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Short-Current Pulse-Based Adaptive Maximum-Power-Point Tracking for a
Photovoltaic Power Generation System
TOSHIHIKO NOGUCHI,1 SHIGENORI TOGASHI,
1 and RYO NAKAMOTO
2
1Nagaoka University of Technology, Japan
2Yuasa Corporation, Japan
SUMMARY
This paper focuses on a maximum-power-point
tracking method of photovoltaics via use of the short-cur-
rent pulse. It has been reported that the optimum operating
current is proportional to the short current and that maxi-
mum-power-point tracking can be performed by detecting
the short current. The proposed method utilizes the inter-
mittent short-current pulse to estimate the optimum operat-
ing current and its operating characteristics have
experimentally been verified. Also, an adaptive mechanism
to identify the parameter between the optimum current and
the short current is discussed. A prototype of the controller
has been set up and the experimental results have demon-
strated excellent performance, proving the feasibility of the
system. © 2002 Scripta Technica, Electr Eng Jpn, 139(1):
6572, 2002; DOI 10.1002/eej.1147
Key words: Photovoltaic; MPPT; short-current
pulse; parameter identification.
1. Introduction
In the context of social demand for renewable energy,
a photovoltaic power generation system that imposes few
installation restrictions is one of the practicable options.
Implementation of such a system, however, is retarded by
low energy density and efficiency. Maximum-power-point
tracking (MPPT) is a technique to maximize the perform-
ance of solar cells. Various MPPT methods for photovoltaic
power generation have been studied [1, 2] but those based
on hill-climbing algorithms are now in the mainstream.
With such an approach, the working voltage (current) of a
power converter connected to the photovoltaic module is
varied directly, and the point where the variation of the
output power becomes zero (that is, the maximum power
point) is sought. This technique exhibits good adaptability
to any environmental changes because the actual output is
measured, but its response is rather slow. Recently, some
new control systems using genetic algorithms [3] or fuzzy
reasoning [4] have been developed; however, these systems,
too, rely basically on hill climbing.
On the other hand, better response is offered by
techniques that involve constant measurement of illumi-
nance and temperature, and using a data table stored in
memory to determine the operating point; however, it is
impossible to pay due regard to aging of solar cells, surface
contamination, and other factors, thus hindering adaptabil-
ity. In addition, multiple sensors are required to measure the
illuminance and temperature, resulting in certain restric-
tions on implementation.
The authors considered a method that determines the
optimum operating current from the short-circuit current of
the photovoltaic module [5]. This method utilizes the fact
that the optimum operating current providing maximum
output power varies directly with the short-circuit current,
thus allowing simple control and good response. However,
a monitor photovoltaic unit (that does not generate any
power) is required for short-circuit current detection, which
leads to redundancy and additional cost. In addition, in this
method MPPT is performed under the assumption of a
constant proportionality factor (ratio of optimum operating
current to short-circuit current) but in real life, this parame-
ter may vary with lighting conditions.
The present study deals with an MPPT system based
on short-current pulses. Specifically, the photovoltaic unit
is short-circuited intermittently for very short periods of
time, and these short-current pulses are employed to find
the optimum operating current [68]. In addition, this study
examines an adaptive mechanism using the active region of
the transistor to correct the proportionality factor (ratio of
optimum operating current to short-circuit current) [9]. In
this paper, the setting of the optimum operating current by
means of short-circuit current is tested in various environ-
© 2002 Scripta Technica
Electrical Engineering in Japan, Vol. 139, No. 1, 2002Translated from Denki Gakkai Ronbunshi, Vol. 121-D, No. 1, January 2001, pp. 7883
Contract grant sponsor: Iwatani Naoji Foundation.
65
ments, and the working principle and operating charac-
teristics of the proposed system are presented.
2. Basic Characteristics of Photovoltaic Modules
and Optimum Operating Point
2.1 Short-circuit current and optimum
operating current
Figure 1 shows voltagecurrent (VI) and power
current (PI) characteristics for a test photovoltaic panel
(monocrystalline silicon GL418-TF by Showa Shell Co.)
under various illuminance and temperature conditions, and
the electrical parameters of the panel are given in Table 1.
As is evident from Fig. 1, the maximum output and
maximum power point vary with the illuminance and tem-
perature. It should be noted that the characteristic curves
depend on the combination of illuminance and temperature,
and multiple operating points offering maximum power
exist. According to Ref. 5, the optimum operating current
Iop that exhibits maximum output at illuminance E is pro-
portional to the short-circuit current Isc as follows:
Iop(E) = kIsc(E) (k = const.) (1)
However, the photovoltaic output characteristic depends
not only on the illuminance but also varies considerably
with the temperature of the panel surface, which is disre-
garded in the above equation. We examined the relation
between the optimum operating current and the short-cir-
cuit current for the test photovoltaic panel while varying the
illuminance over the range 1000 to 32,000 lx, and the
temperature over the range 0 to 60 °C.
The measured results are shown in Fig. 2(a); for
reference, Fig. 2(b) gives the relation between the optimum
operating voltage and the open-circuit voltage. Figure 2(a)
indicates that the optimum operating current varies directly
with the short-circuit current. This confirms that the relation
between the optimum operating current and the short-cir-
cuit current can be approximated to be linear for any illu-
minance and temperature. On the other hand, as shown in
Fig. 2(b), the open-circuit voltage varies positively with
illuminance, but negatively with temperature. The optimum
operating voltage exhibits a wide spread of ±0 to 6%
Fig. 1. VI and PI characteristics (GL418-TF).
Fig. 2. Optimum operating conditions (GL418-TF). (a)
Optimum current for maximum power versus short
current. (b) Optimum voltage for maximum power versus
open voltage.
Table 1. Ratings of GL418-TF
Rated maximum power 6.5 W
Rated output voltage 6 V
66
relative to the open-circuit voltage, and hence cannot be
determined using only the open-circuit voltage. The above
results show that the optimum operating point can be set by
using the short-circuit current alone, irrespective of the
illuminance and temperature. From Fig. 2(a), the propor-
tionality factor for the test panel is 0.92; thus, the optimum
operating current can be found by simply multiplying the
short-circuit current by 0.92.
Similarly, the relation between the optimum operat-
ing current and the short-circuit current was also examined
for other monocrystalline silicon cells (GM1618-MF and
GT144) to determine if the above results have generality.
The electrical parameters of GM1618-MF and GT144 are
given in Table 2, and Fig. 3 presents the characteristics of
GM1618-MF with the illuminance varied in the range 1500
to 20,000 lx, and the temperature in the range 0 to 60 °C.
Figure 4 presents the characteristics of GT144 with the
illuminance varied in the range 6000 to 130,000 lx, and the
temperature in the range 20 to 50 °C. In both cases, the
optimum operating current can be approximated to be linear
versus the short-circuit current, as in Fig. 2(a). Thus, it is
safe to say that for monocrystalline silicon cells, the opti-
mum operating current varies nearly linearly with the short-
circuit current, irrespective of the illuminance and
temperature.
2.2 Variation of k
The above experiments confirmed that the optimum
operating current can be found from the short-circuit cur-
rent; parameter k is the main factor governing the operating
point. The results suggest that k is 0.92 for GL418-TF, and
0.91 for GM1618-MF and GT144. However, the problem
is that k is not necessarily constant under all conditions. For
example, when the illuminance of the photovoltaic panel
surface is not uniform, k varies, and the operating point may
deviate considerably from its optimum. In addition, k dif-
fers for each individual photovoltaic unit, and hence unified
design with constant k would not necessarily prove optimal
in all cases.
Figure 5 shows the variation of k for the test unit
(GL418-TF) with uneven illuminance simulated by several
shade patterns. When the illuminance becomes lower
Table 2. Ratings of GM1618-MF and GT144
Photovoltaic panel GM1618-MF GT144
Rated maximum power 1.4 W 67 W
Rated output voltage 6 V 21.2 V
Fig. 3. Optimum current for maximum power versus
short current (GM1618-MF).
Fig. 4. Optimum current for maximum power versus
short current (GT144).
Fig. 5. Variation of k.
67
(short-circuit current becomes smaller), k drops slightly
even without any shadow; however, this may be disregarded
in the control system. On the other hand, shadows cause
significant variation that cannot be ignored. The problem
arises when MPPT is performed at too high a value of k.
Since solar cells have a declining voltage (current) charac-
teristic, the output drops abruptly as the operating point
exceeds its optimum. Thus, while a negative error of k does
not exert a heavy influence, considerable output loss may
occur due to positive error. Unfortunately, the latter case is
more probable; hence, k must be adjusted when nonuniform
illuminance is assumed.
3. MPPT System Using Short-Current Pulse
3.1 Conventional system based on
short-circuit current detection
The configuration of a conventional system [5] is
shown in Fig. 6(a). With this configuration, a monitor
photovoltaic unit is provided as a part of the control system;
the short-circuit current of this monitor unit is employed as
the command value for MPPT control. The monitor unit
functions as a sensor, thus not contributing to power gen-
eration, and hence installation cost increases. In addition,
the discrepancy in characteristics between the monitor unit
and the main photovoltaic array must be taken into account,
and deterioration of control accuracy is unavoidable when
environmental conditions change.
This study deals with a method of defining the opti-
mum operating point using short-current pulses generated
by intermittent short-circuiting of the photovoltaic module
for a short period. The configuration of the proposed system
is presented in Fig. 6(b). Such a configuration does not
require any monitor unit, thus simplifying the control sys-
tem. Of course, a discrepancy in characteristics between the
monitor unit and main photovoltaic array is no problem
here because the short current is measured directly. More-
over, the transistor used for short-circuiting is also em-
ployed for identification of k, thus providing adaptability to
parameter variation.
3.2 MPPT using short-current pulse detection
The proposed system is presented in Fig. 7. Here
transistor S1 (FET) and a Hall current transformer are added
at the input of an ordinary step-up chopper. Transistor S1
turns on intermittently for a short period to provide short-
circuiting. The short-circuit current pulses thus generated
are detected by the current transformer, and the current
command i* is produced by multiplying the short-circuit
current by k. The current transformer is employed for
chopper current control as well.
In the proposed method, the controlled variable is the
photovoltaic output current i that is obtained by detecting
the average current. The values of i* and i are fed to a
proportional-integral (PI) regulator to provide current con-
trol. Basically, the proposed method requires only a short-
circuiting transistor and a current sensor to ensure MPPT
control, so that the system configuration is very simple.
Now, a capacitor is placed at the output of transistor S1 to
suppress power supply ripple; this capacitor is not short-cir-
cuited with the photovoltaic module because a diode is
provided for reverse-current protection.
Fig. 6. Comparison of system configuration.
(a) Conventional system. (b) Short-current
pulse based system. Fig. 7. Configuration of proposed system.
68
3.3 Identification of parameter k
Parameter k is tuned by operating the short-circuiting
transistor S1 in its active region. Normally, the gate signal
of S1 varies stepwise to provide switching; in active opera-
tion, however, the gate signal is varied rampwise. The load
factor as seen at the photovoltaic module changes continu-
ously from open circuit to short circuit, and the PI curve
can be scanned at the short-circuit state. The drain-to-
source voltage and drain current are detected, and the
instantaneous power P is obtained by multiplying the two
values. This power is fed to the peak detection circuit. The
circuit generates a pulse as P reaches its peak to trigger
measurement of the drain current, that is, the optimum
operating current Iop. Parameter k can be identified by
dividing this Iop by Isc obtained simultaneously. Since com-
plete scanning of the PI curve may prove impossible
depending on the actual load, the load is separated by
turning off transistor S2. In this period, power is not sup-
plied to the load from the photovoltaic array, and the de-
crease of supply must be compensated by a batteries or
other storage devices. Output fluctuation is basically unde-
sirable for a power converter, but considerable variation of
k does not occur often, and thus there is no need for frequent
k tuning. Such tuning might be performed at system start-
up, and then once every few minutes.
4. Experimental Results
Below we give the operating characteristics of the
proposed system implemented using the test photovoltaic
module. The operating conditions are listed in Table 3.
First, consider the identification of parameter k (ratio
of optimum operating current to short-circuit current). Fig-
ure 8 presents waveforms of the detected signals and the Iopsampling trigger signal obtained by PI curve scanning.
The trigger pulse is generated at the moment when P
reaches its peak. The results of parameter identification are
shown in Fig. 9. The photovoltaic installation conditions
are the same as in Fig. 5. The diagram indicates that FET
active operation can be used for PI curve scanning, and k
is identified with good accuracy. As noted in the previous
section, power is not output during k identification, thus
resulting in loss. For this reason, identification must be
performed at sufficiently long intervals to minimize the loss
rate. In the experimental system, parameter identification
takes 25 ms; with an interval of several minutes, the loss is
negligibly small.
Figure 10 shows the waveforms of the short-circuit
current during pulse detection, as well as the FET gate
signals and sampling trigger signal. Immediately after
short-circuiting, ringing occurs due to parasitic capacitance
and line inductance; therefore, sampling should not com-
mence until the current settles down. In order to stabilize
Table 3. Experimental conditions
Test photovoltaic GL418-TF
Rated maximum power 6.5 W
Rated output voltage 6 V
Switching frequency 10 kHz
Sampling interval of Isc 80 ms
Isc pulse width 80 µs
Load 560 Ω
Initial value of k 0.92
Fig. 8. Waveforms in scanning PI curve.
Fig. 9. Results of k estimation.
69
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5. Conclusions
This study deals with determination of the optimum
operating current from the short-circuit current, and verifies
experimentally that such a method tolerates variations of
temperature as well as illuminance. In addition, a simpler
power conversion configuration using detection of short-
circuit current pulses is developed, and a method of improv-
ing the control characteristics under uneven illumination is
proposed. In the latter case, the ratio of the optimum oper-
ating current to the short-circuit current is identified online
using the FET active region. An experimental system was
built to realize maximum-power-point tracking with adapt-
ability to changes of illuminance or temperature as well as
to shadows and other disturbances. In experiments, good
operating characteristics were obtained under various envi-
ronmental conditions.
Acknowledgment
This study received financial support from the
Iwatani Naoji Foundation.
REFERENCES
1. Ooniwa, Sato. MPPT control of photovoltaic power
generation systems. Trans IEE Japan 1987;106-
B:7278.
2. Oonishi, Takada. Comparison of MPPT algorithms
for photovoltaic power generation system, and char-
acteristics of control using step-up chopper. Trans
IEE Japan 1993;112-D:250257.
3. Takahara, Yamanouchi, Matsuda. MPPT control sys-
tem for photovoltaic power generation with adapt-
ability to environmental changes. Trans IEE Japan
1999;119-D:15291534.
4. Kondo, Yasuno et al. Study on fuzzy control in pho-
tovoltaic MPPT. 1999 Conf IEE Japan, No. 3, p
217220.
5. Matsuo H, Kurokawa F. New solar cell power supply
system using a boost type bidirectional DC-DC con-
verter. IEEE Trans Ind Electron 1984;31:5155.
6. Nakamoto, Noguchi. MPP operation of photovoltaic
system using open-circuit voltage and short-circuit
current. IEE Niigata Branch Reports 1998;4:107
108.
7. Togashi, Noguchi, Nakamoto. Maximum power op-
eration of photovoltaic power generation system us-
ing short-current pulse. 1999 Conf IEE Japan, No. 3,
p 221224.
8. Togashi, Noguchi. MPPT in photovoltaic system us-
ing short-circuit currentSimplified system con-
figuration and its characteristics. IEE Niigata Branch
Reports 1999;4:8788.
9. Togashi, Noguchi. Adaptive MPPT for photovoltaic
module using short-current pulse. 2000 IEE Japan
Natl Conf, No. 7, p 10071008.
10. Alghuwainem SM. Matching of DC motor to a pho-
tovoltaic generator using a step-up converter with a
current-locked loop. IEEE Trans Energy Conv
1994;9:192198.
11. Altas IH, Sharaf AM. A novel on-line MPP search
algorithm for PV arrays. IEEE Trans Energy Conv
1996;11:748754.
12. Wasynczuk O. Dynamic behavior of a class of pho-
tovoltaic power systems. IEEE Trans Power Appl
Syst 1983;102:30313037.
13. Hua C, Lin J, Shen C. Implementation of a DSP-con-
trolled photovoltaic system with peak power track-
ing. IEEE Trans Ind Electron 1998;45:99107.
71
AUTHORS (from left to right)
Toshihiko Noguchi (member) received his B.E. degree in electrical engineering from Nagoya Institute of Technology in
1982, and M.E. and Ph.D. degrees in electrical and electronics systems engineering from Nagaoka University of Technology
in 1986 and 1996. He joined Toshiba Corporation in 1982. He served as a lecturer at Gifu National College of Technology from
1991 to 1993 and a research associate in electrical and electronics systems engineering at Nagaoka University of Technology
from 1994 to 1995. Since 1996, he has been an associate professor at Nagaoka University of Technology. His research interests
are static power converters, motor drives, electric vehicles, and photovoltaic power systems. He is a member of IEEE-IAS and
IEEE-IES.
Shigenori Togashi (associate) received his B.E. and M.E. degrees in electrical and electronics systems engineering from
Nagaoka University of Technology, Nagaoka, Japan, in 1999 and 2001, respectively. Since 2001, he has been with Myway Labs
Co. Ltd., where his work is research and development of photovoltaic power systems.
Ryo Nakamoto (member) received his B.Eng. degree in electrical and electronics systems engineering from Nagaoka
University of Technology, Nagaoka, Japan, in 1999. Since 1999, he has been with Yuasa Corporation, Takatsuki, Japan, where
he has been a Research and Development Engineer in the Photovoltaic Power System Group.
72