jyoti pv array

7/29/2019 Jyoti Pv Array

1/5

1-4244-1355-9/07/$25.00 @2007 IEEE

International Conference on Intelligent and Advanced Systems 2007

882 ~

Power Electronic Interface for Grid-Connected PV

array using Boost Converter and Line-Commutated

Inverter with MPPT

T.Shanthi1

and N.AmmasaiGounden2*

1Sri Ramakrishna Institute of Technology, Coimbatore, TamilNadu 641 010, India.

2National Institute of Technology, Tiruchirappalli, TamilNadu 620 015, India.

*[email protected]

Abstract Many methods for interfacing PV system with

utility grid have been reported in the literature. In this paper, a

power electronic interface using boost converter and linecommutated inverter is proposed with maximum power point

tracking (MPPT), for the first time. This paper presents the

design , simulation and implementation of a simple power

electronic interface for grid connected PV array using boost

converter and line commutated inverter. The controller extracts

maximum power from the solar array and feeds it to the single-

phase utility grid. A closed loop scheme employing a PI

controller has been modeled in the power system blockset

platform and the complete system has been simulated. A

prototype of the proposed system is built in the laboratory and

simulated results are experimentally verified.

Index terms boost converter, line commutatedinverter, maximum power point tracking, grid-connected

PV.

I. NOMENCLATURE

Edc output voltage at the inverter(V)Em maximum grid voltage(V)

Idc dc link current(A)Isc short-circuit current of solar arrayKP proportional gain

KI integral gain

L boost converter inductance(H)L2 dc link inductance(H)

Pref reference power from the solar arrayPact actual output power from the grid

R2 internal resistance of dc link ()Vd output voltage of boost converter(V)Vref reference voltage for PI controller (V)

VSA solar array voltage(V)Voc open-circuit voltage of solar array

firing angle of the inverter

duty cycle of the boost converter

II. INTRODUCTIONThe development in renewable energy sources replaces the

other traditional energy sources. Among the renewable energysources, solar energy plays a major role due to its pollution-free nature. For economical reasons the solar energy is not

directly interfaced with the utility grid. Hence a powerelectronic interface is developed to interface the solar systems

to the utility grid [1,2]. This power electronic interfaceconsists mainly an inverter and its output is given to a step-up

transformer primary. The secondary of the transformer isconnected to the grid [3,4].

The use of transformer introduces losses in the system andalso needs more space and leads to noisy operation. Hence aboost converter is introduced between solar system andinverter which eliminates the use of transformer thereby

reducing the losses. Further, recent researches have focused onhow to get maximum power from solar energy [5] - [8]. All

the schemes invariably employ forced commutation for aninverter. In the present paper, a closed loop controller

employing line commutated SCR inverter for extractingmaximum power from solar energy has been proposed. Theinherent advantage of self latching property of SCRs has been

exploited in the proposed scheme.

III. PROPOSED SCHEMEThe block diagram schematic of the proposed solar energy

conversion scheme is shown in Fig. 1. It consists of a solar

array having three solar panels connected in series, interfaced

to the single-phase utility grid through a power electronicinterface. The DC voltage available at solar array is firststepped up to a voltage greater in magnitude to the grid

voltage and converted to AC using the line commutatedinverter in order to transfer the power to the utility grid. The

actual grid voltage and current are sensed and applied to theMPPT controller. These two parameters are multiplied and

this actual power is compared with the reference power P refand the difference between these two powers is fed as input to

the PI controller built inside the MPPT. The output of the PIcontroller modifies the firing angle such that the error gets

minimized.


2/5


~ 883

Fig. 1. The block diagram schematic of the proposed solar energy conversion scheme.

A. Power Electronic InterfaceIt consists of a boost converter, an inductor and line

commutated inverter. The boost converter shown in Fig. 2steps up the DC voltage available from the PV array. The dutycycle of the boost converter is automatically adjusted using PI

controller in order to maintain a constant voltage at its output.This voltage can be calculated by

Vd = VSA/(1 - ) (1)

where VSA is the output DC voltage from the solar array and

is the duty cycle of the boost converter. The can be adjustedby choosing proper values of proportional gain(KP) and

integral gain(KI) in PI controller to give 260V DC at boost

converter output. The KP and KI values chosen here are KP =0.25, KI = 1.

Fig. 2. Circuit for boost converter

The DC link current IDC is governed by the differentialequationdIdc/dt=(1/ L2)(Vd- Edc R2 Idc) (2)where Edc = input DC voltage of the inverter

B. Analysis of Line Commutated InverterA single-phase fully controlled bridge converter shown in

Fig. 3 can be operated either in rectification mode or in an

inversion mode. When the firing angle is between 0 and

90, the converter is said to be in rectification mode and when

is between 90 and 180, it is said to be in inversion mode.In the proposed scheme, the converter is operated as aninverter.

Fig. 3. Circuit for single-phase fully controlled bridge converter.

If the load is capable of supplying power, then the directionof power flow can be reversed by the reversal of the DC

voltage, the current direction being unchanged. The delay

angle must be greater than 90. In the present case, no extra

effort is required to synchronize the inverter output frequencywith that of the grid supply. This of course is possible only

with SCR converters. The average output voltage Edc is hence

given by

Edc = (2Em/) cos (3)where Em = maximum voltage of the single-phase utility grid.

(i) Harmonics:

The rms value of nth

harmonic input current is

)( ) )nIbaI dcnnsn 2/42/12/122=+=

nIdc /22= . (4)


3/5


884 ~

The rms value of the fundamental current is

)/221 dcs II = (5)

The total rms current is given by,

[ ] 2/12dcs )t(d.I2/2I = +

( ) ][ .// dcdc

ItI ==+

(6)

Now, harmonic factor (HF) or total harmonic distortion

(THD) is given by,

[ ( ) ] 2/121 1/ = ss IITHD

483.0= or %3.48 (7)

IV. MAXIMUM POWER POINT TRACKING CONTROLLERIn order to extract the maximum power from the solar

system, the firing angle of the inverter is adjusted in closedloop. The maximum power available at the solar array is used

as the reference power and is given byPref= K1.Voc x K2.Isc (8)

where K1 and K2 are constants given by [9], K1=0.76; K2=0.8;

The actual output power, Pact is compared with the reference

power and any mismatch is used to change the firing angle of the inverter as follows:

= (Pref Pact)[Kp + KI/s] (9)

where Kp and KI are the proportional and integral stage gainsrespectively.

The optimum values for KP and KI have been arrived at bytrial and error method. In the MPPT controller, the P and Icontroller gains are chosen as KP = 0.3; KI = 7;

V. SIMULATION RESULTSThe proposed solar energy scheme is completely modeled

using MATLAB simulink blocks in PSB platform. It consistsof solar array block, boost converter, line commutatedinverter, single-phase power grid and closed loop controllers.

The solar array model that has been proposed by S.ArulDaniel and N.AmmasaiGounden [10] is used in the proposed

scheme. The different parameters of the proposed scheme are:L1=0.01mH; L=1mH; C=1000F; L2=20mH; Rd=0.2.

As the solar radiation increases, the output of the solar array

increases. For any variation in solar irradiation, the output ofthe boost converter is held constant. The closed loop model of

the proposed scheme is simulated and the results are givenalong with experimental readings.

VI. EXPERIMENTAL INVESTIGATIONThe experimental setup of the closed loop scheme consists

of solar array, boost converter, a line commutated inverter,

controller to generate firing pulses to the thyristors andcontroller for adjusting duty cycle of boost converter. Three

solar arrays of 21.2V open circuit voltage and 5.17A shortcircuit current have been connected in series. A blocking

diode is connected to prevent the reverse power flow. Boostconverter has been fabricated with IRFP460 IGBT and

controller for duty cycle variation is constructed using simple

electronic circuits. A single-phase SCR converter has beenfabricated using 50RIA120 SCRs and a microcontroller firingscheme has been developed to trigger SCRs. The firing angle

of the inverter is adjusted to feed maximum power to the grid.Care has been taken to see that the firing angle is kept above

90 in order to facilitate inverter operation. Themicrocontroller PIC16F876A is made use of here. Theprogram for the microcontroller is written in MPLABIDE

software and verified using PROTEUS software and loaded inthe microcontroller chip using PICSTARTPLUS.

The results obtained from the experimental investigationand simulation study of the proposed scheme are furnished inTable I for comparison. It is seen that there is very close

agreement between the two, which ensures the validity of theproposed scheme.

TABLE I

COMPARISON OF SIMULATION AND EXPERIMENTAL RESULTS

= 160

Parameters

Simulation

Results

Experimental

Results

DC link voltage, Vdc -120 V -123.5 V

DC link current, Idc 1.8 A 2.0 A

Grid current, Igrid 0.8 A 1.0 A

Active power fed to the

grid, Pgrid -75.6 W -79 W

The simulated and oscillographic waveforms of DC linkcurrent are shown in Fig. 4. Similarly the observed and

simulated waveforms of voltage and current at the grid aregiven in Fig. 5. The harmonic spectrum of the grid current

obtained is shown in Fig. 6. It can be observed that THD isnearly the same as given by (7). The validity of the controller

can be ascertained by the close agreement betweenexperimental and simulated waveforms shown in Figs. 4 and5. The firing pulses for SCRs corresponding to a maximum

power point are shown in Fig. 7.

(a)


4/5


~ 885

(b)

Fig. 4. Waveforms of dc link current (a) simulated (b) experimental

(a)

(b)

Fig. 5. Voltage and current waveforms at the grid

(a) experimental (b) simulated

Fig. 6. Harmonic spectrum of the grid current.

Fig. 7. Firing pulses to SCRs

VII. CONCLUSION

A simple closed loop scheme employing a boost converterand single-phase line commutated inverter has been developedfor interfacing solar array with the utility grid. Simulationstudies have been carried out to get the various parameters of

the scheme such as active power and reactive powers, DC linkvoltage, current and the firing angle corresponding to the

maximum power for given solar radiation. Experimental setup has been built using a BP1280 solar cells and a PICmicrocontroller has been programmed to generate the trigger

pulses for the SCRs and the firing angle is adjusted to feedmaximum active power to the grid. As the inverter is being

operated as line commutated, the synchronization of outputfrequency with grid frequency does not arise. However due to

losses in the inductor, the output power fed to the grid is fairlysmall. This can be increased by selecting an inductor with lowlosses. Further, the THD of output current waveform is fairlyhigh due to harmonics introduced by switching of the inverter.

This requires a tuned filter to be connected across the gridterminals.

ACKNOWLEDGMENTThe PV panels used in this scheme were procured from the

fund provided by Ministry of Human Resource and

Development, India under the Thrust Area for TechnicalEducation Scheme. The authors gratefully acknowledge the

same. The authors also thank V.Manimaran, M.Prabhu andM.Rajesh for their assistance in conducting the experiment.

REFERENCES[1] Koosuke Harada and Gen Zhao, Controlled power interface between solar

cells and ac source, IEEE transactions on Power Electronics, Vol.8, No.4,October 1993, pp. 654-662,.

[2] S.Yuvarajan and Shanguang Xu, Photo-voltaic power converter with a

simple Maximum power point tracker, IEEE conference proceedings, 2003,

pp. 399-402.

[3] Henry Shu-Hung Chung, Member, IEEE, K.K.Tse, Member, IEEE,

S.Y.Ron Hui, Fellow IEEE, C.M.Mok, and M.T.Ho, Student Member, IEEE,

A novel Maximum Power Point Tracking Technique for solar Panels Using a

SEPIC or Cuk Converter, IEEE transactions on Power Electronics, Vol.18,

No.3, May 2003, pp. 717-724.

[4] N.Patcharaprakiti and S.Premrudeepreechacharn, Member, IEEE,

Maximum power point tracking using adaptive fuzzy logic control for Grid-

connected photovoltaic system, IEEE conference proceedings,2002, pp.372-

377.

[5] M.Veerachary, T.Senjyu and K.Uezato, Maximum Power Point Tracking

Control of IDB converter supplied PV system, IEE proceedings Electric

Power Appl. Vol.148, No.6, Nov 2001, pp.494-502.


5/5


886 ~

[6] F.Huang, D.Tien and James Or, A microcontroller based automatic sun

tracker combined with a new solar energy conversion unit, IEEE conference

proceedings, 1998, pp. 488-492.

[7] Noguchi Toshihiko, Togashi Shigenori and Nakamoto Ryo, Short-

current pulse based adaptive maximum - power point tracking for

photovoltaic power generation system, IEEE conference proceedings, 2000,

pp. 157-162.

[8] R.Leyva, C.Alonso, I.Queinnec, A.Cid-pastor, D.Lagrange andL.Martinez-Salamero, MPPT of photovoltaic systems using Extremum

seeking control, IEEE transactions on aerospace and electronic systems,

Vol.42, No.1, January 2006, pp.249-258.

[9] Dong-Yun Lee, Hyeong-Ju Noh, Dong-seok Hyun and Ick Choy, An

Improved MPPT Converter Using Current Compensation Method for Small

Scaled PV-Applications,IEEE conference, 2003, pp. 540-545.

[10] S.Arul Daniel, N.Ammasai Gounden, A Novel Hybrid Isolated

Generating System Based on PV Fed Inverter-Assisted Wind-Driven

Induction Generators, IEEE transactions on energy conversion, Vol.19,No.2, June 2004, pp. 416-422.

jyoti pv array

Documents