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IET Power Electronics

Research Article

Extended maximum boost control schemebased on single-phase modulator forthree-phase Z-source inverter

IET Power Electron., 2016, Vol. 9, Iss. 4, pp. 669–679& The Institution of Engineering and Technology 2016

ISSN 1755-4535Received on 16th February 2015Revised on 22nd July 2015Accepted on 10th August 2015doi: 10.1049/iet-pel.2015.0124www.ietdl.org

Nassereddine Sabeur1, Saad Mekhilef1 ✉, Ammar Masaoud2

1Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya,

Kuala Lumpur, Malaysia2Department of Electrical and Computer Engineering, Curtin University (Sarawak), Miri, Malaysia

✉ E-mail: [email protected]

Abstract: This study proposes a new control method for Z-source inverter (ZSI) – called the one-dimension ZSI (ODZSI) –based on the single-phase modulator technique. The notable feature of the proposed control compared with the space-vector modulation strategy is its reduced computational processing time, which is attractive for digital implementation.Compared with the maximum boost control (MBC), which uses carrier-based pulse width modulation control methods,the proposed algorithm enhances the output voltage and current quality. In this study, the results of MBC arecompared with those obtained with a single-phase modulator for three-phase ZSI showing its advantage for improvingthe line output current and voltage total harmonic distortion. The obtained simulation and hardware results ensure thefeasibility and validate the performance of the ODZSI modulation method applied to each phase. The proposed methodis easier for digital implementation with less computation, and will be beneficial for further industrial applications ofZSIs. The simulation results are carried out using MATLAB/Simulink, and the hardware performance are provided anddiscussed.

1 Introduction

The conventional voltage source inverter (VSI) is a commoncircuit topology for dc/ac power conversion. The maindrawback of VSI is that the maximum output voltage obtainedcan never exceed the dc-link voltage. To obtain an outputvoltage higher than the input, an additional stage of dc/dcconverter is required, which increases the cost of the systemand decreases the efficiency. To overcome the aforementioneddisadvantages of VSI, Z-source inverter (ZSI) is introduced in[1] where a unique impedance network is coupled between thedc power source and the inverter main circuit. The obtainedefficiency is higher due to the main feature of ZSI, whichcombines the advantages of buck/boost in one-stage powerconversion. Moreover, the reliability is improved due to theinclusion of the shoot-through (ST) interval, which is notallowed in VSI because it destroys the devices. The operationof ZSI has an additional ST state to boost the dc-link voltagebesides the eight switching states in conventional converters,that is, six active and two null vectors [2]. The structure of theZSI and its equivalent circuits are depicted in Figs. 1a–c,respectively. The ZSI has gained increasing attention and hasbeen used in several applications such as wind powergeneration [3, 4], photovoltaic systems [5–10] and electricaldrive systems [11–13]. The basic topology of the two-level ZSIhas been extended to a single- and multiple-phase three-levelinverter in [14–16] with carefully inserted ST to achieve thedesired output performance.

Many control techniques such as carrier-based pulse widthmodulation (CB-PWM) and space-vector PWM (SVPWM),have been proposed for controlling ST in ZSI [17–22]. Sincethe ZSI was proposed in 2003, considerable work has beendone on this subject, especially for the PWM control methods.There are four different control methods for ZSI: namely, thesimple boost control (SBC) [17], the maximum boost control(MBC) [18], the maximum constant boost control [19] and the

modified SV modulation boost control methods [23], whichwere compared in [24, 25]. The MBC using carrier-basedachieves the highest voltage gain by maximising the STinterval time; it turns all traditional zero states into the ST statewhile keeping the six active states unchanged, thus, minimisingthe voltage stress across the switches. The variable ST timeproduces a low-frequency ripple in the inductor current and thecapacitor voltage. The sketch map of this control is depicted inFig. 2. On the other hand, the maximum constant boost controlachieves a maximum boost factor while keeping the ST dutyratio constant, which eliminates the low-frequency harmoniccomponent in the impedance-source network. However, thevoltage stress is relatively higher due to the presence of nullvectors (000) and (111). The range of the modulation index isextended from 1 to 2/

��3

√by injecting a third-harmonic

component with 1/6 of the fundamental component magnitudeto the three-phase-voltage references. A detailed comparison offour (space vector modulations (ZSVMs)) SV modulations forthe three-phase Z-source/quasi-ZSI and SBC is carried out in[26, 27], the results of which show that (ZSVMs) achieve ahigher dc-link voltage utilisation compared with the SBC. Agood summary and review of all the topologies and switchingcontrol types proposed for Z-source converters so far wereprovided by Siwakoti et al. [28, 29].

The voltage SV and flow diagram for implementing theSVPWM-based MBC (SV-MBC) strategy in [21] is shown inFig. 3. It can be clearly seen that the procedure is too complexdue to the difficulty in determining the location of thereference, the calculation of the ON-times for each vector inevery sector and the determination and selection of theswitching states.

The aim of this paper is to present a new time-domainduty-cycle computation technique called one-dimension ZSI(ODZSI). This is based on a single-phase modulator that hasbeen adapted to generate the ST pulses, thereby showing itsconceptual simplicity and its very low computational cost

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Fig. 2 CB-PWM strategy for MBC

Fig. 1 Structure of the ZSI and its equivalent circuits

a Basic topology of ZSIb Equivalent circuit of ZSI during ST statec Equivalent circuit of ZSI during non-ST states

Fig. 3 Voltage SV and flow diagram

a SV for ZSIsb Flow diagram for SV-MBC strategy

compared with SV strategies that have become one of the mostimportant modulation techniques for three-phase converters dueto their easy digital implantation and wide linear modulationrange [30]. The obtained simulation and hardware resultsdemonstrate that the ODZSI applied to each phase are similar tothe conventional techniques and advantageously enhance theoutput voltage and current quality.

2 Proposed ODZSI

The principle of the method in [31] can also be used to developa high switching frequency modulation technique for the ZSI.The OD modulation technique is based on the generation ofthe reference line-to-ground voltage as an average of thenearest voltage levels. The single-phase modulation problem isreduced to very simple calculations, which can easily

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determine the switching sequence (formed by two switchingstates) and the corresponding switching times [32, 33]. Toderive the relationship between the reference inverterline-to-ground voltages VXg_ref and inverter switching states Sx,the volt–second balancing principle is implemented, as shownin Fig. 4.

For a given reference inverter line-to-ground voltage Vxg_ref, thisreference voltage can be generated by the two nearest sequent


Fig. 4 Vag_ref, Vbg_ref and Vcg_ref generated by two nearest voltage levels

voltage levels as

Fig. 5 Proposed ODZSI strategy for MBC

Table 1 Switching states for a three-phase legs ZSI controlled byODZSI

Switching state S1 S2 S3 S4 S5 S6

null (000) → ST 1 1 1 1 1 1active (100) 1 0 0 1 0 1active (110) 1 0 1 0 0 1active (010) 0 1 1 0 0 1active (011) 0 1 1 0 1 0active (001) 0 1 0 1 1 0active (101) 1 0 0 1 1 0null (111) → ST 1 1 1 1 1 1

Fig. 6 Simulation and experimental results

a Hardware prototypeb Control block diagram

Table 2 Simulation model parameters of the ZSI

Parameters Value

desired output line-to-line voltage VLL (rms) 60 VZ-source inductances (L1 and L2) 1 mHZ-source capacitances (C1 and C2) 1.2 mFswitching frequency 10 kHzac load inductance 1.3 mHac load resistance 9.3 Ω

Vxg ref · Ts = V1 · T1 + V2 · T2 (1)

Vxg ref · Ts = Sx∗Vdc · T1 + (Sx + 1) · Vdc · T2 (2)

where x denotes phases a, b or c and

Ts = T1 + T2 (3)


Substitution of (3) in (2) yields

T2 = Ts ×Vxg ref

Vdc− Sx

( )(4)

The switching state of phase x (Sx) is determined using the integerfunction integer (INT) that returns the nearest INT less than orequal to its argument

Sx = INTVxg ref

Vdc

( )(5)

With the switching state of phase x (Sph−x) and its dwell times T1 andT2 are calculated (Sph−x = Sx during T1 and Sph−x = Sx + 1 during T2),the next step is to generate the ZSI switching pulses. The ST isintroduced when the switching state Sph−a = Sph−b = Sph−c = 1 orSph−a = Sph−b = Sph−c = 0 where all zero states are turned to STstate by turning on the upper and bottom switches simultaneouslywithout affecting the active vectors. Table 1 lists the switchingstates for a three-phase legs ZSI controlled by the proposedalgorithm.

To increase the range of the modulation index M, thethird-harmonic injection can be used here. The operation principleof the proposed high switching modulation technique method forZSI is illustrated in Fig. 5.

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3 Simulation and experimental results

To verify the validity and feasibility of the proposed controlstrategy, simulation and experiments have been performed usinga laboratory prototype, as shown in Fig. 6. The three-phaseZSI hardware is built using the parameters summarised inTable 2.

Fig. 7 Simulation waveforms for M= 0.9 and Vdc = 60 V

a Vdc (top) and Vin (bottom)b Inverter VLL (top) and VPh (bottom) voltagesc IL

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The aim of the prototype is to compare the performance of theproposed control algorithm for ZSI under different input voltagesto obtain the same output line-to-line VLL voltage of about 60 V[root mean square (rms)]. The simulation results with modulationindex values M = 0.9, 1.1 and 1.15 are plotted in Figs. 7–9 wherethe corresponding input dc voltage sources (Vdc) are 60, 72 and76.5 V, respectively.


Fig. 8 Simulation waveforms for M= 1.1 and Vdc = 60 V

a 72 V (top) and Vin (bottom)b Inverter VLL (top) and VPh (bottom) voltagesc IL

The simulation waveforms of the inverter dc-link (Vin) and the Vdc

are depicted in Figs. 7a–9a where the corresponding modulationindex values are M = 0.9, 1.1 and 1.15, respectively. Moreover,Figs. 7b–9b show the simulation waveforms of the inverterline-to-line VLL (top) and line-to-neutral Vph (bottom) voltages andthe last part (c) shows the output line current IL. It can be clearlyseen that the controller manages to generate the appropriateswitching gate signals that lead the inverter to output the desiredwaveform. Table 3 summarises the obtained results.

The experimental results obtained under the same operationparameters and the same laboratory prototype as in the


simulation are illustrated in Figs. 10–12 at different modulationindex values M = 0.9, 1.1 and 1.15, respectively. Each figureshows the waveform of Vin and Vdc in figure part (a), the acoutput VLL voltage (top) and the Vph are illustrated in figurepart (b), and the last two figure parts (c and d) depict the acoutput current and total harmonic distortion (THD) spectrum,respectively. The THD graph contains the fundamentalfrequency component of about 60 V followed by 19 harmonicscomponents. It can be clearly seen that there is a greatmatching between the proposed and conventional methods.Moreover, the experimental and simulation results match very

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Fig. 9 Simulation waveforms for M= 1.15 and Vdc = 76.5 V

a Vdc (top) and Vin (bottom)b Inverter VLL (top) and VPh (bottom) voltagesc IL

Table 3 Summary of obtained experimental results under differentconditions

Vdc M VLL (rms), V

60 0.9 60.7872 1.1 59.4576.5 1.15 59.93

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well, which verify the performance of the proposed controlalgorithm.

The variation of THD with the modulation index for the inverterVLL voltage is depicted in Figs. 13 and 14. This illustrates thatTHD is inversely proportional to the modulation index M. In otherwords, a lower THD in the output voltage is experienced at ahigher modulation index. The graph compares the THD of the VLL

voltage for the proposed algorithm and CB-PWM within a range


Fig. 10 Experimental waveforms for M= 0.9 (Vdc = 60 V and VLL(rms) = 60.78 V)

a Vin (top) and Vdc (bottom)b Inverter VLL (top) and VPh (bottom) voltagesc ILd THD spectrum


Fig. 11 Experimental waveforms for M= 1.1 (Vdc = 72 V and VLL(rms) = 59.45 V)


IET Power Electron., 2016, Vol. 9, Iss. 4, pp. 669–679676 & The Institution of Engineering and Technology 2016

Fig. 12 Experimental waveforms for M= 1.15 (Vdc = 76.5 V and VLL(rms) = 59.93 V)



Fig. 13 VLL(THD) versus M for MBC

Fig. 14 VLL(THD) at M = 0.9, 1.1 and 1.15, respectively, using CB-PWM

Table 4 Comparison between the proposed ODZSI and CB-PWM

ODZSI CB-PWM

line current harmonic, % 11.86 16.02VLL(THD), % 26.31 35.46VLL (rms), V 59.45 59.39

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of modulation indices [0.9–1.15]. It can be seen that the THD usingODZSI is about 10% less than the conventional technique, whichaugments the output voltage quality.

Table 4 summarises the obtained results for Vdc = 72 andM = 1.1.

4 Conclusion

In this paper, a new control method has been presented based on thesingle-phase modulator to obtain the maximum voltage gain of theZSI. The ST period is maximised by turning all zero states intothe ST state, whereas the active state remains unchanged. Theresults obtained show that the proposed control strategy achievesthe same performance compared with CB-PWM for the ZSI andminimises the line voltage and current THD. Simulations usingMATLAB/Simulink and experiment were carried out todemonstrate the validity and feasibility of the proposed controlalgorithm under different modulation index values.

5 Acknowledgments

The authors would like to acknowledge the financial support from theMinistry of Higher Education (MoHE), Malaysia, through the UMHigh Impact Research Grant UM-MOHE UM.C/HIR/MOHE/ENG/17and UMRG project No. RP015D-13AET.

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