesinusoidal currents at the inverter AC output and a suffi-

ciently large inductor in the Z-source network with a con-stant inductor current. The method followed by Gao etal.[26] can then be applied to determining the RMS expres-sions for IDClink by first expressing the duty ratios of theactive states, k1 and k2.

k1 = ma

3

2cos

t +

6

k2 = ma

3

2cos

t

2

. (16)

The null duty ratio is then expressed as

k0 = T T0

T k1k2 =

TZ

2 T0

2 . (17)

The null state is divided equally at the start and end ofthe sampling period. This ensures better harmonic profilein the output parameters.

ia = Icos(t )

ib = Icost

2

3

(18)

ic = Icos

t + 2

3

where represents the power flow angle.For a balanced three phase load, the three phase out-

put currents expressed in (18), should satisfy the followingcondition,

ia + ib + ic = 0. (19)

The RMS value of the DC link current can be calculatedby

(IDClink)RMS =

TSTS T0

(

938

ma I2 sin(2 6

))+ (20)

(TS

TS T0 I2(1 3

3

sin(2

6)))

.

By substituting the RMS value of Idclink into (13), theamplitude of current ripples can be calculated. The cur-rent ripples are significantly reduced while applying mod-ified modulation with unequal symmetrical shoot-throughstates and has been shown in the results section.

5 Results and discussion

Simulations have been carried out to verify the effective-ness of the proposed modified shoot-through distributionover the traditional scheme. A LC filter with 1 kHz cut-offfrequency is placed in between the inverter output and ACload.

The system parameters considered for simulation are asgiven below:

Source voltage: VDC = 250V.Z-source inductors: L1 = L2=1mH.Z-source capacitors: C1 = C2 = C=1000F.Load = 5 kW.Power factor = 0.9 (lagging).Modulation index: ma = 0.6.

Shoot-through duty ratio: D0=0.3.Switching frequency: fs=10kHz.Simulation results of the traditional and proposed PWM

for the current and voltage ripples in the Z-source inductorand capacitor respectively, and DC link voltage, AC output(out.) voltage/current waveforms are shown in Figs. 68.From these results, it can be observed that, the continuousswitching of the proposed SVPWM offers better DC linkvoltage boost for the same shoot-through duty ratio thanthe traditional method. The current ripples of the Z-sourceinductor (ind.) current in the proposed SVPWM schemeis reduced by 80% (from 4A to 0.8A), and the voltageripples of the Z-source capacitor (cap.) voltage is reduced

by 60 % (from 1 V to 0.4 V). In the proposed scheme, the


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DC link voltage is boosted to 236V where in the tradi-tional method it is 220 V, hence for the same shoot-throughduty ratio and modulation index an additional 16 V boostis offered by the proposed method. After the filter, thevoltage waveform becomes sinusoidal and having the RMSphase-phase value as 158 V. In traditional scheme the RMS

phase-phase voltage was 148 V. Additional voltage boost isachieved by turning the maximum time period of the tra-ditional zero states into shoot-through zero states by sym-metrical unequal shoot-through distribution. The switch-ing stress across the power devices is directly proportionalto the shoot-through time period and hence the switchingstress is increased while maintaining the shoot-through pe-riod for a significant time. It could again be reduced by themodified modulation scheme which needs only four switchesto be altered while inserting shoot-through state.

Fig. 6 Simulation results of traditional modified SVM for shoot-

through = 0.3 and modulation index = 0.6

The total harmonic distortion (THD) of the output volt-age/current waveforms is also found to be superior to thetraditional SVPWM. In the proposed SVPWM, the THDfor the output voltage is reduced to 0.53 % from 2.33 %and THD for the output current is reduced to 0.63 % from2.43 %. Hence the output voltage and current waveformsare also found to be improved proportionally and holds

the superior harmonics profile when compared with the

traditional SVPWM scheme. Proposed SVPWM providessame advantages for the discontinuous switching scheme forpositive and negative DC rail clamping as the continuousswitching. The results presented in Fig. 8 show 60 dis-continuous switching for positive DC rail clamping. Fig. 9shows the transient response of Z-source capacitor voltage

by traditional modified modulation schemes. It can be seenthat, the proposed modulation scheme provides better DCvoltage boost than the traditional modulation. The outputvoltage and current harmonic spectra of the proposed modi-fied modulation scheme have been shown in Figs.10 (a) and(b), respectively.

Fig. 7 Simulation results of novel modified SVM (continuous

switching) mode SVM for D0 = 0.3 and ma= 0.6

Even though the ripples in the Z-source elements are re-duced significantly than the traditional modulation, stillsome ripples are seen in the current through the Z-sourceinductors as well as the voltage across the Z-source capaci-tors. This is because of increasing the shoot-through dutyratio since shoot-through duty ratio has inverse relationshipwith ripples in the Z-source elements. The graph betweenvoltage gain and modulation index has been depicted inFig. 11 and from which, one could see that the possible op-eration region is extended with the increase of modulationindex. The AC voltage gain is high for a lower range of

modulation indices and less for higher range of modulation


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indices. Comparatively, the proposed modified modulationprovides better voltage gain than the traditional modula-tion technique. Modified control technique maximizes theshoot-through period without effecting the active states byturning maximum time period of the traditional zero statesinto the shoot-through zero state, thus maximum output

voltage could be obtained for a given modulation index.

Fig. 8 Simulation results of novel modified SVM (discontinu-

ous switching mode for +ve DC rail clamping) SVM for shoot-

through = 0.3 and modulation index = 0.6

Fig. 9 Transient response of Z-source capacitor voltage by tra-

ditional SVM and modified SVM

Fig. 10 Harmonics profile of the proposed modulation scheme

(a) Output voltage; (b) Output current

Fig. 11 Graph between voltage gain and modulation index

The results shown in simulation are verified by the exper-iment in the laboratory. The control system is implementedby LM3S611 processor for the voltage control and modifiedspace vector modulation with unequal shoot-through distri-bution. The AC output voltage and current are sensed byisolation devices, amplifiers, and a 12 bit analog-to-digitalconverter within the processor board. The gate drivercircuit is placed together with the power circuit board.The PWM signals coming from the control circuit board(LM3S611) are isolated through an optocoupler (6N135)for separating the control and power grounds and UC3705is used as the IGBT driver. The power circuit components

are selected to minimize parasitic effects. The front end


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diode rectifier supply is realized using low voltage drop40CPQ080 Schottky rectifier and low on state resistanceIGBT (N745AB), respectively. The PWM pulses embeddedwith shoot-through pulses were then sent out through six in-dependent PWM channels to gate the six switches throughthe isolation and driver circuit. The Z-source network is

constructed with L1 = L2 = L=1 mH/10 A inductors andC1 = C2 = C=1000F/600V capacitors.

Figs.12 and 13 show the steady state Z-source induc-tor current (IL) waveform in response to the shoot-throughpulses using traditional and modified modulation respec-tively. When the bridge is under shoot-through state, theZ-source capacitors charge the inductors and IL increases.When the bridge is under non-shoot-through state, the en-ergy stored in the inductors discharges over the load de-creasing IL. Simulation results for inductor current shownin Figs.6 and 7 are given for the duration 0.1 s0.2s whereasin the experimental results they are shown for very smallduration to demonstrate with clarity. From Figs. 12 and 13,

one could note that, the Z-source inductor current wave-form has reduced ripples while applying proposed modifiedmodulation. And also these figures show the shoot-throughtime period (T0) over the total switching cycle (TS). Fig.14and 15 show the experimental results of the traditional andmodified modulation of the ZSI, respectively. The DC linkvoltage, voltage across the Z-source capacitors and the out-put AC voltage waveforms after a LC filter with cut-off fre-quency 1kHz (ma=0.6, D0=0.3, VDC=150V, and switch-ing frequency=5 kHz) are shown.

Fig. 12 Inductor current waveform using traditional modulation

with respect to shoot-through pulses

Fig. 13 Inductor current waveform using modified modulation

with respect to shoot-through pulses

In addition to that mentioned earlier, the following obser-vations are noted when the unequal shoot-through durationbased modified modulation is applied to the three phase Z-source inverter. The DC link voltage has reduced ripplesand the voltage across the Z-source capacitor is boostedwell, and hence the AC output voltage. The Z-source ca-pacitor voltage and inductor current waveforms during 25 %supply voltage sag have been improved well as discussedabove by the proposed modulation as discussed above. The

DC boost is found good for the same shoot-through duty

ratio. The voltage across the Z-source capacitor is main-tained constantly in desired level during the supply voltagesag/fluctuations. It can be observed from the results thatthe experimental results have a close agreement with thesimulation results of the ZSI circuit. This validates that theZSI operates as expected in the theoretical analysis given

in this paper.

Fig. 14 Experimental results for traditional switching (a) DC

link voltage (before filter), (b) Z-source capacitor voltage, (c)

Output voltage

Fig. 15 Experimental results for modified switching (a) DC link

voltage (before filter), (b) Z-source capacitor voltage, (c) Output

voltage


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6 Conclusions

A new modulation scheme with modified shoot-throughdistribution has been presented in this paper for ZSI. Pre-sented scheme has several advantages over the traditionalmodulation scheme:

1) Reduces currant and voltage ripples in the Z-sourceelements wile retaining all the unique features offered bythe traditional methods.

2) Offers additional voltage boost across the DC link.3) Provides optimum harmonic spectrum while modify-

ing the shoot-through states.The theoretical and modulation concepts of continuous

and discontinuous switching modes of the proposed modu-lation have been presented.

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Sengodan Thangaprakash received hisB. Eng. degree in electrical and electronicsengineering and his M. Eng. degree in powerelectronics and drives from BharathiarUniversity and Anna University Chennai,

Tamilnadu, India in 2002 and 2004, respec-tively. He then received his Ph. D. degreein electrical engineering from Anna Univer-sity Chennai, India in 2011. From 2004 tomid-2011, he was working with the KSR

College of Engineering and the Sri Shakthi Institute of Engineer-ing and Technology, Tamilnadu, India. Currently he is workingas a senior lecturer in the School of Electrical Systems Engi-neering, University Malaysia Perlis (UniMAP), Malaysia. Hehas authored more than twenty papers in international journalsand conferences. He is a member of IEEE, IEEE-Power Elec-tronics Society, IEEE-Communications Society and a life mem-

ber of the Indian Society for Technical Education (ISTE). Hewas an organizing Chair for the IEEE sponsored second Interna-tional Conference on Computer Communication and Informatics(ICCCI 2012) held at Coimbatore, India during 1012, January2012. He is an editorial board member for the InternationalJournal of Engineering, Science and Technology, Nigeria and areviewer for the European Transactions on Electrical Power andtechnical program committee member for various IEEE interna-tional/national conferences.

His research interests include power electronics circuits, re-newable power conversion systems and solid state control of elec-trical drives.

E-mail: [email protected] (Corresponding au-thor)

Ammsasi Krishnan received the B. Sc.degree in electrical engineering and theM.Sc. degree in control systems fromMadras University, India in 1966 and 1974,respectively. Then he received the Ph.D.degree in electrical engineering (control,computers) from Indian Institute of Tech-nology, Kanpur in 1979. He has been inthe field of technical teaching and researchfor more than four decades at Government

College of Technology and Coimbatore Institute of Technology,Tamilnadu, India. From 1994 to 1997, he was an associate pro-fessor in electrical engineering at University Pertanian Malaysia(UPM), Malaysia. Currently he is a Dean with K.S.R. Collegeof Engineering, Tamil Nadu, India. He has published more than200 papers in International Journals and Conferences. He is a

senior member of IEEE, life fellow Institution of Engineers ofIndia, IETE of India and Computer Society of India.

His research interests include control systems, power electron-ics and electrical machines.

E-mail: a [email protected]

new switching scheme for z-source inverter

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