8/4/2019 Optimization of a Photo Voltaic System Connected to Electric Power Grid

http://slidepdf.com/reader/full/optimization-of-a-photo-voltaic-system-connected-to-electric-power-grid 1/6

2004 IEEWPESTransmission & DistributionConference8 Exposition: Latin America1

OPTMIZATION OF A PHOTOVOLTAIC SYSTEM

CONNECTED TO ELECTRICO W E R GRIDF.L. Albuquerque,A. J. Moraes, G .C.Guimarks, S.M. R.Sanhueza, A. R.Vaz

Abstmet- An important aspect related to the photovoltaic

system connected to the electr ic grid is tha t it can exercise the

double function of active power generator and reactive powercompensatot. Since it is a type of random generat ion, dependent

on environmental conditions, it can supply reactive power to th e

electrical grid when there is little or no solar radiation. These a re

importa nt during demand peak hours, when the main grid needs

higher amount o f reactive power. Although the photovoltaicsystem does no t generate active power in such period of time, it

ca n supply reactive pow er up to i ts maximum. Thus, within this

context, this work aims to analyze a control proposed to adjust

the power supplied by a photovoltaic system to the electric grid

a t any sunstroke condition. The results show that, when the

system is idle, usually at peak hours, without generating active

power, a capacitor-like operation can be performed.

Index Term--Distributed Generation, Photovoltaic Solar Energy,

Active Power, Reactive Pow er, PW M Inverter.

I. INTRODUCTION

OWADAYS a new energy generation configuration hasappeared, denominated Distributed Generation, which

the generators are located close of the consumers,

offering the electrical utilities a mean to increase the

availability of energy locally. The distributed generation

offers some advantages such as: possibility to produce

reactivepower improving voltage stability, power factor and

power quality, loss reduction, better service capacity,

possibility of locally isolated load operation, smoothing the

system load curve, reduction of grid expansion costs an d

postponement of new investments for construction of large

plants.

Another important advantage is the production of small

blocks of energy through renewable sources, such as small

hydroelectric plants, wind power, fuel cells and photovoltaic

cells.

Most of the distributed generation now works with unit

power factor, just supplying active power, exploring the

capacity of the generators employed. Due t h i s fact, thereactive power consumedby the nearby loads will continue to

be supplied by the central generation and the capacitors

installed in the primary sideof

he distributiongrid.The photovoltaic system is a type of random generation,

that is, dependent of the environmental conditions [1,2,3], and

when it is connected to the elecbic grid, it can exercise the

double function of active power generator an d reactive power

compensator. This is because when there is little or no solar

N

The authors are with the Faculty of Electrical Engineering, FederalUniversity ofUberlbdia, Uberlandia-MG,Brazil, CEP: 38400-902,Tel+55 -34-32394166, Emails: Fabio_irlbuquerqu~ho~mail.corn, jmoraes@ufi.br,

gcaixeta@ufu.br

radiation the system can supply reactive power to the grid,

maintaining the apparent power always constant on its rated

value. Such characteristicis a distinguished aspect to be used

in peak hours (from :OOpm to 900pm,mostly at night) when

the system photovoltaic scarcely generates active power.

During those hours, there is normally an increase in the

reactive power needs to attend the low voltage electric grid,

mainly due to residential and commercial consumers. In t h i s

situation, the photovoltaic system can supply reactive power

up to its rated value, playing an important role of a reactive

compensator device. This local reactive power supply helps to

decrease voltage drops and losses along the distribution grid,

and also avoids unnecessary overcharges in cables and

transformers.

11. THE HOTOVOLTAICODULE

The photovoltaic solar cell is the most important device for

the direct conversion of s o h energy in electricity. When this

cell, connected to an external load, is illuminated, as indicated

in figure 1, a potential difference in the load will be produced.

This will cause a current circulation h m he positive cell

terminal to the external circuit and back to the negative

terminal [4].

Back contactf '

Fig. 1. Structuresof a conventional cell of silicon

To obtain an operation voltage, the cells should be

connected in series to form a module until the desired module

operation voltage is reached. As photovoltaic systems are

commonly operated with multiple values of 12 V, each

module is usually projected to get the best efficiency in this

voltage, so that the output power of the module can be kept

very close to its maximum. As the cells of monocristalino

silicon have open circuit voltage varying from 0.5 to 0.6 V,

each module should consist from 33 to 36 cells connected in

series. Figure 2  shows how the cells are configured in the

module, and how the modules are linked to form a system [4].

0-7803-8775-91041$20.0002004 IEEE 645

8/4/2019 Optimization of a Photo Voltaic System Connected to Electric Power Grid

http://slidepdf.com/reader/full/optimization-of-a-photo-voltaic-system-connected-to-electric-power-grid 2/6

voltage magnitude (Vd and angle (6) .

Fig. 2.Cell, moduleand photovoltaic anays

Figure 3(a) shows a circuit of a solar cell represented by a

current source I,, in parallel witb an ideal p-n diode with

saturation current ID . Some others effects of a real solar cell,

which modify its external behavior, can be considered by a

series resistance and a shunt resistance, as shown in figure

3(b) P I .r I

(4 (b)

Fig. 3. Equivalent circuits of a solar cell

111. OPERATIONAL PRINCIPLES

This work proposes the use of a PWM inverter (VS1 type)

to promote the interface of a photovoltaic system with the ac

system The idea is to make this system to operate as a

controllable voltage source connected in parallel with the

power grid. By controlling the inverter output voltage phase

angle and amplitude in relation to the grid voltage, it is

possible to have thephotovoltaicsystem supplying active andreactive power, independently of the sunstroke level.

The active and reactive power flows in the system are not

uncoupled. In fact, the active power (P) depends

predominantly on the phase angle or load angle (6) between

the inverter (Vi) and system (V,) voltages, and the reactive

power (Q) is a function of the magnitudes of these voltages, as

shown in figure 4 and equations (1) and (2), where LC is the

coupling inductanceandfis the system fkquency.

Fig. 4. Voltagephasor dugram

According to figure 4 and equations (1) and (2), the power

flow adjustment of the inverter unit, connected in parallel with

the main grid, can be performed by controlling the inverter

rv.CONTROL TECHNIQUE k f I 3 POWER CIRCUIT

The control technique used was developed with the

objective of adjusting the inverter active and reactive power

supplied to the electric grid, according to what can be

produced by the photovoltaic system, in order to maintain the

dc side inverter voltage regulated for the best possibleperformance. Therefore, with the variation of the solar

incidence, the photovoltaic system power will change and he

control should act on the inverter active power supply to keep

the dc voltage on the dc side unchanged. In t h i s sense, a

closed loop voltage control is used to act on the load angle

variation, and, consequently, on the dc/ac power adjustment.

This control also modifies the inverter amplitude voltage to

supply reactive power to the grid when there is little or no

solar radiation. In this way, it will function as a generator

andforas a capacitor, according to the system need.

The Control circuit block diagram and the power circuit

proposed for this appIication arepresented in figures 5 and 6.

D I S T R I B B U T I O NF.. I :I I1

PULSES 5.

P U L S E

:ONTROL

Figure6.PowerCircuit

As shown in figure 5, the control cucuit is divided in two

blocks: block 2 extracts the signal fiom the current generated

by the photovoltaic modules (Ip) and compares it with a

reference signal (Iwf), generating an error (e2). This error is

multiplied by the grid voltage signal (V and the result is

added to this voltage (VG), riginating a modified value (y2).Block 1 extracts the signal from the dc-side capacitor voltage

(Vc) and compares it with a reference signal (V&, originating

an error (el) . This error pass through an integral proportional

circuit and it is multiplied by the grid voltage signal WO),which has a 90" phase shift in advance. This new signal

646

8/4/2019 Optimization of a Photo Voltaic System Connected to Electric Power Grid

http://slidepdf.com/reader/full/optimization-of-a-photo-voltaic-system-connected-to-electric-power-grid 3/6

3

fs v, Cac Lf Cr

(kW 0 (PW (W (CIF)10.02 250 1600 1 20

originated (yl) is added to the modified value (y2) from block2, resulting the PW M reference signai (VI).Therefore,block 2

adjusts the voltage amplitude, and block 1 adjusts the load

angle.

Tbe power circuit is composed by a full-bridge inverter, a

dc-side capacitor (Ck) , a low-pass filter (Lf and G),and ac

side coupIing inductors(LR)S,6].

L,

(W2

v. SMULATIONRESULTS

PSpice and MATLAB software packages were used in all

simulations accomplished here which show the results

obtained for voltage and current waveforms, active, reactive

and apparent powers on the ac side supplied to the grid.

The inverter model has being used to supply active power

fiom a dc source to an electric grid in case of lack of energy or

as an additional power in peak hour. For this application, the

inverter was associated to photovoltaic solar modules which

act as dc source, As mentioned, some modifications were

made on its control unit so that the whole system works in a

stable way and with efficient use of the energy generated by

the solar system.

The computer simulation studies, with the new controlimplemented, have as main objectives to analyze, under

severalgeneration conditions,the invertervoltage and current

profiles as well as the active, reactive and apparent power.

Additionally, for the development of this work, the

modeling of a photovoltaic system was used, whch consisted

of 15 modules in series. Th e ratings of each module is 60 W,

16.8 V and 3.57 A, for maxi" power condition, having

1000 W/m2 sunstroke index and 25'C temperature. For full

load system operation, the dc side voltage was fixed on 250V,independently of the sunstroke level. Therefore, for such

conditions, themaximum system power is 900 W. This system

is connected to the secondary of a 127 V single-phase

distribution grid, which is feeding a 1600W resistive load.

The power circuit parameters used in the simulations areshown in table 1.

The results obtained for steady-state operation are shown in

figures 8 to 15, which are the voltage and current profiles for

four generation conditions or sunstroke indexes: 0%, 25%,

50% and 80%.

AAnalysis

of InverterVoltage Results

The graphs of figures 7  to 10 show the inverter voltage

behavior of the photovoltaic system connected to the

distriiution grid, regarding different sunstrokeindexes.

O%Generation

I j , , I I835 a 4 e45 a 5 8 5 5 8 . 6 8 6 5 8.7

tlme(cycle6) 10'

Fig. 7. Inverter voltage with 0%ofphotovoltaicsystem power

50% Geneation

J I I I I I8.45 8.5 8.55 8.6 8 6 5 e 7

lime(cyck4 10'

Fig. 9. Invertervoltage with 50%of photovoltaic systempower

80%Generation

647

8/4/2019 Optimization of a Photo Voltaic System Connected to Electric Power Grid

http://slidepdf.com/reader/full/optimization-of-a-photo-voltaic-system-connected-to-electric-power-grid 4/6

4

Figures 7 to 10 show that the voltages waveforms are very

close to sinusoids for the several generation conditions

analyzed. The small oscillations of high fkquency

characterize harmonics of high orders, however with small

amplitudes when compared to the fundameutal.

B A n ~ I y s i s f Inverter Curren t Results

Figures  11 to 14 show the inverter current to the

distribution grid, regarding different sunstroke indexes.

0% Genentlon

time (cycles) x105

Fig. 11. Invertercurrent with 0%ofphotovoltaicsystempower

25% Generatbn

time (cycles) x I O 5

Fig. 12. Inverter currentwith 25%of photovoltaicsystem power

50% Generatbn

5 5.5 6 6.5 7 7 ~ 5 8

80 % Gemratbn

8.35 8.4 8.45 8.5 8.55 8.6

time [cycles) x 10'

Fig. 14. Inverter current with 80% ofphotovoltaic system power

As can be seen the current behaviors, shown in figures 11 

to 14, are very close to sinusoidal waves for the several

generation conditions. Again, the high frequency oscillations

characterizes high order harmonics which have very small

amplitudeswhen compared to the fundamental.

C Analysis of Generated Active and Reactive Powers

Figures 15  to 18 show the active and reactive powerssupplied by the inverter to the grid. They are divided in four

generation conditions which consider step variations in the

sunstroke index or in the current associated to the photovoltaic

system: Case 1, for a transient sunstroke index from 0% to

100%to 0%;Case 2, fo r a transient sunstroke index ffom 25%

to 100% to 25%; Case 3, for a transient sunstroke index from50% to 100% to 50%; and Case 4, for a transient sunstroke

index from 80% to 100% to SOYO.

Fig. 15. Active and reactive powers suppliedby the inverterwith

0%- 100% - 0%of photovoltaic system generationchanges

bme (cycles) 10'

Fig. 13 . Inverter current with 50% ofphotovoltaic system power

648

8/4/2019 Optimization of a Photo Voltaic System Connected to Electric Power Grid

http://slidepdf.com/reader/full/optimization-of-a-photo-voltaic-system-connected-to-electric-power-grid 5/6

5

12OOr --4 0 40 6b i o Id 0 1 L 140 ,bo

L ! I 1 ! \ !

MlV2(CFkS)

Fig. 16. Active and reactive pow en supplied by the inverter with25% - 1W/' - 25% ofphotovoltaicsystemgenerationchanges

IY

I ! , I

o 20 40 60 ao 100 120 MO 160

hme(CpleS)

Fig. 17.Active and reactive powers supplied by th e inverierwith

50% - 100%- 50% ofphotovoltaicsystem generation changes

! 5 t I \ \ 1

I I I I I II J , I I , I-200

0 20 40 60 6 0 100 120 14 0 160time (cycles)

Fig. 18. Active and reactive powers suppliedby the inverterwith

80% - 100% - 80% of photovoltaic system generation changes

With the results shown, some conclusions can be drawn forthe active and reactive power generated under the several

simulated conditions:

The active power supplied by the photovoltaic system to

the grid,shown in figures 25 to 18,presented a satisfactory

performance in relation to the control response, because,

when there is sunstroke variation in the surface ofmodules, the system adjusted to a new value with very

small osciIlations;

result, since it made the system supply an amount of

reactive power according to the variation of the active

power, As observed in figures 16  to 19, when the active

power was reduced, the control was adjusted to increase

the reactive power supplied;

Moreover, it was observed an interaction between the

active and reactive powers supplied to the system. T h i scaused the system not to stay idle, taking advantage of the

moments of little active power generation to accomplish

the compensation of reactivepower.

With power variation supplied by the solar modules, it can

be verified that the active power generation prevails ia

periods when the sunstroke index is high. On the other

hand, the reactive power generation prevails when the

solar radiation is small or zero,

D Analysis of System Apparent Powers Generafed

The graphs of figures 19 to 22 show the apparent power

suppliedby the inverter to the grid. They are split in the same

four generation conditions stated before.

1400; I I I I I I

I I I I I

lime (qdes)

Fig. 20.Apparent power supplied by the inverter witb

25% - lW% 25%ofphotovoltaic systemgeneration changes

As to the reactive power, the control also exhibited a good

649

8/4/2019 Optimization of a Photo Voltaic System Connected to Electric Power Grid

http://slidepdf.com/reader/full/optimization-of-a-photo-voltaic-system-connected-to-electric-power-grid 6/6

6

The apparent power suppliedmaintainspractically constant,

avoiding overcharges in transformers and cables. This results

in a stable operation, improving the overall electric system

performance, with voltage support through locally reactive

energy supply, loss reduction, power factor improvement.

VT. CONCLUSION

The simulation results revealed that the control developed

to adjust the load angle and the voltage amplitude and,

consequently, to control the active and reactive power

supplied o the grid,presented a v “y satisfactory performance

for the photovoltaic system analyzed.

The use of the arrangement proposed for the interface ofphotovoltaic systemswith the electric grid allows to obtain a

better cost-benefit ratio in the implementation of this type of

altemative energy. This is because it becomes possible to

operate the photovoltaic system in any condition,independently of the sunstroke level, supplying both active

and reactive powers according to the availability of solar

d a t i o n .

[11 Erge, T., H o f f ” , V. V. , Kiefer K; “The German Experience With

Grid-Connected PV Systems ’’ Solat Energy, vol. 70, no6 pp 479-487,2001.

CONTI, S.; RAITI, S . ; “A. 0.;AGLIASINDI, U.; “Integrution of

Multiple PV Units in Urban Power Distribution Systems” Solar Energy

[3] WU, T. F.; CHANG,C. H.; CHEN, Y . K; A Multi-Function

Photovoltaic Power Supply System with Grid-Connection and Power

Factor CorrectionFeatures.” IEEE 2000, p. 1185-1190.

[4] MessengerR, Ventre J.; “Photovoltaic Systems Engineering” CRC Press.[ 5 ] Mohan, N.; Undeland, T.M.; obbins, W. P., “Power Electronics:

Converters, Applications undDesign”. New York: John Wiley & Sons,1989.

Sashida, T. N., Ogasawara, Y. K., and YamasakI, Y . , “Parallel

Processing Inverter System,”IEEE Trans. Power Electron., ol. 6, np3,pp.442450, July 1991.

[ 2 ]

75 (2003) 87-94.

[6 ]

Vm. IOGRAPHIES

Fabio Lima de Albuquerque was born in Muanda,Brazil, in 1974. He graduated in 1999, the U S.degree in 2001, and he IS working in bis doctorate

degree all in eIectncal engbeenng €room the FederalUniversity of Uberlhdia, Brazil. His research

interests are in the areas of Electric Power System,

Distributed Deneration, Renewable Energy andPhotovoltam Solar Energy.

, ---l__.l_*r___a

Addlio Jose de Moraes was bom in Uberlandia,Br a d , in 1955. H e graduated from the FederalUniversity of Uberlandn, Brazll in 1978. He receivedthe M.Sc. degree from the Federal School ofEngineering of Itajuba, Brazil in 1984, and the

Doctorate €ram PUC-RIOe Janeiro,Brazil, in 1992.He is presently a lecturer in the Faculty of ElectricalEngineering. His research interest are in the areas ofPower SystemDynamics, Load Modeling,Ihstributed

Generabon andRenewableEnergy.

Ceraldo Caixeta Guimar les was born in Patos deMinas, Brazil, in 1954. He graduated from theFederal Un iversity of Uberlandia,Brazil in 1977. Hereceived the M.Sc. degree from the FederalUniversity of Santa Catarina, Brazil in 1984,and the

FkD. from University of Aberdeen, Scotland, in

1990. He is presently a lecturer in the Faculty of

Electrical Engineering. His r e s m h interest are inthe areas of Power System DMamics, Distributed

GenerationandApplied Electromagnetism

VII. REFERENCES

65 0