Ain Shams Engineering Journal (2016) 7, 579–592

Ain Shams University

Ain Shams Engineering Journal

www.elsevier.com/locate/asejwww.sciencedirect.com

ELECTRICAL ENGINEERING

Hybrid PV/wind system with quinary asymmetric

inverter without increasing DC-link numberI

q This research has been supported by Azarbaijan Shahid Madani

University.

Abbreviations: MPP, maximum power point; MPPT, maximum power

point tracking; P&O, perturbation and observation algorithm; PMSG,

permanent magnet synchronous generator; PV, photovoltaic; PWM,

pulse width modulation; V–I, voltage–current; V–P, voltage–power* Corresponding author. Tel.: +98 9141186476.

E-mail addresses: a_baghbany@azaruniv.edu (A. Baghbany Oskouei),

m.banaei@azaruniv.edu (M.R. Banaei), sabahi@tabrizu.ac.ir

(M. Sabahi).

Peer review under responsibility of Ain Shams University.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.asej.2015.06.0082090-4479 � 2015 Faculty of Engineering, Ain Shams University. Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Aida Baghbany Oskouei a,*, Mohamad Reza Banaei a, Mehran Sabahi b

a Department of Electrical Engineering, Azarbaijan Shahid Madani University, Tabriz, Iranb Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran

Received 1 November 2014; revised 26 April 2015; accepted 19 June 2015Available online 12 August 2015

KEYWORDS

Hybrid photovoltaic

(PV)/wind system;

Quinary asymmetric inverter;

Coupled inductors;

Battery system;

Boost converter

Abstract This paper suggests quinary asymmetric inverter with coupled inductors and trans-

former, and uses it in hybrid system including photovoltaic (PV) and wind. This inverter produces

twenty-five-level voltage in addition to merits of multilevel inverter, has only one DC source. Then,

it is adequate for hybrid systems, which prevents increasing DC-link and makes control of system

easy. Proposed structure also provides isolation in the system and the switch numbers are reduced in

this topology compared with other multilevel structures. In this system, battery is used as backup,

where PV and wind have complementary nature. The performance of proposed inverter and hybrid

system is validated with simulation results using MATLAB/SIMULINK software and experimental

results based PCI-1716 data acquisition system.� 2015 Faculty of Engineering, Ain Shams University. Production and hosting by Elsevier B.V. This is an

open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

In recent years, renewable energies were expanded because offossil fuel consumption and greenhouse-effect gas emission

[1–3]. Research and development efforts in renewable energies

are required to accurately predict their output reliablyassembling them with other conventional sources. So, use ofbattery in such systems is beneficial solution for increase ofreliability and better operation of those systems. Meanwhile,

hybrid system including different types of renewable energieswas proposed [4,5]. Hybrid system in addition to advantagesof renewable energies, causes reduction in battery size and

increase in reliability of system [6,7]. The renewable energiesin hybrid system can be photovoltaic (PV), wind, fuel cell,etc. In this paper, PV and wind systems are used because of

complementary operation. In hybrid systems, an inverterbetween DC link and load provides ac voltage demand of load,which may have various types.

Nowadays, multilevel inverters receive more attention[8–10], which include some power semiconductors and DCvoltage sources. The most common multilevel inverter topolo-gies are the cascaded, diode-clamped, and capacitor-clamped

types [11–13]. Multilevel inverters with only one DC source

Nomenclature

IPV current of PV cell

VPV voltage of PV cellIph photocurrentIo diode saturation currentT cell temperature

K Boltzmann’s constantq coulomb constantA P–N junction ideality factor

RSmodseries resistance

G real solar radiationGr solar radiation

Iscrcell photocurrent standard test conditionsTr cell absolute temperatureki temperature coefficientNP number of parallel PV cell

NS number of series PV cellq air density

At area swept out by the turbine blades

Vx wind velocityCP power coefficientk tip speed ratiob blade pitch angel

R blade radiusx rotational speedL11 first coupled inductor

L12 second coupled inductorM mutual inductance of coupled inductorsTdwell dwelling time of switch

Vi voltage of DC sources in quinary asymmetric in-verter

NML maximum levels number in quinary asymmetric in-verter

Vout_max maximum generated voltage of quinary asymmet-ric inverter

580 A. Baghbany Oskouei et al.

may be the most desirable topology. Recently [14] presented aninverter called a five-level-active-neutral-point-clamped with

coupled inductor, which uses split of the DC-link capacitor.In [8] inverter was proposed using coupled inductors. Thisinverter can output a novel, single phase, five-level voltage

with only one DC source. Meanwhile, asymmetric structuresuse geometric progression for voltage of DC sources toincrease the output levels. Then, it is desirable to use of new

inverters in asymmetric structures for producing more voltagelevels in output. But increasing DC source numbers in asym-metric structures causes cost increase; also control of systemwith more DC links is more complex.

(a)

(b)

Figure 1 Electrical model of PV; (a) PV cell; (b) solar module.

This paper proposes quinary asymmetric inverter with cou-pled inductors and transformer, and uses it in hybrid system

including PV and wind systems. This inverter has one DCsource which makes it easy and usable in hybrid system.Theoretical analysis and simulation and experimental results

are presented to show the validity of the proposed inverterand proposed hybrid system.

2. Renewable energy systems

2.1. PV system

PV system is based on solar energy, where PV cell is the mostbasic generation part in PV. As Fig. 1(a) shows, the PV cell isformed from a diode and a current source was connected anti-

parallel with a series resistance [15–17].The relation of the current and voltage in the single-diode

cell can be written as follows:

IPV ¼ Iph � IO expqðVPV þ RSmod

IPVÞAKT

� �� 1

� �ð1Þ

Figure 2 Characteristic curves of a typical PV module in certain

solar irradiance and temperature.

Figure 3 A typical turbine output power versus rotor speed with the wind speed as a parameter.

Figure 4 Structures of single-phase five-level inverter with

coupled inductors.

Figure 5 Generation of the dwelling time.

Figure 6 Asymmetric inverter structure (V1 „ V2 „ � � �„Vn).

Hybrid PV/wind system with quinary asymmetric inverter 581

where IPV and VPV are the current and voltage of PV cellrespectively. And, Iph is photocurrent, Io is diode saturationcurrent, T is cell temperature, K is Boltzmann’s constant, q is

coulomb constant, A is P–N junction ideality factor, andRSmod

is series resistance. Photocurrent is directly proportional

to solar irradiation and cell temperature, which is described as

Iph ¼ Iscrcell þ kiðT� TrÞ� �

G=Grð Þ ð2Þ

Table 1 Output voltage of five-level inverter with coupled inductor

v12 +2E +E +E 0

On switches S1, S4, S6 S1, S4, S5 S1, S3, S6 S1, S3

where G is the real solar radiation, Gr, Iscrcell , Tr, and ki are the

solar radiation, photocurrent standard test conditions, cellabsolute temperature, and the temperature coefficient,respectively.

s.

0 �E �E �2E, S5 S2, S4, S6 S2, S4, S5 S2, S3, S6 S2, S3, S5

Figure 7 Proposed quinary asymmetric inverter structure.

Figure 8 Proposed configuration of hybrid system with battery.

582 A. Baghbany Oskouei et al.

Several same PV cells are arranged together in parallel andseries to form PV modules. This work is performed in order toincrease output voltage and current of PV system. The current

versus voltage of the PV module can be formulated as

IPV ¼ NpIph �NpIO expqðVPV þ RSmod

IPVÞNsAKT

� �� 1

� �ð3Þ

Fig. 1(b) shows PV modeling of module, where NP and NS

are number of the parallel and series PV cell, respectively.

For each module, two characteristic curves are defined:voltage–current (V–I) and voltage–power (V–P). These curvesare plotted based on solar irradiance and temperature. Fig. 2shows such curves for typical PV module. From Fig. 2, it is

obvious that in each solar irradiance and temperature, thevoltage, current, and power can have different mounts. But

there is one point with specific voltage and current which pro-duced maximum power [15]. This point is known as maximumpower point (MPP), and operation in it is known as maximum

power point tracking (MPPT).

2.2. Wind system

The mechanical power of a wind turbine is expressed as fol-

lows [18–20]:

Pmech ¼1

2qAtCPV

3x ð4Þ

where q is the air density, At is the area swept out by the tur-bine blades, Vx is the wind velocity, and CP is the power

Figure 9 Control loop of load voltage in hybrid system.

Table 2 Inverter parameters.

VDC 100 V

L11 = L12 = L 3.1 mH

M 3 mH

Nominal frequency 50 Hz

Load resistance 20 OLoad inductance 8.3 mH

(a)

(b)

(c)

Figure 10 Output voltage of proposed inverter with respect to time(s); (a) upper unit; (b) lower unit; (c) total.

Hybrid PV/wind system with quinary asymmetric inverter 583

(a)

(b)

Figure 11 Stepped variation of (a) solar irradiance; (b) temperature, with respect to time(s).

Figure 12 Output power of PV system and DC–DC converter with respect to time(s).

584 A. Baghbany Oskouei et al.

coefficient and represents the efficiency of the wind turbinewhich is expressed as

CP k; bð Þ ¼ 0:5176116

ki

� 0:4b� 5

� �e

�21ki

� �þ 0:006k ð5Þ

1

ki

¼ 1

kþ 0:08b� 0:035

b3 þ 1ð6Þ

where k is tip speed ratio, b is blade pitch angel (deg), R is theblade radius (m), and x is the rotational speed (rad/s). Thetorque obtained from wind energy is transferred via theturbine/generator shaft to the rotor of the generator and drives

the electrical generator, and in this paper the permanent mag-net synchronous generator (PMSG) is used in the output ofwind turbine.

Figure 13 MPPT operation of each PV module in hybrid system.

(a)

(b)

Figure 14 Wind system characteristics; (a) power characteristic curve of the wind turbine; (b) wind speed variation with respect to

time(s).

Hybrid PV/wind system with quinary asymmetric inverter 585

(a)

(b)

(c)

Figure 15 Results of wind system with respect to time(s); (a) rotor speed; (b) output power of turbine; (c) delivered power to DC link.

586 A. Baghbany Oskouei et al.

The power characteristic curve of a typical wind turbine isshown in Fig. 3. It is observed that the maximum power output

occurs at different turbine speeds for different wind velocities.

3. Proposed hybrid system

3.1. Proposed quinary asymmetric inverter with coupledinductors and transformer

Single-phase five-level inverter with coupled inductors and oneDC source was first proposed in [6], which is presented inFig. 4. In this structure no split of the DC capacitor is needed,

avoiding the voltage balancing. The level of the output voltageis half of the DC voltage in all switching conditions, causing

Table 3 Hybrid system parameters.

Load voltage

Load amount

Nominal frequency

Filter capacitance

Filter inductance

much reduced dv/dt. In this inverter, the voltage stresses onall the switches are the same. So, construction of it is easy.

In Fig. 4, 2E is the DC source voltage and L11 and L12 arethe two coupled inductors. Two coupled inductors have thesame number of turns and the mutual inductance of them isM. With attention to Fig. 4 and assuming L11 = L12 = L,

the voltage of coupled inductors can be derived as follows:

Ldibdt�M

dicdt¼ vbn � v2n ð7Þ

Ldicdt�M

dibdt¼ vcn � v2n ð8Þ

And according to Kirchhoff’s current law, the following isobtained:

220 V

Z1 = 35 + 5j

Z1 = 30 + 20j

Z1 = 26 + 25j

50 Hz

30 lF0.5 mH

Figure 16 DC-link voltage of hybrid system with respect to time(s).

Figure 17 Exchanged power between system and battery in proposed system with respect to time(s).

Hybrid PV/wind system with quinary asymmetric inverter 587

ib þ ic þ iL ¼ 0 ð9Þ

From 7–9, v2n can be expressed as

v2n ¼vbn þ vcn þ L�Mð Þ diL

dt

2ð10Þ

The leakage inductance can be designed to be near tomutual inductance of two inductors. Therefore, (10) can be

rewritten as

v2n ¼vbn þ vcn

2ð11Þ

(a)

(b)

(c)

Figure 18 The phase voltage of proposed inverter in hybrid system with respect to time(s); (a) upper unit; (b) lower unit; (c) total.

588 A. Baghbany Oskouei et al.

From Fig. 4 and (11), the output voltage of the inverter isobtained:

v12 ¼ v1n � v2n ¼ v1n �vbn þ vcnð Þ

2ð12Þ

So, the output voltage can have five levels, which are sum-marized in Table 1. Then, it is obvious that the switching state

of S1 is decided by the sign of v12-ref (the reference of v12): S1 is1 if v12-ref P 0 and S1 is 0 if v12-ref 6 0, which is very easy toimplement. However, the switching states of S3 and S5 cannot

be selected without careful study [8]. To decide the switchingstates of (S3, S5), the following PWM algorithm is presented:

The dwelling time for the switch switches within every sam-ple time, and Ts is defined as follows:

Tdwell ¼jv12-refj � Kvi� �

við13Þ

where the integer K is calculated by

K ¼ jv12-refjvi

� ð14Þ

The function [x] rounds x to the nearest integer less than x.Obviously, the dwelling time in (13) can be generated using the

conventional triangle wave which is shown in Fig. 5.Asymmetric structures use geometric progression for DC

source voltages to increase the output levels. Such a structure

produces output voltage with summation of series structuresvoltages with different DC sources, which is displayed in Fig. 6.

Progression factor in quinary asymmetric structure is 5. So,the voltage of DC sources can be written as follows:

Vi ¼ 5i�1� �

VDC; i ¼ 1; 2; 3; . . . ; n ð15Þ

Then, maximum levels number which inverter can be gener-ated is

NML ¼ 5n ð16Þ

and, the maximum generated voltage of inverter can be

obtained as

Vout max ¼1

25n � 1ð ÞVDC

2ð17Þ

According to a bow description, it is obtained that two ser-

ies five-level inverter with quinary asymmetric structure createstwenty-five-level inverter. But, increasing DC sources numberin quinary structure causes cost increase, also control of system

with more DC links is more complex. Then use of transformerin the proposed structure is the solution of represented draw-backs. New proposed structure is shown in Fig. 7. FromFig. 7, two transformers have different turn ratios according

to (15). Then, the turn ratio of first transformer is 1:1, andthe turn ratio of second transformer is 1:5. Use of transformerin this structure, in addition to decreasing DC source number

to one, causes reliability increase and isolation. Using coupledinductor in this structure causes switches number to reduce.Also, the voltage stresses on all the switches on each inverter

section are the same. Then, the switching of two blocks of

(a)

(b)

Figure 19 Output parameters of the load with respect to time(s); (a) load voltage; (b) load power.

Hybrid PV/wind system with quinary asymmetric inverter 589

proposed inverter is based on PWM, which is explained later,

where each block is switched independently and based on out-put level.

3.2. Proposed hybrid system with proposed inverter

The configuration of hybrid system is shown in Fig. 8, wherePV connected to the DC-link through the DC–DC converter

and PMSG wind turbine is connected to the same DC-link

through the diode rectifier and DC–DC converter, and also,battery is connected to it by DC–DC converter. Because oflow voltage generated from PV and wind systems, the boost

DC–DC converter is preferred. The boost converter of PV sec-tion does MPPT of PV system and, in wind section, the dioderectifier acts as the DC source, and its output is controlled by

boost converter to apply MPPT of wind system. The battery

Figure 20 MPPT operation of wind system.

590 A. Baghbany Oskouei et al.

system is used as backup for unpredictable nature of PV andwind. But, in high DC-link voltage, it is necessary to seriesconnection of multiple batteries and connecting them to

DC-link. The other solution is use of bidirectional boost con-verter. This battery with its converter momentarily absorbs orinjects excess power corresponding to abrupt change in the PV

and wind systems. Then, an inverter between DC-link and loadprovides ac voltage demand of load. The inverter may havevarious types. In this paper, it is used from proposed 25-levelinverter, where because of its multilevel nature, it has merits

of multilevel inverter. Also, the presence of one DC source isvery suitable for this hybrid system, where control of onecapacitor voltage is very easy and does not have complexity

of several capacitors voltage as source of multilevel inverter.This structure also provides isolation in the system. The con-trol loop of hybrid system is presented in Fig. 9.

4. Simulation and experimental results

For investigating the proposed structure, results of system are

studied. In this paper, the results include two sections: simula-tion and experimental as follows.

4.1. Simulation results

4.1.1. Proposed inverter

Inverter shown in Fig. 7 has been simulated by

MATLAB/SIMULINK to verify proposed topology. Thesystem parameters with RL load are listed in Table 2.Fig. 10 presents output voltage of the inverter.

4.1.2. Hybrid system with proposed inverter

The proposed hybrid system is simulated, and the results of itare expatiated as follows. Different MPPT algorithms have

been developed. In this paper, the perturbation and observa-tion (P&O) algorithm with the merit of simplicity is used.

4.1.2.1. PV system. In this paper, 10 numbers of 250 W PV

module (in nominal condition of G = 1000 W/m2 andT = 25 �C) are paralleled, where it is considered steppedvariation for solar irradiance and temperature in Fig. 11.The output powers of PV system and DC–DC converter are

shown in Fig. 12. From Fig. 12, it can be seen that there is dif-ference between those powers, which is because of systemlosses. The boost converter increases PV voltage up to DC link

level. MPPT operation of each PV module is presented inFig. 13, where it is obvious that utilization of each PV moduleis almost in MPPT points.

4.1.2.2. Wind system. In this paper, the power characteristiccurve of the wind turbine is shown in Fig. 14(a). The windspeed variation is assumed stepped according to Fig. 14(b).

The rotor speed, output power of turbine, and delivered powerto DC link are presented in Fig. 15.

4.1.2.3. Hybrid system. As represented, the output power of PVand wind systems is transferred to DC-link and then theDC-link power is delivered to load. The parameters of simulated

system are listed in Table 3, where the load is unbalanced.Fig. 16 displaysDC-link voltage which is fixed in 64 V, althoughunbalancing of the load causes little fluctuation in the voltage.

In this system, the battery system is used and its voltageincreases with bidirectional boost converter. The exchangedpower between system and battery is shown in Fig. 17. If theexchanged power is positive, the battery system compensates

deficiency of hybrid power, and if the exchanged power is neg-ative, the excess power is stored in battery.

Then, DC-link voltage is transmitted from inverter and

delivered to ac load. The used inverter in this paper, as repre-sented, is proposed quinary inverter with coupled inverters andtransformer which generate 25-level voltage. Fig. 18 shows the

phase voltage of proposed inverter in this system. The loadvoltage and power are displayed in Fig. 19. Also, Fig. 20 showscorrect MPPT operation of wind system.

(a)

(b)

Figure 21 Experimental results; (a) photograph of prototype; (b) load voltage.

Hybrid PV/wind system with quinary asymmetric inverter 591

4.2. Experimental results

Base segment of proposed inverter according to Fig. 4 was

built using IRFP460 MOSFETS as the switching devices andMUR820G as fast diodes in this topology. A DC voltagesource with amplitude 12 (V) was used to individually supply

the inverter. The switching signals that were obtained fromPWM algorithm were produced with PCI-1716 DAQ. Theswitching signals were interfaced with the inverter powerswitches through driver TLP250. Fig. 21(a) shows pho-

tographs of the prototype. The load voltage of the inverter isshown in Fig. 21(b). Measured five levels are 0, ±6, ±12.

Load voltage total harmonic distortion (THD) was measuredwith power analyzer and it is about 38.2%.

5. Conclusion

This paper proposes hybrid system including PV and wind sys-tems suggested quinary asymmetric inverter with coupledinductors and transformer. This inverter outputs twenty-five-

level voltage with only one DC source. This structure has suchmerits as easy control and isolation, and uses battery systembackup, where PV and wind have complementary nature.

The simulation results showed that the proposed inverter

592 A. Baghbany Oskouei et al.

produces a high-quality load voltage and it demonstrates itsability in hybrid system. Also, the experimental results verifiedthe proportionality of proposed inverter.

Acknowledgement

This research has been supported by Azarbaijan ShahidMadani University.

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Aida Baghbany Oskouei was born in Tabriz,

Iran, in 1988. She received the B.Sc. degree in

electrical engineering from Azarbaijan Shahid

Madani University, Tabriz, Iran, in 2010 and

the M.Sc. degree in electrical engineering from

the University of Tabriz, Tabriz, Iran, in 2012.

She is currently working toward the Ph.D.

degree in electrical engineering at Azarbaijan

Shahid Madani University. Her current

research interests include modeling and

application of power electronic circuits in

power systems and renewable energies.

Mohamad Reza Banaei was born in Tabriz,

Iran. He received his M.Sc. degree from the

Poly Technique University of Tehran, Iran, in

control engineering in 1999 and his Ph.D.

degree from the electrical engineering faculty

of Tabriz University in power engineering in

2005. He is an Associate Professor in the

Electrical Engineering Department of

Azarbaijan Shahid Madani University, Iran,

which he joined in 2005. His main research

interests include the modeling and controlling

of power electronic converters, renewable energy, modeling and con-

trolling of FACTS and Custom Power devices and power systems

dynamics.

Mehran Sabahi was born in Tabriz, Iran, in

1968. He received the B.S. degree in electronic

engineering from the University of Tabriz, the

M.S. degree in electrical engineering from

Tehran University, Tehran, Iran, and the

Ph.D. degree in electrical engineering from the

University of Tabriz, in 1991, 1994, and 2009,

respectively. In 2009, he joined the Faculty of

Electrical and Computer Engineering,

University of Tabriz, where he was an

Assistant Professor from 2009 to 2013 and

where he has been an Associate Professor since 2014. His current

research interests include power electronic converters and renewable

energy systems.