# Global MPPT Scheme for Photovoltaic String Inverters Based on Restricted Voltage Window Search Algorithm -, Dr. Mutlu BOZTEPE - 2014.pdf

8/10/2019 # Global MPPT Scheme for Photovoltaic String Inverters Based on Restricted Voltage Window Search Algorithm -,

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Assoc. Prof. Dr. Mutlu BOZTEPE

Ege University, Department of Electrical and

Electronics Engineering

Global MPPT Scheme for Photovoltaic String

Inverters Based on Restricted Voltage

Window Search Algorithm

3rdRenewable Energy Systems Winter School

21-25 January 2014http://www.reswinterschool03.com

Outline

Introduction to solar cells

Electrical characteristics of Photovoltaic (PV) modules

Effect of non-uniform operating conditions on PV

module electrical characteristics.

Global MPPT tracking methods

Restricted Voltage Window method

Conclusion

3rd Renewable Energy Systems Winter School, 21-25 Jan. 2014 2

Operation of solar cells

P-N junction inside the solar cell induces a built -in

electric field (E).

The photon which has higher energy

than bandgap of material (1.12eV

for Si) can generate electron-hole

pair in the diffusion region.

The generated carriers (electrons

and holes) are separated into opposite sides of junction

by electric field, and became majority carriers.

Carriers are collected by metallic grid and finally the

current flows through the load


P

N+

-E

+ -+ -

Electrical equivalent circuit

4

+

-VPV

IPV

Light generatedCurrent IL

ID

ParallelresistanceRP

RSSeries resistance

IRp

P

SPVPVnkTRIVq

SLRDLPVR

RIVeIIIIII

SPVPV

P

1

q: Electron chargen: Diode ideality factor

K: Boltzmann constantIs: Diode saturation currentT: Cell temperature (K)

3rd Renewable Energy Systems Winter School, 21-25 Jan. 2014


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Current-voltage characteristics

VPV

ISCISC

IPV

VOCOpen circuitvoltage

ShortCircuit

current

Slope Rp

5

P

SPVPVnkT

RIVq

SLPVR

RIVeIII

SPVPV

1

Slope Rs


The effect of irradiation and cell temperature

As solar radiation increases, ISC

increases proportionally and

VOC decreases logarithmically.

As cell temperature increases,

ISCincreases very little, but VOCdecreases linearly (~-1.52

mV/C)


Parallel connection of solar cells

Only equivalent cells can be connected in parallel!


Series connection of solar cells

Only equivalent cells can be connected in series!



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Solar photovoltaic modules

A number of solar cells are usually connected in series

to compose of a solar PV module.


Typical 80W crystal Si PV module

At STC conditions (1000 W/m2, 25C, AM1.5)

36 cells (125x125 mm2)

Pm=80 W @ Vm=18V, Im=4,5A

Voc=22V (0,6Vx36=21,6V)

Isc=4,9A (3Ax1252/1002=4,69A)

Max. installation voltage 600V

8 kg/module

0,65 m2/module

958x680x30 mm

90% power output warranty for 10 yrs.

80% power output warranty for 20-25 yrs.


PV module examples


140Wp72 cells (125x125mm2)

17.2 kg, 1.32m2

300Wp72 cells (156x156mm2)

30.5 kg, 1.94m2

410Wp96 cells (156x156mm2)

34 kg, 2.58m2

PV module current-voltage characteristics


0 0.2 0.4 0.6 0.80

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Voltage [V]

Current[A]

Single cell 36 cells

0 5 10 15 20 250

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Voltage [V]

Current[A]


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PV module current-voltage characteristics


Power-voltage characteristics


VM

IM

VOC

ISC

MaximumPower point(MPP)

Maximum Power Point Tracking (MPPT)


Constant resistance as load

Battery

as load

MPP tracking

1000 W/m2

750 W/m2

500 W/m2

250 W/m2

k gc [W]

Radiation

(W/m2) Constant

R

Battery

MPP

tracking

1000 79,3

(%99)

68,0

(%85)

80,0

(%100)

750 48,0(%81,2)

50,5(%85,4)

59,1(%100)

500 21,3

(%56)

33,3

(%87,6)

38,0

(%100)

250 5,3(%30)

16,3(%92)

17,7(%100)

MPPT realization

By using control input;

Converter (or inverter) input impedance can be controlled

Or PV voltage (or current) can be controlled



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MPPT Methods

Fractional open-circuit voltage: (V=k1*Voc, k1=0.750.8)

Fractional short-circuit current: (I=k2*Isc, k2=0.780.92)

Incremental conductance:


IVP .dV

dIVI

dV

dP

0dV

dP

MPPatdV

dP0

0dV

dP

P-V curve

Perturbation & Observation algorithm:

MPPT Methods


Vstep

Other MPPT Methods

Fuzzy logic control

Neural network

Ripple correlation control

Current sweep, etc

Most widely used ones;

Perturbation&Observation (P&O)

Incremental conductance

Both method are designed to track only single power

peak, and exhibits local maximum tracking behaviour


Non-uniform operating conditions

Irradiation differences

Partial shading, dissimilar inclination&orientation

Temperature differences

Non-unifom heat flow, cooling differences

Parameter differences

Aging, production tolerances

The power-voltage and current voltage curves of a PV

module are affected by these non-uniform operating

conditions drastically!



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Shading problem


Energy losses due to partial shading

German 1000 roofs programme were affected by

shading causing 10% energy loss[Renew. Energy, 33(9), 20482056, 2008]

The power loss due to the improper MPPT may be as

high as 70% according to the real measurements [IEEE T.Ind. Electron., 55(7), 25692580, 2008]

Thus, in recent years,

numerous studies worldwide

have been performed to

mitigate the power loss

due to partial shading


IEEE T.En.Conv. 22(2),2007,439-449

The effect of shading on series solar cells

Shaded cell has lower Isccurrent than other cells.

If Ipv>Isc_shadedthen the cell voltage can reverse, and it

behaves as a load!!!

If reverse voltage exceeds breakdown limit of the cell,

dissipated power increases significantly.


I

A

B

+

-VA

+

-

VB

+

-

VG

A

B

-VB VA

Operating

current

Ipv

Vpv

Hot spot effect

Dissipated power raises the cell temperature, and

creates hot spot effect.

If Tc>Tmax, the module can be damaged permanently!!!


Heat dissipated in ashaded cell caused themodule to crack.

http://pveducation.org/pvcdrom/modules/hot-spot-heating


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How bypass diode affects the I-V curve?


Shaded unshaded.

Without bypass

diode

How bypass diode affects the P-V curve?


Shaded unshaded.

Without bypass diode

Shading problem


Prog. Photovolt Res. Appl., 16,529536, 2008.

Shading problem



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Central inverter



Extracted power: 3230 Watt or 2971 Watt?

String inverter



Extracted power: 1568+2046=3614 Watt?or 529+2046=2575 Watt?

Micro-inverter


Global MPPT methods

Hardware based

Dynamic reconfiguration, Distributed MPPT concept, Module

integrated DC/DC converter, constant input power converter

etc.

Software based (Can be realized only by software

modifications)

Complex methods: Fibonacci search algorithms, ANN or Fuzzy

controllers, Bayesian fusion technics, differential evolution,sequential extremum seeking, ant colony optimization, etc.

Less complex: Two stages MPPT methods, global peak search

algorithm, DIRECT search algorithm, etc..



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1

Global MPPT methods


Global MPPT methods


Global MPPT methods


Global MPPT methods



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1

Choice of the Global Voltage Step VGSTEP

The choice of VGSTEP directly affects both the scanning time

and the algorithm accuracy. However the optimum value

should be around the voltage difference between two adjacent

local power peaks.

MATLAB Simulink

simulations for 1000

random irradiance values

between 1001000W/m2.

Voltage difference between

two adjacent local power

peak larger than 0.5VocP ,

but mostly around of

0.8VocP


Choice of the Global Voltage Step VGSTEP

0.5VocPand 1.0Vocreach

global maximums B and D.

On the contrary, 1.5VocPmisses point B and evolves

towards B.

1.0Voccan miss a peak if

adjacent peaks have nearly

equal power.

So, VGSTEP is selected

between 0.5VocPand

1.0Vocby balancing

tradeoff tracking eff. and

convergence speed


Power operating triangle (POT) and

Voltage Window (VW)


Minimum andmaximum valuefor VW are point

A and Brespectively

The global

power peak mustbe in the rangeof VW, in otherwords between Aand B.

Narrowing the upper limit of the VW

Narrowing the VW

reduce the

searching steps and

therefore decrease

the convergence

time

2000 random

irradiation values

0.9 Voc is selected

as upper limit for

VW



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1

POT update

The proposed algorithm uses datasheet values of both ISC and

VOC to define the first triangle POTSTC.

POTSTCis not closely cover

P-V curve

After measuring current

at Vmin, the algoritm finds

point A.

And then new triangle POT1

is defined which is closely

covers P-V curve

VW=[Vmin Vmax]


VWS algorithm flowchart


Sudden shading detection method


Shading is detected if k: sensitivityparameter

Slope2 isalways higherthan slope1

Algorithm operation

Lets assume

operating point

is Q.

1,2,3,4 and 5

are checked.

a,b,c and d

are omitted



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Algorithm evaluation (Uniformnon-uniform)


Algorithm evaluation (During non-uniform)


Algorithm evaluation (uniformnon-uniform)


Power,voltage andcurrent plotswith respectto time

Algorithm evaluation (non-uniformuniform)



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Algorithm evaluation (during uniform)


Algorithm evaluation (during uniform)


Power,voltage andcurrent plotswith respectto time

Experimental setup


P-V curves programmed in the SAS.



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Proposed VWS Agarwal

algorithm algorithm


Comparison



Documents

# Global MPPT Scheme for Photovoltaic String Inverters Based on Restricted Voltage Window Search Algorithm -, Dr. Mutlu BOZTEPE - 2014.pdf