IEEE PEDS 2011, Singapore, 5 - 8 December 2011
A MPPT Control Method for Stand-Alone
Photovoltaic System in Consideration of Partial
Shadow
Yuya Oshiro, Hikaru Ono, Naomitsu Urasaki
Dept. of Electronics Engineering, University of the Ryukyus
1 Senbaru Nishihara-cho Nakagami Okinawa, 903-0213 Japan
E-mail : urasaki@tec.u-ryuku.ac.jp
decreases, the control input changes in the opposite direction.
Repeating this sequence, the PO method gets up to the MPP
at the top of mountain. The open-circuit voltage control
method adjusts the operating voltage of PV in accordance
with the characteristic in which the maximum power point
voltage (MPPV) of PV is about 80 % of open-circuit voltage.
The PV characteristic alters when the PV array is partially
covered with a shadow. The shadow which is called the
partial shadow may occur by limb and pintle mount. The
output power of PV has multiple local maximal points under
I. INTRODUCTION
the partial shadow. Then, it is difficult for the above
Recently, due to increasing concern about the environment, traditional methods to find the MPP of the PV. The chance of
the attention of renewable energies, such as photovoltaic (PV), occurrence of partial shadow is increasing than before
wind-turbine, and geothermal generations, is increasing because various types of PV have been developed and widely
because they emit no exhaust CO2 in power generation. used at many locations. Thus, it is an important issue to solve
Especially, PV is very useful. It is employed everywhere the partial shadow problem.
This paper proposes a modified MPPT method for a
from small scale application like a cell phone to large scale
application like a mega solar plant. Additionally, the standalone PV system like a camping car equipped to charge
installation of PV is easy. Many researchers have developed battery by PV. In this method, both the open-circuit voltage
and short-circuit current of PV are instantaneously measured
new type solar cells.
They include the colorful dye-sensitised solar cell [1] and to distinguish between irradiance change and the occurrence
the film type solar cell which is light, large, and elaborately of partial shadow when the output power of PV changes. The
designed [2]. PV has rapidly spread thanks to the background method can successfully operate even though a partial
mentioned above and these new type PVs. However, the shadow arises. The superiority of the proposed method is
generating cost of PV is higher than other generators because confirmed by simulation result.
the power density of PV is low [3]. Consequently, it is
important for further spread of PV system to increase
II. SYSTEM CONFIGURATION
conversion efficiency. A maximum power point tracking
(MPPT) control is good solution about this problem. The A. PV ARRAY CONFIGURATION
output power of PV depends on an operating point. In
The proposed MPPT control method for PV system is
addition, the operating point continuously varies with changes evaluated by the simulator PSIM Ver. 9 in this study. Fig. 1
of irradiance, temperature, and so on. A MPPT control shows the PV array configuration used in this study. The PV
maximizes the output power of PV by adjusting the operating array consists of three strings. The string consists of four
point of current and voltage.
modules. The bypass-diode is inserted in parallel with the
The most commonly known MPPT methods are each module. The irradiance and temperature for each module
perturbation and observation (PO) [4], open-circuit voltage can be individually set to in PSIM. In this study, a partial
method [5], and incremental conductance [6]. Especially, the shadow is realized by adjusting the irradiance for modules
PO method has widely spread because it needs no parameters. located in area 1 and 2.
Since the P-V characteristic curve of PV is typically convex,
the PO method can track maximum power point (MPP) by B. Stand-alone PV system
observing the output power of PV to the change in control
Fig. 2 shows the stand-alone PV system to charge battery for
input. If the output power increases, the control input changes camping car. The PV output power is transferred to battery
in the same direction. On the other hand, if the output power
Abstract- This paper presents a modified maximum power point
tracking (MPPT) method for photovoltaic (PV). In this method,
both the open-circuit voltage and short-circuit current of PV are
instantaneously measured to distinguish between irradiance
change and the occurrence of partial shadow when the output
power of PV changes. The method can successfully operate even
though a partial shadow arises. The rapid and accurate operation
of the proposed MPPT method is confirmed by simulation results.
978-1-4577-0001-9/11/$26.00 ©2011 IEEE
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Fig.3. A MPPT control method in consideration of partial
shadow
Fig.1. PV array configuration
Vmpp:Maximum power point voltage [V], Voc:Open-circuit
voltage [V], Impp:Maximum power point current [A], Isc:
Short-circuit current[A]
B. Adaptive partial shadow
Open-circuit voltage is easy to implement. However, this
method often may fail MPPT when a partial shadow occurs
because the MPPV is no longer 80% of the open-circuit
voltage in this situation. To overcome this problem, an
adaptive partial shadow MPPT method is developed in this
paper. The procedure of this method is as follows and
illustrated in Fig.3.
Fig.2. Stand-alone PV system
TABLE 1
PARAMETER OF STAND-ALONE PV SYSTEM
PV
Converter /battery
Open-circuit voltage
20.4V
C1
220uF
Short-circuit current
6.00A
C2
100uF
Maximum power point voltage 26.8 V
L1
10mH
Maximum power point current 5.61 A
Vout
12V
through a buck-converter [7]. The switch SW1 is used for the
detection of short-circuit current. The duty factor of switch
SW2 is adjusted so that the input voltage Vin agrees with the
commanded MPPV VC* . Table 1 lists the parameters for this
system.
III. MPPT CONTROL
A.
Open-circuit voltage method
The open-circuit voltage method relies on the characteristic
in which the MPPV is about 80% of the open-circuit voltage
regardless irradiance. On the other hand, maximum power
point current (MPPI) is about 95% of the short-circuit current
that base on the characteristic of PV. They are showed by the
equation (1) and (2) [5] [8].
Vmpp =0.8Voc
(1)
Impp =0.95ISC
(2)
Firstly, the command voltage VC* is set to 90% of
measured open-circuit voltage when the controller
detects the decrease of the output power of PV.
Experience shows that MPPV is typically smaller than
90% of open-circuit voltage under partial shadow [8].
2) Secondly, the commanded voltage is decreased by Vst
(0.2V) to fine the MPPV. Then, the voltage Vin and
current Iin are measured to evaluate the output power of
PV in this operating point. This operation is repeated
until the input voltage approaches the lower limit, i.e.,
the battery voltage (12V). Since this operation sweeps
down a partial region of I-V curve, it contributes the
rapid search of the MPPV.
3) Finally, the MPPV searched in the item 2 is applied to
the new commanded voltage VC* .
1)
C. Control algorithm
In the proposed MPPT method, both the traditional opencircuit voltage method and the developed adaptive partial
shadow MPPT method are appropriately selected according
to the state of PV. Fig. 4 shows the control algorithm of the
proposed method. When the output power of PV decreases,
the proposed method judges whether irradiance change or the
occurrence of partial shadow.
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Fig.4. Control algorithm
1) (Measuring short-circuit current and open-circuit voltage)
At the time of start-up, short-circuit current Isc and
charging voltage Voc in the capacitor C1 which corresponds
to the open-circuit voltage are measured.
2) (Implementing the open-circuit voltage method)
The command voltage VC* is set at 80% of the open circuit
voltage.
3) (Estimating the output power in evenness irradiance)
The estimated output power Pest which obtained from the
multiplication of 80% of open-circuit voltage and 95% of
short-circuit current in the item 1 is calculated.
4) (Judging whether irradiance change or the partial shadow
occurs)
If the difference of actual and estimated output power is
more than ± 5%, it is necessary to judge whether irradiance
change or the occurrence of partial shadow. Then, the shortcircuit current is measured again.
5) (Distinguishing the state which is whether irradiance
change or the occurrence of partial shadow)
If the difference of the short-circuit current between the
item 1 and the item 4 is more than ± 5%, it is judged as
irradiance change. Then, MPPT method still remains in the
open-circuit voltage method. On the other hand, if the
difference is within ± 5%, it is judged as the occurrence of
partial shadow. Then, the developed adaptive partial shadow
MPPT method is used instead of the open-circuit voltage
method.
Fig.5. Simulation result for irradiance change
Fig.6. P-V characteristic for irradiance change
6) (Updating estimated output power)
When the partial shadow occurs, the estimated output
power Pest is updated to the current output power.
IV. SIMULATION RESULTS
A. Irradiance Change
Fig. 5 shows the simulation result for the response of the
proposed method in the step irradiance change of all areas.
Fig. 6 shows the corresponding P-V characteristics of the
irradiance change. Fig. 5(a) shows irradiance for the PV array.
Firstly the irradiance is set at 1000[W/m2], and steps down to
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Fig.7. Simulation for partial shadow density change
Fig.9. Simulation for partial shadow area spread
Fig.8. P-V characteristic for partial shadow density change
Fig.10. P-V characteristic for partial shadow area spread
875[W/m2] at 0.2s, steps down to 750[W/m2] at 0.4s. Fig.
5(b) shows the input and output currents of buck-converter.
The output current Iout is greater than the input current Iin by
working the buck converter. Fig. 5(c) shows the input and
output voltages of buck-converter. Since the battery is
directly connected to the output of buck converter, the output
voltage Vout is kept at the battery voltage (12V). The transient
phenomenon of input voltage arises due to the charge
operation of the capacitor C1 at start-up. Fig. 5(d) shows the
input and output powers of buck-converter. The input power
Pin instantaneously becomes 0W because of the detection of
short-circuit current. However the output power Pout becomes
never 0W by working the inductor in buck-converter. In this
case, the open-circuit voltage method is through to the end
used as the MPPT method because the partial shadow never
occurs Then, the operating point of PV is rapidly shifted to
the MPP.
B. Partial shadow density change
Fig. 7 shows the simulation result for the response of the
proposed method during an occurrence of partial shadow. The
partial shadow goes on increasing shadow density. Fig. 8
shows the corresponding P-V characteristics of partial
shadow density change. Fig. 7(a) shows the irradiance change
for the modules in area 1 and 2 which is illustrated in Fig. 1.
The irradiance steps down at 0.2s and 0.4s, respectively. As
shown in Fig. 7(d), the occurrence of partial shadow is
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automatically detected and the MPPT method successfully
shifts from the open-circuit voltage method to the developed
adaptive partial shadow MPPT method. The searching time is
within 30ms even though an extreme characteristic change
occurs as shown in Fig. 8. This simulation result shows that
the proposed method is clearly beneficial.
C. Partial shadow area spread
Fig. 9 shows the simulation result for the response of the
proposed method during an occurrence of partial shadow. The
partial shadow goes on spreading shadow area. Fig. 10 shows
the corresponding P-V characteristics of partial shadow area
spread. Fig. 9(a) shows the irradiance change for the modules
in area 1 and 2 illustrated in Fig. 1. The irradiance steps down
at 0.2s in area1 and at 0.4s in area2, respectively. As shown in
Fig. 9(d), the occurrence of partial shadow is automatically
detected and the MPPT method successfully shifts from the
open-circuit voltage method to the developed adaptive partial
shadow MPPT method.
V.CONCLUSIONS
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This paper has proposed a MPPT control method for
standalone PV system in consideration of partial shadow.
This method consists of the traditional open-circuit voltage
control method and the developed adaptive partial shadow
MPPT method. In the case of the irradiance change, it uses
the open-circuit voltage control method because it can track
quickly. In the case of the occurrence of partial shadow, it
uses the developed adaptive partial shadow MPPT method
because it can find MPPV even if the partial shadow occurs.
Simulation results have demonstrated that the proposed
method achieves the MPPT within 30ms even though partial
shadow density and partial shadow area change occur. The
experimental validation of the proposed method is executed
in future works.
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