8
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 1 Nagaoka University of Technology, Japan 2 Yuasa 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, 2002 Translated from Denki Gakkai Ronbunshi, Vol. 121-D, No. 1, January 2001, pp. 7883 Contract grant sponsor: Iwatani Naoji Foundation. 65

Short-current pulse-based adaptive maximum-power-point tracking for a photovoltaic power generation system

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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.

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108.

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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