Download Peak Power Tracking Control for Photovoltaic Array

6/16/2016
Peak Power Tracking Control for
Photovoltaic Array
Charles W. Davidson
College of Engineering
One Washington Square
San José, CA 95192-0080
www.engr.sjsu.edu
Ping Hsu
Electrical Engineering
San Jose State University
Outline
• Basic photovoltaic cell theory of operation
• Characteristic of a photovoltaic cell
• Examples of PV panel power controller
• Maximum Power Point Tracking control
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Basic Photovoltaic Cell Theory
Silicon atom has 14 electrons and 14 protons.
Heat or light
e-
ee-
ee-
e-
14+ 14‐
14+ 14‐
14+ 14‐
14+ 13‐
14+ 14‐
e-eN14+
e-
ee-
ee- e- e
ee-
e-
e-
ee- e-
14+ 14‐
1‐
14+ 14‐
14+ 14‐
14+ 14‐
e-
Freed electron
When sunlight strikes a piece of Silicon, the solar energy
knocks and frees electrons from their atom structure.
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The freed electrons randomly move within the material.
This random motion of charge cannot be utilized for
power generation. In order to utilize the energy from the
sun, this flow of charges must be directed in one
direction.
Heat or light
Freed electron
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Two types of semiconductors are used Positive type (Ptype) and Negative type (N-type)
A small amount of Phosphorus (impurity) is mixed into a
Silicon base and this forms an N-type material. Due to the
impurity, some atoms in this material have one electron
extra for a stable atom configuration. In other words, it
has one loosely bonded electron.
N-type material
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• A small amount of Boron (impurity) is mixed into a Silicon
base and forms a P-type material. With this impurity,
some atoms in this type of material have one electron
short for a stable configuration. In other words, it can
easily accept one electron.
• For simplicity, this characteristic of ‘easily accepting
electron’ is represented by a “hole” with a positive charge
and a corresponding negative charge at the nucleus.
P-type material
hole
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Now put an N-type material in contact with a P-type material.
Before making the contact:
P-type
(neutral)
N-type
(neutral)
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P-type
(Negatively charged!)
boundary
layer
N-type
(Positively charged!)
Note that in the boundary layer, the free electrons in the N-type
materials combine with the holes in the P-type. Consequently, the
P-type side of the boundary layer is negatively charged and N-type
side is positively charged.
This negative charge in P-type material prevents the free electrons in
the rest of the N-type materials continue to migrate into the P-type.
(Negative charge repels negative charged free electrons.)
The boundary lay is called PN-junction or depletion region.
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sunlight
P-type
(Negatively charged)
N-type
(Positively charged)
When sunlight strikes atoms in the P-N Junction and
knockouts more electrons (and creates corresponding
holes), the free electrons are expelled by the negative
charge on the P-type side and hence move to wards the Ntype side.
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If a load is connected across the cell, a complete circuit is
formed. A steady flow of electrons (i.e., current) passes
through the load and hence the energy is transmitted to the
load.
sunlight
P-type
N-type
http://www.youtube.com/watch?v=xLGOagKiXqg
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Photovoltaic cell characteristics
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The current and voltage relationship of a single solar cell can be
expressed by the following equation:

 q (V )  
I  I L  I o exp 
  1
 nKT  

where
I
V
load
IL : Light current, proportional to irradiance (W/m2),
Io : Saturation current q: electron charge n : diode quality factor
K: Boltzman constant T : temperature in oK,
≈0
≈∞
IV curves of a solar cell at different levels of irradiance.
=1000 W/m2
=800 W/m2
=600 W/m2
Cell
current
(Amp)
=400 W/m2
=200 W/m2
I
Cell voltage (volts)
V
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Output power of a solar panel at different levels of irradiance.
Maximum Power Point
=1000 W/m2
=800 W/m2
=600 W/m2
Current
Power
400 W/m2
200 W/m2
Voltage
1.6m
1m
Efficiency 
235W
 15%
(1.6 1.0)m 2  1000(W / m 2 )
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Locus of
peak
power point
=1000 W/m2
=800 W/m2
 W
=600 W/m2
Panel
current
(Amp)
=400
 W
W/m2
 W
=200 W/m2
 W
 W
 W
Panel voltage (volts)
The following figure shows the dependence of the
peak-power locus on the panel temperature. As
shown in the figure, as the temperature rises, the
peak-power locus shifts to a lower voltage level.
50oC
70oC
o
90 C
10
30oC
Panel
current
(Amp) 5
0
20
25
Panel voltage (volts)
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Examples of PV panel power controller
An example of a Grid-tie Photovoltaic System
Panel power
controller
The panel power controller serves the following functions:
(1) Regulating the I-V characteristic to achieve peak power
tracking
(2) Boosting the voltage (DC bus voltage must be at a certain
level for the proper grid-side inverter operation)
(3) Electrical isolation (for safety).
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The boost converter shown in the previous page has limited
voltage boost range and it does not provide electrical isolation.
A more commonly used circuit configuration consists of an Hbridge and a transformer as shown above. This circuit offers
electrical isolation and its voltage boost is achieved by the
transformer’s winding ratio.
Peak Power Tracking Control Methods
(1) Constant voltage with temperature compensation
(2) Constant voltage
(3) Open circuit voltage
(4) short circuit current
(5) Incremental conductance
(6) Perturb-and-observe (P&O)
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(1) Constant voltage with temperature compensation
If the panel voltage is regulated at a certain voltage level, the operating
point will be near the maximum power point at all irradiance levels.
Constant voltage load
=1000 W/m2
Resistive load
=800 W/m2
 W
=600 W/m2
Panel
current
(Amp)
=400
 W
W/m2
 W
=200 W/m2
 W
 W
 W
Panel voltage (volts)
The load characteristic is determined by the panel power
controller’s control law. For example,
For resistive load:
Icmd= K*Vp
(K=1/Req)
For constant voltage load:
Icmd= [Kp+Ki (1/s)] (Vp-Vcmd)
where Vcmd is the set panel voltage.
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Example of constant voltage Mathod
The MPPC circuit controls the inductor current to maintain VIN at the
voltage on the MPPC pin. The MPPC pin voltage is set by connecting a
resistor between the MPPC pin and GND The MPPC voltage is
determined by the equation:
VMPPC = 10μA • RMPPC
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A 2-cell panel Li-Ion battery charger example
VMPPC = 10μA • 75k = 0.75v.
Note that the open circuit voltage of this 2-cell panel is 1v.
The panel voltage can be varied according to the panel
temperature to achieve maximum power control.
Panel voltage is varied according to the panel temperature.
50oC
70oC
o
90 C
10
30oC
Panel
current
(Amp) 5
0
20
25
Panel voltage (volts)
30
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A diode can be used to set the MPPC threshold so that it tracks the cell
voltage over temperature.
(2) Open circuit voltage method
The maximum power operating voltage is closely proportional to the
open circuit voltage of the PV panel. The proportionality constant
is about 76%. The control circuit momentarily draws zero current
from the panel (open circuit) and then measure the voltage. The
reference voltage is then set to 76% of the voltage.
=1000 W/m2
=800 W/m2
=600 W/m2
Panel
current
(Amp)
 W
 W
=400 W/m2
 W
=200 W/m2
 W
 W
 W
Panel voltage (volts)
Open circuit voltage
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(3) Short circuit current method
The maximum power operating current is closely proportional to the
short circuit current of the plane. The proportionality constant is
about 78% ~ 92% of the short circuit current. The control circuit
momentarily short circuit the panel and measure the current. The
reference current is set to 78% of the short circuit current. The
controller in this case should be configured as a current source.
Short circuit
currents
=1000 W/m2
=800 W/m2
 W
=600 W/m2
Panel
current
(Amp)
=400
 W
W/m2
 W
=200 W/m2
 W
 W
 W
Panel voltage (volts)
(4) Incremental Conductance
It is easy to see that at the maximum power point, the derivative
of power (P) with respect to voltage should be zero.
dP
0
dV
=1000 W/m2
=800 W/m2
=600 W/m2
Current
Powe
r
400
W/m2
200
W/m2
Voltage
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(3) Incremental Conductance
dP d VI  dV
dI


I
V 0
dV
dV
dV
dV

I
dI
V 0
dV
dI
I


1 4 2dV4 3
1 44 2V4 43
incremental conductance
conductance
This equation shows that at the maximum power point, the
incremental conductance should be the same as the negative of the
conductance of the panel.
If the operating point is on the ‘left’ of the maximum power point.
dP d VI  dV
dI


I
V 0
dV
dV
dV
dV

I
dI
V 0
dV
dI
I


dV
V
1 4 2 4 3
1 44 2 4 43
incremental conductance
conductance
If the operating point is on the ‘right’ of the maximum power point.
dI
I


1 4 2dV4 3
1 44 2V4 43
incremental conductance
conductance
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Logic flow of the incremental conductance
maximum power point tracking algorithm
Read I(k) and V(k)
dI(k) =I(k)-I(k-1)
dV(k) =V(k)-V(k-1)
I(k-1) = I(k)
V(k-1) = V(k)
dI
I

dV
V
Increase
ref voltage
dI
I

dV
V
decrease
ref voltage
Perturb-and-observe (P&O) method
The P&O algorithm constantly varies the voltage set-point by a
small amount (V). The power level before and after the
perturbation are compared. If the power level is higher after a
positive voltage perturbation V, the operating voltage is
increased by V.
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“Implementation of Maximum Power Point Tracking Control” Boztepe, Colak,
IEEE CPE 2007 International Conference workshop
This is also known as the
Hill climbing method.
Variation of the P&O schemes:
• Dynamically changing perturbation size.
• Three-point P&O.
Drawbacks:
• Oscillation about the maximum power point.
• Confused by fast changing irradiance or noise.
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Simulation results from :
“Energy Comparison of Seven MPPT Techniques for PV Systems”
A. Dolara, R. Faranda, S. Leva, J. Electromagnetic Analysis &
Applications, 2009, 3: 152-162
Two irradiances profiles are used in the simulation.
Case 1
Case 2
Simulation results from :
“Energy Comparison of Seven MPPT Techniques for PV
Systems,” A. Dolara, R. Faranda, S. Leva, J. Electromagnetic
Analysis & Applications, 2009, 3: 152-162
Case 1
Energy (J)
Ideal
4493
P&O
4282~4278
Case 2
Rank
Energy (J)
1~3
3212 ~ 3134
Rank
3298
1~3
Inc. cond.
4215
4
3117
4
Cost. Voltage
4210
5
3100
6
OC voltage
4200
6
3104
5
SC current
4088
7
2942
7
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Thank you!
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