FPGA based new MPPT (maximum power point tracking) method for PV (photovoltaic) array system operating partially shaded conditions

lable at ScienceDirect

Energy xxx (2014) 1e12

Contents lists avai

Energy

journal homepage: www.elsevier .com/locate/energy

FPGA based new MPPT (maximum power point tracking) method forPV (photovoltaic) array system operating partially shaded conditions

Koray Sener ParlakCollege of Technical Science, Department of Electronics and Automation, Firat University, Elazig, Turkey

a r t i c l e i n f o

Article history:Received 18 September 2013Received in revised form19 December 2013Accepted 6 February 2014Available online xxx

Keywords:Photovoltaic panelsPartial shaded conditionsMaximum power point trackerFPGA

E-mail addresses: [email protected], kparlak@fi

http://dx.doi.org/10.1016/j.energy.2014.02.0270360-5442/� 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Parlak KS, Foperating partially shaded conditions, Energ

a b s t r a c t

In PV (photovoltaic) systems, MPPT (maximum power point tracking) methods play highly crucial rolefor its own research domain. Researchers can get effective results using traditional MPPT methods whichwork well under uniform irradiance conditions. On the other hand mentioned methods cannot alwaysrun successfully since there is more than one local maxima in PV characteristics under partial shadingconditions. Under partial shading conditions, one can meet to various approaches in the literatureworking accurately they can, however, reveal complex calculations or requires some of extra parameterssuch as datasheet values, system configuration data. Here, we propose a novel MPPT method providingGMPP (global maximum power point) for PV arrays under any environmental conditions. This method isbased on sensing the current and voltage values of a capacitor connected to the output of the PV arrayduring the charging time. Then it compares instantaneous power values to maximum power, and esti-mates maximum power value and corresponding voltage value. Matlab-Simulink and FPGA based testsystem have been utilized for verification stage of the proposed MPPT method under uniform andpartially shaded conditions, and we got promising results.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Using renewable energy sources have been increased againstharmful of conventional energy sources such as greenhouse effect,prices etc. Among the renewable energy sources, photovoltaic issuch popular. As very well known, PV modules convert the solarenergy into electricity. There are a lot of studies on PV systems,their control with/without main system, and improvement theirefficiency in regarding new methods and manufacturing.

PV modules operate in very low efficiency and they able convertsolar energy into electricity approximately 18e23%. Additionally,their efficiency is highly affected by environmental factors. Effi-ciency of a PV module depends on temperature and mostly irra-diance level. Irradiance level of PV module is great extended byangle of sunshine and shadings from birds, trees, buildings, cloudsand dusts etc. For this reason, in order to gain maximum efficiencyfrom PV system there are developed some method in changingenvironmental condition such as sun trackers and maximum pointtrackers.

Working point a PV module in producing maximum power iscalled as MPP (maximum power point). When there is a change in

rat.edu.tr.

PGA based newMPPT (maximy (2014), http://dx.doi.org/10

environmental condition especially irradiance level or partialshading, MPP of PV module is changing also and this point must beredefined. In order to do this, MPPT (MPP tracker) devices are usedbetween the PV array and the load and ensure that the PV systemoperates at MPP. The MPPT device is basically a dcedc converterand it works a particular algorithmwith adjusting the duty cycle totrack MPP.

There are several MPPTmethods in literature. The early reportedones uses the open circuit voltage and short circuit currentmethods[1,2]. The methods based on that the MPP is approximately 80e90%of the open circuit voltage (Voc) or the short circuit current of thepanel. Implementations of these methods are very easy but theycan’t work under partially shaded conditions because of the MPPpoint vary with irradiance level and temperature. P&O (Perturba-tion and Observation) and IC (Incremental Conductance) are bestknown and widely used MPPT methods [3e7]. In the P&O method,operating point of the module to make changing continuously andtheMPP is located by comparing the power and voltage informationof the PV module to previous values. In the IC method, operationpoint of module is determined which side of MPP by derivation ofpower to voltage and then, this point towards to MPP via tuningduty cycle. Both methods are working effectively under uniformirradiance because of only one MPP formed in this condition.However, in order to occur multiple local maxima under partially

umpower point tracking) method for PV (photovoltaic) array system.1016/j.energy.2014.02.027

Delta:1_given name

mailto:[email protected]

mailto:[email protected]

www.sciencedirect.com/science/journal/03605442

http://www.elsevier.com/locate/energy

http://dx.doi.org/10.1016/j.energy.2014.02.027



IRs

VIpv D

Id

Rp

Fig. 1. Equivalent circuit of a photovoltaic cell.

K.S. Parlak / Energy xxx (2014) 1e122

shading condition these methods can fail to identify which one isGMPP (global MPP). To reduce partial shade effect on PV array, morethan one MPPT is used each PV panel or string of array [8e10]. P&Oor IC based these MPPTs, can operate at MPP where it connectedpart of array. In another study, dcedc bidirectional Cuk convertersare used instead of bypass diodes and used main converter for P&Obased MPPT [11]. An expected disadvantage of the method is theneed for many MPPT devices. Another MPPT method includes twolevel adaptive control processes. Although these methods enhancethe efficiency of system by decreasing ripples with controllingconvertor, it is not used in partially shading conditions [12].

There are some MPPT methods which used heuristic algorithmsare reported and they can work in partial shading condition. PSO(particle swarm optimization) method need to some knowledge ofsystem such as cell number, irradiance and temperature [13].Another PSO algorithm and ant colony optimization based MPPTmethod divide PV array into such submodules and use dcedcconvertor each submodule [14,15]. The MPPT methods that areusing artificial neural network and Genetic Algorithm can workingunder partially shading condition but these methods require serialmodule number and Voc of array [16e18]. Recently, several worksare carried out to employ FLC (fuzzy logic control) for MPPT [19].Although FLC MPP tracking is effective, it requires extensive pro-cesses which include fuzzification, rule base storage, inferencemechanism, and defuzzification operations and some systemknowledge (Isc and Voc) [20]. Consequently, compromise has to bemade between tracking speed and computational cost.

In somedevelopedMPPTmethods,while theoperatingpointofPVarray is changing the current and voltage values are stored andGMPPis determinedwith using stored data [21,22]. Thesemethods use P&Omethod and successively locate the GMPP under partially shadedconditions. However, the methods take long computational time.

Another developed MPPT method uses the association betweenall MPPs and the corresponding voltage values [23]. P&O based thismethod require Voc and the numbers of series panels of array. Inanother study, the MPP in the reference voltage area obtained froman equation solved by using the Voc, the Isc, the number of parallelsubmodules and the number of panels in the strings is shown asGMPP [24]. The drawbacks of these methods are need to the PVpanel catalog values and/or the knowledge of their connectionconfiguration. Therefore they are not really applicable for anyoperation condition.

This studyproposesanewMPPTmethodto locate theGMPPof thearray under partially shaded conditions. A capacitor connected to theoutput of the PV array and obtains the current and voltage data of thearray while it charging time. Then, the GMPP and the correspondingvoltage value at this point are determined with the proposedalgorithm. This voltage is used as the reference voltage informationby the algorithm, and the PV array is operated at this voltage point.

Thescientific contributionsofproposedMPPTmethodaregivenas

� The method can work independently from panel parametersand datasheet values. Additionally it doesn’t require of arrayconfiguration data. Thus, PV modules with different character-istics and power ratings can be used together in array.

� The proposed MPPT method has low hardware requirement,and it needs no complex algorithm.

� The method can find array’s GMPP highly accurate in any irra-diance level and under partial shade conditions.

� The algorithm of proposed method runs quite fast and it causesto locate the GMPP under rapidly changing environmentalconditions.

The validity of the proposed method firstly has been performedin the Matlab-Simulink environment by creating different partial

Please cite this article in press as: Parlak KS, FPGA based newMPPT (maximoperating partially shaded conditions, Energy (2014), http://dx.doi.org/10

shade conditions on the PV array consisting of serially connected3 PV panel. Then, the method has been tested by FPGA controlledexperiment system under different shade conditions.

2. S_IMUL_INK model of the PV panel

To simulate a PV panel, a panel model based on the equivalentcircuit of a PV cell was developed. Therefore, the PV panel model isexplained first. Then, a PV string simulation shown under partiallyshaded conditions is described using the developed PVpanelmodel.

2.1. PV panel model

PV panels are a network of cells, which are the most basic unitsto obtain electrical energy. These cells may have different parallel/series configurations, and the output current and voltage of thepanel depend on this configuration. The most accurate panel modelused in the literature consists of dual diodes [25,26]. Because thismodel has six parameters, the solution requires more complexequations. Therefore, the single diode model known also as thefive-parameter model (Rs, Rp, Ipv, Io, A) has been preferred in thedeveloped PV panel model. The equivalent circuit of the singlediode PV cell is shown in Fig. 1.

Equation (1) shows the output currentevoltage characteristic ofan ideal PV cell in a single diode model.

I ¼ Ipv � I0

�exp

�V þ RsIVtA

�� 1

�� V þ RsI

Rp(1)

To obtain the single diode model mentioned above, five param-eters must be determined. The Rs and Rp values in the model areseries and parallel resistors in the equivalent circuit, respectively,and these parameters vary depending directly on the microstruc-ture of the photovoltaic cell. Several studies in the literaturedescribeways of obtaining these values [27]. However, a detailed account ofthe computation of these values is beyond the scope of this study.

The photoelectrical current (Ipv) in the model depends on theradiation that falls on the photovoltaic cell and the temperature.However, the change in the diode saturation current (Io) isdependent solely on the temperature. As a result, the model iscompleted with Equations (2) and (3) below.

Ipv ¼ GGn

ðIscn þ KiðT � TnÞÞ (2)

Io ¼ Isc þ KiðT � TnÞeVocnþKvðT�Tn Þ

AVt�1

(3)

Here, Iscn, is the short circuit current at standard test conditions(Gn ¼ 1000 W/m2, Tn ¼ 25 �C) and air mass (AM ¼ 1.5). T and Tnrepresent the real and nominal temperatures of the cell, respec-tively; and G and Gn are the real and nominal radiation levels,


10 20 30 40 50 60 700

1

2

3

4C

urre

nt (

A)

0 10 20 30 40 50 60 700

20

40

60

80

100

Pow

er (W

)

Voltage (V)

GM PP LocalMPP

LocalMPP

Fig. 2. IeV and PeV characteristics of the PV array in partial shade conditions.

Fig. 4. Obtaining the IeV and PeV curves by using a capacitor.

K.S. Parlak / Energy xxx (2014) 1e12 3

respectively. Ki is the temperature coefficient of the short circuitcurrent, and Kv is that of the open circuit voltage.

Vt, given in Equation (1), is the thermal junction constant and isequal to the kT/q value. Here, k is the Boltzmann constant(1.38*10�23 J/�K), and q is the electron load (1.602*10�19 c).

The final parameter used in the model is the diode idealityconstant (A), whose value varies between 1 and 2 depending on thetype of the PV panel. The equations given above are valid for onesingle cell. These equations vary depending on the number of seriesand parallel cells used in the panel model [28].

The simulation of the PV panel was built using these equations.In this model, the irradiation level (G e W/m2) and temperature(T-degrees) values can be input separately, and thus the simulationsare very flexible.

It is expected from PV panel model is to obtain partial shadingconditionandto response toconvertor reaction. Therefore themodelis establishedwith some neglects such as Rs variation and avalancheeffect. These effects are important inpointof highaccurate cell/panelmodel nevertheless the model is sufficient to prove MPPT method.

2.2. IeV and PeV characteristics of a PV string under partiallyshaded conditions

In order to get more power from PV array, PV panels are con-nected serial/parallel. Since the number of panels increases so doesthe surface of the array. Hence, possibility of partial shade on thesurface of array is increased and in this situation, more than oneMPP is formed and they are called as “local maxima”.

Fig. 3. Circuit model of the proposed method.


To explain better this situation, the IeV and PeV characteristicsunder partially shaded conditions on serially connected 3 PV panelshave been investigated in the simulation. In the string, each PVpanel has equivalent characteristics; the open circuit voltage isVoc ¼ 22 V, the short circuit current is Isc ¼ 3.85 A, and the powerrating is 55W. Three different irradiation-temperatures values havebeen applied to the PV panels. These are 400 W/m2 e 40� in Panel-1; 700 W/m2 e 55� in Panel-2; and 1000 W/m2 e 70� in Panel-3. Inaddition, each PV panel in the array has bypass diodes. In Fig. 2, theIeV and PeV curves of the PV array in partially shaded conditionsare given. As seen clearly in the PeV curve, there exist three MPPs.Two of these are local maxima, and one is the real MPP or GMPP.

The task of MPPT locates the Global MPP among the severalMPPs formed under partially shaded conditions and is to operatethe PV array at this point. Therefore the MPPT algorithm has tocontinuously redefine the GMPP every changing environmentalcondition. In more powerful PV arrays, the efficiency of system canbe significantly increased with well working MPPT devices that canoperate the system at the GMPP.

3. Definition of the proposed method

This study proposes a new MPPT method to identify the GMPP(global MPP) of the array by using the data on the PeV curve ob-tained by using a capacitor and to ensure that it operates at this

Fig. 5. IeV and PeV curves of the PV panel at different irradiance levels while constanttemperature value (T ¼ 70 �C).

um power point tracking) method for PV (photovoltaic) array system.1016/j.energy.2014.02.027

0 5 10 15 20 250

1

2

3

4

Cur

rent

(A)

0 5 10 15 20 250

20

40

60

Voltage (V)

Pow

er (

W)

70 degree50 degree30 degree

Fig. 6. IeV and PeV curves of the PV panel at different temperature values whileconstant irradiance level (G ¼ 1000 W/m2).


point. In the proposed method, the PeV curve of the array can beobtained according to the currently environmental conditions andits global MPP can be found. Fig. 3 shows the circuit schema of themethod. The PV array, which is formed by serially connected 3 PVpanels, is investigated in the simulation in the following section.

In the proposed method, the switch S2 is used to connect thecapacitor “C” to PV array and the switch S3 is used to discharge itover the resistance “R” for obtaining the PeV curve next time. Thus,every time the environmental conditions change, the capacitor isrecharged, and the current and voltage values are sensed for thiscondition. When S2 is in the “closed” position, the switch S1 isbrought open to separate the converter from the PV array, whichthen feeds the capacitor only. In this way, the PeV curve of the arrayis obtained during charging capacitor according to the exact envi-ronmental conditions at a given time. Because the capacitor ischarged in milliseconds and the MPPT algorithm does not requirecomplex calculations, the load is left without power for a negligibletime. This time and dynamic response will be explained in simu-lation results section.

3.1. Obtaining IeV and PeV curves

In order to get IeV and PeV curves of the PV panel, the outputcurrent and the voltage values are needed. The current and the

Fig. 7. Current (blue), voltage (yellow) and power (red) curves of the PV panel while a capaconditions. (For interpretation of the references to color in this figure legend, the reader is


voltage values on the IeV curve of the PV panel are the data sensedduring the transition of the panel from the short circuit position toopen circuit. To provide this transition, the load fed by the panelmust be changed from a minimal resistance (w0 U) to a very highresistance (wNU). The current and voltage values of the PV systemcan bemeasured during this transition, and the IeV and PeV curvescan be obtained.

There are several methods to obtain IeV and PeV curves asexplained above. Commonly used methods are the connection ofthe rheostat and electronic load at the output of the PV array [29].In the both of these methods, by changing the load resistance of PVmodule as mentioned, these curves can be found.

The other method used in the study is to connect a capacitor atthe output of the PV module. As very well known, in the chargeperiod of a capacitor, it resistance is to vary in the interval of 0 toN.Thus, the IeV and PeV curves can be obtained by measuring theoutput current and the voltage values of the PV array within thischarging time. The equation (4) gives the charging time of the panelor the array of the capacitor depending on the short circuit current(Isc) and open circuit voltage (Voc) values [29]:

ts ¼ 2CVocIsc

(4)

Fig. 4 shows the connection diagram to obtain the IeV and PeVcurves by using a capacitor in the Matlab-Simulink environmental.As the mentioned above, a capacitor is connected to output of PVpanel and current and voltage values are obtained for make to plotIeV and PeV curves in charge period. A PV panel model is estab-lished according to Section 2.1. Here, G and T correspond to theirradiation (W/m2), which falls on the panel and the panel surfacetemperature (�C), respectively. These parameters are used in the PVpanel model.

Fig. 5 shows the IeV and PeV curves for the PV panel model, atdifferent irradiation levels while the temperature value remainsconstant at 70�. Fig. 6 shows the same curves at different temper-ature values with a constant irradiance level of 1000 W/m2.

It is well known that changes of the irradiance level and tem-perature affect to the PV panel characteristics. These effects areclearly shown in Figs. 5 and 6. Decreasing the irradiance level re-duces the short circuit current (Isc) and the maximum power value.The decreasing temperature increases the open circuit voltage(Voc).

An experiment has been performed to obtain the PV panelcharacteristics by using a capacitor. In this experiment, a capacitorhas been connected to the output of panel, and the current, thevoltage and the power have been monitored on an oscilloscope. In

citor is connected to the end of the panel (a) uniform irradiance and (b) partial shadereferred to the web version of this article.)


Fig. 8. Flowchart of the proposed MPPT algorithm.


Fig. 7(a), the PV panel was exposed to uniform irradiance. InFig. 7(b), the panel has been under partial shade conditions. As seenin the figure clearly, the maximum power decreased, and powercurve have two maximum points.

Table 1Irradiation of panels.

Case PV-I PV-II PV-III

Case e I(0e0.2 s)

1000 W/m2

70�1000 W/m2

70�1000 W/m2

70�

Case e II(0.2e0.4 s)

1000 W/m2

70�1000 W/m2

70�500 W/m2

45�

Case e III(0.4e0.6 s)

1000 W/m2

60�600 W/m2

50�300 W/m2

35�

Case e IV(0.6e0.8 s)

1000 W/m2

70�1000 W/m2

70�700 W/m2

55�

Case e V(0.8e1 s)

1000 W/m2

70�1000 W/m2

70�1000 W/m2

70�

Table 2LA 55 PV panel electrical data.

Peak power Pmax [Wp] 55Max. power current Imp [A] 3.3Max. power voltage Vmp [V] 16.8Short circuit current Isc [A] 3.85Open circuit voltage Voc [V] 22.1

3.2. MPPT control algorithm

The algorithm of proposed MPPT method consists of two stages.In stages-1, the PeV curve of array is obtained by sensing currentand voltage data during connecting capacitor to array and deter-mined GMPP and corresponding voltage value (V-GMPP). In stage-2, the converter is controlled by tuning duty cycle to operate the PVarray at V-GMPP. The details of these stages are given as follows.

Stage-1: Assumed that the PV array was initially operating at theGMPP and the power at this point taken as Pmax and the voltage asV-GMPP. When there is a change in irradiance level or environ-mental conditions on PV array, it’s output power will also changeand a difference (DP) will occur between the Pmax and instanta-neous power (actual power-Pact). Considering that tiny fluctua-tions in signals will also cause this difference, the control algorithmstarts to run the new cycle of MPPT algorithm when DP exceeds acertain value (ε). The Pmax is turned to zero to prevent this value inthe previous cycle of the algorithm from affecting its value in thenew cycle. S1 and S3 switches are brought to the “open”, S2 to“closed” position. Then the capacitor “C” is connected to PV array,


and the instantaneous current and voltage data of the PV array aremeasured. These data are sensed by the controller to calculate theinstantaneous power. The instantaneous power values obtainedwhile the capacitor is charged are compared with each other, andthe Pmax value and its corresponding V-GMPP are determined. Thisprocess is performed as long as the capacitor is being charged. Dueto the current drawn by the capacitor will decrease during charged,the algorithm decisions the end of the charge process by using thecurrent, and it terminates the comparison when it nears 0. (In thealgorithm, the termination is executed when the capacitor currentis below 0.1 A). The determined V-GMPP value will also be used inthe Stage-2 as the reference voltage (Vref) of the MPPT or PV array.Thus at the end of the Stage-1, the PeV curve and voltage value ofGMPP are obtained according to the environmental conditions at agiven time.

Stage-2: The purpose of this stage is to adjust the duty cycle ofthe dcedc converter to operate the PV array at the Vref (V-GMPP)that is obtained in Stage-1. For this, S2 is turned off, and S1 and S3are turned on. So the converter is reconnected to array and load isrepowered and the capacitor is discharged over the resistance R sothat it will be ready to restart Stage-1. The controller produces thenecessary PWM (PulseWidthModulation) signals to operate the PVarray at the GMPP by comparing the Vref value and instantaneousoutput voltage of the array.

The flowchart of the proposed control algorithm is shown inFig. 8. In the chart, D and dx represent the duty cycle and howmuchD is changed respectively [30].

4. Simulation results

In this section, the proposed MPPT algorithm has been simu-lated with the 3 serially connected PV panel shown in Fig. 3. In thesimulation, single-diode and 5-parameter PV panel model is used.In this model, irradiation (W/m2) and temperature (degree) valuescan be input separately, which brings great flexibility to simula-tions. The panels in the string have been exposed to differentirradiation levels changing every 0.2 s throughout the 1 s simula-tion, as shown in Table 1. The table also shows that both uniformirradiation and partial shade conditions have been performed onthe PV panel. The electrical characteristics of utilized PV panel aregiven in Table 2.

The parameters of the boost type dcedc converter have beenchosen such that it operated in CCM (Continuously ConductingMode) mode. These parameters as follows:


0 10 20 30 40 50 60 700

20

40

60

80

100

120

140

160

180

Voltage (V)

Pow

er (

W)

case IV

case I - V

case II

case III

Fig. 9. PeV curve of the array at different irradiations.

Fig. 11. Voltage value of GMPP, array voltage and duty curves.


Boost converter and load parameters: L_dc ¼ 5 mH,C_dc ¼ 80 uF, fs ¼ 10 Khz, Load ¼ 50 U.

MPPT control algorithm parameters: dx ¼ 0.00005, ε ¼ 15 W [%10 of Pmax (array)].

The PeV curve of the array according to the changes in irradi-ation in Table 1 is given in Fig. 9. Case I and case V have one MPPbecause they have a uniform irradiation distribution. Case II andcase IV have twoMPPswhile case IV has three. Naturally, cases II, III,IV represent partially shaded conditions.

The results of the proposed MPPT method are given in Fig. 10and Fig. 11. Fig. 10 shows S2 signal shows connection period ofthe capacitor to the PV array. As mentioned previously, this signal isgenerated by the controller when there is a change in the irradiancelevel (Stage-1 in control algorithm). Pmax is found in Stage-1 ofalgorithm, and P-Array shows the output power of the PV arraysystem.

Fig. 11 depicts the V-GMPP values that were found in Stage-1and the array voltage changes in each case. The change in theduty cycle needed to operate the PV string at V-GMPP is also shownin Fig. 11. As explained in the previous section, the duty cycle isdetermined in Stage-2 of the algorithm.

Because cases-I and case-V have similar irradiation, their GMPPvalues are also the same; both are 164W, and the voltage value thatcorresponds to this power is 49.5 V. The case-II Pmax is 108 W, andV-GMPP is 33 V. Because case-III has the lowest irradiation, thearray produced the least power, Pmax is 70W, and V-GMPP is 36 V.

Fig. 10. S2 signal, power value of GMPP and array power curves.


In case-IV, Pmax is 124W, and V-GMPP is 55 V. As shown in Figs. 10and 11, the array voltage and power can track the GMPP point ineach case.

As the figures show, the power at the maximum point of thearray depends on the irradiation. In this case, the proposed MPPTalgorithm gives the power and voltage values at GMPP. It is note-worthy that the proposed algorithm can locate the GMPP amongthe several MPPs formed by the partial shade conditions. Thesimulation has shown that the proposed MPPT method can locatethe GMPP of the array and operate the system at this point underany irradiation condition.

Fig. 12 depicts dynamic response of the system when case-IIoccurs. As shown in the figure, when changing in irradiance levelat instances 0.2 s, the controller brings S2 switch to “1” and thecapacitor is connected to PV array, then the array power is obtainedby measuring current and voltage of the capacitor. As seen in thefigure, this curve is similar to “case-2” in Fig. 10. As mentionedabove, MPPT controller tracks to maximum value of array power(P-Array) during S2 switch in “1” position, and maximum powervalue is defined as power component value of GMPP (Pmax). At thesame time, controller defines the voltage value corresponding toPmax, as voltage of GMPP (V-GMPP). When capacitor is charged, S2is brought to “0” position and, V-GMPP is assigned as referencevoltage of converter. Hence, array voltage traces the reference asseen in Fig. 12.

Fig. 12. Dynamic response of MPPT controller for case-II.


Fig. 14. Experimental system; a) Circuits b) Overview.

Fig. 13. Quartus environment of the control algorithm.


This figure also shows processing time of MPPT algorithm.Scanning of GMPP continues while S2 is at position “1” and forcase-II, which takes around 3.5 ms. In the proposed method, thistime depends on capacitor value, short circuit current, and opencircuit voltage. Hence, it varies with environmental conditions inaccordance with equation-1. For this simulation, this time takesmaximum 5ms for case-III since the array has minimum irradianceor power level. Consequently the computation of GMPP is realizedin a very short while, compared to other MPPT methods [21,23]. In

Fig. 15. Experimental system with electronic load device.


the proposed method, load is disconnected from the array in thisperiod, but it is negligible due to mentioning short time.

5. Experimental results

In this section, the proposed MPPT method is implemented inexperiments. Developed experimental system consists of four maincircuits which are FPGA, ADC, dcedc converter and switches con-trol circuit.

The whole experimental system is controlled by FPGA (CycloneIII - EP3C120F780C7) control board. The control algorithm isdeveloped by Quartus II environment and it consists of some con-trol blocks; for both current and voltage inputs ADC control,multiplication, fix to floating point converter blocks are used.Additionally MPPT algorithm consists of switching control (for S1,S2 and S3 switches), MPP finding (Stage-1), voltage comparing andPWM generator (Stage-2) blocks. All of these control blocks arecoded by VHDL programming language. The Quartus schema of thecontrol algorithm is given in Fig. 13.

ADC circuit, MAX1204 ADC IC, converts the output current andvoltage of PV array to digital signals, and transports them to FPGA inSPI (serial to parallel interface) protocol. Boost type dcedc con-verter is used in the experimental system. The converter parame-ters are chosen to operate in CCM mode. Switch control circuitincludes of S1, S2, S3 switches, their drivers, charge capacitor (C),discharge resistor (R), as shown in Fig. 3. Moreover, this circuitscales the current and voltage to adapt them to ADC. The experi-mental system is shown in Fig. 14.


Fig. 17. Panels shading condition.

Fig. 18. IeV and PeV curves obtained by electronic load device.

Fig. 19. STP screen

Fig. 16. (a) PV panels. (b) Connection structure of cells in the PV panel.

Table 3STP and electronic load values.

P_MAX (W) V_GMPP (V) V [array voltage e (V)] Duty

STP (HEX) 431A3D71 4244EB85 42483333 e

STP (DEC) 154.24 49.23 50.05 %41.1Electronic Load 151 50.94



The experiments are implemented on three serially connectedPV panels and under different partially shading conditions. Theresults are presented as follows: Initially, IeV and PeV curves of PVarray are obtained by electronic load device under formed partiallyshading conditions. In this process, the power and voltagecomponent values belong to GMPP are determined by “Agilent Vee”software packet. The electronic load device is shown in Fig. 15.

Then, the proposed method is performed under the sameenvironmental conditions. In this process, experimental results aredisplayed on the Signal Tap Analyzer (STP) interface of the FPGAsoftware packet. There are steady state values on the STP, which areP_MAX (power component value of GMPP in HEX code), V_GMPP(voltage component value of GMPP in HEX code), S1, S2, S3, V(actual voltage of PV array) and duty (duty cycle of converter). Atthe same time, output voltage of PV array and switching signal ofconvertor are monitored on oscilloscope.

In the experiments Lorentz 55W PV panels are used. The pa-rameters utilized in experiment system and panel parameters are:

L_dc ¼ 3 mH, C_dc ¼ 80 mF, fs ¼ 20 kHz, R_load ¼ 50 U (con-verter parameters), C ¼ 1000 mF e 100 V, R ¼ 80 U.

AM ¼ 1.5, E ¼ 1000 W/m2, cell temperature: 25 �C.

in steady state.




Fig. 23. STP screen



STP (HEX) 42EE147B 42555C29 4257EB85 e

STP (DEC) 119.04 53.34 53.98 %24.8Electronic load 116.6 54.7

Fig. 24. Array voltage and switching signal on oscilloscope screen.




PV panels used in experiments are shown in Fig. 16(a). Each PVpanel has parallel connected 2 strings and each string consist of36 cells connected in series. Connection structure of cells andbypass diodes in the PV panel is given in Fig. 16(b).

5.1. Exp.-1

This experiment is performed under uniform irradiance condi-tion as shown in Fig. 17. In this situation the obtained IeV and PeVcurves of array by electronic load device is shown in Fig. 18. Here,the GMPP values are 151 W and 50.94 V.

In the same environmental condition, the proposed method isrunning on FPGA controller and some of steady state values are dis-played on STP interface packet as shown in Fig. 19. Here, S1eS2eS3switches are brought to 1-0-1positions. P_MAX,V_GMPP,V anddutyvalues which are on STP screen are tabulated in Table 3

As seen in Table 3, the results which are obtained from proposedmethods match to results of electronic load device. Additionally,output voltage curve of array and switching signal are shown inFig. 20 (Notice that scale of the voltage probe is �10). It can be seenfrom figure that the voltage is around in 51 V.

in steady state.



0 10 20 30 40 50 60 700

1

2

3

4

Cur

rent

(A)

0 10 20 30 40 50 60 700

20

40

60

80

100 X: 33.73Y: 79.97

Pow

er (W

)

Voltage (V)




STP (HEX) 42A46148 42023333 420428F6 e

STP (DEC) 82.19 32.55 33.04 %52,4Electronic load 79.97 33.86



5.2. Exp.-2

This experiment is realized that one division of each of twopanels is closed as shown in Fig. 21 and creating partial shadecondition. Fig. 22 shows IeV and PeV curves which are obtained byelectronic load device and two MPPs occur on PeV curve.

Fig. 23 shows STP screen which displays steady state results ofthe system. As seen from P-MAX and V_GMPP values, the algorithmdetermines GMPP of PV array.

Table 4 consists of STP and electronic load device results, andFig. 24 depicts actual voltage of PV array and switching signal.

5.3. Exp.-3

This experiment is performed under partial shade condition asshown in Fig. 25. In this case, two MPP occurs on PeV curve. The IeV and PeV curves are shown Fig. 26.

Fig. 27. STP screen


In this partial shade condition, STP screen includes GMPP valueswhich are found by MPPT algorithm, V and duty parameters areshown in Fig. 27.

STP and electronic load device results are given in Table 5. Actualvoltage of array and switching signal are given in Fig. 28.

5.4. Exp.-4

In this experiment, each PV panel has different irradiance levelas shown in Fig. 29. There are three local maximum points on PeVcurve of array under this condition. GMPP have 65.09 We35.75 Vpower and voltage values respectively. Obtained PV array charac-teristics are displayed in Fig. 30.

The FPGA controller locates the GMPP and its power and voltagevalues among all MPP in HEX code format as shown in Fig. 31. Thenit tunes up duty to operate PV array at GMPP.

As seen in Table 6. P_MAX and V_GMPP which are found by theMPPT algorithm are almost the same electronic load device find-ings. Fig. 32 shows array voltage and switching signal of converteron oscilloscope screen.

in steady state.



0 10 20 30 40 50 60 700

1

2

3

4

Cur

rent

(A)

0 10 20 30 40 50 60 700

20

40

60

80 X: 35.75Y: 65.09

Pow

er (W

)

Voltage (V)




STP (HEX) 42863D71 420AA3D7 420C28F6 e

STP (DEC) 67.12 34.66 35.04 %42.7Electronic load 65.09 35.75



As seen from the experimental results, the proposed MPPT al-gorithm can find accurately GMPP of PV array under uniform anddifferent partially shade conditions. These findings are verifiedwith obtained data from electronic load device. Then FPGA basedMPPT controller operates the system at voltage of GMPP by tuningduty cycle of converter’s switching signal.

6. Conclusion and future works

This study offers a novel Global MPPT method operating underpartially shaded conditions. In the proposed method, a capacitor isconnected to the array as a load, and, its current and voltage pa-rameters are sensing while charging from PV array. The controllerobtain PeV curve using these parameters according to the envi-ronmental conditions affecting the array at that time. The MPPTalgorithm presented here locates the GMPP by using the data onthe curve and then ensures that the system works at this point byadjusting the duty cycle of the boost type dcedc convertor.

Fig. 31. STP screen


The proposed method has been simulated in Matlab-Simulinkunder uniform and partial shade conditions on the PV array sys-tem. The proposed method has been verified experimentally also.The experiments have been performed both uniform irradiance andcreating partial shade conditions on PV array. FPGA controlledMPPT algorithm locates GMPP and operates the PV system at thispoint. Major advantages of the method are ability to find GMPP inpartial shading condition, avoid needing multiple MPPT devices,very short computation time, not requiring datasheet values andarray configuration data, as well as allowing different type andpower rating PV panels together in array.

As a next step, challenges of application for the proposed MPPTmethod will be investigated since there may occur some technicaldifficulties in widely varying irradiance level and for large scalesystem. With this aim, the proposed method will be extended tothe one connecting different capacitors to PV array by usingcapacitor bank, and connecting more than one capacitor to certainparts of array for large powered system.

in steady state.



Acknowledgment

This study has been supported by The Scientific and TechnicalResearch Council of Turkey (TÜB_ITAK-111E076).

References

[1] Lei M, Yaojie S, Yandan L, Zhifeng B, Liqin T, Jieqiong S. A high performanceMPPT control method. In: IEEE Conference on Materials for Renewable Energy& Environment, vol. 1; 2011. pp. 195e9.

[2] Dorofte C, Borup U, Blaabjerg F. A combined two-method MPPT controlscheme for grid-connected photovoltaic systems. In: IEEE European Confer-ence on Power Electronics Application, vol. 1; 2005. pp. 1e10.

[3] Houssamo I, Locment F, Sechilariu M. Maximum power tracking for photo-voltaic power system: development and experimental comparison of twoalgorithms. Renew Energy 2010;35(10):2381e7.

[4] Femia N, Petrone G, Spagnuolo G, Vitelli M. Optimization of perturb andobserve maximum power point tracking method. IEEE T Power Electr2005;20(4):963e73.

[5] Tafticht T, Agbossou K, Doumbia ML, Chériti A. An improved maximum powerpoint tracking method for photovoltaic systems. Renew Energy 2008;33(7):1508e16.

[6] Kakosimos PE, Kladas AG. Implementation of photovoltaic array MPPTthrough fixed step predictive control technique. Renew Energy 2011;36(9):2508e14.

[7] Lalilia D, Mellita A, Lourcia N, Medjaheda B, Berkoukb EM. Input outputfeedback linearization control and variable step size MPPT algorithm of a grid-connected photovoltaic inverter. Renew Energy 2011;36(12):3282e91.

[8] Enrique JM, Andújar JM, Bohórquez MA. A reliable, fast and low costmaximum power point tracker for photovoltaic applications. Sol Energy2010;84(1):79e89.

[9] Tsao P, Sarhan S, Jorio I. Distributed max power point tracking for photovoltaicarrays. In: IEEE 34th Photovoltaic Specialists Conference (PVSC), vol. 1; 2009.pp. 2293e8.

[10] Xiao W, Ozog N, Dunford WG. Topology study of photovoltaic interface formaximum power point tracking. IEEE T Ind Electron 2007;54(3):1696e704.

[11] Chong BVP, Zhang L. Controller design for integrated PV-converter modulesunder partial shading conditions. Sol Energy 2013;92:123e38.

[12] Khanna R, Zhang Q, Stanchina WE, Reed GF, Mao ZH. Maximum power pointtracking using model reference adaptive control. IEEE T Power Electr2014;29(3):1490e9.

[13] Miyatake M, Veerachary M, Toriumi F, Fujii N, Ko H. Maximum power pointtracking of multiple photovoltaic arrays: a PSO approach. IEEE T Aero Elec Sys2011;47(1):367e80.

[14] Sarvi M, Ahmadi S, Abdi S. A PSO-based maximum power point tracking forphotovoltaic systems under environmental and partially shaded conditions.Prog Photovoltaics 2014;1:1e14.


[15] Jiang LL, Maskell DL, Patra JC. A novel ant colony optimization-basedmaximum power point tracking for photovoltaic systems under partiallyshaded conditions. Energ Build 2013;58:227e36.

[16] Chaouachi A, Kamel RM, Nagasaka K. A novel multi-model neuro-fuzzy-basedMPPT for three-phase grid-connected photovoltaic system. Sol Energy2010;84(12):2219e29.

[17] Punitha K, Devaraj D, Sakthivel S. Artificial neural network based modifiedincremental conductance algorithm for maximum power point tracking inphotovoltaic system under partial shading conditions. Energy 2013;62:330e40.

[18] Shaiek Y, Ben Smida M, Sakly A, Mimouni MF. Comparison between con-ventional methods and GA approach for maximum power point tracking ofshaded solar PV generators. Sol Energy 2013;90:107e22.

[19] Alajmi BN, Ahmed KH, Finney SJ, Williams BW. A maximum power pointtracking technique for partially shaded photovoltaic systems in microgrids.IEEE T Ind Electron 2013;60(4):1596e606.

[20] Gounden NA, Peter SA, Nallandula H, Krithiga S. Fuzzy logic controller withMPPT using line-commutated inverter for three-phase grid-connectedphotovoltaic systems. Renew Energy 2009;34(3):909e15.

[21] Koutroulis E, Blaabjerg F. A new technique for tracking the global maximumpower point of PV arrays operating under partial-shading conditions. IEEE JPhotovoltaics 2012;2(2):184e90.

[22] Chin CS, Neelakantan P, Yoong HP, Yang SS, Teo KTK. Maximum power pointtracking for PV array under partially shaded conditions. In: IEEE Third Inter-national Conference on Computational Intelligence, Communication Systemsand Networks, vol. 1; 2011. pp. 72e7.

[23] Patel H, Agarwal V. Maximum power point tracking scheme for PV systemsoperating under partially shaded conditions. IEEE T Ind Electron 2008;55(4):1689e98.

[24] Ji YH, Jung DY, Kim JG, Kim JH, Lee TW, Won CY. A real maximum power Pointtracking method for mismatching compensation in PV array under partiallyshaded conditions. IEEE T Power Electr 2011;26(4):1001e9.

[25] Adamo F, Attivissimo F, Spadavecchia M. A tool for photovoltaic panelsmodeling and testing. In: IEEE Instrumentation and Measurement TechnologyConference, vol. 1; 2010. pp. 1463e6.

[26] Gow JA, Manning CD. Development of a photovoltaic array model for use inpower-electronics simulation studies. IET Electr Power App 1999;146(2):193e200.

[27] Sera D, Teodorescu R, Rodriguez P. PV panel model based on datasheet val-uesIn Proc. IEEE International Symposium Industrial Electronics (ISIE), vol. 1;2007. pp. 2392e6.

[28] Chenni R, Makhlouf M, Kerbache T, Chenni AB. A detailed modeling methodfor photovoltaic cells. Energy 2007;32(9):1724e30.

[29] Mahmoud MM. Transient analysis of a PV power generator charging acapacitor for measurement of the IeV characteristics. Renew Energy 2006;31:2198e206.

[30] Parlak KS, Can H. A new MPPT method for PV array system under partiallyshaded conditions. In: 3rd IEEE International Symposium on Power Elec-tronics for Distributed Generation Systems (PEDG), vol. 1; 2012. pp. 437e41.


http://refhub.elsevier.com/S0360-5442(14)00161-3/sref1

























































































































Documents

FPGA based new MPPT (maximum power point tracking) method for PV (photovoltaic) array system operating partially shaded conditions