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PHOTOVOLTAIC-GRID INTEGRATED SYSTEM

Sameer Khader, Abdel-Karim Daud

Palestine Polytechnic University

emails: Sameer@ppu.edu, daud@ppu.edu

ABSTRACT

This paper proposed solution for directly energizingof ac load throughout Photovoltaic Solar Array duringthe daytime by applying so called variable voltagetracking system (VVT). The main function of VVT isto maintain the average output chopped voltage at fixed

value irrespective of solar radiation rate, in turn thechopped voltage is converted into ac voltage suitablefor grid-connected loads. This solution is realized byintegrating both complementary buck-boost chopperand dc to ac converter. The ac-grid contributes to theload in two cases, first when there is a power shortageduring the daytime due to weak irradiation rates, andsecond during the night time. The power estimator unitis used to determine the grid contribution intervals.

This solution excludes the use of battery bankwhich is the main obstacle in massive use of solarenergy due to their weight, short life time, maintenanceand cost. Matlab/Simulink is used to simulate the

proposed model, where the obtained simulation resultsconfirm and justify the proposed approach for furtherstudy and looking for optimized solutions for costreduction and energy savings.

Index Terms-- Photovoltaic Systems, DC Choppers,Smart Grids, Soft Switching, Inverters, Buck-BoostChopper.

1.INTRODUCTIONPhotovoltaic energy resources presents alternative

and friendly to the environment sources. It presents

unique solution for providing remote area with clean

and sustainable energy during the daytime in heating,lighting, refrigeration and water pumps systems [1-3]

without the need of battery system, while during thenight time the accumulated energy can be fully or

partially used to cover the energy domain.

The output circuit connected to the photovoltaic

system is usually dc-dc converters mainly boost

choppers in order to boost the voltage to the

predetermined levels.The DC/DC converters are widely used in regulated

switch mode power supplies, where the input voltage tothese converters varies in wide range especially in thecase of photovoltaic (PV) supply source due to

unpredictable and sudden change in the solarirradiation level as well as the cell operatingtemperature. Several connection topologies concerningthe switching systems have been proposed [4-8] aiming

at realizing the required voltage level during differentperiods of day for certain application type such aspumps, motors in general and power supplies.

During the design process of PV array poweredsystems; a simulation must be performed for systemanalysis and parameter settings. Therefore an efficientuser friendly simulation model of the PV array with

various control strategies is always needed.The proposed model consists of several modules as

shown in Figure1 with the following functions:-PV Photovoltaic Module (PV) that converts the

solar irradiation into voltage Vpv and current Ipv.

-Complementary Buck-Boost DC ChopperModulethat boosts up the PV voltage to the predetermined

levels. Conversely in case of high Vpv the output

voltage is reduced.

Figure 1. PV-Grid system block diagram

-Variable Voltage Tracking Module that generatesswitching pulses according to the required outputvoltage level in order to maintain Vout at fixed value.

-Grid Adapting Module that converts the ac gridvoltage into dc voltage in case of grid connection.

-Grid Drop Compensation Module that compensatesthe voltage drop according to the drawn load currentand generates reference voltage.

-Power-Status Estimator that detects the availablePpv power, the consumed load power and the value of

power shortage that should be supplied from AC-grid.

Complement-

ary Buck-Boost

Variable

Voltage

Tracking

Grid Adapting

S stem

Load

Inverter

Grid-Drop

Compensation

Iout

Vac

Vg_Q

VoutVref

Power StatusEstimator

Grid

Selector

AC

Grid

PV

Array

Pac

Ppv

Vg_sl

2012 First International Conference on Renewable Energies and Vehicular Technology

978-1-4673-1170-0/12/$31.00 2012 IEEE 60

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The displyed in fig.1 parameters Ppv, Vac, Pac, Iout

are PV power, AC-grid voltage and power, and loadcurrent respectively; Vgsl, Vref and Vgo are grid selector

signal, reference voltage and complementary buck-

boost driving signals.

The remainder of the paper is organized as follows:

Section (2) Modelling & simulation of PV array;Section (3) The behaviours of PV-Grid integratedsystem; Section (4) Discusses the simulation results

and conclusion.

2.MODELING OF PV ARRAY2.1 Characteristics of PV Array

Basically, PV cell is a P-N semiconductor junction that

directly converts light energy into electricity. It has the

equivalent circuit shown in Figure 2 [8-10].

Figure 2. Equivalent circuit for PV cell

Where Iph represents the cell photocurrent; Rp and

Rs are the intrinsic shunt and series resistance of the

cell respectively; Id is the diode saturation current; Vo

and Io are the cell output voltage and current

respectively. The following are the simplified equations

describing the cell output voltage and current:

os

o

odphco I.R

I

IIIlnq

T.K.AV

+= (1)

= 1eII(NI Tc.K.A

Ns/Vo.q

dphpo (2)

= Tc

1

Tr

1

K.B

Eg.q3

r

cord e.

T

TII (3)

{ })TT(I.I.NI rctnscpph += (4)

Where, K- Boltzman constant; Np and Ns are the

number of parallel and series connected cells

respectively; Eg is the band gap of the semiconductor;

Tc and Tr are the cell and the reference temperature

respectively in Kelvin, A and B are the diode ideality

factors with values varies between 1 and 2; n is the

normalized insulation; Isc is the short circuit current

given at standard condition; It and Ior are constants

given at standard conditions.

2.1.1. Photovoltaic I-V Performance

In order to study the I-V performance of the PV

circuit and to look for appropriate dc chopper for

boosting up the output voltage to predetermined value

it is necessary to illustrate the obtained PV voltage andcurrent for boost chopper according to specifications

given in table 1 at reference irradiation 1000W/m2.

Table 1: Data specification for PV Array.

q K I h Id RS RP TC1.602e-

19 C

1.38e-

23J/K4 A 0.2mA 1m 10k 25C

NS NP VO VOC ISC VMPP IMPP

38 4 0.6V 21.5 V 4A 17.5V 3.7A

NSm NPm V v out Rload

6 1 130V 44

The PV Array voltage can be obtained bymultiplying the module voltage and current by N sm and

Npm that represents number of series and parallelconnected modules respectively.

powergui

ontinuous

V2

v+

-

V1

v+

-

T_var

11.2903

T

1

Rf-Cf

R-L

PV Array

T

G

Ns

Np

GND

+Vpv

Output

Nsm

6

Npm

1

Lo

I

i+

-

G_var2

G_T

a) Proposed model for PV Array in simulink

environment

0 5 10 15 20 250

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Ipv,A

I-V performance

Vpv, V

1200W/m2

1000W/m2

800W/m2

600W/m2

400W/m2

b) I-V Performance of PV module.

Figure 3. PV model with I-V performances.

Figure 3 illustrates the proposed PV array built inMatlab/ simulink [11] with R-L load, where theobtained results for different variation levels are

presented. From these performances it is shown that the

total output PV voltage and current varies according toirradiation level with approximated 65W maximumpower at G=1000W/m

2.

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2.2 Double-chopper PV Array

Regulating the output chopped voltage according toreference or grid voltage can be realized by modifyingthe conventional boost chopper into double choppercircuit with buck converter called "Complementary

buck-boost converter" as shown in Figure 4. Powerswitched Q1 and Q2 operates in complementary mode

boosting up the input PV voltage, while Q3 regulatesthis output voltage toward increase or decreaseaccording to Vref.

Figure 4. Complementary-chopper circuit

The obtained output voltage according to thesemodels [12] is illustrated in Figure 5 for differentirradiation levels, and can be presented as follows:

pvO

2Q1Q

VD1

DV

DDD

=

==(5)

Where DQ1 and DQ2 are duty cycles of choppers Q1and Q2 respectively.The actual average voltageVact=Vout' of both choppers operation can be determinedas follows:

( ) ( )

ch.2;ch.1

.1b2Lb;.1b1Lb

1t

0

2t

1t

1Lbpv2Lbpv

ch

'out

T)D1(tTDt

dt

diLV

dt

diLV

dtVVdtVVT

1V

2Lb1Lb

==

==

+++=

(6)Where Lb1 and Lb2 are boost inductances for both

branches respectively, and equals each other; Tch=1/fchis the chopping period.

Introducing variable voltage tracking system VVTcauses voltage regulation and adjustment of outputvoltage as shown in Figure 6 for various irradiation

levels.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

50

100

150

200

250

300

350

Time, S

Vref,Vact,

V

Reference & actual average voltage

G=400W/m2

G=1200W/m2

Vref=220V

Figure 5. Output voltage of complementary chopper

circuit.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2500

1000

1500

2000

G,

W/m2,

V

Solar irradiation

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

100

200

300

400

Vact,

V

Reference & actual chopped voltage

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

2

4

6

8

Time, S

Ich-out,A

Laod current

Vout

Vref

Figure 6. Output chopped voltage and current at various

irradiation rates.

3.PV-GRID INTEGRATED SYSTEMAccording to Figure 1, the generated PV voltage is

adjusted by complementary buck-boost converter andbeing applied to the load via grid selector. The powerstatus estimator generates switching pulses required tooperate the grid selector. The ac-grid contribution can

be described into two approaches:

Fully inverted circuit; Partially inverted circuit.

In case of fully inverted circuit, the ac-grid voltageis converted into dc throughout grid-adapting module,and then added to the output chopped dc voltage asshown in Figure 7.

In partially inverted circuit, the PV voltage isconverted into ac voltage, while the ac-grid voltage isdirectly connected to the load after being synchronized.In present paper first approach will be describedhereinafter.

The consumed by the load effective power and thepower delivered by the PV and ac-grid are by assumingthat the system operates at unity power factor:

( )

gacrmsgac;ooutpvo

,where

invgacpvoRrms

loadinvRrms

I.VPI.VP

.PPP

I.VP

==

+=

=

(7)

Figure 7. Principle PV-Grid connected fully inverted circuit.

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Where Ppvo, Pgac are the effective power provided bythe PV and the ac-grid respectively; Vinv, Io are outputinverted voltage and current of fundamental harmonicrespectively; inv is the inverter efficiency; Vout, Io arethe effective output voltage and current of thecomplementary chopper which are proportional to PVmaximum power respectively; and Vacrms, Ig are theeffective grid voltage and current respectively.

Figure 8. Functional flowchart for power-status estimator.

The reference voltage according to the consumedload current can be determined as follows:

( ) ( )2invf2invlossV

Vinvref

IXIR

;VV

.. +=

+=(8)

Where Vinv, Rloss and Xf are inverter voltage, grid

resistance and circuit reactance including the inductive

filter Lf respectively. According to consumed power,

the power status estimator module estimates wheather

or not the ac-grid contribution. Functional flowchart

illustrating the operation of this module is shown in

Figure 8.The generated pulses required to drive Q4 are

proportional to the rate of power difference, and givesthe status of integrating the grid with the PV system.

4. SIMULATION RESULTSThe proposed simulation model is built in

matlab/simulink environment and consists of severalsub-models. Taking into account main PV-grid datagiven in table 2, the sub-models are as follows:

4.1. Power- Status EstimatorThe simulink model for power status estimator isshown in Figure 9, where the model process the PV and

grif power, and generates the requirred switching pulsesfor grid integration with the PV source.

Table 2. Main parameters of PV-Grid circuit

G,

W/m2Vpv, V Vout, V R,

Lb1,2

mHCf, nF

1200 145

110V 44...250

1.42 16.1

400 77 3.25fch, kHz L , mH Rloss Lf~ Co inv

10 3.25 0.2 2mH 480uF 92%

4.2. Grid-drop compensation

Grid-drop compensation module is required tocalculate the voltage drop of the grid circuit with

purpose generates accurat reference voltage accordingto eq.(8), and generates appropriate switching PWM

pulses that drives buck chopper Q3. The simulink circuitfor this module is illustrated in Figure 10.

..

dp

Ptot

2

Sum(P)

1

Pulse_G

T

Fsmallstep

GoVariableStep

i fStateflow

-C-

c

Z-OH

1

Vref=1

Vdg1

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0 0.5 1 1.5 2 2.5 3100

200

300

Vpv-out,

V

PV output voltage

0 0.5 1 1.5 2 2.5 30

100

200

Vref&

Vch-

avg,

VRef.& out. average voltage

0 0.5 1 1.5 2 2.5 3-200

0

200

Vout-inv,

V

Inverted voltage

0 0.5 1 1.5 2 2.5 3-5

0

5

Time, S

Iout-inv,

A

Out. inverted current

Figure 11. Solar irradiation profile and corresponds PV

voltage.

4.4. Simulation results at various reference voltages

When the reference voltage varies according to loadrequirements at constant irradiation the system regulatesthe output chopped voltage to be equal to the referencevoltage as shown in Figure 12, where the actual outputchopper voltage tracks the reference value with highdegree of accuracy.

4.5. The power contribution profile

According to eq.(7) changing the solar irradiationrate affects the extracted from the PV array power,therefore, in case of power shortages the grid willcontribute with certain amount of watts as shown infig.13 for three levels of solar irradiations (G=400W/m

2, G=1700W/m

2& G=1000W/m

2).

From this figure it is shown that, the region wherethe grid is connected to the circuit throughout transistorswitch Q4.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

0

200

400

600

Vpv-out,V

PV output voltage

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

200

400

Vref&Vch-avg,

VRef.& out. average voltage

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5-500

0

500

Vout-inv,

V

Inverted voltage

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5-2

0

2

Time, S

Iout-inv,A

Out. inverted current

Figure 12: Reference voltage profile and corresponds

inverted voltage and current.

0 0.5 1 1.5 2 2.5 30

1000

2000

G,

W

/m

2

0 0.5 1 1.5 2 2.5 30

100

200

300

Ppv,

Pload,

W

0 0.5 1 1.5 2 2.5 3

-200

0

200

dp,

W

0 0.5 1 1.5 2 2.5 30

0.5

1

P

ulse-Q4

Time, S

PV-Grid Contribution ....

Grid-offGrid-on

Ppv

Pload

Figure 13. PV & Grid power contribution diagram for

various solar irradiation intervals.

5.COMPLETE SIMULINK MODELFigure 14 shows the complete PV-Grid functional

model built in Matlab/ simulink environment, where

several modules are connected and integrated together

resulting in complete simulation process of PV arraybehaviors according to different load requirements.

6.CONCLUSIONIn this work a simulation study for PV-Grid

integrated model has been conducted, where thefollowing conclusions can be drawn:

-The proposed PV model consists of variable trackingmodule and voltage drop compensating module thatcan be used for either dc or ac loads with precise

voltage tracking procedure. The added power-statusestimator modules create new aspect to this model,where the power shortages can be measured anddelivered from alternative sources or main ac-grid.

-The proposed model can be used for simulatingphotovoltaic system individually or combined withbattery charging unit. During the daytime there is noneed of battery unit, resulting in efficiencyenhancement, reliability of the system and long lifetime. Meanwhile, during the night time the load isdirectly energized from the grid, which in turnenhances the system reliability and reduces the totalcost.

-The use of battery bank as alternative power sourceduring the nigh time can be applied when the ac grid

plays the role of standby energy source that could becontribute only in case of energy shortages .

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-The proposed model can be scaled and used for largeenergy converted systems and energy saving with

battery control unit.

7.REFERENCES[1] Ho-sung Kim, Jong-Hyun Kim, Byung-Duk Min,

A highly efficient PV system using a seriesconnection of DC-DC converter output with a

photovoltaic panel", Renewable Energy 34(2009),pp2432-2436.

[2]Tseng S.Y., Li Y.L., Wu J.Y," Buck ConverterAssociated with Active Clamp Flyback Converterfor PV Power System", ICSET 2008, pp.916-921.

[3]Khaligh A., " A Multiple-input dc-dc positive buck-boost converter topology", APEC2008, Twenty-Third Annual IEEE, 24-28 Feb., 2008, pp.1522-1526.

[4]Ahmed N.A.," Modeling and simulation of ac-dcbuck-boost converter fed dc motor with uniformPWM technique", Electric Power systems Research73 (2005), pp363-372.

[5]Balkarishnan A.,Toliyat and Alexander W.C.," Softswitched ac link buck-boost converter", APEC2008, Twenty-Third Annual IEEE, 24-28 Feb.,2008, pp.1334-1339.

[6]Santos J.L, Antunes F, Chehab A., and Cruz C.," Amaximum power point tracker for PV systems using

a high performance boost converter", Solar energy80 (2006) pp.772-778.

[7] Azab M.," Improved circuit model of photovoltaicarray', PWASET, Vol.34, Oct.2008, pp.857-860.

[8] Atlas H., Sharaf A.M.," A Photovoltaic arraysimulation model for Matlab-simulink GUI

environment, IEEE, Trans., 2007, pp.341-345.[9] Chouder A., Silvester S., Malek A., " Simulation of

photovoltaic grid connected inverter in case of grid-failure", Revue des energes Renouveables Vol. 9,

No4, 2006, pp.285-296.

[10]Buresch M.," Photovoltaic energy systems designand Installation", McGraw-Hill, New York, 1983.

[11]Matlab and Simulink, The Mathworks, Inc.,version R2008a, http://www.mathworks.com

[12] Hart D.W, " Power Electronics", ValparaisoUniversity, 2010, McGraw Hill, pp.196-230.

Ipv

Vact_rms

Vgrid_rms

Q4

Grid

PV

Continuous

powergui

RMS(discrete)

Vrms+dv

220

Vref_var

1

Vref1

RMSdiscrete

Vpv_rms

RMSdiscrete

Vout_rms

v+

-

Vout3

v+

-

Vout2

v+

-

Vout-ac

v+

-

Vout

110

Vac_grid_rms

Vact

VrefGate

VVT

G Vg_Q1

VGT

110.1

A

B

+

-

UB

20

T_var

1

T

Sv

Sg

Scope1

R-L

R

g

CE

Q3

g C

E

Q2

g C

E

Q1

Ppv_rms

current

Voltage

Max current

PV-Power

P_status

Pulse_G

Ptot

Power Status Estimater

T

G

Ns

Np

Ipv

GND

+Vpv

PV Array

Output chopper

Vg

DCiDCO

OCP

6

Ns

1

Npm

Ls

Lf

Lb7

Lb4

Lb3

Lb2

Lb1

Irms_load

RMS(discrete)

Iout_rms1

RMS(discrete)

Iout_rms

i+

-

Io2

i+

-

Io1

i+ -

Io

DC

AC2

AC1

Inverter

20

Igrid-max1

g

CE

Grid_connector

[Vg_p]

Goto

821.1984

G_var1

G_var

1

G

[Vg_p]

From

D7

D5

D4

D3

D2

D1

AC Grid Voltage

1 2

1:1

.-

Vpv

Iload_rms/Ipv

Vac_grid_rms

Vout_boost

VG_ch

Vgrid+DV

PV-Grid Compensation

Output inverter

v+

-

NOT

Figure 14. Matlab/ simulink model for PVGrid integrated system.

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