MPPT Algorithm and Photovoltaic Array Emulator using DC/DC Converters

Houda ABIDI Laboratoire des systèmes Electriques (LSE-ENIT)

Ecole Nationale d'Ingénieurs de Tunis

University Of Tunis El Manar ENIT. PB 37, Le Belvédère.

1002 –Tunis – Tunisia Laabidi_houda@yahoo.fr

Afef BENNANI BEN ABDELGHANI

Laboratoire des systèmes Electriques (LSE-INSAT)

Institut National des Sciences Appliquées et de Technologie University Of Tunis Carthage

INSAT. B.P. 676-Centre Urbain Nord.1080 –Tunis -Tunisia

afef.benabdelghani@insat.rnu.tn

Daniel MONTESINOS-

MIRACLE Centre d’Innovaciô Technologica

en Convertidors Astatics I Accioaments (CITCEA-UPC)

d'Enginyeria Elèctrica, Universitat Politècnica de Catalunya.

ETS d'Enginyeria Industrual de Barcelona,Av.Diagonal,

647,PI.2.08028, Barcelona, Spain montesinos@citcea.upc.edu

Abstract—This paper presents a new maximum power point tracking (MPPT) strategy for DC/DC converters used in Photovoltaic field. The main advantages of this strategy are the minimization of the needed electrical sensors and its simple implementation. The considered method uses only the PV current sensor. The performances of this proposed MPPT algorithm are verified using a photovoltaic emulator. This emulator is designed based on power converters and is capable to reproduce, in the laboratory, the electrical characteristics of a real PV module without depending on external weather conditions. The usefulness of proposed system is demonstrated through simulation carried out under PSIM environment.

I. INTRODUCTION Due to petroleum price variation and the pollution

problems caused by conventional power sources, scientific interest towards renewable energy field has significantly increased during the last two decades. Particularly, photovoltaic energy is based on the conversion of solar energy into electrical energy using solar cells [1]. The PV kWh price is sensitively decreasing thanks to, on one hand, the technological progress of the PV cell construction and, on another hand, the efficiency and performances of the power converters used in the PV energy conversion chain.

So, the Maximum Power Point Tracking (MPPT) is one of the major key tasks in the PV applications since it consists in extracting the maximum power from the PV array. It is difficult to test MPPT algorithms using real PV modules due to the weather dependency, the real PV modules cost and weight/volume/space constraints. There are many different methods for evaluating MPPT algorithm of a PV system. Lamps can be used in order to simulate the solar irradiance without depending on weather condition. The relatively great

electric consumption, the mismatch between the sunlight lamp and the sunlight spectrums, and problems of heat

dissipation are the principal limitations of such a method [2, 3].

Power converters-based photovoltaic emulator can be used for reproducing the PV module’s output curve. The design of PV emulator can be done in different ways. The emulator output I-V characteristics are obtained from a lookup table stored in a memory [4] or using a mathematical exponential model [5] or by amplifying the output of a solar cell in order to produce the I-V curve of a solar panel [6].

This paper investigates a novel maximum power point tacking (MPPT) algorithm. The proposed MPPT method is validated using a 4- quadrant chopper photovoltaic emulator. It requires a PV emulator output current in order to seek the maximum power.

II. MAXIMUM POWER POINT TRACKING A. Current-Voltage Characteristic of PV cell

Fig.1 shows the photovoltaic cell characteristics. Each cell is characterized by the non-linear curves (IPV = f (VPV)) and (PPV = f (VPV)) [2]. These curves are determined by the most important parameters given by the cell manufacturer: ISC (Short-circuit current), IMPP (maximum power point current), VOC (Open-circuit voltage) and VMPP (maximum power point voltage). This characteristic can be segmented into three portions. In part (A), the PV cell can be considered as a current generator, while in part (B), it is modeled as a voltage generator. Part (C) is an intermediate zone that includes the nominal operating point for which the cell can deliver the maximum power.

978-1-4673-0784-0/12/$31.00 ©2012 IEEE 567

Fig. 1. Characteristics of the photovoltaic cell

B. Principles of the MPPT algorithms used in literature Several MPPT algorithms have been developed in

literature [7]. Fig.2 shows an algorithm based on the knowledge of the linear relationships between the optimal parameters characterizing the maximum power point (IOPT) and the characteristic parameters of the PV module (ISC). Then this constant current method using short circuit current of PV module is based on

SCOPT IKI .= (1)

where, K is a proportional constant (K<1). The error current is then used to adjust the converter duty cycle, in order to match the two currents. The main problem with this strategy is that the PV source is frequently shorted in order to determine the measurement of short-circuit current ISC. There are some loads may not allow short circuiting of the PV source [8, 9].

DC

DC

Fig. 2. Current control method

Fig. 3. P&O algorithm

One of the commonly used MPPT algorithms is the Perturb and Observe (Fig.3). It perturbs the operating voltage of the PV array around its initial value in order to seek the direction change for maximizing power. If the power increases, then the operating voltage is perturbed in the same direction, while if the power decreases, then the perturbation direction is reversed [10,11,12]. The main inconvenient of this strategy is that uses tow electrical (current and voltage) sensors.

C. The propsed MPPT Fig.4 shows the proposed MPPT algorithm. It requires

only one current sensor and consists in comparing the output current IPV with the reference current Iref in order to adjust continuously the converter switching duty cycle. The reference current corresponds to the maximum power point current given by the PV panel manufacturer. So, the proposed algorithm aims to have the output current, Ipv, equal to the maximum power point current (IMPP). If Ipv increases, then the duty cycle changes in the same direction, whereas if it decreases, the algorithm controls the duty cycle αk in the opposite direction. αk value is calculated with the constant search step size Δα as

αk= αk-1 ±Δα (2)

This method has the advantage of being less expensive, easy to implement and does not need to interrupt energy generation to calculate the current reference as in the ‘Constant Current method’ case.

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Fig. 4. The proposed MPPT strategy

Fig. 5. DC/DC converter with the proposed MPP

The proposed MPPT algorithm is tested using a boost converter (Fig.5).The output voltage and current are given by

( )α−= 1.PVIIch (3)

( )α−=

1PVV

Vch (4)

where VPV and IPV are the input voltage and current of the boost converter and α is the duty cycle of the power switch S.

III. DESIGN OF THE PV EMULATOR In this paper IS-200/32 PV module (from ISOFOTON

[13]) has been emulated. Its characteristics are given on Table.I and Fig.6.

TABLE I: PV module parameters Pmax(W)

VOC

(V)

VMPP

(V)

ISC

(A)

IMPP

(A)

200 57.6 46.08 4.7 4.35

Fig. 6. IS-200/32 PV Panel characteristic [10]

Fig. 7. The proposed PV emulator Block

PV module model

ISC

Modeledcurve

Environmentimpact

IMPP

VMPP

VOC

Solar irradiance (G)

Température (T)

Iref

Fig. 8. The outer-voltage loop

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A. Functionnal description The proposed emulator is given in Fig. 7. It consists on a

power circuit and a control system. The power circuit is divided into two converters (single phase diode rectifier and a four-quadrant chopper used in only one direction) connected via a filter capacitor Cf. The four-quadrant chopper meets the specification needs that is to implement the control system using the module power of microchip.The output current and voltage of the chopper must reproduce the real module characteristics.

The control system inputs are the solar irradiance, the temperature, the voltage and current feedback of the emulator. Two control loops are required: the inner-current loop and the outer-voltage loop. The first is used to generate PWM signals for the 4 quadrant IGBT chopper. The second is designed in order to control the output voltage Vpv of the power circuit and calculate the current reference Iref. Two algorithms are necessary for this loop: the impact of environmental factors and the modeling of the characteristic of the PV module as shown on Fig.8.

B. Impact of environmental factors The ’Environement impact’ bloc calculates the parameters

ISC, VOC , IMPP and VMPP in function of the parameters ISCref, VOCref, IMPPref and VMPPref at the standard solar irradiance Gref=1000W/m² and the standard temperature Tref=25°C using [14, 15, 16].

( )TaG

GIIref

SCrefSC Δ+⎟⎟⎠

⎞⎜⎜⎝

⎛= .1.. (5)

( )( )GbTcVV OCrefOC Δ+Δ−= .1..1. (6)

( )TaGr

GIIef

efMPMPP Δ+⎟⎟⎠

⎞⎜⎜⎝

⎛= .1..Pr (7)

( )( )GbTcVV efMPMPP Δ+Δ−= .1..1.Pr (8)

where: refTTT −=Δ (9)

and 1−=ΔrefGGG (10)

Note that the typical values of coefficient a, b, and c is as follows. a=0.0025/°C, b=0.5/°C and c=0.00288/°C.

C. Modeling the characteristic of the PV module For, the ‘Modeled curve’ bloc, the linear mathematical

model is used for modeling the PV module characteristic, thanks to its simplicity [17]. The Fig.9 shows that the IPV-VPV curve is divided into four segments obtained by joining the four PV module parameters (ISC, VOC, VMPP, IMPP). This method can minimize the errors between the modeled curve and that given by the manufacturer.

When VPV ∈ [0, 0. 95VMPP], the equation between the point 0and 0.95VMPP is

SCPVMPP

SCMPPPV IV

VII

I +×−

=95.0

05.1 (11)

When VPV ∈ [0.95VMPP, VMPP], the equation between the point 0.95VMPP and VMPP is

MPPPVMPP

MPPPV IV

VI

I 2+×−

= (12)

When VPV ∈ [VMPP, 1.05VMPP], the equation between the point VMPP and 1.05VMPP is

MPPPVMPP

MPPPV IV

VI

I 2+×−

= (13)

When VPV∈ [1.05VMPP, VOC], the equation between the point 1.05VMPP and VOC is

MPPOC

MPPMPPMPPPV

MPPOC

MPPPV VV

VIIVVV

II05.1

9975.095.005.1

95.0−

×++×−

−= (14)

In order to test the PV emulator, a variable resistor is used as a load. For each value of the resistor, an operation point defined by (Vpv, Ipv) is obtained. Fig.10 shows the modeled Ipv-Vpv curve (red) and the simulation results of the Ipv-Vpv emulator curve (blue).

Fig. 11 gives the modeled Ppv-Vpv curve (red) and the simulation results of the Ppv-Vpv emulator curve (blue). Fig.10 and 11 prove the good performance of the proposed emulator

Fig. 9. Modeling the I-V curve of PV module

Fig. 10. Modeled and Emulated IPV-VPV curves.

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Fig. 11. IPV-VPV curves generated by the PV emulator

IV. SIMULATION RESULTS The PSIM-model with a dynamic link library DLL based

on C language is used to simulate the proposed MPPT algorithm. Its performances towards a changing of the load and of the solar irradiance are tested. First, the MPPT is simulated with a variable load (R=30 Ω, R=117 Ω and R=300 Ω) in standard climatic conditions (1000W/m², 25°C). Fig.12 shows that the power supplied by the emulator stabilizes around 200 W which is the maximum power set by the manufacturer. The current and voltage stabilize respectively at 4.3 A and 46 V.

To evaluate the performance of the proposed MPPT the variation of the solar irradiance is introduced [18]. It increase from 1000W/m² to 700W/m² and then decrease from700W/m² to 1000W/m². Fig.12 and Fig.13 show that the power reaches the optimal value corresponding to the new weather condition and the system within 25 µs. When the solar irradiance is 1000W/m² (respectively, 700W/m²), the power is equal to 200W (respectively, 100W). It can be deduced that the proposed MPPT guarantees effectively the extraction of the maximum power from the PV emulator with acceptable response times as shown in Fig.13 and Fig.14

.

Fig. 12. IPV, VPV and PPV according to changing load

Fig. 13. PV power (solar irradiance change from 1000W/m² to 700W/m²)

Fig. 14. PV power (solar irradiance change from 700W/m² to 1000W/m²)

V. CONCLUSION In this paper, an MPPT algorithm for PV converters has

been proposed using a single current sensor. This MPPT was validated using a PV emulator. The design of this emulator was described and its performances were proved when variation of temperature or solar irradiance occurs. In addition, the validity of the MPPT method was demonstrated by simulation results.

ACKNOWLEDGMENT

This work was supported by the Tunisian-Spanish collaboration projet PCI A1/037435/11 Of the AECID-Ministerio de Asuntos Exteriores de España and the Tunisian Ministry of High Education and Scientific Research under Grant LSE-ENIT- LR 11ES15.

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