EPE_Experimental Study of the Shoot-Through Boost Control Methods for the Z-Source Inverter.pdf

8/12/2019 EPE_Experimental Study of the Shoot-Through Boost Control Methods for the Z-Source Inverter.pdf

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Omar Ellabban, Joeri Van Mierlo, and Philippe Lataire

EPE Journal Vol. 21 no 2 June 201118

Introduction

The demand for the electric energy is never ending and with thisrapid increase in energy demand, more generation resources areneeded. Because of environmental concerns and limited fossil fuelresources, the renewable energy sources such as: fuel cells, PVand Wind etc., which are environmentally clean, are gainingincreasing impetus across the globe. The output voltage of theserenewable energy sources has a wide voltage variation rang. Inthese situations an additional d.c.-d.c. boost converter or step-uptransformer is usually needed, increasing the system complexityand cost. The ZSI is an emerging topology for power electronicsconverters with very interesting properties such as buck-boostcharacteristics and single stage conversion. As a result, the ZSIhas received a lot of attention recently [1].

The special Z-network, comprising two capacitors and two induc-tors, connected to the well known three phase bridge, as shown inFig. 1, allows working in buck or boost modes using the ST state.The ZSI advantageously utilizes the ST states to boost the d.c.-link voltage by gating on both the upper and lower switches of aphase leg. In addition, a ST state caused by electromagnetic inter-ference (EMI) noise does not destroy the circuit. Therefore a morereliable single stage power converter for both buck and boostpower conversion is obtained.

Many PWM control methods have been developed and used forthe voltage source inverter (VSI). The VSI has six active states,when the d.c. voltage is impressed across the load, and two zerostates when the load terminals are shorted through the lower orupper switches. The ZSI has an additional zero state, the ST state.How to insert this ST state becomes the key point of the PWMcontrol methods for the ZSI. Since the ZSI was proposed in 2003

[1], lots of work have been done in this subject, especially for ST

control methods [2-12]. Four different ST boost control methodshave been proposed, which are: simple boost control [2], maxi-mum boost control [3], maximum constant boost control [4], andmodified space vector modulation (MSVM) boost control [5, 6].In [7, 8], the authors present a simulation efficiency comparisonbetween two different ST control methods, which are: two STstates insertion per switching cycle and six ST states insertion perswitching cycle. Therefore the authors did not distinguish betweenthe simple boost, the maximum boost, and the maximum constantboost control methods. In [9], the authors present a simu-lationcomparison between the above four mentioned ST control meth-ods for the different ZSI topologies (the basic, the bidirectionaland the high-performance ZSI). In [10], the author proposed theapplication of the discontinuous PWM modulation methods forimproving the efficiency of the ZSI, but the paper did not present

the effect of the application of these modulation methods on thevoltage gain, switch voltage stress and output current andvoltage harmonic content. A simulation comparison of the voltagegain of different ST control methods of the ZSI, without the modi-

Experimental Study of the Shoot-Through Boost Control Methods

for the Z-Source Inverter

Omar Ellabban, Joeri Van Mierlo, and Philippe Lataire: The department of Electrical Engineering and Energy Technology (ETEC), VrijeUniversiteit Brussel (VUB), 1050 Brussels, Belgium

Keywords: Z-source inverter, shoot-through control, simple boost, maximum boost, maximum constant boost and modified space vec-tor modulation boost

Abstract

This paper presents a simulation and experimental comparative analysis of the Z-source inverter (ZSI) with four diffe-rent shoot-through (ST) control methods, namely: the simple boost control, the maximum boost control, the maximumconstant boost control and the modified space vector modulation boost control methods. A review of these methods is

presented with a summary of all expressions. A prototype of a 30 kW ZSI is designed and implemented. The eZdspF2808 evaluation board is used for the realization of the shoot-through control methods and the real time workshop

(RTW) is used for automatic code generation. The paper compares between the different four shoot-through control methodsin terms of: the line voltage harmonic, the phase current harmonic, the d.c. link voltage ripples, the switch voltage stress,the inductor current ripples, the obtainable a.c.output phase voltage and the overall efficiency with detailed simulationand experimental results. In addition, the paper presents the effect of varying the d.c. input voltage and the shoot-throughduty ratio on the overall inverter efficiency.

The maximum constant shoot-through boost control method seems to be the most suitable shoot-through control method for the ZSI. Also, it is shown that the efficiency of the ZSI improves with increasing the d.c. input voltage and degradeswith increasing the shoot-through duty ratio.

Fig. 1 : The basic ZSI topology


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Experimental Study of the Shoot-Through Boost Control Methods for the Z-Source Inverter

EPE Journal Vol. 21 no 2 June 2011 19

fied space vector modulation method, was presented in [11]. Thesimple and the maximum constant ST boost control methods arecompared by simulation in terms of voltage gain and voltagestress in [12].

The aim of this paper is to present a review of the four ST boostcontrol methods and presents a simulation and experimental com-parison of them. In addition, loss model based calculation and

experimental measurement of the ZSI efficiency with the differentST control methods for different output power levels are presentedand compared, Also, the effects of varying the input voltage andthe ST duty ratio on the ZSI efficiency are theoretically and exper-imentally studied.

Operation of the ZSI

Fig. 1 shows the basic ZSI topology, which consists of inductors( L1 and L2) and capacitors ( C 1 and C 2) connected in X shape tocouple the inverter to the d.c. voltage source. The ZSI can produceany desired a.c. output voltage regardless of the d.c. input voltage.Because of this special structure, the ZSI has an additional switchingstate, when the load terminals are shorted through both the upper

and lower switching devices of any phase leg, which called theshoot-through (ST) state besides the eight traditional non-shootthrough(NST) states. The ZSI has two operating modes: non-shoot-through mode and shoot-through mode, as shown in Fig. 2.

During the ST switching state, the input diode is reverse biased;the input d.c. source is isolated from the load, and the two capaci-tors discharge energy to the inductors and to the load. During theNST switching states, the input diode turns ON, and the d.c. inputvoltage source as well as the inductors transfer energy to the loadand charge the capacitors, as a result the d.c.-link voltage of bridgeis boosted.

In steady state, the capacitor voltage, the d.c.-link voltage and theoutput a.c. peak phase voltage of the ZSI are given by [1]:

(1)

(2)

(3)

where D0 = T 0 / T s is the ST duty ratio, T 0 is the ST time per theswitching period, T s, B = 1/1 2 D 0 is the boost factor and M is themodulation index.

From (3), the peak a.c. output phase voltage can be controlledboth by adjusting the modulation index or ST time, and it can belarger than the input d.c. voltage by adjust the ST time. That is themain advantage of the ZSI.

Review of PWM control methods for ZSI

Simple ST boost control method

The simple ST boost control method, [2], uses two straight linesequal to or greater than the peak value of the three phase refe-rences to control the ST duty ratio in a traditional sinusoidal PWM,as shown in Fig. 3. When the triangular waveform is greater thanthe upper line, V p, or lower than the bottom line, V n, the circuit

V M V

M B V

aci in= =

2 2

V B V V

Di in

in= =1 2 0

V V

DC

in=1 0

turns into ST state. Otherwise it operates just as traditional carrierbased PWM. This method is very straightforward; however, theresulting voltage stress across the switches is relatively highbecause some traditional zero states are not utilized.

Maximum ST Boost Control Method

Reducing the voltage stress under a desired voltage gain becomesmore important to control the ZSI, this can be achieved by makingthe ST duty ratio as large as possible. The maximum ST boostcontrol, [3], turns all the traditional zero states into ST state. Asshown in Fig. 4, the circuit is in ST state when the triangular car-rier wave is either greater than the maximum curve of the refe-rences ( V a, V b and V c) or smaller than the minimum of thereferences. The ST duty ratio varies at six times of the output fre-quency. The ripples in the ST duty ratio will result in ripple in theinductor current and the capacitor voltage. This will cause a higher

(a)

(b)

Fig. 2: The ZSI operation modes: (a) non-shoot-through state,(b) shoot-through state

Fig. 3: Simple ST boost control method waveforms


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requirement of the passive components when the output frequencybecomes very low. Therefore, the maximum ST boost controlmethod is suitable for applications that have a fixed or relativelyhigh output frequency.

Maximum constant ST boost control method

In order to reduce the volume and the cost, it is important alwaysto keep the ST duty ratio constant. At the same time, a greatervoltage boost for any given modulation index is desired to reducethe voltage stress across the switches. The maximum constant STboost control method achieves the maximum voltage gain while

always keeping the ST duty ratio constant [4]. Fig. 5 shows thesketch map of the maximum constant ST boost control with thirdharmonic injection. Using the third harmonic injection, only twostraight lines, V p and V n, are needed to control the ST time with1/6 of the third harmonic injected.

Modified space vector modulation ST control method

The space vector PWM (SVPWM) techniques are widely used inindustrial applications of the PWM inverter because of lower cur-rent harmonics and a higher modulation index. The SVPWM issuitable to control the ZSI. Unlike the traditional SVPWM, themodified space vector modulation (MSVM) has an additional STtime T 0 for boosting the d.c.-link voltage of the inverter beside thetime intervals T 1, T 2 and T z. The ST states are evenly assigned to

each phase with T 0 /6 within zero voltage period T z. The zerovoltage period should be diminished for generating a ST time, and

the active states T 1 and T 2 are unchanged. So, the ST time does notaffect the PWM control of the inverter, and it is limited to the zerostate time T z.

The MSVM can be applied using two patterns. The MSVPWM1as shown in Fig. 6 (a), at this switch pattern, the ST time T 0 is limi-ted to (3/4) T z, because the period ( T z /4-2 T s) should be greater thanzero. The MSVPWM2 as shown in Fig. 6 (b), where the distribu-tion of zero state time is changed into ( T z /6) and ( T z /3). Themaximum ST time is increased to the zero state time T z, [5, 6].

Table 1 shows a summary of all relations for the different ST

boost control methods, where D 0 is the ST duty ratio, M is themodulation index, B is the boost factor, G is the voltage gain, andV s is the voltage stress across the switch. Fig. 7(a) shows thevoltage gain versus the modulation index and Fig. 7(b) shows thevoltage stress versus the voltage gain for different ST boostcontrol methods. At high voltage gain, the MSVPWM1 has thehighest voltage stress.

Efficiency comparison

Efficiency evaluation is an important task during inverter design.The losses of the inverter mainly distributed on the semiconductordevices. The semiconductor device losses mainly include conduc-tion losses and switching losses.

Fig. 4: Waveforms of the maximum ST boost control method

Fig. 5 : Maximum constant ST boost control method withthird harmonic injection

(a)

(b)

Fig. 6: Switching pattern for the MSVPWM: (a) MSVPWM1,(b) MSVPWM2


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The unique operation principle makes the losses evaluation of theZSI is complex and different from that of the VSI, and differentST boost control methods have a great influence on the lossesevaluation. When The ZSI is operating in buck mode, it operateslike a VSI and the losses of the IGBTs and the freewheelingdiodes (FWDs) are calculated in the same way as the VSI. Goinginto the boost mode, the ST states are required to boost the inputvoltage. During the ST state all six IGBTs (for simple, maximumand constant maximum ST boost control methods) or two IGBTs(for modified space vector modulation ST boost control method)are conducting simultaneously and the d.c.-link is short circuited.In this case the current through one IGBT is the superimpositionof the sinusoidal a.c. current and the high-frequency ST current.Therefore, there is a different way to calculate the losses for theZSI when operating in boost mode [13, 14]. The conduction lossesare calculated according to the well known equations [15]:

(4)

(5)

where V CE0 and V F0 constitute the IGBT's and diode's thresholdvoltages, r CE and r F constitute their differential ohmic resistance

P V i r iCDiode

F0 av F rms= + 2

P V i r iCIGBT

CE0 av CE rms= + 2

and iav, irms are the average and RMS current passing through theIGBT or the FWD.

Using a linearized model of the switching losses, the switchinglosses are directly proportional to the switching frequency andthey depend on the chosen ST boost control method and theswitching loss energies of the IGBT ( E on, E off ) and the diode( E rec), can be given by [15]:

(6)

(7)

In datasheets, the switching loss energy are only given for a refe-rence voltage V ref and a current I ref .

In this paper, the 1200 V IGBT module SKM 400GB12T4 is cho-sen as the switch devices for the inverter bridge and the inputdiode. The device parameters specified for junction temperature T = 150 C are extracted from the datasheet as indicated in Table 2.

P f E V

V I

I SDiode

s recv

ref

v

ref

= 1

P f E E V

V I

I SIGBT

s on off v

ref

v

ref

= + 1

( )

Table 1 : Summary of the different ST boost control methods expressions [9]

ST boost control Simple Max. Max. Con. MSVPWM1 MSVPWM2method

1 M

1

2 1 M

2 1 M

M

2 1G

G

2 1G V ( ) in

2 3 3

2

3 3

3 3

3 3

3 3

M

M

M

M

G

G

GV in

2 3

2

1

3 1

3 1

3 1

3 1

M

M

M

M

G

G

G V ( )in

3

4

2 3 3

2

4

9 3 2

4

9 3 2

2

9 3 4

9 3 4

M

M

M M

GG

G(

)

2 V in

2 3 3

2

3 3

3 3

3 3

3 3

M

M

M

M

G

G

GV

( )in

D 0

B

G

M max

V s

(a) (b)Fig. 7: (a) Voltage gain versus modulation index, (b) Voltage stress versus voltage gain for different PWM control methods [9]


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Simulation and experimental results

The dynamic performance of the ZSI with the four different STboost control methods has been compared using MATLAB simu-lation and experimental verification using the parameters in Table3. A 30 kW prototype of the ZSI, as shown in Fig. 8, has beendesigned and implemented. For the realization of the ST control

methods the evaluation board eZdsp F2808 based on theTMS320F2808 DSP by Texas Instruments has been chosen [16].The eZdsp evaluation board is connected to a PC using USB cableand the PC has MATLAB, real time workshop (RTW) and codecomposer studio (CCS) installed on it for automatic code genera-tion and real time control, as shown in Fig. 9. Fig. 10 shows theSimulink model used for automatic code generation.

Figs. 11-14 compare the simulation and the experimental resultsof the ZSI performance using the different four ST boost controlmethods. Each figure shows the waveforms of the a.c. output linevoltage and its harmonic spectrum, the a.c. output phase currentand its harmonic spectrum and the d.c. input voltage, the Z-net-work capacitor voltage, the d.c. link voltage and the Z-network inductor current waveforms in terms of output and switchingfrequencies. Where, the figure parts a, c, e and g represent the sim-ulation results and the parts b, d, f and h represents the experi-mental results. Fig. 15 shows the obtainable a.c. output phasevoltage with constant ST duty ratio for different ST control methods.Fig. 16 shows the measured and calculated efficiency of the ZSIat different load levels with different ST boost control methods.The calculated efficiency is based on the conduction and switchinglinearized loss models, and it is calculated at constant inputvoltage. Fig. 17 shows the losses distribution of the ZSI at nomi-nal input and 10 kW output power, where the input diode conduc-tion and switching losses are included, which are neglected inmost publications, the extra losses of the MSVM boost methodmainly come from the switching losses of IGBTs and reverserecovery losses of the input diode which are about three times of other methods. Table 4 presents a comparison between the differ-ent four ST control methods.

By comparing the simulation and experimental results, one findsa great matching between the simulation and experimental results.But the measured efficiency is lower than the calculated one,because the measured efficiency included the Z-network inductorsand capacitors which are not included in the calculated efficiency.These results show that the constant maximum ST boost controlmethod seems to be the most suitable ST control method for theZSI. It requires less inductor value and results in less switchvoltage stress, less output current total harmonic distortion, betterZ-network behavior, high obtainable ac output voltage and higherefficiency.

Table 2 : Characteristics of SKM 400GB12T4 IGBT module

IGBT FW Diode

V CES (v) I C (A) V CES (v) r CE (m ) E on (mJ) E off (mJ) I F (A) V F0 (V) r F (m ) E rr (mJ)

1200 610 0.7 3.8 33 42 440 0.9 3 30.5

E on, E off and E rec are given at V ref = 600 V, I C = 400 A.

Fig. 8: Experimental setup of a 30 kW ZSI

Fig. 9: DSP based control circuit of the proposed ZSI

Fig. 10: Simulink model used for code generation for theTMS320F2808 DSP

Table 3 : Experimental prototype and simulation modelparameters of the ZSI

Parameter ValueInput voltage 220 VInductance 650 HInductance internal resistance 0.22

Capacitance 320 FCapacitance internal resistance 0.9 m

Switching frequency 10 kHzSteady state shoot-through ratio 0.24AC load inductance 340 HAC load resistance 8.3


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Fig. 18 shows the relationship of the input d.c. voltage and the ZSIefficiency at 10 kW output power. One can find that lower theinput d.c. voltage brings lower efficiency, this because the lowerinput d.c. voltage means the larger ST duty cycle and higher ST

current, as indicated in Fig.19, the efficiency of the simple STboost control method is strongly affected by increasing the STduty ratio.

(a)

(c)

(e)

(g)

(b)

(e)

(f)

(h)

Fig. 11: Simulation and experimental results of the ZSI topology using the simple ST boost control method


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(a)

(c)

(e)

(g)

(b)

(e)

(f)

(h)

Fig. 12: Simulation and experimental results of the ZSI topology using the maximum ST boost control method


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(a)

(c)

(e)

(g)

(b)

(e)

(f)

(h)

Fig. 13: Simulation and experimental results of the ZSI topology using the maximum constant ST boost control method


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(a)

(c)

(e)

(g)

(b)

(e)

(f)

(h)

Fig. 14: Simulation and experimental results of the ZSI topology using the MSVM ST boost control method


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Fig. 15: Output phase voltage of the ZSI for different STboost control methods

Fig. 17: Losses distribution of the ZSI at 10 kW output power

(a)

(b)Fig. 16: Calculated, (a), and measured, (b) ZSI efficiency fordifferent ST boost control methods

Table 4: Different ST control methods comparison - (+, 0 and -) represents the best, the moderate and the lowest perfor-mance, respectively

Simple Boost Max. boost Max. Cont. boost MSVM boost

Line voltage harmonic - + 0 +Phase current harmonic 0 0 + -dc link voltage ripples 0 - + 0Switch voltage stress 0 + 0 -Inductor current ripples 0 - + -Efficiency 0 + + -Obtainable ac voltage 0 0 + -Total - +++-- +++++ +----

(a) (b)

Fig. 18: The ZSI efficiency for different input voltage: (a) calculated efficiency, (b) measured efficiency using the maximum con-stant ST boost method


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DSP-based system control and switched reluctance motor control forautomotive applications.

Joeri Van Mierlo obtained his Ph.D. inElectromechanical Engineering Sciences fromVrije Universiteit Brussel in 2000. He is now aFull Time Professor at this university, where heleads the MOBI - Mobility and AutomotiveTechnology Research Group. His currentresearch interests include the development of hybrid propulsion systems (converters, super-caps, energy-management, etc.) as well as theenvironmental comparison of vehicles with dif-ferent kinds of drive trains and fuels (LCA,WTW). He is the author of more than 100 scien-

tific publications. He chairs the EPE chapter on Hybrid and ElectricVehicles (www.epe-association.org). He is the Secretary of the Boardof the Belgian section of AVERE (ASBE) and is a Board Member of

AVERE. He is a Co-Editor of the Journal of Asian Electric Vehicles. Heis an active Member of EARPA - the Association of Automotive R&DOrganizations. Furthermore he is a Member of Flanders Drive and of VSWB - the Flemish Cooperative on Hydrogen and Fuels Cells. Prof.Van Mierlo is Chairman of the International Program Committee of theInternational Electric, Hybrid and Fuel Cell Symposium (EVS24).

Philippe Lataire received a degree inElectromechanical Engineering in 1975 and hisPh.D. in 1982, both from Vrije UniversiteitBrussel (VUB), Brussels, Belgium. He ispresently a Full Time Professor at VUB. His cur-rent research interests include electric drives,power electronics and control.

Authors: Bin Wu, Yongqiang Lang, Navid Zargari, Samir Kouro

IEEE Press Series on Power Engineering and John Wiley & Sons, Inc.,Publication

ISBNBook: 978-1-118-02900-8PDF: 978-1-118-02899-5Pub: 978-1-118-02898-8

The conversion of wind kinetic energy into electric energy is of a multi-

disciplinary nature, involving aerodynamics, mechanical systems,electric machines, power electronics, control theory, and power systems.In the past, a number of books have addressed some of these subjects.This book explores the power conversion and control of wind energyconversion systems (WECS) from the electrical engineering perspec-tive. It provides a comprehensive and in-depth analysis of wind genera-tors, system configurations, power converters, control schemes, anddynamic/steady-state performance of various practical wind energysystems.

The book contains nine chapters. Chapter 1 provides a market surveyand an overview of wind turbine technology, wind energy systemclassifications, costs, and grid codes for wind power integration.Chapter 2 introduces the fundamentals and control principles of windenergy systems, including wind turbine components, aerodynamics,stall and pitch controls, and maximum power tracking schemes. Chapter3 presents commonly used wind generators, including squirrel cageinduction generators, doubly fed induction generators, and synchronousgenerators. The dynamic and steady-state models of these generators arealso derived to facilitate the analysis of wind energy systems in thesubsequent chapters.

Chapter 4 discusses various power converters and PWM schemes usedin wind energy systems. Both voltage and current source converters arepresented with an emphasis on high-power wind energy system.Chapter 5 presents a general overview of configurations and character-istics of major practical WECS. Chapter 6 focuses on fixed-speed,inductor generator based wind energy systems; important issues such asgrid connection, two-speed operation, and reactive power compensationare discussed.

Chapter 7 deals with wind energy systems with variable-speed, squirrelcage induction generators (SCIG), in which typical systems configura-tions and advanced control schemes such as field oriented control(FOC) and direct torque control (DTC) are elaborated. Chapter 8discusses doubly fed induction generator (DFIG) systems, where thesubsynchronous modes of operation are investigated. Chapter 9 is dedi-cated to variable-speed, synchronous generator wind systems, in whichvarious control schemes, including zero d-axis current (ZDC) control,maximum torque per ampere (MTPA) control, and unity power factor(UPF) control, are analyzed in detail.

Throughout, important concepts are illustrated with simulations andexperiments, and design guidance is provided with tables, charts, andgraphs. To help the reader understand the principle and operation ofvarious WECS, more than 30 case studies are given in various chaptersand more that 100 solved problems are included in a dedicatedappendix. This book not only serves as a valuable reference foracademic researchers, practicing engineers, and consultants, but also asa textbook and senior-year undergraduate students.

BIN WU, PhD, is a Professor in the Department of Electrical andComputer Engineering at Ryerson University (Canada). He is anNSERC/Rockwell Automation Industrial Research Chair and thefounder of the Laboratory for Electric Drive Applications and Research(LEDAR). He is a Fellow of the Institute of Electrical and ElectronicsEngineers, Engineering Institute of Canada, and Canadian Academy of Engineering.

YONGQIANG LANG, PhD, is a Senior Engineer in the State GridElectric Power Research Institute (China). He worked in the LEDARWindTech Laboratory at Ryerson University from November 2006 toJune 2009 as a postdoctoral fellow in the area of wind power.

NAVID ZARGARI, PhD, is a leading expert in medium voltage drivesand a practicing engineer for fifteen years. He has been with theMedium Voltage Business of Rockwell Automation Canada since 1994,and has in-depth knowledge of many aspects of adjustable speed drivesand wind energy systems.

SAMIR KOURO, PhD, is a research academic in the ElectronicsEngineering Department of Universidad Tecnica Federico Santa Maria(Chile).

Book

New publication: Power Conversion and Control of Wind Energy Systems

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