Comprehensive Design and Implementation of a ...

First International Conference on Engineering Sciences’ Applications, ICESA College of Engineering University of Kerbala

Paper Designation No. E24-8

Comprehensive Design and Implementation of a MPPT Controller for a PV Module Based on dSPACE Microcontroller Ali J. Mahdi Department of Electrical Engineering & Electronics, University of Kerbala, Iraq Mobile: +9467811961473, E-mail: [email protected] ABSTRACT However, the voltage supplied by a solarAphotovoltaic (PV) module can vary significantly depending upon solar irradiance and load. In order to operate a solar PV power system for optimum performance, a maximum power point tracking (MPPT) controller is needed to regulate the PV voltages at maximum power points. In this paper, the main steps in implementing a MPPT algorithm are demonstrated using dSPACEADS1104 microcontroller. The system consists of a PV module and a pulseAwidth modulation (PWM) buck-boost converter, a MPPT controller based on hill-clamping algorithm. The system has been tested under various solar irradiance levels and the experimental results of the MPPT controller are evaluated and the advantage power is improved to 37% at 100AW/m2. Keywords – photovoltaic powerAsystem, maximum power point tracking, hill-climbing algorithm, buck-boost converter, dSPACEAmicrocontroller

INTRODUCTION Solar energy is a neat solution to the electrical power generation due to abundance of solar energy and minimal environmental damage. Currently, solar power systems are used to reduce the crisis of power generation and to supply the loads that are not connected to the national grid [1]. The generated PV power depends on its voltage, which varies with the solar irradiance levels and the equivalent load resistance. For each solar irradiance, there is a unique peak power at the PV voltage at maximum power point. Thus, to track these power pecks, the solar PV module must be connected with a PWM DC-DCAconverter. The optimal operating points of the PV module are adjusted by varying duty cycle of the switching element of the buck-boost converter. The main contributions of this paper are as follows: (i) Design a solar power system testAbench, which includes of a PV module and a boost converter and (ii) A digitalAimplementation of the proposed MPPT algorithm based on hill-climbing method using a dSPACE microcontroller under the variation of solar irradiance levels. The outline of this paper is as following: Section II demonstrates the mathematical modeling of the PV module and the buck-boost converter. The equations for selecting the components of the buck-boost converter are also presented. The digital MPPT algorithm isAillustrated in Section III. This section discusses the procedures of implementing the MPPT algorithm based on hill-climbing method. In Sections (IV) the features of the dSPACEAmicrocontroller is discussed. In Section (V), the simulation and experimental performance results is presented. To confirm the robustnessAof the proposedAMPPT algorithm, it is tested to different solar irradiance levels. Finally, a summary of finding is given in Section (VI).

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PV POWER MODELLING AND DESIGN 1. PV MODULE

A PV module is the entire assembly of solar cells, which are fabricatedAfromAsemiconductor materials (typically silicon). Figure 1 shows the equivalent circuit of a solar cell. It includes a photocurrent source Iph, a diode, a parallel resistance Rp and a series resistance RS. The PV voltage and current can be represented as follows [3] [4]: I PV  I ph  I D  ( I ph  I s c ( I D  I r (e

VD ) .....(1) RP



) .....(2) 1000 {

q (VPV  I PV RS ) } AkT c 1

) .....(3)

V D  V PV  I PV R S .....(4)

Fig. 1: The equivalent circuit of a solar PV cell 2. BUCK-BOOST CONVERTER

A DC-DC buck-boost (i.e. a step-down / step-up converter) is connected between a PV module and a load to adjust the PVIvoltage at maximumIpower point (MPP) voltages. The load voltage can be either higher or lower than the PV voltage, and its polarity is oppositeIof the input voltage. The input current for a buck-boost converter is discontinuousIorIpulsating due to the power switch

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(i.e. IGBTIorIMOSFET). For each switching period, Tsw, the current level changes from zero to the maximumIinductor current IL for each switching period Ts. In addition, the load current is also discontinuousIdueIto the diode, which only conducts during the OFFItime [3]. The power circuit diagram of theIbuck-boost converter with is illustrated in Fig. 2. The main components are: (i) an IGBT, Q1, (ii) a diode, Q2, (iii) an inductor, L, as storage device, (iv) a capacitor, C, as an output filter, and a resistive load, R. PWM

g E C

Q2 Q1

PV Module

Ipv

Vpv

L

C

Vo

R

Fig. 2: The power circuit of theIbuck-boostIconverter connected with the PVIModule The operation of theIbuck-boost converter can be summarisedIas the following: (i) during ON period, the inductorIconnects to the PV module and its voltage steps-up to maximum inductor voltage, Vlmax, which is equalIorIless than the PV voltage; (ii) during OFF period, the inductor connected in parallel with RC load and its voltage steps-down to minimum output voltage. AccordingItoIprevious operation modes, there are twoIstates per switching period. The first state is called ONIstate, where Q1 is ON and Q2 is OFF. WhilstIthe second state is calledIOff State, where Q1 is OFFIand Q2 is ON. The ON period, TON, can be represented as follows [5]:

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Whilst during OFFIperiod, IL is given as follows:

dI L Vo  .....(9) dt L I LOff  

(1 D )Ts

0

dI L 

Vo (1  D)Ts .....(10) L

In steady-state,Ithe average inductor currentIvariation in ON and OFF periods is zero and the relationship between the input and the output voltage of the converter is obtained as the following:

I LOn  I LOff  0 .....(11) VPV DTs Vo (1  D)Ts   0 .....(12) L L Vo D  .....(13) VPV (1  D) 3. SELECTING THE COMPONENTS OF THE BUCK-BOOST CONVERTER

In order toIoperate a buck-boost converter in CCM, the load current must exceed the full load current (normally 5%-10% of fullIload). The inductor value is selected to ensure a continuous conductionIoperation. The steps of selecting the components of the buck-boostIconverter can be surmised as follows:

where Iomax is theImaximumIoutput current, Dmax the maximum dutyIcycle, FSW the switching frequency and ΔVO the ripple voltage. MPPT ALGORITHMS

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Basically, the output power of a PV module changes withIsolarIirradianceIvariations, as shown in Fig. 3, there is a uniqueImaximum power point (MPP) for each solar irradiance. To operate the PV module at MPP, it is needed to adjust the PV voltage to follow theIvoltage at MPPs. There are many techniques have been proposed forItracking MPPIsuch as: fractionalIopen-circuit voltage and short-circuitIcurrent strategies, which are based on measuring the open-circuit voltage or short-circuit current of a PV module to obtainIreference voltage. The drawbackIof these strategies is requiring the knowledgeIof the PVIcharacteristics. In addition, hill-climbingIalgorithm, which is an iterative algorithmIthat begins with an initial voltage to approach the referenceIvoltage (i.e. the voltage at MPP) by incrementallyIchanging load voltage. In this paper, hill-climbingIalgorithm is adopted due to its advantages, which are simplicity and easy implementation in dSPACE microcontroller. It is based on perturbing the PVIoperating voltage to obtain the reference voltage [7]. Figure 3 shows all possibleIpowerIpoints in each side of MPP. Thus,Ithe general steps of implementing the proposed MPPTIcontrol algorithm are summarized as below:

TABLE 1: Selecting ofIincremental step Power Point

Errors

Incremental Step

Location

∆P

∆V

Vstep

Left Side

+,-

+,-

+0.01

Right Side

-,+

+,-

-0.01

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Fig. 3: Peak powerIpoint of P-V curve EXPERIMENT SETUP

The experimental prototypeIincludes a PV module, a buck-boost converter,Ithe dSPACEIDS1104 DSP microcontroller and a resistiveIload. The block diagram of the implementedIPV power system is shown in Fig. 4.

Fig. 4: Block diagram of the implemented PV power system The MPPT algorithm is built usingItheIbasic Simulink and dSPACE blocks. It is worth noting that the dSPACE microcontroller is a powerful tool toIadjust the parameters of the MPPT controller in real-time and to illustrate the actual power-voltage curves. It hasImany advantages such as: 1. It works on Matlab/SimulinkIplatform, which is familiarIengineering software.

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TABLE 3: The specifications of the solar buck-boost converter Buck-Boost Converter: InductorISize

15.86 mH

CapacitorISize

2200 µF

SwitchingIFrequency

1 kHz

SamplingITime

1 msec

The MPPT algorithm is implementedIinIthe MATLAB/Simulink with platform dSPACE as shown in Fig. 5 and the experimentalIsetup is illustrated in Fig. 6. ?

?

?

?1

PWM Channel 1

RTI Data Duty_Cycle

Controller

PWM Channel 2

Pulses Controller

PWM Channel 3

PWM Channel 4

DS1104SL_DSP _PWM

DeltaP

DeltaP

1 Vref

z DeltaV

DeltaV

1 z

In1

Out 1

1 V_o [V]

z

Vo Out 1

In1

MUX ADC

In1

DS1104MUX_ADC

1 V_i [V]

Out 1

1 I_o [A]

Out 1

In1

Ii

Po

Vi

Pi

Io

Ro

Ii

Ri

z

Io

Demux

Vo

z

Vi

Demux

-1

In

RMS

1 I_i [A]

z

P_o [W]

P_i [W]

R_o [ohm ]

R_i [ohm ]

Measure

Algorithm _OK

Po

Pi

Ro

Ri

Measure 1

Fig. 5: Simulink block diagramIin dSPACE

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-1

V_ref [V]


AD C

PWM Driver IGBT

dSPAC E

VT & CT Sensors

Resistive Load Buck-Boost Converter

Fig. 6: Experimental setup V. SIMULATION AND EXPERIMENTAL RESULTS

The simulation and experimental PV module characteristics (power versusIvoltage) are respectively shown in Figures 7 and 8 under the step variations of solar irradianceIlevels (200 W/m2 - 1000 W/m2). Based on PV moduleIcharacteristics, the advantage power AP has been calculated from (19) and listed in TABLE 4.

AP %  [1 - (

Ppv Pmpp

)]  100.....(19)

where Ppv is the PV power without MPPTIalgorithm and Pmpp the PV power with MPPT algorithm. AsIshown in TABLE 4, the efficiency of the solar powerIsystem is improved by using theIMPPT algorithm especially at 800IW/m2 and 1000IW/m2.

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Fig. 7: Simulation PV ModuleICharacteristics

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Fig. 8: ExperimentalIPVImodule characteristics TABLE 4: The simulationIand experimentalIresults Power [W]

Irradiance Level

AP [%] [W/m2]

Pth

Ppv

Pmpp

200

2.879

2.38

2.7

10

400

10.185

9.53

10

4.7

600

17.907

16.58

17.4

4.7

800

25.222

18.51

24

23

1000

32.023

19.47

30.7

37

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