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Solar Energy 84 (2010) 2056–2067

A grid-connected photovoltaic power conversion systemwith single-phase multilevel inverter

Ersoy Beser, Birol Arifoglu, Sabri Camur, Esra Kandemir Beser ⇑

Department of Electrical Engineering, Kocaeli University, Umuttepe Campus, 41380 Kocaeli, Turkey

Received 7 May 2010; received in revised form 26 September 2010; accepted 28 September 2010Available online 20 October 2010

Communicated by: Associate Editor Nicola Romeo

Abstract

This paper presents a grid-connected photovoltaic (PV) power conversion system based on a single-phase multilevel inverter. The pro-posed system fundamentally consists of PV arrays and a single-phase multilevel inverter structure. First, configuration and structuralparts of the PV assisted inverter system are introduced in detail. To produce reference output voltage waves, a simple switching strategybased on calculating switching angles is improved. By calculated switching angles, the reference signal is produced as a multilevel shapedoutput voltage wave. The control algorithm and operational principles of the proposed system are explained. Operating PV arrays in thesame load condition is a considerable point; therefore a simulation study is performed to arrange the PV arrays. After determining thenumber and connection types of the PV arrays, the system is configured through the arrangement of the PV arrays. The validity of theproposed system is verified through simulations and experimental study. The results demonstrate that the system can achieve lower totalharmonic distortion (THD) on the output voltage and load current, and it is capable of operating synchronous and transferring powervalues having different characteristic to the grid. Hence, it is suitable to use the proposed configuration as a PV power conversion systemin various applications.� 2010 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic; Photovoltaic power conversion; Grid-connected; Single-phase multilevel inverter

1. Introduction

Recently, renewable energy resources have been becom-ing popular due to the decrease of fuel sources and theirdamages to the environment. As a result of the negativeeffects of global warming and climate changes, the interestof renewable resources has been increased gradually.

Solar energy is one of these alternative energy resources.It is converted to the electrical energy by photovoltaic (PV)arrays. PV arrays do not generate any toxic or harmfulsubstances that pollute the environment (Kang et al.,2005a,b). Another considerable feature of them is the

0038-092X/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2010.09.011

⇑ Corresponding author. Tel.: +90 2623033466; fax: +90 2623033003.E-mail addresses: ebeser@kocaeli.edu.tr (E. Beser), barif@kocaeli.

edu.tr (B. Arifoglu), scamur@kocaeli.edu.tr (S. Camur), esrakandemir@kocaeli.edu.tr (E.K. Beser).

requirement of low maintenance. Depending on the devel-opment in photovoltaic technologies, the efficiency of thePV arrays has been improved. Therefore, studies on PVsystems have increased gradually.

PV systems are occasionally operated in stand-alonemode and they feed fixed loads by stand-alone PV inverters(Myrzik, 2001; Kang et al., 2005a,b; Daher et al., 2008;Lalouni et al. 2009; Saravana Ilango et al., 2010). PV sys-tems are also interconnected to the grid. Interconnectinga PV system to the grid has been the popular design trendand grid-connection types of PV inverters have been pro-posed (Calais et al., 1999; Myrzik, 2001; Kuo et al., 2001;Alonso et al., 2003; Yu et al., 2005; Wu et al., 2005; Patcha-raprakiti et al., 2005; Lee et al., 2008; Hassaine et al., 2009;Rahim et al., 2010). Therefore various power electronicstechnologies are improved to convert the dc to ac powerfor PV applications.

Nomenclature

Ck capacity of the kth capacitor group (lF)f grid frequency (Hz)I inverter current (A)IPM maximum power current value (A)Ls inductance of the self inductor (H)m number of the level modulesn output level numberPLMk power of the kth level module (W)Pref reference real power (W)P0 system output power (W)Qk(t) the kth switching signalQref reference reactive power (VAr)Rs resistance of the self inductor (X)S sum of the energy ratiost instantaneous time value (s)tmax maximum value of the sample time (s)tsample value of the sample time (s)Vb voltage of the base level module (V)Vbus bus voltage (V)

Vbusref the reference bus voltage (V)VCk voltage of the kth capacitor group (V)Vg grid voltage (V)Vmax maximum value of the required voltage (V)VPM maximum power voltage value (V)Vref reference voltage (V)Vz voltage of the self inductor (V)V0 output voltage (V)WCk energy of the kth capacitor group (J)WLMk energy of the kth level module (J)XLs reactance of the self inductor (X)Zs impedance of the self inductor (X)Dt time between ti and ti+1 (s)d angle between V0 and Vg (�)dref reference angle between Vref and Vg (�)h the angle between Rs and XLs (�)u power angle (�)x angular speed (rad s�1)

E. Beser et al. / Solar Energy 84 (2010) 2056–2067 2057

In addition, it is important to operate PV energy conver-sion systems near the maximum power point to increase theoutput efficiency of PV arrays (Kuo et al., 2001; Alonsoet al., 2003; Yuvarajan et al., 2004; Yu et al., 2005; Patcha-raprakiti et al., 2005; Lee et al., 2008). Thus, power elec-tronics inverters are required for maximum power pointtracking (MPPT) algorithm, which provides maximumPV power. They are also needed for transferring the PVpower to a load or to the grid.

Multilevel inverters are suitable choices for realizing thisobjective. Various multilevel inverter topologies have beenintroduced and studied in the literature. The most consid-erable of these types are the diode clamped, the flyingcapacitor, the cascaded H-Bridge, the magnetic coupledand the full bridge with cascaded transformers inverters.The remarkable feature of these inverters is generating lessharmonic components on both output voltage and loadcurrent. By increasing the number of output levels, thequality of the output voltage and load current is increasedstep by step (Calais et al., 1999; Rodriguez et al., 2002;Kang et al., 2005a,b; Daher et al., 2008; Rahim et al.,2010; Beser et al., 2010). Due to the production of less har-monic components, the PV power is transferred to the loador to the grid in a high-quality form by multilevel inverterstructures.

Related to these developments, this study presents a PVassisted multilevel inverter system for the conversion of PVpower to the electrical power. The proposed structure ofthe inverter system is quite suitable for the use of PVarrays. Owing to the use of a multilevel inverter structure,more sinusoidal shaped output voltage waves are obtained.Therefore, the THD of the output voltage is considerablyreduced. Placement of the PV arrays in the system is the

significant point of the study because of loading the arraysin the same condition. To accomplish this, a simulationstudy is performed and the number and connection typesof the PV arrays in the system are determined. Accordingto the order of the PV arrays a configuration is formedfor the PV assisted inverter system. The system is simulatedwhile operating synchronous and transferring variouspower values having different characteristics to the grid.The validity of the proposed system is also verified throughan experimental study. The measured THD values of theoutput voltage and current are quite low. The systemprovides good performance as a PV energy conversionsystem.

2. Proposed PV assisted single-phase multilevel inverter

system

2.1. Configuration and structural parts of the proposed

system

Fig. 1 shows the base configuration of the proposed PVassisted multilevel inverter. It consists of PV modules, levelmodules (LM) and a conventional H-Bridge module. Thebase configuration of the multilevel inverter generates a7-level shaped output voltage wave. However, the pro-posed system can be easily expanded and the number ofoutput voltage levels is increased by adding level modulesto the system (Beser et al., 2010).

The proposed inverter structure provides an advantagein point of switching element number compared to someinverter types in the literature. A switch number compari-son related to output level number (n) is made between dif-ferent inverter types in Table 1.

Fig. 1. Configuration of the proposed PV assisted single-phase multilevel inverter system.

Table 1Switch number comparison related to output level number (n).

Inverter type Switch number

Diode clamped 2(n � 1)Flying capacitor 2(n � 1)Cascaded H-Bridge 2(n � 1)Magnetic coupled 4 log3(n)Full-bridge with cascaded transformer 4 log3(n � 2) + 4Proposed 2 log2(n + 1) + 2

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It can be seen from Table 1 that 60 switches are used indiode clamped, flying capacitor and cascaded H-Bridgeinverters to obtain 31-level output voltage. The magneticcoupled inverter type uses 12 switches and shapes 27-levelvoltage wave. The full bridge with cascaded transformerinverter type uses 16 switches and forms 29-level shapedoutput voltage. However, the proposed inverter achieves31-level output voltage by using only 12 switches.

Another considerable feature of the proposed inverter isthat the system configuration allows operating regenera-tive. So this feature provides to transfer both active andreactive power to the grid.

2.1.1. PV module

Fig. 2 shows the configuration of the PV module. It canbe seen from Fig. 2 that it consists of PV arrays and acapacitor group.

Fig. 2. Configuration of the PV module.

Depending on the increase of the output level number,PV arrays are suitably connected serially and PV moduleis expanded according to the number of level modules.The configuration of the expanded PV module is shownin Fig. 3.

The voltage between the points Pk and Nk is the voltageof serially connected PV arrays. Therefore the voltage ofthe capacitor group should be chosen in respect of thePV module voltage. The voltage of the capacitor groupVCk is expressed as

V Ck ¼ 2ðk�1ÞV b ð1Þ

k ¼ 1; 2; 3; . . . ;m ð2Þ

2.1.2. Level module

Level module (LM) consists of two switching devicesand a dc source input. The points Pk and Nk in the dcsource input are connected to output of the PV module.

Fig. 3. Configuration of the serially connected PV arrays in the PVmodule.

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The first level module (LM1) is named as the base levelmodule. The configuration of the level module is illustratedin Fig. 4.

2.1.3. H-Bridge module

Conventional H-Bridge structure is used in the proposedsystem. The numbers of output voltage levels can beincreased by varying the number of PV and level modules.However, no modification is made in the structure of theH-Bridge module. Therefore, H-Bridge module is definedas the stable part of the proposed system (Beser et al.,2010). The configuration of the H-Bridge module is shownin Fig. 5. It can be seen from Fig. 5 that the grid connectioncan be made between the points A and B. A load can alsobe connected to the points A and B, and thus the system isoperated in stand-alone mode.

2.2. Switching strategy

Switching strategy in the PV assisted inverter structure isto generate gate signals by calculating switching angles. Inorder to calculate switching angles, first, the reference out-put voltage Vref is determined as follows:

Fig. 4. Configuration of the level module.

Fig. 5. Configuration of the H-Bridge module.

V ref ¼ V max sinðxt þ dref Þ ð3Þ

A sample reference output voltage wave is illustrated inFig. 6.

Dt seen in Fig. 6 gives the sample time of the switchingsignals. The maximum value of the sample time is relatedto the frequency of the reference output voltage and thelevel module number. In Eq. (4) the maximum value ofsample time (tmax) is given

tmax ¼ sin�1 1

2ðmþ1Þ � 2

� �� ð2pf Þ�1 ð4Þ

tsample ¼ Dt ¼ tiþ1 � ti ð5Þtsample � tmax ð6Þ

During operation, the sample time (tsample) should bechosen smaller than tmax. The smaller tsample is chosen,the better results are obtained for the output voltage. Thus,the output voltage more closely resembles the reference sig-nal. But the speed of the microcontroller in the system canmake a lower limitation for tsample. According to the micro-controller speed, tsample is chosen as tmax/10 in this study.Vref value at any time is taken from the curve and usedin the switching equations. The switching equations areexpressed as

Q1ðtÞ ¼ V ref ðtÞ mod 2 ð7Þ

Q2ðtÞ ¼V ref ðtÞ� V ref ðtÞ mod 2

� �2

� �mod 2 ð8Þ

By using the equations, the switching signals areobtained. Switching equations can be generalized and thegeneral switching function related to the level module num-ber is defined as follows:

QkðtÞ ¼V ref ðtÞ� V ref ðtÞ mod 2ðk�1Þ� �

2ðk�1Þ

� �mod 2 ð9Þ

By using Eq. (9), the switching signals are obtained forthe proposed multilevel inverter structure including fourlevel modules. Therefore, the reference output voltage wavein Fig. 6 is configured as a 31-level shaped output voltagewave. The switching signals and the simulated output volt-age wave are given in Figs. 7 and 8, respectively.

Switching strategy changes related to PV module andlevel module number in the system. In order to obtain more

Fig. 6. A sample reference output voltage wave.

Fig. 7. Switching signals in the proposed multilevel inverter including fourlevel modules.

Fig. 8. 31-Level shaped output voltage of the proposed multilevel inverterincluding four level modules.

Fig. 10. Output voltage waves for various amplitudes and frequencies inthe multilevel inverter.

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quality output voltage, module number is increased. So,the PV arrays are suitably connected serially in a PV mod-ule to obtain suitable dc voltage values. In this case theswitching strategy changes. However, if the system poweris required to increase, PV arrays are connected parallelin the PV module. The switching strategy does not changeat this time.

The proposed system is able to produce output voltagewaves having different amplitude. The amplitude of theoutput voltage is easily regulated without modifying thenumber of level modules. So, the system can easily toleratethe changes on the load. The multilevel inverter structure isalso capable of producing output voltage waves having dif-ferent frequency. Experimental output voltage waves forvarious amplitudes and frequencies are shown in Figs. 9and 10, respectively.

Fig. 9. Output voltage waves for various amplitudes in the multilevelinverter.

3. Control algorithm and operational principles of the

proposed system

A control algorithm is improved for generating the ref-erence signals at the inverter output and transferring PVpower to the grid. A simplified electrical circuit of the PVassisted system can be drawn as in Fig. 11.

A self inductor is used between the multilevel inverteroutput and the grid. The impedance of the inductor canbe defined as

Zs ¼ Rs þ jX Ls ¼ jZsj\h ð10Þ

Beside the determination of the reference signals, thealgorithm achieves MPPT feature, which provides maxi-mum utility from the PV power. Fig. 12 shows the controlblock diagram of the proposed PV assisted inverter system.

For determining the reference real power Pref, DC busvoltage Vbus is first subtracted from the reference voltageVbusref, and then, the error is evaluated by the controllerand Pref is generated. To operate the system for therequired power factor, the reactive power Qref is enteredto the control system by the user. Therefore, energy trans-ferring is realized for the required power factor. Thecurrent I and the power angle u are derived from the Eq.(11) and Eq. (12) as follows:

u ¼ a tanQref

P ref

� �ð11Þ

I ¼ P ref

V g cos uð12Þ

Then, voltage of the self inductor Vz is calculated as

V z ¼ I\u � Zs\h ¼ jV zj\ðuþ hÞ ð13Þ

Fig. 11. Simplified electrical circuit of the PV assisted system.

Fig. 12. Control block diagram of the proposed PV assisted inverter system.

Fig. 14. I–V and P–V characteristic of the PV arrays.

Table 2Specifications of PV arrays.

Rated power (PM) 5 WMaximum power voltage (VPM) 17.1 VMaximum power current (IPM) 0.30 AOpen-circuit power (VOC) 21.7 VShort-circuit current (ISC) 0.31 A

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and the reference output voltage Vref and the reference an-gle dref comments can be determined by adding Vz to thegrid voltage Vg as

V ref\dref ¼ V g\0þ V z\ðuþ hÞ ð14Þ

The calculated Vref and dref values are used in Eq. (9)and switching signals are obtained as it has been mentionedin Section 2.

Control algorithm is easily provided by a PIC18F452microcontroller in the digital control unit. A principlescheme of the digital control unit is shown in Fig. 13.Owing to zero crossing sensing (ZCS) circuit, zero crossingpoints of the Vg are determined. The points are sensed anda timer is operated by the microcontroller. By the datataken from the timer output, a sinus shaped output voltagewhich is synchronous to the grid is obtained. The outputvoltage can be shifted forward or backward by the angledref. Vg, Vbus and I are sampled by the analogue inputs ofthe microcontroller. By these data, Vref and dref arecalculated and switching signals are obtained for theswitching devices. Thus, the multilevel inverter structuregenerates the output voltage based on the switching signals.

By means of the proposed control algorithm, PV arraysare operated at the maximum power transfer point. Thisfeature is provided by the embedded MPPT algorithm inthe system. The MPPT procedure does not require anyextra equipment, it consists of an algorithm. The algorithm

Fig. 13. Principle scheme of

tracks the maximum power transfer voltage (VPM). Vbusref

in the control algorithm represents the maximum powertransfer voltage (VPM).

the digital control unit.

Fig. 15. Calculation of the level module energy.

Fig. 16. Calculation of the load energy.

Fig. 17. Simulation results of the transferred output power in theproposed system including four level modules where P1 = 960 W,Q1 = 290 VAr and P2 = 540 W, Q2 = 180 VAr.

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4. Arrangement of the PV arrays in the system

In the proposed system, setting only the voltage of thedc sources is not sufficient for operating the system. Whenthe system is arranged only according to the voltage values,PV arrays are not loaded in the equal load conditions.Some of them are forced to operate over capacity andthe others are forced to operate under capacity. Overloaded PV arrays can not transfer the power and outputvoltage of the PV arrays decreases. On the other hand,under loaded PV arrays can not generate the rated power

Table 3Transferred energy to the load and energy ratios of the level modules for vari

PV array side

WLM1

(J)WLM2

(J)WLM3

(J)WLM4

(J)WLMT

(J)WLM2/WLM1

WLM3/WLM1

0.488 1.116 2.549 5.897 10.049 2.288 5.2280.975 2.230 5.097 11.790 20.092 2.288 5.2301.946 4.457 10.190 23.572 40.165 2.291 5.2370.299 0.686 1.568 3.629 6.182 2.292 5.2410.847 1.938 4.430 10.248 17.463 2.289 5.231

Table 4Arrangement of PV arrays and calculated capacity values in the PV modules

LM Level module capacity PV connection ty

Label Capacity (mF) Voltage (V) Series Paral

LM1 C1 30,000 25 1 10LM2 C2 17,000 50 2 12LM3 C3 10,000 100 4 13LM4 C4 6000 200 8 15

value and output voltage of the arrays increases. This situ-ation can be seen from the I–V and P–V characteristic ofthe PV arrays in Fig. 14. As a result, required voltage levelsare not provided for the multilevel inverter and a sinusoidalshaped voltage wave can not be generated. To prevent thisproblem, the required energy transferred by each levelmodule and the connection types of the PV arrays shouldbe determined. To determine the number and arrangementof the PV arrays in the PV modules, a simulation study isfirst implemented. The specifications of each PV array aregiven in Table 2.

System output power (P0) is determined as 1 kW and PVarrays are placed into the PV modules with respect to theoutput power. In order to determine the amount of thetransferred energy to the load by each dc source in levelmodules, the multilevel inverter system is simulated for var-

ous load conditions in the proposed system including four level modules.

Load side

WLM4/WLM1

WL

(J)PL

(W)QL

(VAr)Series load

R

(W)XL

(W)XC

(W)

12.093 10.048 502 0 65.76 – –12.099 20.089 1004 0 32.88 – –12.115 40.157 2008 0 16.44 – –12.129 6.180 309 464 32.88 49.32 –12.100 17.460 873 �338 32.88 – 12.73

for the proposed inverter system including four level modules.

pe Total PV number Total PV number (W) LM voltage (V)

lel

10 50 17.124 120 34.252 260 68.4

120 600 136.8

Fig. 18. Simulation results of the inverter output voltage and invertercurrent in the proposed PV assisted multilevel inverter: (a) P1 = 960 W,Q1 = 290 VAr and (b) P2 = 540 W, Q2 = 180 VAr.

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ious loads which have a different value and characteristic(resistive, inductive and capacitive). To determine the pow-ers obtained from the level modules, dc sources are usedinstead of the PV arrays in the simulation study. It is seenthat the transferred energy of each level module (WLMk) tothe load is different from one another (WLM1 – WLM2 –WLM3 – WLM4). The calculation of the level module andload energy is shown in Figs. 15 and 16, respectively.

In addition, to determine the energy ratio of the levelmodules ðW LM2

W LM1; W LM3

W LM1; W LM4

W LM1Þ, the energy of each level module

is divided by the energy of the base level module (LM1)and a ratio is obtained for the dc source in each level mod-ule. It is determined that the ratio between LM1 and theother level modules does not change related to characteris-tic and value of the load, each ratio is calculated as a differ-ent constant value for a specific level module number.Table 3 shows the transferred energy to the load andenergy ratios of the level modules for various loadconditions.

In order to determine the PV power in the level modules,the sum of the energy ratios of the level modules are firstobtained as

S ¼ 1þ W LM2

W LM1

þ W LM3

W LM1

þ W LM4

W LM1

ð15Þ

and then power values of the base level module and theother level modules are calculated as follows

P LM1 ¼P 0

Sð16Þ

P LMk ¼ P LM1

W LMk

W LM1

ð17Þ

In this study, PLM1 is calculated as 48 W for the outputpower 1 kW and accepted as 50 W. PLM2, PLM3 and PLM4

are obtained as 114 W, 260 W and 604 W, respectively andthey are accepted as 120 W, 260 W and 600 W.

According to the calculated values, PV number and con-nection types are determined and PV arrays are placed intothe level modules. Therefore, each PV array is operated inthe same load condition and the required voltage levels areachieved for the inverter structure.

The current value transferred from the PV arrays variesin relation to the instantaneous magnitude of the sinusshaped current and on/off conditions of the PV arrays.On/off conditions of the PV modules are given in Fig. 7for a period of time. Due to their characteristic, PV arrayscan not realize power transfer in case of exceeding the max-imum power current (IPM) value. When the PV arrays arefirst connected to the system, a current value higher thanIPM is occurred and PV arrays can not realize power trans-fer at this duration. This problem can be solved by usingproper capacitors in the PV module. The capacitors com-pensate the instantaneous over currents and prevent thePV array from the short-circuit situation. The capacitorsare charged from the PV arrays in case the PV arrays donot feed the system. However, they are discharged, whenthe PV arrays are connected to the system. Voltage ripples

occurring during the charging and discharging (V1 � V2)are directly related to the transferred energy from thecapacitors. The transferred energy by the capacitor groupis defined as

W ck ¼1

2Ck V 2

1 � V 22

� �ð18Þ

The required energy is considered for a period of time inthe simulation study and the required capacity values arecalculated by Eq. (18). Table 4 shows the arrangement ofPV arrays and calculated capacity values in the PV mod-ules for the proposed inverter system including four levelmodules.

5. Simulated and experimental results and discussion

After determining the situation of PV arrays, the pro-posed PV power conversion system was simulated andimplemented. It was first operated synchronous to the grid(110Vac, 50 Hz) and two different states were performed inthe simulation study. First, approximately all of the gener-ated power was transferred, and then, less than the gener-ated power was transferred to the grid. The parameters

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Rs = 0.39 X and Ls = 4 mH are used for the self inductor inthe simulation and experimental study. Fig. 17 shows sim-ulation result of the transferred real power and reactivepower values for two different states (P1, Q1; P2, Q2).The simulation results of the voltage and current wave-forms at the power values P1, Q1 and P2, Q2 is shown inFig. 18a and b.

If the power is not transferred as much as the generationof the PV arrays, voltages of the capacitors approach theopen circuit voltage of the PV arrays and the system cannot be operated at the maximum power transfer point. Incase of transferring the power to the grid as much as thePV arrays generate, output voltage of the PV arraysdecreases to the maximum power voltage (VPM). Simula-tion results of the voltages of the capacitors are shown inFig. 19 for transferring approximately all of the generatedpower and less power than generated.

It can be seen from Fig. 19 that when the less power (P2)than generated is transferred, the capacitor voltages reachhigher values than the maximum power voltage and thesystem diverges from the maximum power transferringpoint. In case of transferring approximately all of the gen-erated power (P1) by the PV arrays, the capacitor voltagesreach to maximum power voltage value. As it can be seenfrom Fig. 19, the voltage of base level module is obtainedas 17.4 V. It can be also seen from Table 2 that the real

Fig. 19. Simulation results of the capacitor voltages in the PV

maximum power voltage value (VPM) of the PV arrays is17.1 V. The simulated and real values are so close to eachother. Therefore, the base level module operates at themaximum power transfer point. The voltages of the othermodules are calculated as 35.8 V, 70 V and 140 V in thesimulation study. These values are approximately equalto 2VPM, 4VPM and 8VPM. Thus, it is observed that theother level modules operate at the maximum power trans-fer point.

After the simulation study, experimental tests wererealized by multilevel inverter structure. The proposedinverter system includes four level modules, an H-Bridgemodule, a digital control unit and zero crossing sensingcircuit. PIC18F452 microcontroller is preferred in the dig-ital control unit. A maximum of 31-level output voltagewaveform can be obtained by six level modules. IRFP460mosfets (500 V, 20 A) are used in level and H-Bridgemodules.

The proposed system was tested in the laboratory condi-tions. Similar to the simulation study, the system was con-nected to the grid (110Vac, 50 Hz) in synchronous modeand power transfer is realized. The THD values of the out-put voltages and currents are also measured by a harmonicanalyzer. Experimental wave forms are shown in Figs. 20and 21 for different operation points, respectively. Mea-sured THD values are given in Table 5.

modules while transferring the powers P1, Q1 and P2, Q2.

Fig. 20. Experimental results of inverter output voltage and grid voltagewhile operating synchronous to the grid for different operation points: (a)Vg = 114 V, P = 405 W, Q = 68 VAr, d = 0; (b) Vg = 113 V, P = 415 W,Q = �806 VAr, d = 10; (c) Vg = 113 V, P = 105 W, Q = �525 VAr,d = �10; and (d) Vg = 76 V, P = 295 W, Q = 185 VAr, d = 0.

Fig. 21. Experimental results of inverter output voltage and current whileoperating synchronous to the grid for different operation points: (a)Vg = 114 V, P = 405 W, Q = 68 VAr, d = 0; (b) Vg = 113 V, P = 415 W,Q = �806 VAr, d = 10; (c) Vg = 113 V, P = 105 W, Q = �525 VAr,d = �10; and (d) Vg = 76 V, P = 295 W, Q = 185 VAr, d = 0.

E. Beser et al. / Solar Energy 84 (2010) 2056–2067 2065

Grid voltage (Vg) and inverter output voltage (V0) wavesare given in Fig. 20. The curves are so close to each other.This shows that the proposed system operates synchronousto the grid.

It can be seen from the experimental results that therequired real power and reactive power values are trans-ferred to the grid in sinusoidal form as mentioned in the

proposed control algorithm. In addition Figs. 20d and21d show that the system can operate for the different gridvoltage values. Hence, the system is flexible and capable ofoperating for different points.

According to the IEEE 519-1992 standard, THD valueof the voltages under 69 kV should not be over 5%. Table5 shows that THD value of the output voltage is acceptablefor the IEEE standard in each operation point.

Table 5Measured THD values for different operation points.

P (W) Q (VAr) Delta (�) Grid Multilevel inverter

Voltage (V) THD (%) Voltage (V) THD (%) Current (A) THD (%) Efficiency (%)

405 68 0 114 1.87 114 1.86 3.66 5.03 97.79415 �806 10 113 1.71 112 3.36 8.25 4.94 97.87105 �525 �10 113 1.28 117 2.4 4.69 6.34 97.67295 185 0 76 2.01 80 3.28 4.55 3.36 92.57458 �1090 10 131 1.81 119 3.37 9.08 6.45 98.75452 �565 �10 141 1.39 143 1.83 5.07 5.07 99.00

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6. Conclusions

A PV power conversion system based on a single-phasemultilevel inverter was proposed in this paper. The config-uration of the proposed system was designed first, andthen, the system was simulated and implemented. The sim-ulated and experimental results are presented andexplained. Presentable results of the proposed system aresummarized as follows:

(1) The inverter structure in the proposed system pro-duces multilevel shaped output voltage waveforms.Therefore, it reduces dv/dt stresses imposed on theswitching devices and generates less harmonic com-ponents on the output voltage and current.

(2) The inverter can be easily expanded by increasinglevel modules. Thus, number of the output levels isincreased and the inverter generates higher-qualityoutput voltage waveforms.

(3) The switching strategy is considerably simple. Byonly using Vref and dref, switching signals are easilydetermined.

(4) A control algorithm is improved for generating thereference signals at the inverter output and transfer-ring PV power to the grid. A considerable advantageof the control algorithm is being realized by a simplemicroprocessor structure. By means of the algorithm,the proposed system operates near the maximumpower point. Thus, the system does not require anyMPPT unit as the algorithm achieves MPPT feature.

(5) To include PV arrays into the system suitably, a sim-ulation study was made and PV arrays were arrangedfor the inverter system. Owing to this configuration,each PV arrays are loaded in equal conditions. As aresult, the required voltage levels are achieved forthe inverter structure.

(6) The proposed system was verified through simula-tions and experimental study. The system was oper-ated in synchronous mode and the PV power wastransferred to the grid. It is seen that the results areconsiderably similar to the reference signals.

(7) It is seen from the experimental results that the THDvalues of the output voltage and current are quitelow. Thus, it can be said that the transferred powerto the grid is in good quality.

(8) These results show that the proposed system can suit-ably achieve the transfer of the PV power to the grid.Consequently, this study provides an efficientemployment of PV energy since there is a shortageof the energy resources and an increasing importanceof renewable energy resources in these days.

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