06743312

Modeling of grid connecting photovoltaic generation system with improved MPPT algorithm

Qiu Gefei Faculty of Electric Power Engineering

Kunming University of Science and Technology China Kunming

[email protected]

Gao Xia Foreign language teaching department

Yunnan University of finance and economics China Kunming [email protected]

Abstract—In this paper, photovoltaic maximum power point tracking (MPPT) algorithm based on duty cycle disturbing method is improved with a modified self-adopting step value calculation algorithm, the convergence of this modified tracking algorithm is speeded up compared with the original one and in the same time, numeric oscillation in the calculation process is avoided and the stability of convergence of algorithm is enhanced. A grid connecting photovoltaic generation system model has been developed with matlab/simulink, to fit for the using in power system phasor analysis, the converter is modeled in a reduced way in this system ensures that no power transferring variation will be led to. The MPPT algorithm is implemented with Matlab Simulink S 2 function and this will accelerate the calculating speed of simulation. To test the usability of modeled PV generation system in power system slow dynamic analysis, a simple power grid connected with PV system is constructed and simulation results show the effectiveness of this model.

Keywords- photovoltaic generation, power system, slow dynamic

I. INTRODUCTION (HEADING 1)In recent years, the presentation of photovoltaic

generating system is increasing in high rate due to its merits of zero emissions and inexhaustible nature of solar energy. With the penetration of PV generation system, operation mode, parameters and characters of power grid are different with that of classic one. A proper PV generation unit model is needed when we study operation characteristics of power system connected with PV generation units. In recent years, several models with little differences have been constructed [1~5], but most of these model studies put their focus on research and verification of characteristics of photovoltaic cell. In the research of power grid character, like the research on Electromechanical transient characteristics of power grid connected with PV units for example, these types of models cannot satisfy the need of research and should be modified. In this paper, a power grid connecting PV generation unit model is developed based on Matlab/Simulink in which a modified MPPT algorithm based on duty cycle disturbing method is employed and implemented with Simulink S2 function. Reduced model of converter ensuring no power variation is employed such that the modeled system will be easy to be used in power system slow dynamic analysis.

II. MODEL OF PV CELL ARRAY

General PV cell model is shown in figure 1.

Fig1. General model of PV

The output of PV cell is determined by solar irradiation level, ambient temperature etc., the source character of it like that of constant current source and can be molded as:

- [exp(( / / )/ ) 1] ( / )P PH P S S S P C P S S SHI N I N I qV N IR N kT A N V N IR R= + − − +(1)

In which,IPH is output current of PV cell,IS is saturation current,q is electron charge,k is Boltzmann’s constant,TC is ambient temperature,RS is series resistance ,RSH is parallel resistance, NP is number of parallel cells,NS is number of series cells.

The value of PV output current is influenced by solar irradiation level, ambient temperature:

Ref[ ( )]λ= + −PH SC I CI I K T T (2) ISC is the cell’s short circuit current at temperature TC

Kelvin, and irradiation (W/m2), KI is short circuit current temperature coefficient of cell, TRef is the cell’s reference temperature

The value of reverse saturation current is

Ref Ref3( / ) exp[ (1/ 1/ )/ ]= −S RS C G CI I T T qE T T kA (3)

IRS is reverse saturation current,EG is band gap of semiconductor material,A is ideal factor determined by technology

The IRS can be represented as: /[exp( / ) 1]= −RS SC OC S CI I qV N kAT (4)

III. THE DUTY CYCLE DISTURBING MPPT ALGORITHM WITH ADAPTIVE STEP CONTROL

In the operation process of PV cell, there will be different power output in different operation points with the same solar irradiation and ambient temperature. The process of

2013 2nd International Symposium on Instrumentation and Measurement, Sensor Network and Automation (IMSNA)

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maximum power tracking is adjusting operation parameters of generation unit to make the output of PV cell to be the maximum. The often been used MPPT algorithm include disturbing observing method, conductance increment method etc. Considering the MPPT algorithm has often been implemented in the DC/DC block of generation unit, a disturbing duty cycle MPPT algorithm is proposed in [5], this algorithm is improved in [6] with adaptive step, but in the process of calculation, there may be numeric oscillation. So, disturbing duty cycle MPPT algorithm is modified in this paper to steady the variation of adaptive step to improve the convergence of algorithm.

The principle of DC/DC segment using BOOST circuit is shown below. The conversion of input and output voltage is implemented by on off control of switch Q.

Fig2. The principle figure of Boost circuit

Fig3. Boost circuit when Q is ON

Fig4. Boost circuit when Q is OFF

When switch Q is on as indicated by figure 3, all the source voltage is applied across the boost reactance and the diode is cut off, the load current is supplied by capacitor C, the current of reactance will increase linearly with time.

LdiL V

dt= 5

The operation condition of circuit after Q is cut off is shown in figure4, the energy stored in reactance charges capacitor and the charging current is shown in figure 5.

Fig5. Current and voltage of reactance L

LO

diL V V

dt= − 6

The current increment of reactance charging is equal to decrease of discharging in a cycle:

( )(1 ) 0S O SV D T V V D T+ − − = 7(1 ) OV D V= − 8

Adaptive duty cycle disturbing algorithm can be constructed according to equation (8), by adjusting duty ratio D of boost circuit the referring impedance is adjusted to match that of power source so to get the maximum power output.

An adjusting factor is introduced in adaptive step control:

1kk

Pλ ε

λ+

Δ= 9

1kλ + is next step value of duty ratio adjustment, kλ is current step value, PΔ is the increment power value after last disturbing ε is sensitive parameter of adjustment

With the adjustment factor, adaptive duty ratio can be derived as:

+1 1k k k iD D dλ += + ∗ (10) di is step adjustment value,the magnitude of it can be

adjust,the sign of it is determined according to if the research result exceeds maximum point.

In the process of calculation, the calculated step factor often varies drastically, especially when the research is going near the point of maximum. This variation characteristic of step decelerates the calculation convergence speed and deteriorates convergence of algorithm. To solve this problem, the original algorithm is modified, the step adjustment value di will not remain as a constant, the search step will become finer when search process proceeds closer to maximum:

i i n

Pd d

PαΔ

=Δ0 (11)

di0 is initial step value, >1,is step adjustment factor,n is the number of exceeding maximum point. The flow chart of algorithm is listed in figure 6. in this paper, is0.005,convergence error is 2e-4.

IV. SIMULATION RESULTS

The model developed in this paper can be used in power system slow dynamic analysis in which phasor analyzing method is adopted. In this type of analysis, convertor can be reduced modeled as long as the power is the same before and after reducing. Simulink model of PV system is shown in Figure A1.

To verify the applicability of developed model in power system simulation, a simple grid connected with PV generation unit is employed as figure 7(only single line figure is shown here). The source is a 10kV system and PV supply the load with 400V voltage. The ambient temperature is T=35 .

The MPPT system is engaged after 0.4s of beginning, the load is a 10kW, 1kVar constant KVA load, The maximum power that PV unit can supply is 3000W assuming the


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initial solar irradiation isG=900W/m2, after 2.5s it turns to G=1000W/m2,the powers and some voltage, current of buses are shown below:

Fig6. Self-adaptive duty cycle disturbing MPPT algorithm

Fig7. Single line figure of test system

Fig8. P/V character of generation unit

9100

9150

9200

9250

9300

9350

9400P(kW)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5910

915

920

925

930

935

940Q(kVar)

Fig9.Power output of bus1

6000

6500

7000

7500

8000

8500

9000

9500P(kW)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 51600

1800

2000

2200

2400

2600

2800Q(kVar)


-3500

-3000

-2500

-2000

-1500

-1000

-500

0P(kW)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-50

-40

-30

-20

-10

0

10

20

30Q(kVar)


312

312.5

313

313.5

314

314.5

315

315.5

316Voltage(V)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

1

2

3

4

5

6

7I(A)

Fig12.Magnitudes of voltage and current of bus3

0.32

0.34

0.36

0.38

0.4angle V(rad)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-3

-2.5

-2

-1.5

-1angle I(rad)

Fig13.Phase angles of voltage and current of bus3

From figures above, it can be seen the convergence time of MPPT algorithm is less than 0.1s.

V. CONCLUSION

A duty cycle disturbing MPPT algorithm is improved and implemented by Simulink S 2 function in this paper. a photovoltaic generation system model which can be applied in phasor analysis of power system is constructed and tested using matlab/Simulink. The simulation results in line with theory expectation, it proves that the model constructed is correct and can be easily applied in slow dynamic power system analysis. The maximum power points have been found quickly, this proves that the convergence speed and calculation stability of MPPT algorithm is satisfying.


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REFERENCES

[1] Huan-Liang Tsai, Ci-Siang Tu, Yi-Jie Su. “Development of Generalized Photovoltaic Model Using MATLAB/SIMULINK,” Proceedings of the World Congress on Engineering and Computer Science 2008, WCECS 2008, October 22-24,San Francisco, USA.

[2] Debasis das, Shishir Kuma. “Modeling and simulation of PV array with boost converter: an open loop study,” Graduating thesis[D], 2011.

[3] Caisheng Wang. “Modeling and control of hybrid wind/photovoltaic/fuel cell distributed generation systems,” PHD thesis[D], 2006.

[4] LianCheng. Zhou, LiWei.Y,QiangGang. Wang “Research on dynamic characers of PV generation connecting micro grid,” Protection and control of power system, vol. 38, Dec. 2010, pp. 120-126.

[5] Koutroulis E , Kalaitzakis K, Voulgaris N C, “Development of a microcontroller - based , photovoltaic maximum power point tracking control system” [ J ] . IEEE Transaction on Power Electronics , vol. 16 Nov. 2001: pp. 46 - 52.

[6] Qiuhua Li, Lin Zhou, Qian liu etc.,”New MPPT algorithm and simulation of PV generation system”, Automatic equipment of power system, vol. 28, July, 2008, pp. 21-24M. Young, The Technical Writer’s Handbook. Mill Valley, CA: University Science, 1989.

1Io

Timer

TC

lambda

reset

mpptebl

mpptctrl

U

I

Uo

Dmin

VFeed

complete

1-D

Io

mppt

Dmin

1

[VDCout]

[oneND]

[Enable]

[lambda]

[VFeed]

[TC]

Timer1

>=

Switch

Product

TC

lambda

VFeed

VI

II

Psiganl

Pane

[VDCout]

From4

[Enable]

[lambda]

[TC]

[VFeed]

From

Dref

Drefr Add

3VDCout

2lambda

1TC

Fig. A1 Photovoltaic generating system


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06743312

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