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Solar Energy 123 (2016) 102–115

Design and implementation of a dynamic FPAA basedphotovoltaic emulator

Marco Balato, Luigi Costanzo ⇑, Daniele Gallo, Carmine Landi, Mario Luiso,Massimo Vitelli

Department of Industrial and Information Engineering, Second University of Naples, Italy

Received 2 September 2015; received in revised form 27 October 2015; accepted 3 November 2015

Communicated by: Associate Editor Mario Medina

Abstract

In this paper, a new Photovoltaic (PV) emulator is presented and discussed. Its main feature is represented by the use of a Field Pro-grammable Analog Array (FPAA) on which the desired current vs. voltage (I–V) PV characteristic can be implemented. The FPAA pro-vides a suitable analog time varying reference signal for the output current control of a proper DC/DC converter whose output portemulates the PV I–V curve. The proposed emulator allows to track time varying irradiance values and therefore it allows also to emulatetypical scenarios of automotive applications or involving fast time varying weather conditions (e.g. the ones which usually occur in trop-ical locations). Additional, not less important advantages of the proposed solution are the following ones: (1) no numerical interpolationsand no storage of big amount of data in memory are required; (2) the FPAA is characterized by a great ease of reconfiguration andprogramming with respect to FPGA or DSP based implementations; (3) no DAC or ADC converters are needed; (4) not only uniformbut also mismatching operating conditions can be easily emulated; (5) power sources different from PV sources can be easily emulated byusing the same architecture. The presented experimental results allow to confirm the validity of the proposed FPAA based architecture.� 2015 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic emulator; FPAA; DC/DC converter control

1. Introduction

In the scientific literature of the last ten years, a hugeamount of papers has been published on Maximum PowerPoint Tracking (MPPT) algorithms Liu et al., 2014;Miyatake et al., 2011; Sera et al., 2013; Reisi et al., 2013;Al Nabulsi and Dhaouadi, 2012 and on advanced staticor dynamic PV architectures (e.g. distributed MPPT appli-cations, array reconfiguration strategies, etc.) (Qin et al.,2014; Balato and Vitelli, 2014; Feng et al., 2014; LaManna et al., 2014; Spagnuolo et al., 2015).

http://dx.doi.org/10.1016/j.solener.2015.11.006

0038-092X/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: luigi.costanzo@unina2.it (L. Costanzo).

Whichever the MPPT algorithm or the PV architectureor the reconfiguration technique to analyze and test, theexecution of the experimental test activities by using realPV modules is nearly always unpractical or even impossi-ble. In fact, there are many limitations that have to betaken into account when using real PV modules. First ofall, the availability of enough solar radiation in correspon-dence of the planned experimental activity cannot bealways guaranteed because, of course, the external climaticconditions are not under control. Secondly, the repeatabil-ity of the experimental tests cannot be ensured, once againbecause of the variability of weather conditions. Moreover,a huge variety of solar panels exist. It would be pro-hibitively expensive to purchase each type of panel and test

M. Balato et al. / Solar Energy 123 (2016) 102–115 103

it individually if needed, for example for efficiency compar-ison purposes, by the specific study to be carried out.Moreover, the cost, the physical dimensions of the PVarray and the limited space on the laboratory roof forthe installation, may represent a hard problem for Univer-sities or for small/medium Companies research laborato-ries. In addition, the outdoor installation of themeasurement station is not easy, especially in presence ofunfriendly environmental conditions. Last, but not least,if tests concerning mismatching conditions are needed, inorder for example to examine the effect on the overall effi-ciency of PV sources with portion of cells which arecracked, damaged or shaded, such tests should be destruc-tive or might cause, for example in case of shading, theaccelerated aging of the PV source under test. In fact, itis well known that mismatching operating conditionsunavoidably lead to nonuniform aging and to damagingof the PV cells (Report IEA-PVPS T13-01, 2014).

The need of PV Emulators (PVEs) is the natural conse-quence of the above drawbacks. A PVE allows to repro-duce the current vs. voltage (I–V) characteristic of agiven PV field by means of a suitable controllable powersource. A non exhaustive list of commercial PVEs whichare available on the market is the following one: Keysight(Agilent) E4360 Modular Solar Array Simulators (http://literature.cdn.keysight.com/litweb/pdf/5989-8485EN.pdf),Magna-Power Photovoltaic Power Profile Emulation(PPPE) http://www.magna-power.com/files/datasheet/pppe/datasheet_pppe_2.0.pdf, Newdoll Enterprices Photo-Voltaic Emulator (http://www.accuratesolarpower.com/PVE_User_Guide_V1.5.pd).

The main drawback which prevents the widespread useof commercial PVEs in nearly all the University laborato-ries or in small/medium Companies research laboratoriesis represented by their quite high cost. In fact, it is worthnoting that in many cases, PV arrays with a relatively largenumber of PV modules need to be tested and therefore asmany PVEs are needed.

For such reasons, many papers in the literature aredevoted to the development of low cost PVEs (Sanchiset al., 2007; Lu and Nguyen, 2012; Rana and Patel, 2013;Ickilli et al., 2012; Zeng et al., 2002; Midtgard, 2007;Mukerjee and Dasgupta, 2007; Martin-Segura et al.,2007; Chavarria et al., 2014; Gadelovits et al., 2014;Balakishan and Sandeep, 2014; Woojoo et al., 2011; DiPiazza and Vitale, 2010; Nagayoshi, 2004; Mellit et al.,2010; Dolan et al., 2011; Tornez-Xavier et al., 2013;Schofield et al., 2011; Roncero-Clemente et al., 2013). Inmost papers, the PVE consists of a DC/DC power con-verter which is controlled by means of a FPGA or aDSP-based unit. More specifically, the PVEs can begrouped in three main families. The first and largest onecollects static PVEs, that is emulators which are only ableto operate with a fixed I–V curve which must be set beforethe beginning of any test and that cannot be changedduring the test itself. If different operating conditions, asconcerns irradiance and temperature values, need to be

analyzed, then the system must be turned off and a newI–V curve can be set (Sanchis et al., 2007; Lu andNguyen, 2012; Rana and Patel, 2013; Ickilli et al., 2012;Zeng et al., 2002; Midtgard, 2007; Mukerjee andDasgupta, 2007; Martin-Segura et al., 2007).

The second family collects instead semi-dynamic PVEs,that is emulators which are able to switch, during the test,from a starting I–V curve to other different I–V curveswhich have been a priori evaluated and stored in the mem-ory (Chavarria et al., 2014; Gadelovits et al., 2014;Balakishan and Sandeep, 2014; Woojoo et al., 2011; DiPiazza and Vitale, 2010; Nagayoshi, 2004; Mellit et al.,2010; Dolan et al., 2011; Tornez-Xavier et al., 2013). Ofcourse, only a limited number of curves can be storedand, due to such a limitation, only relatively simple testscan be carried out. A few PVEs are indeed able to evaluatethe actual I–V curve in real time, during the test, but a notnegligible time is required to carry out the necessary calcu-lations; so that, in any case, the actual, continuous trackingof rapid fluctuations of the irradiance is not possible. Thismay be acceptable in some cases (e.g. emulation of station-ary PV applications on buildings in case of slowly time-varying weather conditions) but it is not acceptable at allduring tests on mobile applications such as the automotiveones which can be characterized by irradiance rate of vari-ations as high as 40 kW/m2 s (Stracke et al., 2014) or whenfast time-varying weather conditions (e.g. the ones usuallycharacterizing tropical locations) need to be accounted for(Ye et al., 2013).

The last family of PVEs is composed by fully dynamicdevices. They are able to continuously track, during thetests, time-varying irradiance behaviors. In fact, such PVEsare characterized by the presence of an analog input portwhere a time-varying voltage signal, which is proportionalto the irradiance, can be applied. The input voltage signalrepresenting the irradiation can be characterized by a quitehigh speed of variation since the only limitation is repre-sented by the bandwidth of the control circuitry of theDC–DC power supply. Such a bandwidth is in turn limitedby the switching frequency and by stability considerations(Erikson and Maksimovic). As concerns the ambient tem-perature, which, together with the irradiance value andthe position of the operating point dictate the PV moduleoperating temperature, due to its relatively slow variations,no particular fast PVE dynamic capabilities are insteadrequired. Therefore, except that in very rare, particularcases concerning tests carried out with PVEs and character-ized by very long durations, the modules temperature canbe nearly always considered constant due to their relativelylarge thermal inertia.

It is worth noting that, to the best of the authors’ knowl-edge, only very few papers dealing with PVEs belonging tothe third family have been presented in the literature(Schofield et al., 2011; Roncero-Clemente et al., 2013). In(Schofield et al., 2011) the implementation of a very lowpower (up to 4 W) analog circuitry which is able to approx-imate (single diode model) the I–V characteristic of a PV

DC-DC POWER SUPPLY

LOAD

vpan(t)

ipan(t)

ipan(t)

FPAA{ IscSTC, VocSTC, IMPP, VMPP, Tmodule, αV, αI}

CURRENTCONTROLLER

ipan_ref(t)

vpan(t) Sin(t)

Fig. 1. Block diagram of the proposed architecture.

104 M. Balato et al. / Solar Energy 123 (2016) 102–115

cell is discussed. Such an analog circuitry is not useful forexperimental activities requiring higher powers (over4 W). In principle, the above circuitry could be used inorder to provide the reference control signal for a properpower stage of a high power emulator. But, since welldefined analog components, such as diodes, operationalamplifiers (OP-AMP), and bipolar junction transistors(BJT), are adopted, there is little or no flexibility at all asconcerns the final shape and characteristics of the desiredI–V curve. That is, little or no tuning capabilities can beobtained. Therefore it is nearly impossible to test perfor-mances associated to PV systems adopting different PVmodules. Moreover it is not possible to obtain I–V curveswhich exhibit the classical shape deformations appearingwhen mismatching conditions occur.

Another example of PV array emulator belonging to thethird family is presented in Roncero-Clemente et al. (2013).The power conversion stage is represented by a three-phasesynchronous rectifier (Semikron SEMISTACK SKS 230FB8CI 190 V12). The control strategy is implemented in aPC which is also equipped with data acquisition cards; Halleffect sensors measure the DC-link current and the corre-sponding currents drawn from the grid. Such an emulatoris specifically devoted to the testing of commercial PVinverters at quite high power levels.

In this paper, a new emulator belonging to the thirdfamily is proposed. Its main feature is represented by theuse of a Field Programmable Analog Array (FPAA)http://www.anadigm.com/an231e04.asp on which thedesired I–V characteristic is implemented. In particular,the FPAA has an input port where the analog voltage sig-nal Sin(t), which is proportional to the time-varying solarirradiance, is applied. In turn, an output port provides asuitable analog time varying reference signal ipan_ref(t) forthe output current control of a proper DC/DC converter.The output port of such a converter emulates the PV I–Vcurve. The main advantages of the proposed solution are:

– Both static and dynamic irradiance conditions can beemulated; in particular it is possible to emulate any typeof dynamic irradiance condition; this is highly desirablewhen testing MPPT techniques that should be able toproperly work under time varying environmentalconditions.

– The signal ipan_ref is obtained without numerical interpo-lations and without the need of the storage of bigamount of data in memory in order to emulate time-varying climatic conditions.

– As discussed in Selow et al. (2009), the FPAA is charac-terized by a great ease of reconfiguration with respect toFPGA or DSP based implementations. A very userfriendly graphical interface is provided by the manufac-turer and allows to take advantage of the various avail-able FPAA analog building blocks (adders, multipliers,integrators, PIDs, look up table, filters, etc.). Thereforethe designer programming is strongly simplified.

– Not only uniform but also mismatching operating con-ditions can be easily emulated.

– Power sources different from PV sources can be easilyemulated by using the same architecture; it is only nec-essary to program the desired I–V characteristic on theFPAA.

– Since both FPAA output and input signals are analog,no DAC or ADC converters are needed. Therefore allthe drawbacks associated to the handling of digital sig-nals are avoided.

The paper is organized as follows: Section 2 describesthe programming of the FPAA to generate the referencesignal and contains some preliminary results. Section 3reports the description and the experimental resultsobtained by using a Commercial Power Supply basedPVE which emulates a Solar World SW225 PV module(http://www.solarworld-usa.com/�/media/www/files/data-sheets/sunmodule-poly/sunmodule-solar-panel-225-poly-ds.pdf). Section 4 reports the description and theexperimental results of a Buck DC–DC converter basedPVE which emulates the same PV module. Section 5reports additional experimental results concerning theemulation of PV sources which operate in mismatchingconditions. In Section 6, the conclusions are reported.

2. Generation of the reference signal ipan_ref(t)

2.1. FPAA programming

The block diagram of the proposed PVE architecture isshown in Fig. 1. It consists of a power stage (DC/DC con-verter) whose output current is regulated by means of aproper controller. The FPAA input signals are: ipan(t)(PV output current), vpan(t) (PV output voltage) and Sin(t)(time varying signal which is proportional to the desiredirradiance). The FPAA output signal is ipan_ref(t). The inputsignal of the controller is represented by the error signalipan_ref(t)–ipan(t). Once the desired PV module type has beenselected, a set of 7 parameters must be provided to theFPAA programming tool. Such a set, as indicated in

M. Balato et al. / Solar Energy 123 (2016) 102–115 105

Fig. 1, is composed by the following parameters which aretypically provided by photovoltaic modules manufacturers:STC short circuit current (IscSTC), STC open circuit voltage(VocSTC), STC Maximum Power Point voltage and current(VMPP and IMPP), voltage and current temperaturecoefficients aV ([%/K]) and aI ([%/K]). Without loss ofgenerality, in the following we will refer to SolarWorld SW225 PV modules whose characteristics arereported in Table 1 (http://www.solarworld-usa.com/�/m-edia/www/files/datasheets/sunmodule-poly/sunmodule-sol-ar-panel-225-poly-ds.pdf).

The single diode circuital model of a PV module and itstypical I–V curve are shown in Fig. 2 (Amrouche et al.,2012). The values of the circuital components appearingin Fig. 2 can be found on the basis of the PV module char-acteristics and of the values of ambient temperature Tamb

and solar irradiance S (Kadri et al., 2012; Eichker, 2003).Eqs. (1)–(5) rule the working of the circuit of Fig. 2:

ipanðtÞ ¼ iphðtÞ � idðtÞ � iRpðtÞ ð1Þ

iphðtÞ ¼ I scSTC � SðtÞSSTC

� 1þ aI100

� Tmodule � T STCð Þh i

ð2Þ

idðtÞ ¼ I sat � evdðtÞV T � 1

� �ð3Þ

iRpðtÞ ¼vpanðtÞ þ RsipanðtÞ

Rp

ð4Þ

I sat ¼ I satSTCTmodule

T STC

� �3

e1k�

EgSTCTSTC

� EgTmodule

� �� �ð5Þ

where S is the irradiance in W/m2, Tmodule is the moduletemperature in K (in the following it has been assumedTmodule = 313 K), Isat is the diode reverse bias saturationcurrent, VT is the thermal voltage, IsatSTC is the STC diodereverse bias saturation current, vd is the voltage across thediode, k is the Boltzmann constant, EgSTC is the STC bandgap in eV, Eg is the band gap at Tmodule in eV, TSTC = 298 K,SSTC = 1000 W/m2. In the sequel, with the symbol Voc

we will indicate the open circuit voltage at Tmodule givenby the following expression (Eichker, 2003):

V oc ¼ V ocSTC � 1þ aV100

� ðTmodule � 298Þh i

ð6Þ

In order to emulate the operation of the PV module, theabove equations need to be implemented on the FPAA.The adopted FPAA is the AN231E04 by ANADIGM(http://www.anadigm.com/an231e04.asp). Such a device

Table 1Solar world SW225 PV module characteristics in STC(1000 W/m2, 25 �C, AM 1.5).

VocSTC 36.8 VIscSTC 8.17 AVMPP 29.5 VIMPP 7.63 AaI 0.034%/KaV �0.34%/K

can be programmed by means of the software ANADIGMDESIGNER 2 (http://www.anadigm.com/anadigmdesigner2.asp). A schematic block diagram of the architecture whichhas been implemented in the FPAA is shown in Fig 3.

As it is evident from Fig. 3, only some gains, adders anda Look Up Table (LUT) are necessary. Such blocks canbe easily obtained by using the Configurable AnalogBlocks (CABs) composed by sets of switched capacitors,OP-AMPs, Configuration SRAMs, etc. which are availablein any FPAA. The CABs can be easily arranged by meansof the user programming interface. In the following, themeaning and the role of the various blocks of Fig. 3 willbe explained. Preliminarily, it is worth noting that thesignals which are marked in Fig. 3 with an asterisk symbolsimply are scaled versions of the original signals. As anexample, vpan* (t) is obtained from the original voltagevpan(t) by means of a proper scaling. Such a scaling is nec-essary because of the limited range of the FPAA signals. Inparticular, the allowed range of the input and output sig-nals is [0, 3]V. Instead, internally, the FPAA can work onlywith signals belonging to the range [�3, 3]V. Indeed, forsimplicity, for the internal signals (ir*(t), iph* (t), id*(t), ipan* (t))the same scaling of the input and output signals has beenconsidered so that all the signals have been confined inthe range [0, 2.8]V. 2.8 V has been chosen, instead of 3 V,in order to ensure the right working of the FPAA, withenough margin with respect to the actual internal upperlimit. Such a choice leads to a negligible accuracy reduc-tion, in fact, as it will be evident in the following, theobtained results are very accurate for the consideredapplication. The only internal signal for which the wholerange [�3, 3]V needs to be considered during the scalingprocess is vd*(t), that is the scaled version of vd(t). Thereason of such a need will be explained in the following.The scaled signals are defined as:

v�panðtÞ ¼ 2:8 � vpanðtÞV MAX

ð7aÞ

i�panðtÞ ¼ 2:8 � ipanðtÞIMAX

ð7bÞ

S�ðtÞ ¼ 2:8 � SðtÞSSTC

ð7cÞ

v�dðtÞ ¼ 12 � vdðtÞV OC

� 9 ð7dÞ

i�dðtÞ ¼ 2:8 � idðtÞIMAX

ð7eÞ

i�RpðtÞ ¼ 2:8 � iRpðtÞ

IMAX

ð7fÞ

i�phðtÞ ¼ 2:8 � iphðtÞIMAX

ð7gÞ

i�pan refðtÞ ¼ 2:8 � ipan refðtÞIMAX

ð7hÞ

where VMAX = 48 V and IMAX = 15 A are the maximumallowed values of the power stage output voltage and cur-rent. The LUT is used in order to get the I–V characteristic

(a) (b)

Rp

Rsid iRpipan

vpan

iph

vd

ipan

Vpan

Fig. 2. Single diode circuit model of a PV module (a) and corresponding I–V curve (b).

Look Up Table

+

+

-

vd*(t)

G1

G2

9

+

+

G3

G4

-

-

+

G5

Filter

ipan*(t)

vpan*(t)

S*(t)

iRp*(t)

id*(t)

iph*(t)

ipan_ref*(t)

2.8/VMAX

2.8/IMAX

2.8/SSTC

vpan(t)

ipan(t)

S(t)

FPAA

Fig. 3. Schematic block diagram of the architecture which has been implemented in the FPAA.

106 M. Balato et al. / Solar Energy 123 (2016) 102–115

of the diode of Fig. 2. The LUT provides a specified outputvoltage in response to the value of the input voltage. TheFPAA’s LUT can implement a user specified voltage trans-fer function with 256 quantization steps. Such a transferfunction is provided to the FPAA by uploading a proper‘‘.csv” file where a single column containing 256 samplesof the output variable (in our case id*) are recorded. Suchsamples represent the output values in correspondence of256 values, taken in ascending order, of the input signal(in our case vd*) belonging to the range [�3, 3]V. The rangeof the input signal of the LUT is fixed and cannot be chan-ged by the user. The I–V diode characteristic depends onthe PV module temperature. In the proposed emulator,during the generation of the ‘‘.csv” file provided to theLUT, such a temperature is considered as a fixed parameter(in particular Tmodule = 313 K has been assumed). Ofcourse, as discussed in Section 1, such an assumption doesnot represent any loss of generality. In fact it is worth not-ing that, although the PV module temperature has a greatinfluence on the shape and on the values of the I–V charac-teristic, in all realistic scenarios, PV modules are character-ized by a more or less large thermal inertia so that theirtemperature can be considered constant in nearly all shortor medium duration experimental tests. This is the reasonwhy nearly all the PV emulators presented in the literatureassume a fixed PV module operating temperature. What isreally important in a PVE is the capability of taking intoaccount dynamic variations of the irradiance rather thanof the ambient temperature. In any case, if a different oper-ating temperature needs to be considered, the LUT can beeasily and quickly reloaded by means of a new ‘‘.csv” file.

Of course, in case of experimental activities character-ized by longer durations, also the module temperatureneeds to be handled by the FPAA as a time-varying signal.In such a case, the number of CABs which are available ina single FPAA are not enough and two FPAAs are indeednecessary. But, we explicitly remark here, that the increaseof complexity and of hardware resources required in orderto take into account time-varying temperatures is not at allnecessary in nearly all realistic cases. Therefore, withoutany further specification, all the results shown in the fol-lowing sections refer to a constant module temperatureequal to 313 K.

It is worth noting that, as shown in Fig. 3, the irradianceS(t) is instead entered in the FPAA as a time varying volt-age signal. As shown in the following, the FPAA basedcontrol architecture is fully able to track realistic irradiancetemporal variations.

The voltage vd(t) across the diode can assume only val-ues belonging to the interval [0, Voc]. In order to improvethe accuracy, by lowering the quantization step, it has beenassumed that id(t) = 0 for vd(t)I[0, Voc/2] and id(t) > 0 forvd(t)I[Voc/2, Voc]. Therefore it is necessary to store in theLUT only the values of id(t) for vd(t)I[Voc/2, Voc] sincethe LUT will automatically output a value equal to zerofor vd(t)I[0, Voc/2]. In this way, the LUT resolution DVd

changes from the value reported in Eq. (8) to the valuereported in Eq. (9):

if vdðtÞ 2 ½0; V OC� ! DV d ¼ V OC

256ð8Þ

if vdðtÞ 2 V OC

2; V OC

� �! DV d ¼ V OC=2

256¼ V OC

512ð9Þ

M. Balato et al. / Solar Energy 123 (2016) 102–115 107

The above assumption means that we are consideringequal to zero all the diode currents lower than

idðtÞjV oc2¼ I sat � e

V oc2�V T � 1

� �¼ 6:85 � 10�5 A. Such a thresh-

old is reasonable for practical purposes since, in theconsidered case, at vd(t) = Voc/2 the photocurrent iph(t) isnearly equal to 8.17 A, current iRp = 7.4 _s 10�2 A, currentiRs = 8.096 A and therefore idðtÞjV oc

2can be considered neg-

ligible with respect to all other currents flowing in the cir-cuit of Fig. 2.

In order to better clarify the meaning of the aboveassumption, the output of the LUT is shown in Fig. 4 incorrespondence of an input voltage belonging to the range[�3, 3] V. In particular, Fig. 4(a) has been obtained byusing Eq. (10a) while Fig. 4(b) has been obtained by usingEq. (11a). It is evident that, by using Eq. (11a), a consider-ably greater accuracy can be obtained.

if v�dðtÞ ¼ 6 � vdðtÞV OC

� 3 2 ½�3; 3�V ð10aÞ

8vdðtÞ 2 0; V OC

2

� ! v�dðtÞ 2 ½�3; 0�V8vdðtÞ 2 V OC

2; V OC

� ! v�dðtÞ 2 ½0; 3�V

8<: ð10bÞ

if v�dðtÞ ¼ 12 � vdðtÞV OC

� 9 2 ½�9; 3�V ð11aÞ

8vdðtÞ 2 0; V OC

2

� ! v�dðtÞ 2 ½�9;�3�V8vdðtÞ 2 V OC

2; V OC

� ! v�dðtÞ 2 ½�3; 3�V

8<: ð11bÞ

On the basis of the above considerations, the gains ofthe scheme in Fig. 3 assume the following values:

G1 ¼ 12 � V MAX

2:8 � V oc

ð12aÞ

G2 ¼ 12 � Rs � IMAX

2:8 � V oc

ð12bÞ

G3 ¼ V MAX

Rp � IMAX

ð12cÞ

G4 ¼ Rs

Rp

ð12dÞ

G5 ¼ 1þ aI100

� ðTmodule � T STCÞh i

� I scSTCIMAX

ð12eÞ

Fig. 4. Comparison among the theoretical diode characteristic and the LUT

The model shown in Fig. 3 has been implemented on theFPAA by loading, by means of the software ANADIGMDESIGNER 2, the configuration shown in Fig. 5.

2.2. Preliminary test results

A preliminary test activity has been carried out on theFPAA. The results are represented by the oscilloscopescreenshots of Fig. 6. In particular, a periodic (frequency100 Hz) ramp signal growing from 0 V to 2.15 V (corre-sponding to a PV module ramp voltage vpan(t) from 0 toVOC) has been applied at the FPAA input vpan* (t) ofFig. 5. The output signal ipan_ref* (t) has been connected tothe FPAA input port ipan* (t). Moreover, in Fig. 6(a), theinput signal S*(t) is equal to 2.8 V (corresponding to S(t)= 1000 W/m2), while in Fig. 6(b) the input signal S*(t) isequal to 1.12 V (corresponding to S(t) = 400 W/m2). InFig. 6, the input ramp vpan* (t) and the corresponding outputsignal ipan_ref* (t) of the FPAA are reported in the two con-sidered irradiance conditions. The obtained results are veryaccurate as confirmed by Fig. 7 where the comparisonamong the emulated curves (non scaled versions) and thetheoretical curves are shown. It can be observed that emu-lated and theoretical curves are nearly overlapped, thusconfirming the goodness of the characteristics providedby the FPAA. The main motivation behind the preliminarytest activity which has been discussed in this section is tocheck the results which could be obtained in an idealPVE characterized by a perfect current control in theDC/DC converter representing its power stage. In fact,the FPAA output signal ipan_ref* (t) has been directly fedback to the FPAA input port ipan* (t). In a real applicationinstead, the actual input signal at the input port ipan* (t),rather than ipan_ref* (t), would be the scaled version of theactual current sensed at the output of the power stage.But, in case of perfect control, such a current would justbe perfectly coincident with ipan_ref* (t).

3. Experimental results obtained by means of a commercial

power supply

In order to test the potentialities of the FPAA based PVemulator, the system shown in Fig. 1 has been assembled inthe laboratory (Fig. 8). In particular, the set composed by

output characteristic obtained by using Eq. (10a) (a) and Eq. (11a) (b).

vpan*(t)

ipan*(t)

S*(t)

ipan_ref*(t)

Fig. 5. Architecture of the Configurable Analog Blocks (CABs) of the FPAA.

Fig. 6. I–V curves provided by the FPAA and displayed by the oscilloscope, (a) S = 1000 W/m2, Tmodule = 313 K and (b) S = 400 W/m2, Tmodule = 313 K.

Fig. 7. Comparison among emulated and theoretical curves, (a) S = 1000 W/m2, Tmodule = 313 K; (b) S = 400 W/m2, Tmodule = 313 K.

108 M. Balato et al. / Solar Energy 123 (2016) 102–115

the DC/DC converter and the current controller has beenobtained by means of a commercial power supply. Theadopted power supply is a Kepco BOP 36-12M, a deviceable to work in all four quadrants of the current voltageplane. It is a linear power supply with two bipolar controlchannels (voltage or current mode), selectable and individ-ually controllable either by front panel controls, or by

remote signals. The positive and negative current orvoltage limit points can be manually set or remotelyprogrammed simultaneously or individually. The BOPcan act as either a source or a sink (http://www.kepcopower.com/support/opmanls.htm#bop). It is worthnoting that, any other equivalent controllable powersupply, which in case is available in the laboratory, can

Fig. 8. Commercial power supply based PV emulator system.

M. Balato et al. / Solar Energy 123 (2016) 102–115 109

be used. Of course, a PV emulator architecture based onthe use of a commercial power supply is not cheap, but itallows a consistent saving of time and efforts. Thereforewe explicitly remark here that, before showing the experi-mental results obtained by using a homemade Buck DC/DC converter, it is useful also to preliminarily show thatthe use of a FPAA grants great flexibility and great simplic-ity of implementation whichever the I–V characteristic tobe emulated. It is enough to obtain by the FPAA, after afew, easy and fast programming steps, a proper analog out-put reference signal which can be used to control the powersupply.

The picture of the whole system which has been assem-bled in the laboratory is shown in Fig. 8. In such a system,two commercial power supplies have indeed been used. Thefirst one (POWER SUPPLY PS1, a Kepco BOP 36-12M)represents the power stage of the PVE and it is used as avoltage controlled current generator. The second one(POWER SUPPLY PS2, a Kepco BOP 100-10MG(http://www.kepcopower.com/support/opmanls.htm#bop)is instead used as a controlled load in order to scan thewhole I–V characteristic of the PV module to emulate.

In addition, in Fig. 8 it is also possible to observe thepresence of a Teledyne Lecroy oscilloscope for the visual-ization of the emulated PV characteristic, a NationalInstruments data acquisition board for uploading theobtained experimental data in Labview, a signal generatorwhich is used to reproduce different dynamic irradianceconditions, a LEM current transducer and an OPAMPbased FPAA-Power Supply interface (which is used toadapt the allowed ranges of the FPAA output and of thePS1 input) with their DC voltage supply.

The power supply PS2 has been used to apply a periodicramp voltage at the output of PS1. Such a voltage grows

from 0 V to VOC with a frequency of 1 Hz. It has been ver-ified that the adoption of scan frequencies higher than afew Hz unavoidably leads to the distortion of the obtainedcurrent waveform. This is of course a drawback associatedto the more or less limited bandwidth of PS1, since it hasbeen previously shown in Section 2 that the FPAA isinstead fully able to correctly work at scan frequencies upto 100 Hz.

The oscilloscope screenshots of Fig. 9 show the scaledscan voltage vpan* (t) (periodic ramp provided by PS2 andcharacterized by a repetition frequency equal to 1 Hz)and the corresponding scaled sensed output current ipan* (t)of the PVE (PS1). In particular, Fig. 9(a) refers to a staticirradiance level S(t) = 1000 W/m2, while in Fig. 9(b) it is S(t) = 400 W/m2. In practice, the results shown in Fig. 9 arethe waveforms which have been obtained in the powerstage and which directly correspond to the FPAA signalsshown in Fig. 6. The obtained results are very accurateas confirmed by Fig. 10 where the comparison among theunscaled emulated curves (PS1 output) and the theoreticalcurves are shown. Also in this case it can be observed thatthe emulated and the theoretical curves are nearly superim-posed, thus confirming the goodness of the proposed sys-tem. It is worth noting that, in nearly all the paperspresented in the literature and dealing with PV emulators,usually, to the best of the authors’ knowledge, the compar-ison among theoretical results and experimental results iscarried out by showing the position, in the I–V plane, ofa more or less limited number of experimental points withrespect to the continuous theoretical I–V characteristic. Inthe case shown in Fig. 10 instead, not only the theoreticalI–V curve but also the experimental I–V curve iscontinuous. This confirms the excellent dynamic behaviorof the proposed system. In other words, the proposed

Fig. 9. I–V curves emulated by the power supply PS1 and displayed by the oscilloscope, (a) S = 1000 W/m2, Tmodule = 313 K, (b) S = 400 W/m2,Tmodule = 313 K).

Fig. 10. Comparison among emulated and theoretical I–V curves, (a) S = 1000 W/m2, Tmodule = 313 K, (b) S = 400 W/m2, Tmodule = 313 K.

110 M. Balato et al. / Solar Energy 123 (2016) 102–115

system is very fast so that it is not necessary to wait formore or less relatively long transients when changing froma steady-state operating point to another steady-state oper-ating point and the proposed system is able to track a rel-atively fast time-varying PV module ramp voltage signal(up to 1 Hz).

Also the case of time varying irradiance levels has beenof course examined, in order to assess the dynamic capabil-ities of the proposed emulator. The results of the corre-sponding experimental tests are shown in Fig. 11. Inparticular, the results shown in Fig. 11(a) refer to an irra-diance characterized by a periodic (frequency 0.1 Hz) ramplike time domain behavior which leads to an irradiancechange from 200 W/m2 to 1000 W/m2 in 10 s (constantirradiance rate of change equal to 80 W/(m2 s)). The resultsshown in Fig. 11(b) refer instead to an irradiance character-ized by a periodic (frequency 0.1 Hz) square wave signalwith a lower level equal to 400 W/m2 and an higher levelequal to 1000 W/m2. The oscilloscope screenshots ofFig. 11 show both the scaled input irradiance signal S*(t)(blue curve1) and the PV emulator output current ipan* (t)(green curve) which is obtained while the PV emulator out-put voltage is periodically swept (frequency 0.5 Hz) bymeans of PS2 between 0 and Voc.

1 For interpretation of color in Fig. 11, the reader is referred to the webversion of this article.

In both cases the obtained results are in perfect agree-ment with the expected theoretical results; the comparisonamong experimental and theoretical curves has not beenreported here for the sake of brevity.

4. Experimental results obtained by means of a Buck DC/DC

converter

The results which are reported in this section refer to apower stage of the PVE which has been obtained by meansof a current controlled Buck DC/DC converter (which hasbeen built in the laboratory) rather than by a commercialpower supply. The resulting PVE system is of coursecheaper, with respect to the adoption of a commercialpower supply (Table 2), but of course more time and addi-tional efforts have been required in order to design andbuild both the power stage and the control circuitry ofthe Buck converter. The block diagram of the Buck basedPVE is shown in Fig. 12. The objective of the PWM controlcircuitry is of course to get an output current ipan* (t) whichcorrectly tracks the reference signal ipan_ref* (t) generated bythe FPAA device.

The design of the PI compensator’s transfer function Gc

and a preliminary analysis of the scheme of Fig. 12 arereported in Barra et al. (2014). The experimental setup isshown in Fig. 13. It can be observed that two FPAAs havebeen used. The first one (FPAA PV Reference, FPAA1) isused to generate the reference signal ipan_ref* (t) by adopting

Fig. 11. Constant irradiance rate of change equal to 80 W/(m2 s) (a), periodic square wave irradiance with a lower level equal to 200 W/m2 and an higherlevel equal to 1000 W/m2 (b).

Table 2Costs of the main commercial photovoltaic emulators and of the proposed emulator.

Model Cost per unit Total cost

FPAA PVE based on �11 € (FPAA) �111 €

Buck DC/DC converter �100 € (Buck)FPAA PVE based on �11 € (FPAA) �3211 €

Kepco BOP 36-12M �3200 € (Kepco)Keysight (Agilent) E4360 modular solar array simulators (http://literature.cdn.keysight.com/litweb/pdf/

5989-8485EN.pdf)�10,000 € �10,000 €

Magna-Power (PPPE) (http://www.magna-power.com/files/datasheet/pppe/datasheet_pppe_2.0.pdf) �441 € (PPPE software) �3441 €

Emulation software for Magna-Power DC power supply �3000 € (Magna-power DCpower supply)

Chroma ATE 62150H programmable DC power supply for solar array simulation (http://www.mhzelectronics.com/ebay/manuals/chroma_62000h-series_power_supply_datasheet.pdf.)

�7000 € �7000 €

vpan (t)Vg

L

C

ipan _ref*(t)

+-Gc

Compensator e(t)PWM

vc(t)Transistor gate driver

δ(t)

ipan (t)

Load

FPAA

ipan*(t)

vpan*(t)

Current sensor

Voltage sensor

Fig. 12. PWM current control of the Buck PVE.

M. Balato et al. / Solar Energy 123 (2016) 102–115 111

the scheme of Fig. 5. The second one (FPAA compensator,FPAA2) is instead used to implement the PI compensatorGc and the PWM block in order to generate the duty cyclesignal d(t) driving the MOSFET of the Buck. The FPAA2is programmed by adopting the scheme shown in Fig. 14.Of course the PI compensator and the PWM block couldalso be obtained by using discrete analog componentsrather than the FPAA2. The choice to use the FPAA2has been once again dictated by the possibility to get thedesired control blocks in a very easy way, after only sometrivial and rapid FPAA programming steps. The resultsshown in Fig. 15 (white curves) have been obtained byuploading in the Labview environment the experimental

results obtained by setting at the output port of the Buckconverter, by means of PS2, a ramp voltage growing from0 to Voc in 1 s. The black curves of Fig. 15 representsinstead the theoretical results. In particular, Fig. 15(a)refers to a static irradiance level S(t) = 1000 W/m2, whilein Fig. 15(b) it is S(t) = 600 W/m2. A very good matchingamong experimental and theoretical curves is evident.

5. Experimental results concerning mismatching operating

conditions

The adoption of a FPAA based PVE control circuitrymakes quite easy also the emulation of I–V characteristics

Fig. 13. Buck DC–DC converter based PV Emulator.

Fig. 14. Implementation of the PI compensator Gc and of the PWM block on the FPAA2.

Fig. 15. Experimental and theoretical I–V curves; S = 1000 W/m2, Tmodule = 313 K (a); S = 600 W/m2, Tmodule = 313 K (b).

112 M. Balato et al. / Solar Energy 123 (2016) 102–115

M. Balato et al. / Solar Energy 123 (2016) 102–115 113

associated to mismatching conditions. As an example, withthe architecture which has been proposed in this paper, it isvery easy to emulate a PV array composed by N PV mod-ules connected in parallel and working in mismatching con-ditions. It is enough to use the instantaneous sum of the Nreference signals provided by the N different FPAAs as areference signal for the output current of a unique PVEpower stage. In other words, no change of the power stageis required in order to emulate N PV modules connected inparallel, under mismatching conditions. This is of coursetrue provided that the voltage and/or the current ratingsof the power stage are enough high for the applicationsunder study, that is, provided that the power stage is ableto emulate the same N PV modules connected in parallel,under uniform operating conditions. Instead, in order toemulate a PV array composed by N PV modules connectedin series and working in mismatching conditions, it is nec-essary to use N FPAAs programmed with a ‘‘dual imple-mentation”. In fact, while the implementation which hasbeen proposed in this paper is based on the relationipan_ref = f(vPAN, iPAN), the ‘‘dual implementation”, usefulin order to emulate a PV array composed by N series con-nected PV modules, must be based on the relationvpan_ref = g(vPAN, iPAN), where vpan_ref is the reference sig-nal for the output voltage of the PVE power stage. Suchan implementation is of course not discussed in this paperfor the sake of brevity and also because of the fact that itcan be easily derived from the original implementation bymeans of obvious ‘‘dual” principles and considerations.

With such a dual implementation, it is very easy to emu-late a PV array composed by N PV modules connected inseries and working in mismatching conditions. It is enoughto use the instantaneous sum of the N reference signals pro-vided by the N different FPAAs as a reference signal for theoutput voltage of a unique PVE power stage.

In conclusion, the emulation of N PV modules undernon-uniform operating conditions does not imply any sub-stantial additional practical difficulty. In both cases of par-allel connection of N PV modules under mismatchingconditions or of series connection of N PV modules under

Fig. 16. Oscilloscope screenshots with the I–V curves provided by the twoSB = 500 W/m2, TB = 303 K.

mismatching conditions an analog adder is additionallyrequired. It is needed just in order to sum the signals whichare provided by the N FPAAs and which represent the NPV currents, in the case of parallel connection, or the NPV voltages, in the case of series connection. Obviously,a proper scaling of the signals is also needed in order tocomply with the input range of the control circuitry ofthe power stage.

In this section experimental results concerning the emu-lation of non-uniform operating conditions are reported.In particular, two test cases have been considered with ref-erence to a couple of PV modules which operate in mis-matching conditions. One PV module (module A)operates at an irradiance level SA and at a moduletemperature TA. The other PV module (module B) operatesat an irradiance level SB and at a module temperature TB.SA = 1000 W/m2, TA = 363 K, SB = 500 W/m2, TB =303 K. In Fig. 16 the oscilloscope screenshots containingthe output signals ipan_ref* (t) of the two FPAAs emulatingthe characteristics of modules A and B are reportedtogether with the input ramp vpan* (t) used to scan suchcharacteristics. The first test which has been performedconcerns the parallel connection of such two modules. InFig. 17(a) the oscilloscope screenshot with the referencesignal ipan_ref_tot* (t) (sum of the two current reference signalswhich are provided by the two FPAAs) together with theramp voltage vpan* (t) used to carry out the scan is reported.The second test case concerns the series connection of thetwo modules. In Fig. 17(b) the oscilloscope screenshot withthe output signal vpan_ref_tot* (t) (sum of the two voltage ref-erence signals which are provided by the two FPAAs)together with the ramp signal ipan* (t) used to carry out thescan is reported. As written above, ipan_ref_tot* (t) andvpan_ref_tot* (t) can be respectively used as reference signalsfor the output current (in the case of the parallel connec-tion) or for the output voltage (in the case of the series con-nection) of the PVE power stage. The experimental I–Vcurves (Fig. 17) are in very good agreement with thecorresponding theoretical curves. The comparison is notreported here for the sake of brevity.

FPAAs, (a) module A, SA = 1000 W/m2, TA = 363 K, (b) module B,

Fig. 17. Oscilloscope screenshots, (a) I–V curve of the parallel connection of modules A and B, (b) V–I curve of the series connection of modules A and B.

114 M. Balato et al. / Solar Energy 123 (2016) 102–115

6. Conclusions

In this paper, a new fully dynamic PVE has been pro-posed. Its main feature is represented by the use of a FPAAon which the desired I–V characteristic can be imple-mented. The main advantages of the proposed solutionare the following ones:

– both static and dynamic irradiance conditions can beemulated;

– no numerical interpolations and no storage of bigamount of data in memory are required;

– the FPAA is characterized by a great ease of reconfigu-ration and programming with respect to FPGA or DSPbased implementations;

– no DAC or ADC converters are needed;– not only uniform but also mismatching operating condi-tions can be easily emulated;

– power sources different from PV sources can be easilyemulated by using the same architecture.

The obtained experimental results allow to assess thevalidity of the proposed FPAA based PVE architecture.A further advantage of the proposed PVE is representedby its quite low cost. For the sake of completeness, inTable 2 the costs of the main commercial devices and ofthe proposed PVE are reported.

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