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ECE1750, Spring 2018 Power Electronics Applications in Photovoltaic Power Generation Photovoltaic Power Generation 1

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ECE1750, Spring 2018

Power Electronics Applications in Photovoltaic Power GenerationPhotovoltaic Power Generation

1

Photovoltaic modules

• Photovoltaic (PV) modules are made by connecting several PV cells. PV arrays are made by connecting several PV modules.

• Although the sun will eventually die as a white dwarf star in about 4.5 Billion years, solar power can be considered a renewable source of energy because we can expect that for the next couple of billion years the sun will still radiate

ith t ki th E th i h bit blpower without making the Earth inhabitable.

• Solar power is radiated through space.

• Solar power is generated by nuclear fusion.

• Photons are created at the center or the Sun It takes an average of 10 millionPhotons are created at the center or the Sun. It takes an average of 10 million years for the photons to emerge (they collide many times in the Sun interior). Then it takes 8 minutes for a photon to reach the Earth.

• Once they reach earth some photons are scattered and absorbed in the atmosphere.

Photons’ Journey into Electricity

• Finally, the photons reach ground on Earth

US Solar Insolation Map: NREL© A. Kwasinski, 2017

Photons’ Journey into Electricity

• The incident power has 3 components depending on the final photons path.

Diffuse radiation

Direct-beam radiation

Reflected radiation

© A. Kwasinski, 2017

Photons’ Journey into Electricity

• Sun’s location terms

© A. Kwasinski, 2017

Photons’ Journey into Electricity

• Magnetic vs. celestial poles:• Magnetic poles:

• Caused by Earth’s magnetic field• Can be located with a compass• They move along Earth’s surface!

Celestial poles: • Caused by Earth’s rotation.• They are two imaginary stationary• They are two imaginary stationary

points in the sky.• Important for PV system applications.

Geological Survey of Canada

© A. Kwasinski, 2017

Sun Moves Throughout the YearSu o es oug out t e ea

June 21

7December 21

Photons’ Journey into Electricity

Edge of

• Impact of the sun’s position for the calculation of the direct-beam radiation with respect to the incidence angle and the air mass ratio

June 21Austin’s Latitude: 30o 30o

Edge of PV module

(for incidence angle

calculation)

March 21September 21

Tropic of CancerLatitude 23.45o 23.45o

calculation)

September 21

Equator23.45o

December 21Tropic of CapricornLatitude -23.45o

Earth’s surface(for air mass ratio(for air mass ratio

calculation)© A. Kwasinski, 2017

Photons’ Journey into Electricity

Solar Zenith versus Azimuth at Austin

22 d D f J Jl A S O t N D

• Sun’s position in the sky throughout the day and during an entire year.

22nd Day of Jun, Jly, Aug, Sep, Oct, Nov, Dec(Sun hrs/day. Jun=13.9,Jly=13.6,Aug=12.8,Sep=12.0,Oct=11.0,Nov=10.3,Dec=10.0)

0 30 60 90 120 150 180 210 240 270 300 330 360

Azimuth (South = 180)

0

10

20

30Verti

cal)

NOON 1 PM

3 PM

Jun

30

40

50

60

h (D

egre

es fr

om V 3 PM

Sep

70

80

90

Zeni

th

Dec

© A. Kwasinski, 2017

Photons’ Journey into Electricity

• The direct-beam insolation IBC depends on the PV module orientation with respect to the sun. If the PV module is fixed, this insolation will change in a deterministic way throughout the day and the year:

if the incident angle θ is given bycos cos cos( )sin sin cos

S C

• Then, the direct-beam insolation is

I I cosBC BI I

© A. Kwasinski, 2017

Panel Orientation is Importanta e O e tat o s po ta t

tiltpanel

Line perpendicular to horizontal plane

Line perpendicular to panel surface

tiltpanel Horizontal plane

Edge of panel

Figure 6. Panel Tilt Angle

• Best all-year tilt = Latitude

• Best winter tilt = Latitude + 15°

• Best summer tilt = Latitude – 15°

11

Photons’ Journey into Electricity

At ’ d l• Atom’s energy model:Conduction band(partially filled)

Conduction band(Empty at T = 0K)

Eg Forbidden band

Filled band

n E

nerg

y Eg Forbidden band

n E

nerg

y

Gap

Filled bandEle

ctro

n

Gap

Filled bandEle

ctro

n

M l i d

• Photons energy is quantized. The energy of a photon with a wavelength of λ(or a frequency of υ) is

Metals semiconductors

hcE h

where h is Planck’s constant© A. Kwasinski, 2017

Photons’ Journey into Electricity

• After a long journey, photons are converted into electricity in semiconductors:

• Whenever a photon with enough energy hits an atom, an electron may jump the energy gap into the conduction band Once in the conduction band thethe energy gap into the conduction band. Once in the conduction band, the electron is free to move in an electric circuit.• If the circuit is open or if the load requires less current (charge per time) than the one being produced, the free electrons will eventually decay again.g p , y y g• Since it is assumed a continuous slow varying incident solar energy, electrons are freed at a constant rate (direct current). Hence, a constant voltage is produced.

PV Cells TechnologiesUni-Solar solar shingle

BP SX170B Polycrystalline BP SX170B Monocrystalline

Uni-Solar Laminate PVL-136 Amorphousp

Mitsubishi PV-TD 190MF5 Multicrystalline

Various types of PV Modules© A. Kwasinski, 2017

PV Applications

• More conventional applications (not all necessarily for microgrids)

© A. Kwasinski, 2017

PV Applications• Less conventional applications (not all necessarily for microgrids)

© A. Kwasinski, 2017

The p-n junction diode

n-type substrate dqV

p-type substrate

yp

Bias voltage 0 1dq

kTdI I e

I

Ideal diode

Id

• Vd is the diode voltage• I0 is the reverse saturation current caused by Ideal diode

Real diode0 y

thermally generated carriers• At 25 C:

dV 0.026

0 1dV

dI I e

I0

© A. Kwasinski, 2017

PV Cells physics

I

The current source shifts the reversed diode curve upwardsISC

VOC

diode curve upwards

Same curve

The bias source

Reverse v-i f th

ISC

p-n junction is equivalent to a diode

The bias source (voltage source) is replaced by a current source

d b th curve for the diode

powered by the photons

PV Cell steady state characteristic

• From Kirchoff’s current law:

1dqV

kTI I I I I

• The open circuit voltage is

0 1kTPV SC d SCI I I I I e

0

( 0) ln 1SCOC PV

IkTV V Iq I

Power

Maximum power point

0q Power

Pmax 0.7 • Voc • Isc

PV PVP I V

OCV

Current

I

© A. Kwasinski, 2017

SCI

PV Cell steady state characteristic

• Dependence on temperature and insolation:

© A. Kwasinski, 2017

PV Cell steady state characteristic

• More on the dependence on temperature and insolation:

© A. Kwasinski, 2017

• 36 Cells in Series Make a 12V-Class Panel (Voc 19V)

9 cells x 4 cells is acommon configuration

• Two 12V-Class Panels in Series Make a 24V-Class Array (Voc 38V)

22

I-V Curve

17770 VIsc

100524.034.5)( 1777.0 VeVI

PV Station 13, Bright Sun, Dec. 6, 2002

6 P t 30V

4

5

6

s

IscPmax at approx. 30V

Pmax 0.7 • Voc • Isc

1

2

3

I - a

mps

0

1

0 5 10 15 20 25 30 35 40 45

V(panel) - volts Voc

23

Voc

The Maximum Power Pointe a u o e o t

PV Station 13, Bright Sun, Dec. 6, 2002

100.0

120.0

140.0

tsPmax

40 0

60.0

80.0

P(pa

nel)

- wat

t

0.0

20.0

40.0

0 5 10 15 20 25 30 35 40 45

P=0 at short circuit P=0 at open circuit

V(panel) - volts

24

(S S

Grid tied inverters• Traditional architecture (SMA Sunny Boy, PV Powered,

Fronius, Xantrex, and others):

“CONNECTION BOX” (NO

• Two main issues need to be addressed:– Operate PV modules at their maximum power.

ELECTRONICS)

Operate PV modules at their maximum power.– Control inverter’s power output.

25

Maximum Power Point Tracking

Power

Maximum power pointmaxP

Current

Power

ILP

Current

maxVLload

VR

LImaxI

When we connect a resistance R directly to a PV panel the power output is

maxmax

max

VRI

loadLI

LV maxV

• When we connect a resistance RL directly to a PV panel the power output is PL. In general

maxLP P

• Ideally, we would like to have the PV panel always operating at Pmaxregardless of the load. Also, take into account that Pmax changes due to various factors.

Impedance matching

Iout = Iin / DIin

outout

load IVR DC−DC Buck

Converter

+

Vin−

+

Vout = DVin−

Source

Iin

equivR+

VinEquivalent from source perspective

RVDV

V l dout

iSo, the buck converter makes the load

2722 D

RDI

VDI

DIVR load

out

outoutin

inequiv

makes the load

resistance look larger to the source

Example of drawing maximum power from l lsolar panel

PV Station 13, Bright Sun, Dec. 6, 2002g

5

6

IscPmax is approx. 130W (occurs at 29V, 4.5A)

2

3

4

I - a

mps

For max power from panels at this solar intensity level, attach

0

1

2

0 5 10 15 20 25 30 35 40 45

44.65.4

29A

VRload

y

But as the sun conditions0 5 10 15 20 25 30 35 40 45

V(panel) - volts Voc

I-V characteristic of 6.44Ω resistor

But as the sun conditions change, the “max power resistance” must also change.

Also, the load changes based on the user needs.

Example of a directed connected load different to that yielding maximum power

PV Station 13, Bright Sun, Dec. 6, 2002

6 130W55W

3

4

5

amps

1

2

I - a

00 5 10 15 20 25 30 35 40 45

V(panel) - volts

Consider that the user wants to connect a 2 Ohm load If it is connected directly itConsider that the user wants to connect a 2 Ohm load. If it is connected directly it consumes 55 W. To draw maximum power (130W), connect a buck converter between the panel and the load resistor, and use D to modify the equivalent load resistance seen by the source so that maximum power is transferred

56.044.62 ,2

equivloadload

equiv RRD

DRR

D is adjusted automatically by a maximum power point tracker (MPPT) so maxequivR R

Impedance matchingImpedance matchingIin inout IDI 1

outout

load IVR DC−DC Boost

Converter

+

Vin−

+

−Source D

VV inout

1

Iin

equivR

+

VinEquivalent from source perspective

l doutoutin

i RDVDVDVR 22 111

30

loadoutoutin

equiv RDI

D

DII

R 11

1

Example of drawing maximum power from l lsolar panel

PV Station 13, Bright Sun, Dec. 6, 2002g

5

6

IscPmax is approx. 130W (occurs at 29V, 4.5A)

2

3

4

I - a

mps

29V

For max power from panels, attach

0

1

2

0 5 10 15 20 25 30 35 40 45

44.65.4

29A

VRload

But as the sun conditions0 5 10 15 20 25 30 35 40 45

V(panel) - volts Voc

I-V characteristic of 6.44Ω resistor

But as the sun conditions change, the “max power resistance” must also change.

31Also, the load changes based on the user needs.

Example of a directed connected load different to that yielding maximum power

PV Station 13, Bright Sun, Dec. 6, 2002

6 130W

3

4

5

ampsSo, the boost converter

reflects a high load

1

2I -

a

14W

gresistance to a low resistance on the source side

00 5 10 15 20 25 30 35 40 45

V(panel) - volts

C id th t th t t t 100 Oh l d If it i t d di tl itConsider that the user wants to connect a 100 Ohm load. If it is connected directly it consumes 14 W. To draw maximum power (130W), connect a boost converter between the panel and the load resistor, and use D to modify the equivalent load resistance seen by the source so that maximum power is transferred

75.0100

44.611 ,1 2 load

equivloadequiv R

RDRDR

y pD is adjusted automatically by a MPPT controller so maxequivR R

Impedance matchingImpedance matchingIin

D

DII inout

1

outout

load IVR DC−DC SEPIC

+

Vin−

+

−Source D

DVV inout

1

Iin

equivR

+

VinEquivalent from source perspective

out

outin RDVDD

VDVR

22 111

33 load

out

outoutin

inequiv R

DIDD

DID

IR

1

Impedance matchingImpedance matching

outVD22

1

load

out

outout

out

in

inequiv R

DD

IV

DD

DDI

DIVR

22 11

1

For any Rload, as D → 0, then Requiv → ∞ (i.e., an open circuit)

For any Rload, as D → 1, then Requiv → 0 (i.e., a short circuit)

Thus, the SEPIC can sweep the entire I-V curve of a solar panel in order to achieve the MPP regardless of actual

load used or received solar energy

34

load used or received solar energy

Grid-tied inverter control• As it has been explained, we can place a dc-dc converter between the PV array and the inverter so the PV array output is controlled so that it provides itsarray and the inverter so the PV array output is controlled so that it provides its maximum power output regardless of the actual load or other condition.• The output of the dc-dc converter is the power input of the inverter. Output voltage of the inverter can then be controlled to synchronize with the grid.

Vdc

g y g• But what if we want to control the power output of the inverter even without a dc-dc converter at the output of the PV array? The, consider the following….

A+ B+ invZ

Equivalent Circuit

Mot Mot

inv

+ )cos( tVinv −

A– B–

dVa

dcinv m

VV

2invZ

invZ is mostly resistive unless a large inductor is addedAt 60 Hz, inverter impedance

The Electrical Circuit Model

invZ

+

gridZ

+

QPSI , , ,

+ )cos( tVinv −

)cos( tVgrid −

Inverter Grid

• I is the phasor current

• S (complex power) = P + jQ

• P is the active power

• Q is the reactive power

36

• Impedances Z can be expressed as R + jX

• Zinv depends on circuit parameters and on the inverter controller action

The Electrical Circuit Model, cont.

For typical 60Hz systems, the circuit resistance R’s are much higher than the inductive reactance X’s (otherwise the inverter can be controlled so it is a mostly resistive impedance). y p )

Also, voltage angle δ is zero because the inverter control signal is assumed to be a replica of the grid voltage (also, current standards require that the power factor of grid tiedcurrent standards require that the power factor of grid tied inverters is 1).Then, it can be found that

gridinvgrid VV

RV

P 0Q

Active power Reactive power

gtotR

Thus, we control the direction

37

and amount of P by adjusting this difference

Effect of real and reactive power from PV invertersinverters

38

Effect of real and reactive power from PV inverters

• With grid tied inverters, when PV power increases, the real power provided by the grid is reduced by the reactive power provided by the grid is not significantly changed. Hence, power factor from the grid is reduced.

39

Grid Tied InvertersGrid Tied Inverters• Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex,

and others):

Grid Tied InvertersGrid Tied Invertersand others):

Fronius

PV Powered

40

Islanding invertersIslanding invertersIslanding invertersIslanding inverters• Alternative to traditional architecture to provide backup and operation

without grid connection (SMA Sunny Boy – Sunny Island 5048U).

41