Parallel Operation of Extended Boost Quasi Z- Source Inverters for … · 2015. 9. 11. · usable...

Parallel Operation of Extended Boost Quasi Z-

Source Inverters for Photovoltaic System

Applications

Ali Zakerian and Daryoosh Nazarpour Department of Electrical Engineering, Urmia University, Urmia, Iran

Emails: [email protected], [email protected]

Abstract—Nowadays, more and more distributed

generations and renewable energy sources, such as wind,

solar and tidal power, are connected to the public grid by

the means of power inverters. They often form microgrids

before being connected to the public grid. Due to the

availability of high current power electronic devices, it is

inevitable to use several inverters in parallel for high-power

and/or low-cost applications. So, inverters should be

connected in parallel to provide system redundancy and

high reliability, which are important for critical customers.

This paper presents a new structure based on combination

of extended-boost quasi z-source converters and parallel

inverters to increase the output voltage in a wide range,

usable in photovoltaic (PV) systems. Additionally, the

proposed inverter has all the advantages of parallel power

converters. The proposed topology and its performances are

validated using simulation results which are obtained in

Matlab/Simulink. 1

Index Terms—voltage gain, parallel operation, Z-source

inverter (ZSI), extended boost quasi Z-source inverter

(EBQZSI), modulation index

I. INTRODUCTION

In recent years, due to energy crisis, renewable energy

sources, such as wind, photovoltaic (PV) and fuel cell are

becoming more popular in industrial and residential

applications. Today, PV cells are used in many

applications due to the advantages like less maintenance,

free from pollution and zero fuel cost [1]. Standalone

solar modules are commonly used for house hold lighting,

street lighting, etc. Large scale solar generating stations

are being installed for supplying power to utility grid. Fig.

1 and Fig. 2 show the structure of photovoltaic systems

and the equivalent circuit of a solar cell, respectively. A

photovoltaic module is formed by connecting many solar

cells in series and parallel. In PV generation, power

converter interface is the major part in the overall system.

Power converter topologies employed in the PV power

generation systems are mainly characterized by two- or

single-stage inverters. The single-stage inverter is an

attractive solution due to its compactness, low cost, and

reliability. However, its conventional structure must be

oversized to cope with the wide PV voltage variation

Manuscript received January 7, 2015; revised April 14, 2015.

derived from changes of irradiation and temperature. The

two-stage inverter topology applies a boost dc/dc

converter to minimize the required kilovoltampere rating

of the inverter and boost the wide-range input voltage to a

constant desired output value. However, the switch in the

dc/dc converter will increase the cost and decrease the

efficiency [2].

The Z-source inverter (ZSI) presents a new single-

stage structure to achieve the voltage boost/buck

character in a single power conversion stage, which has

been reported in applications to PV systems [3]. The ZSI

has gained popularity as a single-stage buck-boost

inverter topology among many researchers. However, its

boosting capability could be limited and therefore it may

not be suitable for some applications requiring very high

boost demanding of cascading other dc-dc boost

converters. This could lose the efficiency and demand

more sensing for controlling the added new stages.

Over the recent years, many researchers have given

their focus in many directions to develop ZSI to achieve

different objectives [3]-[10]. A new family of extended

boost quasi ZSI (EBQZSI) proposed to fill the research

gap left in the development of ZSI [11]. The most

important profits of this topology are increasing output

voltage in a wide range and its expandability as that for

each added new stage the boost factor can be increased

by a constant factor.

LO

AD

DC/AC

Inverter

DC/DC

Converter

PV array

VpvVdc Vo

Figure 1. The structure of photovoltaic systems.

Iph

Id Ish

RshVd

I Rs

V

Figure 2. Equivalent circuit of a solar cell.

On the other hand, many industrial systems demand a

reliable power supply. One way to increase the reliability

International Journal of Electronics and Electrical Engineering Vol. 4, No. 1, February 2016

©2016 Int. J. Electron. Electr. Eng. 17doi: 10.18178/ijeee.4.1.17-23

is to increase the number of sources. Another way to

increase the reliability is to have parallel inverters and

this would increase the redundancy as well as the

maintainability of the inverters. Moreover, the parallel

connected inverters effectively offer a significantly

higher level of availability than conventional approaches.

Commercially available ratings range from several kVA

to hundreds of kVA. Parallel connection techniques for

inverters have been gaining increasing attentions in

motor-drive systems, converter systems, and distributed

generation systems [12]-[16].

Paralleled inverters can be built in numerous ways.

First and the most obvious way is to have independent

inverters with separate dc sources [17] and the other

possibility is to connect the inverters into a common dc

source [18]. First method is common as it is simple.

However, it requires more than one power source. In the

context of Z-source inverter, this method requires more

than one independent Z-source impedance network.

Therefore, this paper proposes a new hybrid structure

based on EBQZS converters and parallel inverters which

share one dc-input voltage to increase the output voltage

in a wide range which is not limited to the dc-output

voltage of the PV array.

The main point in this study is proposing a structure in

order to link the PV dc-output voltage to the load, and for

this reason the PV array is substituted with a DC source.

This paper is started with a short summary about

EBQZSI and its different types in Section II. Operating

principles and the equivalent circuit of the proposed

topology are presented in Section III. Section IV presents

the simulation results and finally, conclusions are drawn

in Section V.

II. EXTENDED BOOST QUASI Z-SOURCE INVERTER

TOPOLOGIES

Extended Boost Quasi Z-Source (EBQZS) converter as

z-source converter can utilize shoot-through (short circuit)

states to boost the dc-bus voltage by gating on both the

upper and lower switches of a phase leg. There are four

topologies for EBQZSI in [11]. These topologies can be

mainly categorized as diode assisted boost or capacitor

assisted boost and can be further divided into continuous

current and discontinuous current topologies. All these

topologies can be modulated using the modulation

methods proposed for the original ZSI. The other main

advantage of these topologies is their expandability. This

was not possible with the original ZSI, i.e. if one needs to

increase the boosting range, another stage can be

cascaded at the front end without increasing the number

of active switches. Fig. 3, Fig. 4, Fig. 5 and Fig. 6 show

the diode assisted boost and capacitor assisted boost,

respectively. All figures are drawn for the second

extension of these structures.

All the topologies show higher boost and lower voltage

stress across the capacitors compared to those of

traditional ZSI. The following equations are obtainable

according to Fig. 7(a) that shows the first extension of the

capacitor assisted extended boost discontinuous current

quasi Z-source inverter. This converter has three

operating states similar to those of the traditional ZSI

topology. For simplicity, it can be simplified into shoot-

through and nonshoot-through states.

VinVdc

L3 L2 L1D2 D1

C3

C2

C1

LOAD

C4

D4L4

D3

D5

Figure 3. Diode-Assisted extended-boost continuous-current qZSI.

VinVdc

L3 L2 L1D2 D1

C3 C2

C1

LOAD

C4

D4L4

D3

D5

Figure 4. Diode-Assisted extended-boost discontinuous-current qZSI.

VinVdc

L3 L2 L1D2 D1

C3

C2

C1C4

LOAD

C5

D4L4

C6

Figure 5. Capacitor-Assisted extended-boost continuous-current qZSI.

VinVdc

L3 L2 L1D2 D1

C3 C2

C1C4

LOAD

C5

D4L4

C6

Figure 6. Capacitor-Assisted extended-boost discontinuous-current qZSI.

L3 L2 L1D2 D1

C3 C2

C1C4

loadIdcV inV

(a)

L1L2L3

C1C4

C2C3

loadIinVdcV dV

(b)


©2016 Int. J. Electron. Electr. Eng. 18

L1L2L3

C1C4

C2C3

dVdcV

(c)

Figure 7. (a) Structure of the capacitor-assisted extended-boost discontinuous current quasi Z-source first extension (b) Non-shoot-

through state. (c) Shoot-through state.

The inverter’s action is replaced by a current source

plus a single switch. First, consider the nonshoot-through

state, which is represented with an open switch. Fig. 7(b)

shows the equivalent-circuit diagram for nonshoot-

through state. Then, by applying KVL, the following

steady-state equations can be observed:

1 1C LV V (1)

2 2C LV V (2)

3 3C LV V (3)

3dc C dV V V (4)

Consider equivalent circuit of shoot-through state

shown in Fig. 7(c):

2 1 0d C LV V V (5)

1 2 0d C LV V V (6)

3dc C dV V V (7)

Considering the fact that the average voltage across the

inductors is zero and by defining the shoot-through duty

ratio as SD and nonshoot-through duty ratio as

AD ,

where 1A SD D , the following equations can be

derived:

sh

S

TD

T (8)

ns

A

TD

T (9)

1 2

1 3

S

d dc

S

DV V

D

(10)

1

1 3in dc

S

V VD

(11)

where, shT is the total shoot-through state period and

nsT

is the total nonshoot-through state period during all

period of switching, T. According to the above equations,

boost factor for one stage capacitor-assisted extended-

boost is as follows:

1

1 3 S

BD

(12)

The following equation is obtained for both structures

of capacitor-assisted extended-boost in which the boost

factor will increase while the number of stages increases:

1

(1 ( 2) )S

Bn D

(13)

where, n is the number of capacitor-assisted stages. The

added stages provide the boosting function without

disturbing the operation of inverter.

Similar argument, boost factor relation for both

topologies discontinuous-current and continuous-current

for diode-assisted can be written as:

1

(1 2 )(1 )n

S S

BD D

(14)

III. PROPOSED TOPOLOGY

Parallel operation of inverters has many advantages

such as modularity, ease of maintenance, (n+1)

redundancy, high reliability, etc. [19]-[22]. In addition to

these, output current ripple of the paralleled inverter can

be reduced significantly by virtue of interleaving effect

[23]. The basic concept of the proposed Extended Boost

Quasi Z-source with N-parallel inverters is shown in Fig.

8.

Extended

Boost

Quasi

Z-Source

VinVdc

Inverter 2

Inverter 1

Inverter N

Load

1bL

1aL

1cL

2aL

2bL

2cL

aNL

bNL

cNL

Figure 8. Basic concept of the proposed Extended-boost Quasi Z-

source with N-parallel inverters.

In the proposed topology (Fig. 8) both the diode-

assisted and capacitor-assisted extended-boost quasi Z-

Source topologies can be used. However, capacitor-

assisted topology is used in this paper, because it would

produce high boost with smaller shoot-through as well as

it would apply lower voltage stress on devices [11]. It is

assumed that all the inverters share the same dc bus that

is fed from EBQZS converters.

According to (13) and (14), different gains due to

different boost factors can be achieved for both the diode

and capacitor assisted extended boost qZSI:

G MB (15)

Then the peak ac-output voltage ˆoutV can be written as

follows:

ˆ ( )2

dc

out

VV B M (16)



Since the voltage at the point of common connection is

derived from the switching of different power

semiconductors, an intermodule reactor is absolutely

necessary to interconnect the different inverters. Fig. 9

shows a simplified equivalent circuit of the system

illustrated in Fig. 8. It is assumed that each inverter

develops a balanced three phase voltage and its

intermodule reactor consists of identical phase inductors.

However, differences may exist between different

inverter modules, both in voltage and reactance.

Under such assumptions, the equivalent circuit

illustrated in Fig. 9, may be further simplified into a

phasor equivalent circuit shown in Fig. 10. Here, each

inverter is represented by its internal phasor voltage

source, and one inductor and one resistor representing the

equivalent series resistance of the circuit.

o Load

AI

BI

CI

1AI

1BI

1CI

2AI

2BI

2CI

ANI

BNI

CNI

AV

BV

CV

1AV

2AV

ANV

2BV

1CV

1BV

2CV

BNV

CNV

O

1AL

1BL

1CL

2AL

2BL

2CL

ANL

BNL

CNL

Figure 9. Simplified equivalent circuit of N three-phase inverters connected in parallel.

1I

2I

NI

CV

LI

1L

2L

NL

1R

2R

NR

1V

2V

NV

O

Figure 10. Simplified phasor model for N three-phase inverters connected in parallel.

Inverter pole voltage and current vectors can be

defined as 1 2( ... )T

NV V V and 1 2( ... )T

NI I I

respectively, corresponding to the labels illustrated in Fig.

10. Now, the thevenin impedance of thi inverter, iZ

may be identified to be:

, = 1, 2, ... , i i iZ R j L i N , , = 1, 2, ... , i i iZ R j L i N (17)

And the equivalent thevenin impedance of all inverters

in parallel, THZ , may be determined as:

1 2Z ...TH NZ Z Z (18)

Furthermore, the voltage at the common coupling point

can be calculated to be:

1

( )N

j

C TH L

j j

VV Z I

Z

(19)

Now, the output current of the thi inverter can be

evaluated as:

1

( ) , N

jTH i TH

i L

ji i j j

VZ V ZI I i

Z Z Z Z

, 1

( ) , N

jTH i TH

i L

ji i j j

VZ V ZI I i

Z Z Z Z

(20)

It may be observed from (20), that various iZ play an

important role in determining the sharing of current

between the different modules. In order to quantify this

phenomenon, the sensitivity of the current to the thevenin

impedance, iZ can be determined to be:

2 2 32 , i L i TH

TH i

i i i i

I I V ZZ V i

Z Z Z Z

,

2 2 32 , i L i TH

TH i

i i i i

I I V ZZ V i

Z Z Z Z

(21)

Therefore, small deviations in equivalent thevenin

impedances of inverters can result in uneven current

distributions as suggested by (20). From (21), we can also

observe that, if the thevenin impedance is increased, the

dependence of output current on the output impedance is

also decreased leading to better current distribution.

As mentioned previously, this topology can be

modulated using the modulation methods such as the

simple control [3], maximum boost control [24] and

third-harmonic injection control [25]. Table I is the

summary of different PWM control methods for basic

ZSI. Fig. 11(a) shows voltage gain versus modulation

index and Fig. 11(b) shows voltage stress versus

modulation index for different PWM control methods

which have been used in basic ZSI. It is obvious from

these figures, the operation region of the third-harmonic

injection method is more than the others and by using this

method, voltage gain versus modulation index for both

the diode-assisted and capacitor-assisted EBQZSI topologies are drawn in Fig. 12(a) and Fig. 12(b),

respectively. These figures show how increasing in the

number of stages (n) will increase the operation region.

TABLE I. SUMMARY OF DIFFERENT PWM CONTROL METHODS

EXPRESSIONS

Control Method

Simple Max. Boost 3rd harmonic

injection

0D 1 M

2 3 3

2

M

2 3

2

M

B 1

2 1M

3 3M

1

3 1M

G 2 1

M

M

3 3

M

M

3 1

M

M

maxM 2 1

G

G

3 3

G

G

3 1

G

G

SV (2 1) inG V 3 3

in

GV

( 3 1) inG V



0.6 0.8 1 1.20

5

10

15

20

25

30

Modulation Index (M)

Vo

lta

ge G

ain

(G

)

Simple

Max. Boost

3rd harmonic injection

(a)

0.6 0.8 1 1.20

5

10

15

20

25

30

Modulation Index (M)Vo

lta

ge s

tress

/ E

qu

iva

len

t D

C v

olt

ag

e

Simple

Max. Boost

3rd harmonic injection

(b)

Figure 11. (a) Voltage gain versus modulation index, and (b) voltage

stress versus modulation index for different PWM control methods of basic ZSI.

0.6 0.7 0.8 0.9 1 1.10

5

10

15

20

25

30


Vo

lta

ge G

ain

(G

)

n=1

n=2

n=3

n=4

(a)

0.8 0.9 1 1.10

5

10

15

20

25

30


Vo

lta

ge G

ain

(G

)

n=1

n=2

n=3

n=4

(b)

Figure 12. Voltage gain (G) versus modulation index (M) (a) diode assisted mode (b) capacitor assisted mode.

IV. SIMULATION RESULTS

Extensive computer simulation using MATLAB-

Simulink has been performed to prove performance of the

proposed inverter. The simulation schematic is shown in

Fig. 13. The results are obtained for the operation of

discontinuous current capacitor-assisted extended boost

qZSI topology. The structure parameters are presented in

Table II. The modulation method which is proposed in

[24] is used in this simulation.

It is considered that the converter is operated with zero

boost in the beginning and at t = 250ms the shoot-through

is increased to 0.2 while the modulation index is kept

constant at 0.967.

The simulation results for the voltage across the

inverters (Vx) and output voltage are shown in Fig. 14

and Fig. 15. The output current of the top and down

inverters and the load current are shown in Fig. 16(a), Fig.

16(b) and Fig. 16(c), respectively. All simulation results

comply with the equations derived in Sections II and III.

Inverter 1

Inverter 2

Vdc

Vx

Figure 13. Structure of two paralleled operation of discontinuous current capacitor assisted extended boost qZSI.

TABLE II. THE PARAMETERS OF THE PROPOSED STRUCTURE

Input voltage 100 V

Extend Capacitor 1 mF

Extend Inductor 3.5 mH

Output filter capacitor 20 µF

Output filter inductor 15 mH

Load Resistor 9.7 Ω

Load Inductor 25 mH

Switching frequency 10 KHz

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.60

100

200

300

400

500

600

700

800

Time (S)

DC

Lin

k V

olt

ag

e V

x (

V)

Figure 14. Voltage across the inverters (Vx).

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

-300

-200

-100

0

100

200

300

Time (S)

Ou

tpu

t V

olt

ag

e (

V)

Figure 15. Output voltage of the simulated circuit.



0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6-15

-10

-5

0

5

10

15

Time (S)

Cu

rren

t W

av

efo

rm o

f th

e in

ver

ter

1 (

A)

(a)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6-15

-10

-5

0

5

10

15

Time (S)

Cu

rren

t W

av

efo

rm

of

the i

nv

erte

r 2

(A

)

(b)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6-30

-20

-10

0

10

20

30

Time (S)

Lo

ad

Cu

rren

t (A

)

(c)

Figure 16. Current waveforms of the simulated circuit.

As mentioned, increment in the number of EBQZS

converter stages will increase the amplitude of voltage to

a desired value. Additionally, in conditions where the

load requires higher current, in order to prevent the

probable damages to the switches, by increasing the

number of inverters in parallel we can supply the load by

desired current.

V. CONCLUSION

This paper has presented a new structure based on

combination of extended-boost quasi z-source converters

and parallel inverters to expand the range of system's

voltage gain and supply high load currents. With this

structure, the proposed inverter has the following features:

a) The proposed inverter has all the advantages of

paralleling power converters such as modularity,

ease of maintenance, (n+1) redundancy, high

reliability, reducing of the output current ripple by

virtue of interleaving effect, etc.

b) It can be short- or open-circuited without

damaging switching devices. Therefore, it is very

resistant to EMI noise and therefore its robustness

and reliability are significantly improved.

c) Voltage gain of the proposed inverter can be

increased by increasing in the number of

extended-boost quasi z-source converter stages.

d) Since here the load current can be divided between

all inverters, in conditions where the load requires

high current, the probable damages to the switches

which can be caused by high currents are

preventable.

The steady state operation is performed to analyze the

boosting capability then it is validated by simulation

results.

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Z-source inverter,” IEEE Trans. Power Electron., vol. 20, no. 4, pp. 833-838, Jul./Aug. 2005.

[25] M. Shen, J. Wang, A. Joseph, F. Z. Peng, L. M. Tolbert, and D. J. Adams, “Constant boost control of the Z-source inverter to

minimize current ripple and voltage stress,” IEEE Trans. Ind.

Appl., vol. 42, no. 3, pp. 770-778, May/Jun. 2006.

Ali Zakerian was born in Andimeshk, Iran, in 1990. He received the B.Sc. degree in Power

Electrical Engineering from Urmia University,

Urmia, Iran in 2013 and is currently a M.Sc. student in Power Electrical Engineering in the

same university. His current research interests include power electronic converters, multilevel

converters, Z-source converters, Application of

Power Electronics in Renewable Energy Systems, Flexible AC Transmission Systems

(FACTS), Hybrid and Electric vehicles.

Daryoosh Nazarpour was born in Urmia,

Iran in 1958. He received his B.Sc. degree from Iran University of Science and

Technology, Tehran, Iran in 1982 and the M.Sc. degree from Faculty of Engineering,

University of Tabriz, Tabriz, Iran in 1988. He

received the Ph.D. degree from Tabriz University, in 2005 in Electrical Power

Engineering. He is now an Associate Professor in Urmia University, Iran. His

research interests include Power Electronics, Flexible AC Transmission

Systems (FACTS), Power Quality in Power systems, Transients in Electrical Power Systems, Renewable Energy systems, Power System

Stability and Control.



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