analysis and design of a photovoltaic system

4354 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 11, NOVEMBER 2009

Analysis and Design of a Photovoltaic SystemDC Connected to the Utility With a

Power Factor CorrectorYu-Kang Lo, Member, IEEE, Huang-Jen Chiu, Senior Member, IEEE, Ting-Peng Lee,

Irwan Purnama, and Jian-Min Wang, Member, IEEE

Abstract—This paper presents a photovoltaic (PV) system par-allel connected to an electric power grid with a power factor cor-rector (PFC) for supplying the dc loads. The operation principlesand design considerations for the presented PV system are ana-lyzed and discussed. The balanced distribution of the power flowsbetween the utility and the PV panels is achieved automatically byregulating the output dc voltage of the PFC. Experimental resultsare shown to verify the feasibility of the proposed topology, whichcan effectively transfer the tracked maximum power from the PVsystem to the dc load, while the unity power factor is obtained atthe utility side.

Index Terms—Grid connection, maximum power point tracking(MPPT), power factor correction.

I. INTRODUCTION

DUE TO the increasingly detrimental greenhouse impactson the living conditions of human beings, the develop-

ment of renewable energy has become a serious worldwide con-cern [1], [2]. Consequently, substantial efforts are being exertedto cut down the emission of carbon dioxide. Likewise, varioussolutions to the greenhouse effects are being considered. A va-riety of renewable energies, such as wind power, fuel cells, tidalenergy, geothermal energy, biomass energy, and solar cells,have been widely utilized and advocated. Among these energyresources, the photovoltaic (PV) system is considered one of themost appropriate and primary renewable energies, owing to thefollowing features: 1) It has abundant sources; 2) it is clean andpollution free; and 3) it does not have any ostensible pitfall suchas noise from wind shear while using wind turbines nor does itemit any by-products such as carbon dioxide which is the by-product of biomass energy. A conventional grid-connected PVsystem is shown in Fig. 1(a) [3]–[5]. In this system, the load isfed by the utility and a PV inverter that is parallel connectedat the ac side. The power capacity of the PV system is notlimited by load conditions because the excessive energy canbe fed back to the utility. This is the main advantage of thistopology. However, this system presents some drawbacks whenapplied to dc loads. These can be summarized as follows.

1) Islanding protections are necessary [6].

Manuscript received December 31, 2008; revised August 6, 2009. Firstpublished August 28, 2009; current version published October 9, 2009.

Y.-K. Lo, H.-J. Chiu, T.-P. Lee, and I. Purnama are with the Department ofElectronic Engineering, National Taiwan University of Science and Technol-ogy, Taipei 10607, Taiwan (e-mail: [email protected]).

J.-M. Wang is with the Department of Vehicle Engineering, NationalFormosa University, Yunlin 632, Taiwan (e-mail: [email protected]).

Digital Object Identifier 10.1109/TIE.2009.2030216

Fig. 1. Topologies of (a) a conventional grid-connected PV system,(b) a hybrid PV system, and (c) the proposed grid-connected PV system.

2) The control algorithm and topology are complicated.3) For a dc load, the power needs to be transferred through

a two-stage (dc/ac and ac/dc) converter. Both the cost andconversion efficiency of the grid-connected PV systemhave to be carefully taken into account.

In order to diminish the influences of atmospheric conditionsand provide sufficient power to the dc load, the auxiliary powersources such as the rechargeable batteries and the fuel cells [7]

0278-0046/$26.00 © 2009 IEEE

LO et al.: ANALYSIS AND DESIGN OF PV SYSTEM DC CONNECTED TO UTILITY WITH PFC 4355

Fig. 2. Schematic diagram of the proposed parallel-connected PV system.

are required in most PV system applications, as shown inFig. 1(b) [8]–[10]. The main function of auxiliary powersources is to supply energy to the dc load on rainy or cloudydays. However, the battery bank has some disadvantages,namely, limited life cycles, high costs, bulky size, possibleenvironmental pollution, and safety consideration. Based on theaforementioned drawbacks, a multi-input converter has beenproposed with power factor correction and maximum powerpoint tracking (MPPT) [11], which used the utility sourceto substitute auxiliary power sources to achieve power man-agement between two different energy sources. Nevertheless,the presented power management is too complicated, and itscost has also increased due to the adoption of two full-bridgeconverters and a transformer. To simplify the aforementionedtopology and reduce the cost for supplying dc load systems,a new grid-connected PV system is proposed in this paper, asshown in Fig. 1(c). The utility source is parallel connected withthe PV system through a power factor corrector (PFC) and anMPPT tracker. The proposed PV system can be used in someapplications such as LED traffic lights, street lights, and otherdc loads which always consume higher constant power than thePV system can offer under normal operation conditions. It fea-tures less power loss and lower circuit cost by means of parallelconnection at the dc side. In the following section, the opera-tion principles and steady-state analysis of the proposed grid-connected PV system are discussed. Furthermore, the designconsiderations and experimental results of a prototype systemthat is capable of supplying a 1-kW load power are presented.

II. OPERATION PRINCIPLES AND

STEADY-STATE ANALYSIS

The schematic diagram of the proposed grid-connected PVsystem is shown in Fig. 2. It is observed that an MPPT converterand a PFC drawing power from solar panels and the utility,respectively, are parallel connected at the dc side. The maxi-mum power point tracker is a dc/dc power electronic converter

Fig. 3. Control flowchart of the proposed PV system.

inserted between the PV module and its load to achieve opti-mum power matching [12]–[15]. Commonly used nonisolateddc/dc converter topologies [16]–[18] include the buck, boost,and buck–boost converters. The presented PV system in thispaper is parallel connected with the PFC. The output voltageof the PFC is set higher than that of the solar cell. Therefore,the boost converter is adopted as the MPPT in the proposedsystem. In Fig. 2, Microchip’s PIC16F877 microcontroller isprogrammed to achieve the MPPT function [19], [20].

Maximum power transfer and power factor correction canbe simultaneously achieved by using the proposed control


Fig. 4. Discussed PV system.

Fig. 5. Proposed PV system parallel connected with a PFC fed by the utility.

algorithm. The output voltage and current of the solar panelsVpv and Ipv are sensed to perform the MPPT function. Fig. 3shows a control flowchart of the proposed PV system. The dcoutput voltage Vo is sensed to determine the operation mode ofthe PV system. When Vo is lower than its high threshold voltageVo,h, it is regulated by the PFC controller to keep at its nominalvalue Vo,n. As long as the PV array outputs current, the MPPTcontrol function is enabled. When there is no current suppliedby the PV array at night, the MPPT function is disabled, and thedc load is solely fed by the utility. On the other hand, Vo rises toreach its high threshold voltage Vo,h when the maximum powerproduced by the PV array is higher than the load power. TheMPPT function will be disabled, and the load power is solelyprovided by the PV array. PI control is adopted by the PVconverter for regulating the dc output voltage Vo to retain atthe threshold voltage Vo,h. When the load power is changed tobe higher than the maximum power of the PV array, Vo willdrop to its nominal value Vo,n and be regulated by the PFCcontroller again. Based on the atmospheric and load conditions,the proposed grid-connected PV system has the following threeoperation modes.

1) Mode I. At night, the solar panels’ output power willbe reduced due to the insufficient sun light. The outputcurrent of solar panels is measured to determine whetherthe MPPT function is disabled or not. During this mode,the dc load is solely fed by the utility with a nearly unitypower factor at the ac side.

2) Mode II. In this mode, the dc load, which consumeshigher power than the PV system can offer, is fed simul-taneously by the utility and PV system. The dc output

voltage Vo is regulated by the PFC controller to keep atits nominal value Vo,n. The MPPT algorithms are used toensure that the maximum power from the PV system istransferred to the dc load. The utility then supplies part ofthe dc load power with a nearly unity power factor at theac side.

3) Mode III. In this mode, the maximum power producedby the PV array is higher than the load power. TheMPPT function of the PV system is disabled, and theload power is solely provided by the PV system. The dcoutput voltage Vo is regulated by the PV converter withPI control to retain at the threshold voltage Vo,h.

Fig. 4 shows some important circuit variables of the dis-cussed grid-connected PV system. The steady-state voltagegain of the MPPT boost converter under the continuous con-duction mode is used to analyze the relations between thetransferred power and the duty cycle. By neglecting the circuitparasitics and defining the PV arrays’ output resistance as Rth,the following can be obtained:

Rth =Vpv

−Ipv(1)

Ri =Vo(1 − D)

Io

1−D

= Ro(1 − D)2. (2)

From (2), it is known that the input resistance of the boostconverter Ri can be tuned by varying the MPPT duty cycle.According to the maximum power transfer theory, as long asRi is equal to the solar panels’ output resistance Rth, the solar


Fig. 6. Typical I–V and P –V characteristics of a solar cell.

Fig. 7. Sign of dP/dV at different positions on the P –V curve.

Fig. 8. ACC of the boost PFC.

TABLE ISPECIFICATIONS OF THE PROPOSED PV SYSTEM

panels can send out its maximum power to the load. Thatis, at that duty cycle, the MPPT function of the PV array isfulfilled.

Fig. 5 shows the presented PV system parallel connectedwith a PFC fed by the utility. The equivalent output resistance

TABLE IISPECIFICATIONS OF THE PV PANEL AT 25 ◦C

TABLE IIICIRCUIT PARAMETERS AND COMPONENT VALUES

Fig. 9. Measured waveforms at (a) Pmpp = 180 W and (b) Pmpp = 450 W.


Fig. 10. Measured waveforms of (a) MPPT and (b) PFC converters in mode I(Po = 100 W).

of the MPPT Rop is now expressed as

Rop =Vo

Ip=

Vpfc

Io − Ipfc. (3)

Therefore, from (2), the equivalent input resistance of theMPPT Rip is now

Rip = Rop(1 − D)2 =Vpfc(1 − D)2

Io − Ipfc. (4)

It can be observed in (4) that the dc-bus voltage can beregulated by the PFC. By tuning the duty cycle of the MPPT, thePV system can still produce its available maximum power whenRth is equal to Rip. If this available maximum output poweris less than the consumed load power, which is related to theoperation modes I and II as stated earlier in this section, then theload power is supplied simultaneously by both the PV systemand the PFC. According to the amount of the PV system’smaximum output power, which is proportional to the amplitudeof Ip, the dc-bus voltage may tend to increase or decrease.The PFC then starts to extract the excessive power from orsupply the insufficient power to the load by regulating its outputvoltage. When the sum of Ipfc and Ip is equal to the required dcload current Io, a balanced power distribution can be reached.On the other hand, should the maximum output power of the

Fig. 11. Measured waveforms of (a) MPPT and (b) PFC converters in mode I(Po = 1000 W).

PV system be larger than required, which is related to operationmode III, the dc-bus voltage will increase. Then, the PFC keepslowering its output power while trying to regulate its output dcvoltage. Once the PFC is operated at no load, the only way toregulate the dc-bus voltage is to shut down the MPPT functionof the PV system. At this stage, the required load power is solelyprovided by the PV system.

III. DESIGN CONSIDERATIONS

The PV array consists of many multimodule series- andparallel-connected unit cells. Fig. 6 shows the I–V and P–Vcharacteristics of a solar cell. It is seen that the output powerof a solar cell is not proportional to the voltage. Moreover, itis a function of temperature and irradiance intensity. In Fig. 6,Voc and Isc are the open-circuit voltage and the short-circuitcurrent, respectively, of the solar cell. Vmpp and Impp are thevoltage and current at its maximum peak power (MPP) Pmpp.The MPPT is achieved when the slope of the operating point onthe P–V curve is zero.

Well-developed MPPT algorithms include the powerfeedback method, the perturbation and observation (P&O)method, the incremental conductance (I&C) method, thethree-point weight comparison P&O method, the fuzzy logiccontrol, and the neural network approach [21]. Even thoughthe P&O method is relatively simple, a perturbation will cause


Fig. 12. Measured waveforms of (a) MPPT and (b) PFC converters inmode II (Pmpp = 180 W).

the sway at the maximum output power point [22, 23]. In orderto improve the defect in the P&O method, the I&C methodis used to achieve MPPT in the proposed system. The I&Cmethod is based on the zero-slope feature at the maximumoutput power revealed on the solar cell’s P–V curve. It is alsoobserved that positive slopes appear on the left of the MPP andnegative slopes on the right. Fig. 7 shows the change in the signof the slope on the P–V curve [24], [25]. The partial derivativeof the cell power P with respect to the cell voltage V can beexpressed as

dP

dV=

d(I × V )dV

= V × dI

dV+ I. (5)

When dP/dV = 0, (5) can be rearranged as follows:

dI

dV= − I

V. (6)

From (5) and (6), the MPP can be tracked by determining theimmediate conductance I/V and the I&C dI/dV .

The boost PFC converter is popularly used in achieving ahigh input power factor due to the following advantages [26]:1) It is easily controlled; 2) the electromagnetic interferenceis reduced because the input current of a boost converter iscontinuous; 3) the current spike will be suppressed by the input

Fig. 13. Measured waveforms of (a) MPPT and (b) PFC converters inmode II (Pmpp = 450 W).

inductor; and 4) it can be used in the high-power application.The adopted controller IC, Unitrode UC3854 [27], is operatedunder average current-mode control (ACC) [28], [29]. TheACC features a fixed switching frequency and lower inputcurrent harmonics. Fig. 8 shows the ACC used in a boost PFC[30]–[34]. The main purpose of the ACC is the use of a two-loop compensation to achieve input current waveform shapingand output voltage regulation.

IV. EXPERIMENTAL RESULTS

A laboratory prototype for the proposed grid-connected PVsystem was built to perform some experiments. The specifica-tions of the proposed PV system are listed in Table I. Table IIgives the electrical characteristics of the PV panels.

In this paper, the PV array is composed of three parallelstrings of four 75-W PV panels in series. A solar simulatorAgilent E4360A with 1.2-kW rated power is also used in theconverter design stage. Table III shows the circuit parametersand component values of the prototype system. Fig. 9(a) and(b) shows the related waveforms of the presented PV system atmaximum powers of 180 and 450 W, respectively. It is obviousthat the MPPT performance under the I&C method can befulfilled.

At night, the load power is fed along by the utility throughthe PFC. The MPPT function of the PV system is disabled.


Fig. 14. Measured waveforms of (a) MPPT and (b) PFC converters inmode III.

Figs. 10 and 11 show the related waveforms under the loadpower of 100 and 1000 W, respectively, in mode I. Unity powerfactor can be achieved at the utility side, and the measuredconversion efficiency is above 93%.

As the sun rises, the MPPT begins to produce the maximumsolar power. The load is simultaneously fed by the utility andthe MPPT. Figs. 12 and 13 show the related waveforms of theproposed PV system with 1000-W load power. In Figs. 12(a)and 13(a), the maximum output powers of the MPPT are180 and 450 W, respectively. Figs. 12(b) and 13(b) show thecorresponding PFC waveforms. It can be observed that theinsufficient load power is provided by the utility while the PVpanel is operated at its maximum power point.

When the load level is reduced below the maximum power ofthe PV panel, the system enters mode III. The MPPT functionof the PV system will be disabled, and the power point of thePV panel will be shifted from MPP. The load power will besolely provided by the PV panel. Fig. 14 shows the relatedwaveforms of the proposed PV system under the load powerof 110 W. The original maximum PV power is 480 W. Fig. 15shows the measured transient waveforms during the transitionbetween different operation modes. It can be observed that thereis a small output voltage fluctuation during such mode changes.During modes I and II, the dc output voltage Vo is regulatedby the PFC controller to keep at its nominal value Vo,n of200 V. During mode III, Vo is regulated by the PV converter

Fig. 15. Measured transient waveforms for (a) mode I → mode II,(b) mode II → mode III, and (c) mode III → mode I.

with the PI control to retain at the threshold voltage Vo,h

of 210 V.

V. CONCLUSION

In this paper, we have studied a PV system that is parallelconnected to an electric power grid with a PFC for supplyingthe dc loads. Based on the atmospheric and load conditions, theproposed grid-connected PV system has three operation modes.The balanced distribution of the power flows between the utilityand the PV panels can be achieved automatically by regulatingthe output dc voltage of the PFC. A laboratory prototype wasbuilt and tested. The experimental results were shown to verifythe feasibility of the proposed topology.


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Yu-Kang Lo (M’96) was born in Chia-Yi,Taiwan, in 1969. He received the B.S. and Ph.D.degrees in electrical engineering from NationalTaiwan University, Taipei, Taiwan, in 1991 and 1995,respectively.

Since 1995, he has been with the faculty of the De-partment of Electronic Engineering, National TaiwanUniversity of Science and Technology, Taipei, wherehe is currently a Professor and in charge of the PowerElectronics Laboratory and the Power ElectronicsTechnology Center. His research interests include the

design and analysis of a variety of switch-mode power converters and powerfactor correctors.

Dr. Lo is a member of the IEEE Power Electronics and IEEE IndustrialElectronics Societies.


Huang-Jen Chiu (M’00–SM’09) was born inI-Lan, Taiwan, in 1971. He received the B.E. andPh.D. degrees in electronic engineering from the Na-tional Taiwan University of Science and Technology,Taipei, Taiwan, in 1996 and 2000, respectively.

From August 2000 to July 2002, he was an As-sistant Professor in the Department of ElectronicEngineering, I-Shou University, Kaohsiung, Taiwan.From August 2002 to July 2006, he was withthe Department of Electrical Engineering, Chung-Yuan Christian University, Chung-Li, Taiwan. Since

August 2006, he has been with the Department of Electronic Engineering,National Taiwan University of Science and Technology, Taipei, where he iscurrently a Professor. In 2009, he joined the Future Energy Electronic Center,Virginia Polytechnic Institute and State University, Blacksburg, as a VisitingProfessor. His research interests include renewable energy conversion, high-efficiency LED drivers, soft-switching techniques, electromagnetic compatibil-ity issues, power factor corrector topologies, and electronic ballasts.

Dr. Chiu is a Senior Member of the IEEE Power Electronics Society. He wasthe recipient of the Young Researcher Award in 2004 from the National ScienceCouncil, Taiwan.

Ting-Peng Lee was born in Taipei, Taiwan, in 1982.He received the M.S. degree from the Department ofElectronic Engineering, National Taiwan Universityof Science and Technology, Taipei, Taiwan, in 2007,where he is currently working toward the Ph.D.degree.

His research interests include design and develop-ment of photovoltaic (PV) systems, grid-connectedPV systems, DSP control applications, and electroniccircuit design.

Irwan Purnama was born in Bandung, Indonesia,in 1978. He received the B.S. degree in physicsfrom Universitas Padjadjaran, Bandung, in 2002.He is currently working toward the M.S. degree inthe Department of Electronic Engineering, NationalTaiwan University of Science and Technology,Taipei, Taiwan.

From December 2002 to July 2008, he was a Su-pervisor of an electrical laboratory in the TechnicalImplementation Unit for Instrumentation Develop-ment (UPT BPI), Indonesian Institute of Sciences

(LIPI). From September 2007 to July 2008, he was a Teaching Assistantin the Department of Physics, Universitas Padjadjaran. His research interestsare in instrumentation and measurement, renewable energy conversion, powerelectronics, and control systems.

Jian-Min Wang (M’09) was born in Kaohsiung,Taiwan, in 1976. He received the M.S. and Ph.D.degrees in electronic engineering from the Na-tional Taiwan University of Science and Technology,Taipei, Taiwan, in 2001 and 2007, respectively.

Since February 2008, he has been an AssistantProfessor in the Department of Vehicle Engineering,National Formosa University, Yunlin, Taiwan. Hisresearch interests include the design and analysisof photovoltaic inverters, arc welding machines, andpower converters.

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