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1070 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 3, MAY 2007

Multi-Input Inverter for Grid-ConnectedHybrid PV/Wind Power System

Yaow-Ming Chen, Senior Member, IEEE, Yuan-Chuan Liu, Shih-Chieh Hung, and Chung-Sheng Cheng

Abstract—The objective of this paper is to propose a novelmulti-input inverter for the grid-connected hybrid photovoltaic(PV)/wind power system in order to simplify the power systemand reduce the cost. The proposed multi-input inverter consists ofa buck/buck-boost fused multi-input dc–dc converter and a full-bridge dc–ac inverter. The output power characteristics of thePV array and the wind turbine are introduced. The perturbationand observation method is used to accomplish the maximumpower point tracking algorithm for input sources. The operationalprinciple of the proposed multi-input inverter is explained. Thecontrol circuit is realized by using a digital signal processor andauxiliary analog circuits. For practical applications, functions ofsoft-start and circuit protection are implemented. Experimentalresults have shown the performance of the proposed multi-inputinverter with desired features.

Index Terms—Inverter, photovoltaic (PV), wind energy.

I. INTRODUCTION

APPLICATIONS with photovoltaic (PV) energy and windenergy have been increased significantly due to the rapid

growth of power electronics techniques [1]–[3]. Generally, PVpower and wind power are complementary since sunny days areusually calm and strong winds are often occurred at cloudy daysor at nighttime. Hence, the hybrid PV/wind power system there-fore has higher reliability to deliver continuous power than ei-ther individual source [4], [5]. Traditionally, a substantial energystorage battery bank is used to deliver the reliable power and todraw the maximum power from the PV arrays or the wind turbinefor either one of them has an intermittent nature [6]. However,the battery is not an environmental friendly product because ofits heavy weights, bulky size, high costs, limited life cycles, andchemical pollution. Therefore, it is very common to utilize thesolar or wind energy by connecting them to the ac mains directly.

Usually, two separated inverters for the PV array and the windturbine are used for the hybrid PV/wind power system [7]. Analternative approach is to use the multi-input inverter for com-bining these renewable energy sources in the dc end instead ofthe ac end. It can simplify the hybrid PV/wind power systemand reduce the costs.

The objective of this paper is to propose a novel multi-inputinverter for grid-connected hybrid PV/wind power system. The

Manuscript received January 18, 2005; revised April 10, 2006. This paper waspresented at the IEEE Applied Power Electronics Conference and Exposition,Austin, TX, 2005. Recommended for publication by Associate Editor Z. Chen.

Y.-M. Chen is with the Elegant Power Application Research Center,(EPARC), Department of Electrical Engineering, National Chung ChengUniversity, Chia-Yi 621, Taiwan, R.O.C. (e-mail: [email protected]).

Y.-C. Liu is with the NuLight Technology Company, Tainan 741, Taiwan,R.O.C.

S.-C. Hung is with Anpec Co., Ltd., Hsinchu 886, Taiwan, R.O.C.C.-S. Cheng is with Richtek Technology Co., Hsinchu 302, Taiwan, R.O.C.Digital Object Identifier 10.1109/TPEL.2007.897117

proposed multi-input inverter has the following advantages:1) power from the PV array or the wind turbine can be deliveredto the utility grid individually or simultaneously, 2) maximumpower point tracking (MPPT) feature can be realized for bothsolar and wind energy, and 3) a large range of input voltage varia-tion caused by different insolation and wind speed is acceptable.

II. OPERATION PRINCIPLE OF THE PROPOSED

MULTI-INPUT INVERTER

The schematic diagram of the proposed multi-input inverteris shown in Fig. 1. It consists of a buck/buck-boost fused multi-input dc–dc converter and a full-bridge dc/ac inverter. The inputdc voltage sources, and , are obtained from the PVarray and the rectified wind turbine output voltage. By applyingthe pulse-with-modulation (PWM) control scheme with appro-priate MPPT algorithm to the power switches and , themulti-inputdc–dcconvertercandrawmaximumpower fromboththe PV array and the wind turbine individually or simultaneously.The dc bus voltage, , will be regulated by the dc/ac inverterwith sinusoidal PWM (SPWM) control to achieve the input-output power-flow balance. Details of the operation principlefor the proposed multi-input inverter are introduced as follows.

A. PV Array

The PV array is constructed by many series or parallel con-nected solar cells [8]. Each solar cell is form by a – junctionsemiconductor, which can produce currents by the photovoltaiceffect.Typicaloutputpowercharacteristiccurvesfor thePVarrayunder different insolation are shown in Fig. 2. It can be seen that amaximum power point exists on each output power characteristiccurve. Therefore, to utilize the maximum output power from thePV array, an appropriate control algorithm must be adopted [9].

B. MPPT Algorithm

Different MPPT techniques have been developed [10]–[12].Among these techniques, the perturbation and observation(P&O) method with the merit of simplicity is used in this paper.The perturbation of the output power is achieved by periodicallychanging (either increasing or decreasing) the controlled outputcurrent. The objective of the P&O method is to determine thechanging direction of the load current. Fig. 3 shows the flowchart of the MPPT algorithm with P&O method for the pro-posed multi-input inverter. Since there are two individual inputsources, each one of them needs an independent controller.However, both of the controllers can be implemented by usingone integrated controller.

At the beginning of the control scheme, the output voltageand output current of the source (either the PV array or thewind turbine) are measured, then the output power can be

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Fig. 1. Schematic diagram of the proposed multi-input inverter.

Fig. 2. Typical output power characteristic curves of the PV array under dif-ferent insolation.

calculated. By comparing the recent values of power and voltagewith previous ones, the P&O method shown in the flow chartcan determine the value of reference current to adjust the outputpower toward the maximum point.

C. Wind Turbine

Among various types of wind turbines, the permanent-magnet synchronous wind turbine, which has higher reliabilityand efficiency, is preferred [13]–[16]. The power of thewind can be derived as

(1)

where is the air density (kg/m ), is the area (m ) sweptby wind blades, and is the wind speed (m/s). It had beenproven that the energy conversion efficiency, , of the windturbine is a function of the tip speed ratio, , which is definedas

(2)

where is the rotational speed (rad/sec) of wind turbine blades,is the radius of the area swept by wind turbine blades. A typical

Fig. 3. Flow chart of the MPPT algorithm with P&O method.

curve of the versus is shown in Fig. 4, where the max-imum value of is only achieved at a particular tip speed ratio.Since the speed of the wind is not constant, the rotational speedof the wind turbine must be adjustable to ensure a constant tipspeed ratio to gain the maximum . The output current changeof the wind turbine will cause of the rotational speed as well as

to change. Since is a function of , the output powerof the wind turbine will change, too. Therefore, by controllingthe output current of the wind turbine, the rotational speed of the



Fig. 4. Typical curve of the C versus � .

Fig. 5. Conceptual block diagram of the experimental setup for wind powergeneration system.

wind turbine blades can be adjusted to achieve the appropriatetip speed ratio. Eventually, the maximum value of can beobtained and the maximum power can be transferred from theairstream to the wind turbine to produce the maximum electricalpower.

For the convenience of experiment, instead of natural wind,a controllable dc motor is used to drive the wind turbine to sim-ulate the actual operation situation under the natural air-stream.Fig. 5 shows the conceptual block diagram of the experimentalsetup for wind power generation system. For different dc voltageprovided by the dc motor controller, the dc motor will receive alimited maximum power to drive the directly coupled wind tur-bine. When the output power of the wind turbine is small, thedc motor will request small power from the dc source to drivethe wind turbine. When the output current of the wind turbineis increased, the output voltage and the rotational speed will bedecreased. Also, the dc motor needs to provide a larger torque tothe wind turbine. Since the dc motor has limited maximum inputpower predetermined by the control box, it can only providelimited maximum torque to the wind turbine which can onlygenerate a limited maximum power to the load. Fig. 6 showstypical output power characteristic curves of the wind turbineunder different driving power from the controllable dc motor.These curves have same characteristics with those driven by thenatural air-stream. Each one of the curves represents a constantdriving power from the dc motor. The output power of the windturbine is drawn by an electronic load. The load current is grad-ually increased, and then the output power can be measured.Output power characteristic curves shown in Fig. 6 imply thatthe wind turbine will generate different maximum output powerfor different wind speed. Because the output power character-istic curves of the wind turbine are similar to those of the PVarray shown in Fig. 2, the P&O method is adopted as the MPPT

Fig. 6. Typical output power characteristic curves of the wind turbine for dif-ferent wind speed.

Fig. 7. Equivalent circuits for the multi-input dc–dc converter. (a) Mode I.(b) Mode II. (c) Mode III. (d) Mode IV.

algorithm for the wind turbine. Therefore, a power electronicconverter with appropriate controller is needed to process thewind energy which varies considerably according to the meteo-rological conditions such as wind speed.

D. Multi-Input DC–DC Converter

The proposed multi-input dc–dc converter is the fusion of thebuck-boost and the buck converter [17]. Syntheses of the multi-input dc–dc converter are done by inserting the pulsating voltagesource of the buck converter into the buck-boost converter. Inorder not to hamper the normal operation of the buck-boost con-verter and to utilize the inductor for the buck converter, the pul-sating voltage source of the buck converter must be series-con-nected with the output inductor.

Base on the conduction status of the switches and ,the multi-input dc–dc converter has four operation modes.Fig. 7(a)–(d) show the equivalent circuits for Mode I throughMode IV, respectively. When switches or are turnedoff, diodes and will provide a free-wheeling path forthe inductor current. If one of the voltage sources is failed, theother voltage can still provide the electric energy, normally.Therefore, it is very suitable for renewable energy applications.Details of the circuit operation principle can be found in [17].



Fig. 8. Conceptual control block diagram of the proposed multi-input inverter.

The input-output voltage relationship can be derived from thesteady-state volt-second balance analysis of the inductor. Ifhas longer conduction time than has, then the equivalent op-eration circuit for one switching cycle will follow the sequenceof Mode I, Mode III, and Mode IV. On the other hand, ifhas longer conduction time than has, the sequence becomesMode II, Mode III, and Mode IV. In either case, the outputvoltage can be expressed as

(3)

where and are the duty ratio of switches and ,respectively. Similarly, the average input and output current canbe obtained

(4)

(5)

From the above derived steady-state voltage and current equa-tions, different power distribution demands of the multi-inputdc–dc converter can be achieved.

E. Control Scheme

The conceptual control block diagram of the proposed multi-input inverter is shown in Fig. 8. The hardware implementa-tion of the control circuit is realized by using a central control

unit, digital signal processor (DSP) TMS320F240, and auxil-iary analog circuits. The sensed voltage and current values forthe PV array and the wind turbine are sent to the DSP where theMPPT algorithm will determine the reference current and

for the PV array and the wind turbine. The PWM Com-parator1 and Comparator2 will generate desired gate signals forpower switches and according to the current error sig-nals and , respectively.

The dc/ac inverter will inject a sinusoidal current into the acmains. The SPWM gate signals of switches throughfor producing sinusoidal ac current is generated by the DSPwhere the amplitude of the ac current is determined by the errorsignal of the measured dc bus voltage and the referenceone, . If the measured dc bus voltage is less than the ref-erence value, then the amplitude of the ac output current will bedecreased in order to increase the dc bus voltage. On the con-trary, if the dc bus voltage is higher than the reference one, thenthe amplitude of the ac output current will be decreased. On theother point of view, the dc bus voltage is regulated by the dc–acinverter and the input-output power balance can be achieved.

For practical operation considerations, functions of soft-start, over-voltage protection, over-current protection, andunder-voltage protection are all realized by the controller, too.Since both of the input currents for the PV array and the windturbine is controlled by the MPPT algorithm with P&O method,the starting current is gradually increased and the soft-startfunction of the input current is naturally obtained. Also, duringthe starting transition, the ac output current for the utility line islimited by the reference current command and the small amountof input power. Therefore, the soft-start demand for the outputcurrent is achieved naturally. In order to control the proposedmulti-input inverter properly, the central control unit, DSP,need to sense the input voltages, input currents, dc bus voltage,output voltage, and output current, continuously. Therefore, noextra sensor is needed to realize these protection functions.

III. HARDWARE RESULTS

To verify the performance of the proposed multi-input in-verter for grid-connected hybrid PV/wind power system shownin Fig. 1, a prototype with the specifications, shown at thebottom of the page, is implemented. The PV array is formedby 24 series-connected Solarex Maga SX-60 solar panels whilethe Bergey BWC 1500 is used for the wind turbine. Each PVarray has peak power of 60W and open voltage of 21V. Thewind turbine is a three-phase permanent magnetic generatorwith rated power of 1.5 kW at rated wind speed of 12.5 m/s.

Input voltages V DC

V DC

DC bus voltage V DC

Output voltage V AC Hz

Output power kW

Switching frequency kHz and kHz



Fig. 9. Measured waveforms of gate driving signals, v and v , and in-ductor current i .

Fig. 10. Measured waveforms of voltage, current, and power for the PV arrayand the wind turbine.

The prototype of the multi-input inverter is built and testedstep by step. First, the multi-input dc–dc converter with MPPTfeature is tested. Fig. 9 shows the measured waveforms of gatedriving signals, of and of , and inductor cur-rent . Because switches and have different duty ra-tios, the inductor current has two different charging slopes. Itreveals that the multi-input dc–dc converter can deliver powerfrom both of the two energy sources to the dc bus simultane-ously. The measured waveforms of voltage, current, and powerfor the PV array and the wind turbine are shown in Fig. 10. InFig. 10, the PV array is first connected to the multi-input dc–dcconverter. Output current of the PV array is then increased grad-ually because of the MPPT control algorithm with the P&Omethod. Eventually, the output power of the PV array will reachits maximum power point and stay around that place. The max-imum output power of the PV array is very close to the pre-determined one where the power is drawn by using the elec-tronic load. On the other hand, the wind turbine is connectedto the multi-input dc–dc converter, too. The output current andvoltage waveforms of the wind turbine are very similar to thePV array’s. Due to the measurement limitation of the oscillo-scope, the power waveform for the wind turbine is not shown inFig. 10. However, Fig. 10 shows that the MPPT feature for thePV arrays and the wind turbine are both achieved.

The incoming power from the PV array or/and the wind tur-bine will cause the rise of the dc bus voltage. The value of dcbus voltage is regulated by adjusting the amplitude of the acoutput current. If the value of the dc bus voltage is higher thana pre-set range, then the ac output current of the dc/ac inverterwill be increased in order to lower the dc bus voltage. On thecontrary, if the dc bus voltage is smaller than the pre-set range,the amplitude of the ac output current must be reduced to boostup the dc bus voltage. Fig. 11 shows the waveforms of the dc

Fig. 11. Measured waveforms of the dc bus voltage and the ac output current.

bus voltage and the ac output current. It can be seen thatthe input-output power balance of the proposed multi-input in-verter can be achieved. Also, the dc bus voltage ripple pat-tern and the envelope of the ac output current are related sincethe dc bus voltage is regulated by controlling the magnitudeof the ac output current.

Fig. 12(a) shows the waveforms of the dc bus voltage and theac output current when only the PV array is connected to theproposed multi-input inverter. At the beginning, the controllerwill send out the PWM gate signal with MPPT feature to switch

when the utility line voltage is detected. Once the dc busvoltage reaches its pre-set range, the dc/ac inverter will beginto inject ac output current into the utility line. Since the inputpower is not a constant, a small variation of the ac output currentamplitude can be found in Fig. 12(a). The ac output voltageand current waveforms of the proposed multi-input inverter areshown in Fig. 12(b). Using Power Analyzer Voltech 100, themeasured power factor is 0.99 and the total harmonic currentdistortion is less than 3%. Fig. 13(a) and (b) show the similarresults when only the wind turbine is connected to the proposedmulti-input inverter. The measured power factor is 0.99 and thetotal harmonic current distortion is less than 3.5%. Because theresponse of the wind turbine is slower than the PV array’s, it hasrelatively larger dc bus ripple voltage.

Fig. 14(a) shows the waveforms of the dc bus voltage and theac output current when both of the PV array and the wind turbineare connected to the proposed multi-input inverter. A stable acoutput current with small variation can be obtained when bothof the input sources reach their MPPT condition. The ac outputvoltage and current waveforms are shown in Fig. 14(b), wherethe measured power factor is 0.99 and the total harmonic currentdistortion is about 4.5%. The experimental results clearly showthat the proposed hybrid PV/wind power system can draw powerfrom the PV array and the wind turbine individually or simul-taneously with MPPT feature. Also, almost unity power factorand very low harmonic current distortion can be achieved.

For practical applications, many circuit protection functionsare included in the hardware realization. Some of the testingwaveforms are shown in Fig. 15(a)–(c). The testing waveformsfor under-voltage and over-voltage protection are shown inFig. 15(a). The proposed multi-input inverter can inject acoutput current when the value of the dc bus voltage is insidethe pre-set range. Outside this range, there will be no ac outputcurrent. It should be noticed that there is an initial value forthe ac output current reference signal, . Fig. 15(b) showsthe testing waveforms for the utility line fault protection. The



Fig. 12. Waveforms of the proposed multi-input inverter when only the PV array is supplying power. (a) The dc bus voltage and the ac output current waveforms.(b) The expanded ac output voltage and current waveforms.

Fig. 13. Waveforms of the proposed multi-input inverter when only the Wind turbine is supplying power. (a) The dc bus voltage and the ac output current wave-forms. (b) The expanded ac output voltage and current waveforms.

Fig. 14. Waveforms of the proposed multi-input inverter when both of the PV array and the wind turbine are supplying power. (a) The dc bus voltage and the acoutput current waveforms. (b) The expanded ac output voltage and current waveforms.

utility line voltage is indicated by . When the utility linevoltage is detected, will change from high voltage levelto low voltage level gradually and stay at low level. It can beused to limit the maximum duty ratio of the power switchesand of the multi-input dc–dc converter during the start-uptransient and to achieve the soft-start function. Once the acmains is off-line, will change its status to high voltagelevel, immediately. Gate driving signals for power switches ofthe proposed multi-input inverter will be turned off immediatelyto realize the shutdown protection function. Fig. 15(c) showsthe waveforms of the dc bus over-voltage protection. When thedc bus voltage is over the limitation, the gate driving signals

for the power switches and will be turned off to stopthe incoming power.

IV. CONCLUSION

A novel multi-input inverter for the grid-connected hybridPV/wind power system is proposed. It has the following ad-vantages: 1) power from the PV array or the wind turbine canbe delivered to the utility grid individually or simultaneously,2) MPPT feature can be realized for both PV and wind energy,and 3) a large range of input voltage variation caused by dif-ferent insolation and wind speed is acceptable. In this paper,the operation principle of the proposed multi-input inverter has



Fig. 15. Some testing waveforms for circuit protection functions. (a) Theunder-voltage and over-voltage protection. (b) The utility line fault protection.(c) The dc bus over-voltage protection.

been introduced. The perturbation and observation method isadopted to realize the MPPT algorithm for the PV array andthe wind turbine. The control circuit is implemented by usinga DSP and auxiliary analog circuits to accomplish the desiredcontrol functions and circuit protection. Experimental resultsunder different operation conditions were shown to verify theperformance of the proposed multi-input inverter with desiredfeatures.

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[13] O. F. Walker, Wind Energy Technology. New York: Wiley, 1997.[14] P. Gipe, Wind Energy Comes of Age. New York: Wiley, 1995.[15] K. Amei, Y. Takayasu, T. Ohji, and M. Sakui, “A maximum power con-

trol of wind generator system using a permanent magnet synchronousgenerator and a boost chopper circuit,” in Proc. Power Conv., 2002,vol. 3, pp. 1447–1452.

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Yaow-Ming Chen (S’96–M’98–SM’05) receivedthe B.S. degree from National Cheng-Kung Univer-sity, Tainan, Taiwan, R.O.C., in 1989 and the M.S.and Ph.D. degrees from the University of Missouri,Columbia, in 1993 and 1997, respectively, all inelectrical engineering.

From 1997 to 2000, he was with I-Shou University,Dashu, Taiwan, as an Assistant Professor. In 2000,he joined National Chung Cheng University, Chia-Yi,Taiwan, where he is currently an Associate Professorin the Department of Electrical Engineering. His re-

search interests include renewable energy, power electronic converters, powersystem harmonics and compensation, and intelligent control.

Yuan-Chuan Liu was born in Chia-Yi, Taiwan,R.O.C., in 1973. He received the B.S. and Ph.D.degrees from National Chung Cheng University,Chia-Yi, in 1996 and 2006, respectively, both inelectrical engineering.

In 2006, he joined the NuLight TechnologyCompay, Tainan, Taiwan, as a Senior Engineer. Hisresearch interests include developing and designingof converter topologies, power-factor correctors, andelectronic ballasts.



Shih-Chieh Hung was born in Changhwa, Taiwan,R.O.C., in 1980. He received the B.S. and M.S.degrees from National Chung Cheng University,Chia-Yi, Taiwan, in 2002 and 2004, respectively,both in electrical engineering.

In 2004, he joined the Anpec Co., Ltd., Hsinchu,Taiwan, as a Senior Engineer. His research interestsinclude analog IC application systems and micropro-cessor-based application systems.

Chung-Sheng Cheng was born in Tainan, Taiwan,R.O.C., in 1980. He received the B.S. and M.S.degrees from National Chung Cheng University,Chia-Yi, Taiwan, in 2003 and 2005, respectively,both in electrical engineering.

In 2006, he joined the Richtek Technology Co.,Chupei, Hsinchu, Taiwan, as a Senior Engineer. Hisresearch interests include power management IC ap-plications.


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