grid-connected photovoltaic generation system

IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 3, APRIL 2008 953

Grid-Connected Photovoltaic Generation SystemRong-Jong Wai, Senior Member, IEEE, and Wen-Hung Wang

Abstract—This study addresses a grid-connected photovoltaic(PV) generation system. In order to make the PV generationsystem more flexible and expandable, the backstage power circuitis composed of a high step-up converter and a pulsewidth-modu-lation (PWM) inverter. In the dc–dc power conversion, the highstep-up converter is introduced to improve the conversion effi-ciency of conventional boost converters and to allow the paralleloperation of low-voltage PV modules. Moreover, an adaptive totalsliding-mode control system is designed for the current control ofthe PWM inverter to maintain the output current with a higherpower factor and less variation under load changes. In addition,an adaptive step-perturbation method is proposed to achieve theobjective of maximum power point tracking, and an active suntracking scheme without any light sensors is investigated to makePV plates face the sun directly in order to capture maximumirradiation and enhance system efficiency. Experimental resultsare given to verify the validity of the high step-up converter, thePWM inverter control, the ASP method, and the active sun trackerfor a grid-connected PV generation system.

Index Terms—Active sun tracking scheme, adaptive step-pertur-bation (ASP) method, high step-up converter, photovoltaic (PV)generation system, pulsewidth-modulation (PWM) inverter, totalsliding-mode control (TSMC).

I. INTRODUCTION

I N THE PAST century, global surface temperatures haveincreased at a rate near 0.6 C/century because of the

global warming taking place due to effluent gas emissionsand increasing CO [1], [2]. Problems with energy suppliesand use are related not only to global warming but also tosuch environmental concerns as air pollution, acid precip-itation, ozone depletion, forest destruction, and radioactiveemissions. To prevent these effects, some potential solutionshave evolved including energy conservation through improvedenergy efficiency, a reduction in fossil fuel use and an increasein environmentally friendly energy supplies. Recently, energygenerated from clean, efficient and environmentally friendlysources has become one of the major challenges for engineersand scientists. Among them, photovoltaic (PV) applicationhas received a great attention in research because it appearsto be one of the most efficient and effective solutions to thisenvironmental problem [3]–[7].

Dc–dc converters with high voltage gain are required in manyindustrial applications, such as front-end stages for clean-en-ergy sources, dc back-up energy systems for uninterruptible

Manuscript received January 5, 2006; revised April 27, 2007, June 26, 2007,and July 6, 2007. This work was supported in part by the National ScienceCouncil of Taiwan, R.O.C. through Grant Number NSC 95-2221-E-155-070-MY3. This paper was recommended by Associate Editor D. Masimovic.

The authors are with the Department of Electrical Engineering, Yuan Ze Uni-versity, Chung Li 32003, Taiwan, R.O.C. (e-mail: [email protected];[email protected]).

Digital Object Identifier 10.1109/TCSI.2008.919744

power supplies (UPS), high-intensity discharge lamps forautomobile headlamps, and the telecommunications industry[8]–[11]. The conventional boost converters cannot providesuch a high dc voltage gain, even for an extreme duty cycle.It also may result in serious reverse-recovery problems, andincrease the rating of all devices. As a result, the conversion ef-ficiency is degraded and the electromagnetic interference (EMI)problem is severe under this situation [12]. In order to increasethe conversion efficiency and voltage gain, many modifiedboost converter topologies have been investigated [13]–[17].Although voltage-clamped techniques are manipulated in theconverter design to overcome the severe reverse-recoveryproblem of the output diode in high-level voltage applications,there still exist switch over-voltage stresses, and the voltagegain is limited by the turn-on time of the auxiliary switch [13],[14]. Wai and Duan [17] investigated a novel coupled-inductorconverter strategy to increase the voltage gain of a conven-tional boost converter with a single inductor, as well to dealwith the problem of the leakage inductor and demagnetizationof the transformer in a conventional coupled-inductor-basedconverter. In this study, the high step-up converter topology in[17] is introduced to boost and stabilize the output dc voltageof PV modules for the utilization of a dc–ac inverter.

Developments in microelectronics and power devices haveprovided widespread applications of pulsewidth-modulation(PWM) inverters to industries. A PWM inverter used for agrid-connected scheme is controlled in order to produce anoutput current in phase with the utility voltage for obtaining aunity power factor (PF). The performance is evaluated by the PF,the transient response, and the efficiency. Thus, much attentionhas been paid to the closed-loop regulation of PWM inverters toachieve good dynamic response for the grid-connected schemein the past decade [18]–[21]. Variable structure control (VSC)with sliding mode, or sliding-mode control (SMC), is one of theeffective nonlinear robust control approaches since it providessystem dynamics with an invariance property to uncertaintiesonce the system dynamics are controlled in the sliding mode[22], [23]. The insensitivity of the controlled system to uncer-tainties exists in the sliding mode, but not during the reachingphase, i.e., the system dynamic in the reaching phase is stillinfluenced by uncertainties. Recently, some researchers haveadopted the idea of total SMC (TSMC) to get a sliding motionthrough the entire state trajectory [24]–[26]. Since there is noreaching phase in TSMC, the motion of the controlled systemis never influenced by uncertainties. This study attempts toextend an adaptive TSMC (ATSMC) from [25] to the currentcontrol of a PWM inverter. Up to now, this is the first time toinvestigate the application of TSMC to the power electronicscontrol.

In general, PV modules have nonlinear voltage-current char-acteristics, and there is only one unique operating point for a PV

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954 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 3, APRIL 2008

Fig. 1. Configuration of grid-connected PV generation system.

generation system with a maximum output power under a par-ticular environmental condition. However, the maximum powerpoint varies with irradiation and temperature, so that the max-imum power point tracking (MPPT) at all atmospheric situa-tions is a challenging problem. In the past decade, many MPPTmethods have been applied to PV generation systems for ex-acting maximum available powers from PV modules, e.g., in-cremental conductance method [4], perturbation and observa-tion (P&O) method [27]–[29], and voltage-and current-basedmethod [30], [31], etc. The P&O method, which measures thevariations of power and voltage to judge the momentary re-gion and change the reference voltage for operating close tothe maximum power point, is often used because of its simplestructure and fewer measured parameters [28], [29]. Althoughthe P&O method is easy to realize, the reference voltage stillvaries periodically when the MPPT is reached, so that it maycause oscillation phenomena around the maximum power point,causing extra power losses. For this reason, an adaptive step-per-turbation (ASP) method modified from the conventional P&Omethod is proposed to obtain faster tracking response and stableoperation by perturbing the voltage of PV modules with an adap-tive voltage step.

According to different irradiations, the output power of a PVmodule is substantially changed. For example in Taiwan, thedirection with maximum average irradiations during one yearis the South, and the corresponding angle of inclination is 23.5so that many PV modules are installed in this posture. However,it could not capture maximum irradiations persistently by thismethod so that the performance of the PV generation systemcan not be improved effectively. Nowadays, many researchershave focused on sun tracking investigations [32]–[35]. Con-ventional sun tracking strategies have light sensors equippedon the terminals of PV plates. When the feedback signals fromlight sensors are equal, it means that the PV plate directly facesthe sun and has the maximum irradiation at the correspondingposition. Unfortunately, the initial proofreading and correctingof light sensors are time consuming and the devices’ proper-ties are easily varied under different operational conditions. Inorder to overcome the aforementioned drawbacks, this studyinvestigates an active sun tracking scheme without light sensorsvia the property of open-circuit voltage of PV modules propor-tional to the corresponding irradiation, to follow the trail of thesun.

II. SYSTEM DESCRIPTION

In this study, the configuration of a grid-connected PV gen-eration system is depicted in Fig. 1. The system connected witha utility power is mainly composed of PV modules, an activesun tracker, a high step-up converter, a full-bridge inverter, anda system controller. Due to the photo-voltaic effect, the voltageof a PV cell is not very high. Because PV panels in a seriesstring are constrained to all conduct the same current, the leastefficient cell sets this string current, which may spell failurewhen one cell of a string is inactive. The overall efficiency ofthe PV array is reduced to the efficiency of this least efficientcell. It means that PV panels in a series string must be given thesame orientation and be of identical size for obtaining a higheroutput voltage. Besides, the corresponding output voltageis varied easily with respect to the variation of loads. In order tosatisfy the requirement of high-voltage demand, a dc–dc con-verter with high voltage gain is one of the essential mechanismsin the grid-connected PV generation system. In this study, a highstep-up converter [17] is implemented to reduce the series-con-nected numbers of PV modules, to maintain a constant dc busvoltage for the inverter utilization, and to decouple andsimplify the inverter control design.

A unipolar PWM full-bridge inverter, including four powersemiconductors and an output inductor, is regarded as thedc–ac power conversion circuit to meet the requirement of gridconnection. Since the PWM inverter is expected to produce anoutput current in phase with the utility voltage for obtaininga unity PF, an ATSMC system [25] is introduced by way ofswitching four power semiconductors in this inverter to main-tain an output current with a higher PF and less variationunder load changes.

PV modules exhibit nonlinear voltage-current characteris-tics, and the maximum power point varies with the irradiationand temperature. At a particular environmental condition, onlyone unique operating point exists to provide the PV generationsystem with maximum output power. In this study, an ASPmethod is proposed to inject an adaptive step perturbationinto the voltage of PV modules. Since the step perturbation ispersistently adjusted in accordance with the variations of thevoltage and power of PV modules, the ASP method boosts theMPPT speed and stability much higher than the conventionalmethods, to further reduce extra power losses in the trackingprocess.

WAI AND WANG: GRID-CONNECTED PV GENERATION SYSTEM 955

Fig. 2. Architecture of high step-up converter.

Generally speaking, the output power of PV modules is sub-stantially changed according to different irradiations. To furtherenhance the capability of the PV generation system, an activesun tracker actuated by a synchronous motor is investigated onthe basis of the open-circuit voltage of a PV module, for main-taining the PV plate in the face of the sun in order to improvethe generation efficiency of the fixed-installation PV module,and to lower the cost of conventional sun trackers with light sen-sors. Incidentally, it is unnecessary to modify the original circuitframework of the grid-connected PV generation system becauseof the sole requirement of the open-circuit voltage of PV mod-ules in the active sun tracking scheme.

In this study, the PWM inverter control, the ASP method, andthe active sun tracking scheme are carried out using Turbo C lan-guage inserted into a system controller, i.e., a digital-signal-pro-cessor (DSP) development module. This development modulehas: Texas Instruments TMS320LF2407A central processingunit with an evaluation module (EVM), 16 channel 10-bitanalog-to-digital, 4 channel 12-bit digital-to-analog convertersand programmable I/O ports. The central processing unit has: a40MIPS 16-bit fixed point DSP core, 16 PWM channels, fourgeneral purpose timers and two encoder channels. The detailedfunctions of the main components in the grid-connected PVgeneration system are described in the following sections.

III. HIGH STEP-UP CONVERTER

The architecture of a high step-up converter introduced from[17] is depicted in Fig. 2, where it contains seven parts: a PVmodule input circuit, a primary-side circuit, a secondary-sidecircuit, a passive regenerative snubber circuit, a filter circuit,a dc output circuit, and a feedback control mechanism. In thisstrategy, a coupled inductor with a lower-voltage-rated switchis used for raising the voltage gain whether the switch is turnedon or turned off. Moreover, a passive regenerative snubber isutilized for absorbing the energy of stray inductance so that theswitch duty cycle can be operated under a wide range; the re-

lated voltage gain is higher than that of other coupled-inductor-based converters. In addition, all devices in this scheme alsohave voltage-clamped properties and their voltage stresses arerelatively smaller than the output voltage. Thus, it can selectlow-voltage low-conduction-loss devices, and there are no re-verse-recovery currents within the diodes in this circuit. Fur-thermore, the closed-loop control methodology is utilized toovercome the voltage drift problem of the power source underthe load variations. As a result, this converter topology can in-crease the voltage gain of a conventional boost converter witha single inductor, and deal with the problem of the leakage in-ductor and demagnetization of the transformer for a coupled-in-ductor-based converter.

The major symbol representations are summarized as follows.and denote dc input voltage and current, and is an

input filter capacitor in the PV module input circuit. andrepresent individual inductors in the primary and secondary sidesof the coupled inductor , respectively. is a switch in theprimary-side circuit; and are the output voltage commandand the trigger signal in the feedback control mechanism, respec-tively. and denote a clamped capacitor, a clampeddiode and a rectifier diode in the passive regenerative snubber cir-cuit. is a high-voltage capacitor in the secondary-side circuit.

and are the output diode and the filter capacitor in thefilter circuit, respectively. and describe dc output voltageand current, respectively, in the dc output circuit.

The coupled inductor in Fig. 2 is modeled as an ideal trans-former, a magnetizing inductor , and a leakage inductor

. The turns ratio and coupling coefficient of thisideal transformer are defined as

(1)

(2)

where and are the winding turns in the primary and sec-ondary sides, respectively. The voltages across the switch, the


primary and secondary winding of the ideal transformer, andthe leakage inductor are denoted as and , re-spectively. Moreover, the primary current of the coupledinductor is composed of the magnetizing current and theprimary induced current . The secondary current isformed by the primary induced current through the idealtransformer, and its value is related to the turns ratio . Inaddition, the conductive voltage drops of the switch and alldiodes ( and ) are neglected in order to simplify cir-cuit analyses.

According to the detailed circuit analyses in [17], the voltagegain of the high step-up converter and the correspondingswitch voltage can be represented as

(3)

(4)

where is the duty cycle of the switch . Because the voltagegain is less sensitive to the coupling coefficient , (3)and (4) can be rewritten with as

(5)

(6)

According to (5) and (6), one can obtain

(7)

By analyzing (7), the switch voltage is not related to theinput power source and the switch duty cycle if thevalues of the output voltage and the turns ratio arefixed. Thus, it can ensure that the maximum sustainable voltageof the switch is constant. As long as the input voltage is nothigher than the switch voltage-rated, the high step-up convertercan be applied well to low-voltage PV power sources even withlarge voltage variations.

IV. PWM INVERTER CONTROL

A. Dynamic Model Description

Fig. 3 illustrates the PWM inverter framework including fourpower semiconductors and an output inductor. In Fig. 3, isthe equivalent series resistor of the output inductorand are the output voltage of the full-bridge inverter andthe utility voltage; is the output current of the full-bridgeinverter; and the voltage source emulates the disturbanceincurred by load variations. In order to analyze conveniently,the following assumptions are made in this study: i) the value of

is small enough to ignore; ii) the conduction and switchinglosses are zero since all power switches are assumed to be idealdevices; iii) the delay time between the switch turn-on andturn-off states is small enough to neglect; and iv) the controlsignal and input/output voltages are taken as constant valuesbecause the switching frequency is greater than the systemdynamic frequency.

Due to the symmetry property of the positive-half and neg-ative-half period in the unipolar PWM switching, the dynamicequation during the positive-half period can be represented via

Fig. 3. PWM inverter framework.

Fig. 4. Equivalent dynamic model of PWM inverter.

the state-space average method [12] and the linearization tech-nique as

(8)

where is the duty cycle of the switches and duringone switching period. Define the duty cycle and the power gainas and , where is asinusoidal control signal and is the amplitude of a triangularcarrier signal , then the dynamic equation of the PWMinverter can be given by

(9)

By way of the Laplace transformation of (9), the equivalent dy-namic model of the PWM inverter is depicted in Fig. 4, where

is the Laplace operator.By choosing the ac output current as the system state

and the control signal as the control input, (9) can berearranged as

(10)

whereand and denote

the nominal values of and , respectively; andrepresent the system parameter variations; is called thelumped uncertainty and defined as

(11)

Here, the bound of the lumped uncertainty is assumed to begiven by

(12)


Fig. 5. ATSMC system for PWM inverter.

where is the operator of an absolute value, and is a givenpositive constant.

B. ATSMC System

The objective of the PWM inverter control is to force thesystem state to track a reference output current

, which is designed by the ASP method introduced later.An ATSMC system, as shown in Fig. 5, is introduced for thecurrent control of the PWM inverter, where the control error ischosen as . Define a sliding surface [25]as

(13)

where is the initial value of and is a nonzero positiveconstant.

The ATSMC system is divided into three main parts. The firstpart addresses performance design. The objective is to specifythe desired performance in terms of the nominal model, and itis referred to as the baseline model design . Following thebaseline model design, the second part is the curbing controllerdesign to totally eliminate the unpredictable perturbationeffect from the parameter variations and external disturbanceso that the baseline model design performance can be exactlyensured. Finally, the third part is the adaptive observation de-sign to estimate the upper bound of the lumped uncertaintyfor alleviating the chattering phenomenon caused by the inap-propriate selection of a conservative constant control gain inthe curbing controller. The entire control methodologies of theATSMC system are summarized in the following theorem.

Theorem 1: If the PWM inverter scheme shown in (10) is con-trolled by the three-part ATSMC system described by (14)–(16)with the adaptive observation design shown in (17), then the sta-bility of the ATSMC system for the current control of the PWMinverter can be guaranteed

(14)

(15)

(16)

(17)

where is a positive constant.

Fig. 6. Control flowchart of ASP method.

Proof: Consider a Lyapunov function candidatewith and take the

derivative of with respect to time; there exists anATSMC system described by (14)–(16) with the adaptiveobservation design shown in (17) such that . Accordingto the Lyapunov stability theorem and Barbalat’s lemma [22],[23], it can imply that the function will converge to zero as

. Moreover, the parameter estimation error can beguaranteed to be bounded. The detailed proof of Theorem 1can be referred to [25]. As a result, the stable behavior for thecurrent control of the PWM inverter can be ensured.

V. ASP METHOD

Due to the characteristic of PV modules with nonlinearvoltage-current relation and the variation of the maximumpower point with respect to irradiation and temperature, anASP method is proposed in this study to adjust the referencevoltage by injecting a step perturbation into the voltage ofPV modules and judging the momentary operation region forraising the operating point close to the maximum power point.In this way, the major drawback concerned with the tradeoffbetween tracking speed and response in the conventional P&Omethod can be effectively improved. The corresponding controlflowchart of the proposed ASP method is illustrated in Fig. 6.

In Fig. 6, and represent the terminalvoltage, current and power of PV modules, respectively, inwhich denotes the iteration number; and representthe reference voltage and its step perturbation, respectively;


Fig. 7. Block diagram of current command for PWM inverter.

and denote the variations of and, respectively. Define an ASP control law as

(18)

where is a positive constant. Because the step perturbationof the reference voltage, , is proportional to the ratio ofPV power with respect to voltage , it means thatthe ASP method has the ability to adjust the reference voltageaccording to instantaneous operational conditions.

When the control process starts, the terminal voltage andcurrent of PV modules are first measured, then the power ofPV modules can be obtained from the product of and

. If the variations of and both are not equal to zero,the reference voltage could be produced by (18), otherwise, thereference voltage will be maintained at the previous value. Theblock diagram of the current command produced by the indirectvoltage control of PV modules for the PWM inverter is depictedin Fig. 7, where the voltage control error is defined as

is a proportional gain; is a time delay;is the amplitude of the current command, , inwhich is the utility frequency sensed by a phase-lock loop(PLL) circuit [4]. In other words, the current commandfor the PWM inverter is composed of the product of and

so that the inverter current will be in phase with thegrid voltage when the PWM inverter control error

converges to zero by the proposed ATSMC system.

VI. ACTIVE SUN-TRACKING SCHEME

Because the movement of the sun is slow and monotonous,and the variation range of the climbing angle is within ,it is unnecessary to adjust the inclined angle of the PV platefor simplifying the mechanical framework. Only by way of thesingle axis direction control, it can immediately achieve the goalof collection of maximum irradiation. In this study, an active suntracking scheme actuated by a synchronous motor is used for thesun tracking via the information of the open-circuit voltage ofthe PV module, and the corresponding control flowchart of theactive sun tracker is depicted in Fig. 8.

In Fig. 8, and represent the present and pre-vious open-circuit voltages, respectively; denotes the vari-ation of the open-circuit voltage. Because the sun only movesfrom the East to the West during one day, the PV plate is ro-tated by the unit angle for a time at clockwise (CW) in thebeginning of the control process to disturb the correspondingopen-circuit voltage. In this way, it can adjust the rotating direc-tion by observing the variation trend of the open-circuit voltageto capture more irradiations because the open-circuit voltage ofthe PV module is proportional to the corresponding irradiation.If the condition of holds, the PV plate is rotated bythe unit angle for a time at counterclockwise (CCW), i.e.,

Fig. 8. Control flowchart of active sun tracker.

it is returned to the previous location. After that, the controlprocess will wait for a time to further ensure whether or notthe reason for decreasing disappears. If the condition of

holds, the control process also waits for a timefor the next CW rotation. Note that, the function of the waitingtime is helpful for alleviating the extra power consumption inback and forth motion. According to the aforementioned actionprinciple, the control target of the active sun tracking schemecan be achieved. In this study, the implementation of the MPPTcontrol is faster than in the case of the sun tracker. Because thesun moves slowly in general, it is reasonable to ignore the inter-action of the ASP control and the active sun tracker.

VII. EXPERIMENTAL RESULTS

The validity of the high step-up converter, the PWM invertercontrol, the ASP method, and the active sun tracker in the grid-connected PV generation system are verified by the followingexperimental results.

A. Experimental Results of High Step-Up Converter

In order to verify the effectiveness of the high step-upconverter, the input side consists of six 75-W PV modulesmanufactured by the MOTECH Company (F-MSN-75W-R-02)connecting in parallel as a low-voltage power source. Thespecifications of a single PV module for the standard condi-tion (100 mW/cm , 25 C) are rated 76.78 W, rated

17.228 V, rated 4.4567 A, open-circuit21.61 V, short-circuit 4.9649 A, and PV

%.In the experimentation, the high step-up converter is designed

initially to operate from the variability dc input of PV modules,to deliver a constant dc output, 200 V. Assume that themaximum value of the switch voltage is clamped at 34 V; theturns ratio is according to (7).From (6), the related duty cycle, , is reasonable in prac-tical applications if the minimum input voltage is assumed to be10 V. In order to solve the problem of the output voltage of PVmodules varying with the load variations, this converter withdc voltage feedback control is utilized to ensure the system sta-bility, and a PWM control IC TL494 is adopted to achieve the


goal of feedback control. The prototype with the following spec-ifications is designed to illustrate the design procedure given inSection III.

Switching frequency:

kHz

Coupled inductor:

H

H

Capacitor:

F V

F V

F V

F V

Switch:

V

Diode:

V

V

The experimental voltage and current responses of the highstep-up converter operating at 320 W-output power aredepicted in Fig. 9. From Fig. 9(a), the switch voltageis clamped at 34 V, which is much smaller than the outputvoltage V, and the curve of the switch currentis similar to a square wave so that it can further reduce theconduction loss of the switch . By observing Fig. 9(b) and(c), the primary current keeps about 30A; thus, only asmaller core capacity is necessary for H. According toFig. 9(d)–(j), the reverse-recovery currents in all of the diodes( and ) can be alleviated effectively, and the voltagesof the clamped capacitor and the high-voltage capacitor

are close to constant values. Therefore, it can alleviatethe reverse-recovery problem and exhibit the voltage-clampedeffect for further raising the conversion efficiency. FromFig. 9(i), the selection of the output diode with 200-V blockingvoltage is enough in this application because the voltage of theoutput diode is limited below the output voltage ( 200 V)at all times. Fig. 10 summarizes the experimental conversionefficiency of the high step-up converter under different outputpowers. As can be seen from this figure, the conversion effi-ciency at light powers is over 95% and the maximum efficiencyis over 96.5%, which is comparatively higher than conventionalconverters with the same voltage gain.

By the same assumption of the maximum value of the switchvoltage to be clamped at 34 V in this study, the turns ratio is

Fig. 9. Experimental voltage and current responses of high step-up converterwith � �320 W and � �200 V.

according to (7). From (6), therelated duty cycle, , is reasonable in practical appli-cations if the minimum input voltage is assumed to be 8.614 V(i.e., when is decreased by a factor of two). Even though thesolar array voltage changes from 17.228 V to less than 8.614 Vdue to clouding, snow or dirt, the step-up converter can compen-sate for such a drop and provide the inverter with rated voltageof 200 V regardless of irradiation changes as long as the solararray voltage is not higher than the switch voltage rating andthe clamped switch voltage is appropriately preset. Although thecombination of a step-down/step-up converter also can providean alternative solution handling the voltage variation problem


Fig. 10. Conversion efficiency of high step-up converter with � � ��Vunder different output powers.

due to irradiation changes, it may fail under excessive voltagevariations and the multistage conversion loss is a latent problemto be overcome.

The experimental inductor current and switch voltage curvesof the high step-up converter under the condition of 24-Vinput voltage, 200-V output voltage, and 1-kW output powerare shown in [17, Fig. 14] to examine the coupled-inductorcapability for a high power application. As can be seen fromthis figure, the proposed converter with the same size of thecoupled inductor in [17] can be operated over 1-kW power.Moreover, the conversion efficiency of the high step-up con-verter for 24-V input voltage and 200-V output voltage underdifferent output powers are shown in [17, Fig. 15] to verifyits superiority over that of the coupled-inductor in [16]. Ac-cording to these comparisons in [17], one can conclude that thishigh step-up converter with the appropriate design of circuitcomponent specifications is still suitable for the application ofkilowatt-level power conversion. Although the efficiency ofthe high step-up converter would be reduced because of thelosses in the inductors, the decrease of winding turns and theincrease of air gap in the coupled inductor are helpful to solvethis problem efficiently.

B. Experimental Results of PWM Inverter Control

The circuit components of the PWM inverter schemeare IRFP264 (250 V/38 A) and

mH; the switching frequency is kHz.Moreover, the parameters of the ATSMC systems for the PWMinverter scheme are given as follows:

(19)

All the parameters in the ATSMC system are chosen to achievethe best transient control performance by considering the re-quirement of stability. The experimental results of the ATSMCfor the PWM inverter of the grid-connected PV generationsystem under light load and heavy load are depicted in Fig. 11.It can be noticed that the output current is almost in phase withthe utility voltage, and the PF of the PWM inverter is higherthan 0.98 that satisfies the PF demand in industrial applications.

The experimental results of the grid-connected PV generationsystem with the ATSMC system for the PWM inverter under

Fig. 11. Experimental results of grid-connected PV generation system withATSMC for PWM inverter. (a) Light load. (b) Heavy load.

different step load changes are given to examine the load vari-ation effect. In Fig. 12(a), the load is changed from light loadto heavy load; reversely, the load is changed from heavy load tolight load in Fig. 12(b); and in Fig. 12(c)–(d) the load changedfrom no load to heavy load and heavy load to no load, respec-tively. As can be seen from this figure, the control performanceof the ATSMC system for the PWM inverter is insensitive tothe abrupt load changes. Generally speaking, inverters may gen-erate small dc voltages/currents due to somewhat asymmetricgating. In conventional way, transformers are generally con-nected between inverter and power system to prevent these dccurrents from entering the power system. Due to the powerfulcontrol ability of the proposed ATSMC system, a transformeris omitted in this study to reduce the energy transformation lossand lower manufacturing cost. If it is desirable to use a trans-former for electric isolation under safety considerations, the pro-posed control strategies still can work well by additionally con-sidering the turn ratio of the transformer.

C. Experimental Results of ASP Method

Figs. 13–15 are the experimental results for the purpose ofverifying the effectiveness of the ASP method. The atmosphericcondition is the irradiation level 88 mW/cm and the moduletemperature 53 C dated on the afternoon of November 6, 2005.The control flowchart as shown in Fig. 6 is implemented via aDSP with 0.166 ms time step and the parameters of the ASPmethod are given as follows:

(20)

The experimental results of conventional P&O method withfixed step perturbations 0.15 and 0.3 V are illustrated in Figs. 13and 14, respectively. It is obvious that the smaller step perturba-tion (0.15 V) results in slower tracking response, but it has stable


Fig. 12. Experimental results of grid-connected PV generation system withATSMC for PWM inverter under load change. (a) Light load to heavy load.(b) Heavy load to light load. (c) No load to heavy load. (d) Heavy load to noload.

operation around the maximum power point. Although the largerstep perturbation (0.3 V) can provide faster tracking response,oscillations around the maximum power point occurred. Forcomparison, Fig. 15 shows the experimental results of the ASPmethod. By observing Fig. 15, the operating point could be con-trolled to locate at the maximum power point rapidly and stably.As a result, the proposed ASP method indeed yields superiorperformance to that of the conventional P&O method.

Fig. 13. Experimental results of grid-connected PV generation system withconventional P&O method (0.15-V step). (a) Tracking response. (b) TransientV–P and V–I curve. (c) Steady V–P and V–I curve.

D. Experimental Results of Active Sun Tracker

In order to verify the validity of the sun tracking scheme byway of realistic experimentations, a single PV module actuatedby a synchronous motor manufactured by TUSHING Company(GL-301) is used to form the active sun tracker, and the cor-responding rotational angle is . The atmospheric circum-stance is the irradiation level 67 mW/cm and the module tem-perature 30 C dated on October 5, 2005 (Local time PM 3:00,Taiwan). The control flowchart shown in Fig. 8 is implementedusing a DSP with 1-ms sampling interval, and the parameters ofthe active sun tracking scheme are given as follows:

(21)

Two conditions of irradiation are examined here: one is thenominal condition, and the other is the shading condition byplacing a plastic plate abruptly above a PV plate. The experi-mental results of the grid-connected PV generation system withthe active sun tracker at nominal and shading conditions aredepicted in Fig. 16. In Fig. 16(a), the open-circuit voltage isincreased from V to V when the ac-tive sun tracker was started to rotate the PV plate, so that itwould result in the increasing of output powers. In Fig. 16(b),the open-circuit voltage suddenly decreased because the shadingcondition occurred at 34 s such that the PV plate was returnedto its previous location for waiting a span. When the shadingcondition was removed at 62s, the PV plate was rotated againafter the waiting time to track the sun’s direction, so that the


Fig. 14. Experimental results of grid-connected PV generation system withconventional P&O method (0.3V-step). (a) Tracking response. (b) TransientV–P and V–I curve. (c) Steady V–P and V–I curve.

open-circuit voltage increased to a steady state. According tothe experimental results in Fig. 16, the expected goal of the ac-tive sun tracker can be realized perfectly, and this simple ac-tive sun tracking mechanism could be taken as a ‘supervisor’to further provide the adjustable command for PV plates in alarge-scale PV generation system. However, the proposed activesun tracking scheme can not handle well more than one powermaximum (i.e., power versus voltage functions with two powerpeaks due to partially shaded solar arrays with partial bypassingof solar cells by diodes). How to jump over the local maximumpower point is worthy of investigation in the future research.

VIII. CONCLUSION

This study has successfully developed a grid-connected PVgeneration system. The effectiveness of the high step-up con-verter, the PWM inverter control, the ASP method, and the ac-tive sun tracker for a grid-connected PV generation system wasverified by realistic experimentations. According to the experi-mental results, the conversion efficiency of the high step-up con-verter at rated power is over 95%, and the overall efficiency in-cluding the inverter and losses in the sun tracker is over 85%.Moreover, the output current of the PWM inverter can almost bemaintained in phase with the utility voltage. The correspondingPF under different loads are higher than 0.98, satisfying the PFstandards in industrial applications. In addition, the realizationof the ASP method provides faster tracking response with 3 s

Fig. 15. Experimental results of grid-connected PV generation system withASP method. (a) Tracking response. (b) Transient V–P and V–I curve. (c) SteadyV–P and V–I curve.

Fig. 16. Experimental results of grid-connected PV generation system with ac-tive sun tracker. (a) Nominal condition. (b) Shading condition.


settling time and overcomes the oscillation problem in the con-ventional P&O method for reducing extra power losses. Fur-thermore, the implementation of the active sun tracking schemeon the basis of the open-circuit voltage of PV modules, is forimproving the generation efficiency of the fixed-installation PVarray, and lowering the cost of the conventional sun trackerwith light sensors. This system-integration research providesdesigners with an alternative choice to convert the PV energyefficiently, and it also can be extended easily to a large-scale PVgeneration system.

REFERENCES

[1] S. R. Bull, “Renewable energy today and tomorrow,” Proc. IEEE, vol.89, no. 8, pp. 1216–1226, Aug. 2001.

[2] S. Rahman, “Green power: What is it and where can we find it?,” IEEEPower Energy Mag., vol. 1, no. 1, pp. 30–37, Jan. 2003.

[3] J. A. Gow and C. D. Manning, “Photovoltaic converter system suitablefor use in small scale stand-alone or grid connected applications,” Proc.IEE Electr. Power Appl., vol. 147, no. 6, pp. 535–543, Jun. 2000.

[4] T. J. Liang, Y. C. Kuo, and J. F. Chen, “Single-stage photovoltaic en-ergy conversion system,” Proc. IEE Electr. Power Appl., vol. 148, no.4, pp. 339–344, 2001.

[5] S. J. Huang and F. S. Pai, “Design and operation of grid-connectedphotovoltaic system with power-factor control and active islanding de-tection,” Proc. IEE Gen. Trans. Distrib., vol. 148, no. 3, pp. 243–250,2001.

[6] Y. K. Chen, C. H. Yang, and Y. C. Wu, “Robust fuzzy controlled pho-tovoltaic power inverter with Taguchi method,” IEEE Trans. EnergyConv., vol. 38, no. 3, pp. 940–954, Mar. 2002.

[7] T. Shimizu, O. Hashimoto, and G. Kimura, “A novel high-performanceutility-interactive photovoltaic inverter system,” IEEE Trans. PowerElectron., vol. 18, no. 2, pp. 704–711, Feb. 2003.

[8] J. E. Harry and D. W. Hoare, “Electronic power supplies for high-den-sity discharge (HID) lamps,” IEE Eng. Sci. Educ. Jo., vol. 9, no. 5, pp.203–206, 2000.

[9] I. Barbi and R. Gules, “Isolated dc–DC converters with high-outputvoltage for TWTA telecommunication satellite applications,” IEEETrans. Power Electron., vol. 18, no. 4, pp. 975–984, 2003.

[10] O. Abutbul, A. Gherlitz, Y. Berkovich, and A. Ioinovici, “Step-upswitching-mode converter with high voltage gain using a switched-ca-pacitor circuit,” IEEE Trans. Circuits Syst. I, Fundam. Theory Appl.,vol. 50, no. 8, pp. 1098–1102, Aug. 2003.

[11] K. C. Tseng and T. J. Liang, “Novel high-efficiency step-up converter,”Proc. IEE Electr. Power Appl., vol. 151, no. 2, pp. 182–190, Feb. 2004.

[12] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics:Converters, Applications, and Design. New York: Wiley, 1995.

[13] M. M. Jovanovic and Y. Jang, “A new soft-switched boost converterwith isolated active snubber,” IEEE Trans. Ind. Appl., vol. 35, no. 2,pp. 496–502, Mar. 1999.

[14] C. M. C. Duarte and I. Barbi, “An improved family of ZVS-PWM ac-tive-clamping dc-to-dc converters,” IEEE Trans. Power Electron., vol.17, no. 1, pp. 1–7, Jan. 2002.

[15] E. S. Da Silva, L. Dos Reis Barbosa, J. B. Vieira, L. C. De Freitas, andV. J. Farias, “An improved boost PWM soft-single-switched converterwith low voltage and current stresses,” IEEE Trans. Ind. Electron., vol.48, no. 6, pp. 1174–1179, Dec. 2001.

[16] Q. Zhao and F. C. Lee, “High-efficiency, high step-up dc–DC con-verters,” IEEE Trans. Power Electron., vol. 18, no. 1, pp. 65–73, Jan.2003.

[17] R. J. Wai and R. Y. Duan, “High step-up converter with coupled in-ductor,” IEEE Trans. Power Electron., vol. 20, no. 5, pp. 1025–1035,May 2005.

[18] P. G. Barbosa, L. G. B. Rolim, E. H. Watanabe, and R. Hanitsch, “Con-trol strategy for grid-connected dc–ac converters with load power factorcorrection,” Proc. IEE Gen. Trans. and Distri.b, vol. 145, no. 5, pp.487–491, 1998.

[19] T. Erika and D. G. Holmes, “Grid current regulation of a three-phasevoltage source inverter with an LCL input filter,” IEEE Trans. PowerElectron., vol. 18, no. 3, pp. 888–895, Mar. 2003.

[20] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of single-phasegrid-connected inverters for photovoltaic modules,” IEEE Trans. Ind.Appl., vol. 41, no. 5, pp. 1292–1306, Sep. 2005.

[21] M. Armstrong, D. J. Atkinson, C. M. Johnson, and T. D. Abeyasekera,“Low order harmonic cancellation in a grid connected multiple invertersystem via current control parameter randomization,” IEEE Trans.Power Electron., vol. 20, no. 4, pp. 885–892, Apr. 2005.

[22] J. J. E. Slotine and W. Li, Applied Nonlinear Control. EnglewoodCliffs, NJ: Prentice-Hall, 1991.

[23] K. J. Astrom and B. Wittenmark, Adaptive Control. New York: Ad-dison-Wesley, 1995.

[24] R. J. Wai, “Adaptive sliding-mode control for induction servomotordrive,” Proc. IEE Electr. Power Appl., vol. 147, no. 6, pp. 553–562,2000.

[25] R. J. Wai and K. M. Lin, “Robust decoupled control of direct field-oriented induction motor drive,” IEEE Trans. Ind. Electron., vol. 52,no. 3, pp. 837–854, Jun. 2005.

[26] C. C. Kung and K. H. Su, “Adaptive fuzzy position control for electricalservodrive via total-sliding-mode technique,” Proc. IEE Electr. PowerAppl., vol. 152, no. 6, pp. 1489–1502, 2005.

[27] C. Hua, J. Lin, and C. Shen, “Implementation of a DSP-controlled pho-tovoltaic system with peak power tracking,” IEEE Trans. Ind. Electron.,vol. 45, no. 1, pp. 99–107, Feb. 1998.

[28] E. Koutroulis, K. Kalaitzakis, and N. C. Voulgaris, “Development ofa microcontroller-based, photovoltaic maximum power point trackingcontrol system,” IEEE Trans. Power Electron., vol. 16, no. 1, pp. 46–54,Jan. 2001.

[29] T. T. Yun, D. S. Kirschen, and N. Jenkins, “A model of PV generationsuitable for stability analysis,” IEEE Trans. Energy Conv., vol. 19, no.4, pp. 748–755, 2004.

[30] M. Veerachary, T. Senjyu, and K. Uezato, “Voltage-based maximumpower point tracking control of PV system,” IEEE Trans. Aero. Elec-tron. Syst., vol. 38, no. 1, pp. 262–270, Jan. 2002.

[31] M. A. S. Masoum, H. Dehbonei, and E. F. Fuchs, “Theoretical and ex-perimental analyses of photovoltaic systems with voltage- and current-based maximum power-point tracking,” IEEE Trans. Energy Conv.,vol. 17, no. 4, pp. 514–522, Apr. 2002.

[32] A. Konar and A. K. Mandal, “Microprocessor based automatic suntracker,” Proc. IEE Sci. Meas. Technol., vol. 138, no. 4, pp. 237–241,1991.

[33] B. Koyuncu and K. Balasubramanian, “A microprocessor controlledautomatic sun tracker,” IEEE Trans. Consumer Electron., vol. 37, no.4, pp. 913–917, Apr. 1991.

[34] A. Ferriere and B. Rivoire, “An instrument for measuring concentratedsolar-radiation: A photo-sensor interfaced with an integrating sphere,”Sol. Energy, vol. 72, no. 3, pp. 187–193, 2002.

[35] J. D. Garrison, “A program for calculation of solar energy collection byfixed and tracking collectors,” Sol. Energy, vol. 72, no. 4, pp. 241–255,2002.

Rong-Jong Wai (M’99–A’00–M’02–SM’05) wasborn in Tainan, Taiwan, R.O.C., in 1974. He receivedhis B.S. degree in electrical engineering and thePh.D. degree in electronic engineering from ChungYuan Christian University, Chung Li, Taiwan,R.O.C., in 1996 and 1999, respectively.

Since 1999, he has been with the Department ofElectrical Engineering, Yuan Ze University, ChungLi, Taiwan, R.O.C., where he is currently a Professor.He is also the Director of the Electric Control andSystem Engineering Laboratory at Yuan Ze Univer-

sity, and the Energy Conversion and Power Conditioning Laboratory at the FuelCell Center. He is a chapter-author of Intelligent Adaptive Control: IndustrialApplications in the Applied Computational Intelligence Set (CRC, 1998) andthe coauthor of Drive and Intelligent Control of Ultrasonic Motor (Tsang-HaiPress, 1999), Electric Control (Tsang-Hai Press, 2002) and Fuel Cell: New Gen-eration Energy (Tsang-Hai Press, 2004). He has authored numerous publishedjournal papers in the field of control system applications. His research interestsinclude power electronics, motor servo drives, mechatronics, energy technology,and control theory applications.

Dr. Wai received the Excellent Research Award in 2000, and the Wu Ta-YouMedal and Young Researcher Award in 2003 from the National ScienceCouncil, R.O.C. In addition, he was the recipient of the Outstanding ResearchAward in 2003 and 2007 from the Yuan Ze University, R.O.C.; the ExcellentYoung Electrical Engineering Award in 2004 from the Chinese ElectricalEngineering Society, R.O.C; the Outstanding Professor Award in 2004 and


2008 from the Far Eastern Y. Z. Hsu-Science and Technology MemorialFoundation, R.O.C.; the International Professional of the Year Award in 2005from the International Biographical Centre, U.K., the Young Automatic ControlEngineering Award in 2005 from the Chinese Automatic Control Society,R.O.C., and the Yuan-Ze Lecture Award in 2007 from the Far Eastern Y. Z.Hsu-Science and Technology Memorial Foundation, R.O.C.His biographywas listed in Marquis Who’s Who in Science and Engineering in 2004–2009,Marquis Who’s Who in 2004–2007, and Leading Scientists of the World (Inter-national Biographical Centre) in 2005, Marquis Who’s Who in Asia, MarquisWho’s Who of Emerging Leaders in 2006–2009, and Asia/Pacific Who’s Who(Rifacimento International) in Vol. VII and VIII.

Wen-Hung Wang was born in Taichung, Taiwan,R.O.C., in 1981. He received his B.S. and M.S.degrees in electrical engineering from Yuan ZeUniversity, Chung Li, Taiwan, R.O.C., in 2003 and2006, respectively.

His research interests include photovoltaic genera-tion system, power electronics, and adaptive control.

grid-connected photovoltaic generation system

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