A New MPPT Control for Photovoltaic Panels by Instantaneous Maximum PowerPoint Tracking

DAIKI TOKUSHIMA,1 MASATO UCHIDA,2 SATOSHI KANBEI,1 HIROKI ISHIKAWA,1 and HARUO NAITOH1

1Gifu University, Japan2Toshiba Industrial Products Manufacturing Corp., Japan

SUMMARY

This paper presents a new maximum power pointtracking control for photovoltaic (PV) panels. The controlcan be categorized into the Perturb and Observe (P&O)method. It utilizes instantaneous voltage ripples at PV paneloutput terminals caused by the switching of a chopperconnected to the panel in order to identify the direction forthe maximum power point (MPP). The tracking for theMPP is achieved by a feedback control of the averageterminal voltage of the panel. Appropriate use of the instan-taneous and the average values of the PV voltage for theseparate purposes enables both the quick transient responseand the good convergence with almost no ripples simulta-neously. The tracking capability is verified experimentallywith a 2.8-W PV panel under a controlled experimentalsetup. A numerical comparison with a conventional P&Oconfirms that the proposed control extracts much morepower from the PV panel. © 2006 Wiley Periodicals, Inc.Electr Eng Jpn, 157(3): 73–80, 2006; Published online inWiley InterScience (www.interscience.wiley.com). DOI10.1002/eej.20255

Key words: photovoltaic; MPPT; instantaneousmaximum power point tracking control; perturb and ob-serve method.

1. Introduction

In the context of global environment problems, thedevelopment and implementation of photovoltaic powergeneration and other alternative energy instead of fossil fuelis an urgent task. The technological components of photo-voltaic power generation can be divided into those relatedto the materials of photovoltaic cells (below referred to as

PV), and those related to the operation of photovoltaicpower systems. As regards the former, the most importantissue is to develop materials offering high conversion effi-ciency at low cost. On the other hand, the problem of betterefficiency applies to the latter as well. Maximum powerpoint tracking (MPPT) control is a control method to oper-ate PV systems so that the operating point corresponds tothe maximum output power [maximum power point(MPP)].

Several MPPT algorithms have been proposed so far,including methods using functional approximation of PVcharacteristics [1, 2], methods that match the internal im-pedance of the PV panel and the input impedance of theconnected converter to MPP conditions [3, 4], binary con-trol [5, 6], and so forth.

One of the proposed techniques is the perturb andobserve method (P&O, or hill climbing method). Its prin-ciples can be formulated as follows.

• An oscillation of fixed magnitude is always ap-plied to the operating-point voltage (current) ref-erence.

• The corresponding power variation is detected.• This variation is used to determine the direction

that moves the operating point toward the MPP.• The operating point is moved in that direction.

In this method, the operating point oscillates at a certainamplitude, no matter whether the MPP is reached or not.That is, this oscillation remains even in the steady state afterreaching the MPP, which leads to power loss. This is anessential drawback of the P&O method.

The method proposed in this study is a variety ofP&O. In particular, we use the instantaneous voltage vari-ation caused by the switching operation of a converter(chopper) connected to PV in order to determine the direc-tion of movement of the operating point. Any converter maybe employed provided that the instantaneous voltage vari-

© 2006 Wiley Periodicals, Inc.

Electrical Engineering in Japan, Vol. 157, No. 3, 2006Translated from Denki Gakkai Ronbunshi, Vol. 124-D, No. 12, December 2004, pp. 1182–1188

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ation caused by switching can be detected. The averagevoltage at the operating point is used for MPPT control. Fasttransient response and convergence to the MPP with verysmall ripple are obtained due to the proper distinctionbetween instantaneous and average values.

Below we explain the principles of the proposedmethod and present an experimental verification.

2. Principles of Instantaneous MPPT Control, andOperating Voltage Control System

A typical example of PV output characteristics isshown in Fig. 1. The diagram shows the basic current–volt-age (I–V) characteristic and the power–voltage (P–V) char-acteristics derived from the I–V characteristic.

The MPP is at the vertex of the P–V curve. When nocontrol is applied, there is no guarantee that the operatingpoint coincides with the MPP. Moreover, it is quite possiblethat the operating point is located in a position resulting inrather small output. In addition, the characteristics shownin Fig. 1 are not invariant: they vary with insolation, tem-perature, and other weather conditions. Therefore, the MPPis not a fixed point. In order to obtain maximum poweroutput from PV arrays, a power converter must be con-nected to the PV, and control must be performed so that theoperating point of the PV follows the drifting MPP. This isreferred to as MPPT control.

2.1 Principle of instantaneous MPPT control

In the MPPT considered in this study, a chopper isused as the PV converter. The chopper is connected to thePV panel and maintains the operating-point voltage at itsreference value. This reference value is the average voltageat the chopper input during the sampling period of voltagecontrol (below referred to as ∆T).

The PV operating voltage inevitably fluctuatesaround the reference (average value) because of the chop-per’s switching operation. A corresponding ripple also oc-curs in the PV output power, as shown in Fig. 2(b). In theP–V characteristic given in Fig. 2(a), this is expressed bythe operating point range shown by the two-pointed arrow.This ripple is utilized in the proposed control method.

Let ∆T denote the control sampling period, PIMAX itsmaximum instantaneous power, and VIMAX the correspond-ing instantaneous voltage. Now let VAVE denote the averagevoltage at the operating point during ∆T. The followingrelations are true when the voltage at the operating point islower or higher than that at the MPP, respectively [see Fig.2(a)]:

As is obvious from Fig. 2(a), VIMAX is always closerto MPP than VAVE. Therefore, MPP can be approached ifcontrol is applied so that VAVE follows VIMAX.

Consider the retention of such tracking when theMPPT is reached, as shown in Fig. 3. As is evident from theP–V characteristic shown in the diagram, the ripple of theinstantaneous voltage, that is, the range of variation of theoperating voltage, extends to both sides of VIMAX, in contrastto Fig. 2(a). Therefore, MPPT control can be implementedby following VIMAX. This also applies when the operatingpoint has a lower voltage than MPP.

The proposed MPPT control is called instantaneousmaximum power point tracking control (IMPTC).

2.2 Calculation of voltage reference atoperating point

Thus, VIMAX is the tracking target for PV operatingpoint. A flowchart of the VIMAX calculation is shown in Fig.4.

Fig. 1. Characteristics of typical PV.

(1)

(2)

Fig. 2. Ripple of PV output power (tracking principle).

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First, the instantaneous voltage v(t) and instantaneouscurrent i(t) are found and are multiplied to find the instan-taneous power p(t). Then the maximum value PIMAX of p(t)during ∆T is found, and the corresponding VIMAX is deter-mined. This value is set as the voltage reference V* to thechopper:

2.3 Control system for operating-point voltage

The control system is configured so that the PVoperating voltage follows V*. The actual physical quantitythat is specified to the chopper as the reference is the dutyratio. This duty ratio is implemented by on/off control atthe switching frequency (in this study, 10 kHz). In the

short-circuit state during switch-on, the output current ofthe PV panel increases and output voltage drops accordingto the I–V characteristic shown in Fig. 1. The opposite takesplace during switch-off. This voltage variation is the fluc-tuation of the instantaneous value due to switching. Asstated above, the ripple is utilized for MPPT in this study.The reference V* is associated not with the instantaneousvalue but with the average over the interval ∆T. The feed-back system is configured so that the average PV voltageVAVE follows V*. A block diagram of such control is shownin Fig. 5.

In the diagram, IMPTC logic is used for the calcula-tion of the voltage reference described in Section 2.2. Hereintegral logic is used in order to maintain the duty cycle atthe moment when tracking is fulfilled, so that the voltagedifference ∆V is zero, that is, to keep the operating voltageat its reference.

3. Experimental Results and Discussion

In this section, we present an experimental confirma-tion of the IMPTC response under varying operating con-ditions. We show that it is more effective than theconventional P&O, and explain the reasons.

The experimental conditions are given in Table 1. Inthe experiments, a halogen lamp with variable intensity wasused as the light source.

3.1 Circuit configuration of experimentalsystem

The configuration of photovoltaic generation systemconsidered in this study is shown in Fig. 6. The chopperkeeps the average voltage VAVE at the PV terminals at thereference value V*.

Fig. 3. Ripple of operating point around MPP inIMPTC.

Fig. 4. Flowchart of maximum instantaneous powercalculation.

(3)

Fig. 5. Block diagram of IMPTC controller.

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The output stage of the chopper is connected directlyto the load. Since we only intend to estimate the MPPTperformance, a battery is connected instead of an inverterand load. The PV voltage (instantaneous value) is detectedby an isolation amplifier, and is used to control the averagevoltage in the IMPTC. The PV current is detected by a HallCT, and is then used to calculate the instantaneous power.

3.2 Outline of P&O used for comparison

The conventional perturb and observe (P&O) methodused for comparison is explained below.

The PV operating voltage is varied at a fixed rate atintervals ∆T. The PV output before and after variation iscompared, and the operating point is moved so as to in-crease the output power, thus approaching the MPP. Denot-ing the voltage and power before and after variation as VA,PA and VB, PB, respectively, and the voltage variation by ∆V,the operating point is updated by the following criteria:

3.3 Operation at constant illumination andtemperature

Consider the experimental results obtained with theinsolation and temperature maintained at a fixed level.Since the P–V characteristic is determined by a singlecurve, this is suitable for estimating the basic performanceof MPPT control. In our experiments, the initial operatingpoint was located off the MPP. Then MPPT control wasapplied and the power response was observed. We assumedthat at the initial operating point, the voltage was lower thanat the MPP, and that output power was 65% of the maxi-mum.

Figure 7 shows the power response when the IMPTCresponse is set at its highest, and the voltage variation ∆Vin P&O is adjusted so as to obtain a response of the samelevel. In particular, the IMPTC parameter KI in Fig. 5 is setto 0.06. Comparing the waveforms after reaching steadystate, there is hardly any ripple in the IMPTC, but there isconsiderable ripple in P&O. The power generated in onecycle of this ripple (interval A in the diagram) is comparedin Table 2. As is evident from the table, IMPTC outperformsP&O by about 4%. The reason is that, as explained above,the reference voltage at the operating point in P&O must bevaried after reaching the MPP, while in IMPTC the voltagereference is maintained due to integral control. Since mostPV operation during a day is in steady state, the increase in

Fig. 6. Experimental setup.

(4)

(5)

(6)

(7)

Fig. 7. PV power response at constant illumination andtemperature.

Table 1. Experimental conditions

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generated power in steady state contributes to the overallpower output.

In Fig. 8, the IMPTC setting is unchanged and theP&O setting is adjusted so that the pulsation in steady stateis of the same level as in IMPTC. In particular, the voltagevariation in P&O is reduced by a factor of 6 compared toFig. 7. This resulted in smaller ripple in steady state, but thetransient response speed is 0.7 s worse. The power gener-ated in one transient response cycle (interval B in thediagram) is compared in Table 3. It is evident that IMPTCoutperforms P&O by about 20%.

It may be concluded from the above comparison thatwhen the power response of MPPT control in P&O is setfaster, the ripple increases after reaching MPP (that is, insteady state), resulting in a greater power loss. On the otherhand, when the focus is on steady-state characteristics andthe response speed is set slow, the power loss too is large.We see that IMPTC offers both better PV power responsespeed and better steady-state characteristics.

In particular, a power drop in steady state is a fataldrawback. Comparisons under different conditions are pre-sented below; however, the response speed of P&O is setwith regard to steady-state characteristics, that is, as shownin Fig. 8.

3.4 Operation at fixed temperature and variedillumination

Here we present experimental results obtained whilestepping the source voltage of the halogen lamp up anddown so as to vary the illumination from 0.5 to 1.0 kW/m2

while maintaining a constant temperature. We examined thepower response before reaching MPP.

The power response waveforms in the case of increas-ing illumination for IMPTC and P&O are shown in Fig. 9.The loci of the operating point fluctuations on the P–Vcharacteristics are shown in Figs. 10 and 11, respectively.In Fig. 9, IMPTC reaches the MPP 0.15 s faster than P&Obecause P&O deviates from the MPP at instant T. Compar-ing Figs. 10 and 11, this phenomenon is not observed inIMPTC. The reason is that P&O deals only with the averagevalue in ∆T. Thus, even though the operating point is movedaway from the MPP, the average power increases due toincreasing illumination. In IMPTC, this phenomenon doesnot occur because the instantaneous maximum power pointis always tracked. The power generated in one transientresponse cycle (interval D in Fig. 9) is compared in Table4. It is seen that IMPTC outperforms P&O by about 14.6%under increasing illumination.

Table 2. Comparison of generated energy duringinterval A

Fig. 8. PV power response at constant illumination andtemperature.

Table 3. Comparison of generated energy duringinterval B

Fig. 9. PV power response under rising illumination.

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The power response waveforms under decreasingillumination for IMPTC and P&O are shown in Fig. 12. Theloci of the operating point fluctuations on the P–V charac-teristics are shown in Figs. 13 and 14, respectively. In Fig.12, too, IMPTC reaches the MPP faster than P&O. Consid-ering the operating point trajectories, a perfect trackingresponse is obtained by IMPTC in Fig. 13. On the otherhand, in P&O as shown in Fig. 14, the operating point tendsto deviate from the MPP as the illumination decreases. Theexplanation is similar to that for the case of increasingillumination. The power generated in one transient responsecycle (interval E in Fig. 12) is compared in Table 5, whereIMPTC outperforms P&O by about 7.65%.

Fig. 10. Locus of operating point under risingillumination in IMPTC.

Fig. 11. Locus of operating point under risingillumination in P&O.

Table 4. Comparison of generated energy duringinterval D

Fig. 12. PV power response under decreasingillumination.

Fig. 13. Locus of operating point under decreasingillumination in IMPTC.

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As follows from the above comparison, IMPTC isvery responsive to changes of illumination.

4. Conclusions

The principles of IMPTC are as follows.

• Ripple of the instantaneous power caused byswitching operation of the converter used to con-trol the PV operating point is utilized to determinethe direction in which the operation point shouldbe moved.

• Adjustment of the operating point to the MPP andthen maintenance of the operating point are as-sured by feedback control using the average volt-age at the operating point, based on integral logic.

This method is applicable to any converter so long aspulsations of the instantaneous power caused by switchingcan be detected. The essential difference from the conven-tional P&O method is that an appropriate distinction ismade between instantaneous and average values. Due tothis distinction, as soon as MPPT is achieved under certainconditions, the variation of average-based voltage referencecan be reduced considerably. At the same time, informationabout the MPP drift caused by changing illumination and

other conditions is acquired without delay by using thevoltage ripple at the PV operating point, which occursconstantly because of the chopper’s switching operation.

An experimental comparison with conventional P&Oconfirms that IMPTC offers fast transient response andconvergence with very little ripple around the MPP.

REFERENCES

1. Kislovski A, Redl R. Maximum-power-tracking us-ing positive feedback. Proc IEEE Power ElectronSpec Conf, p 1065–1068, 1994.

2. Yamaguchi Y, Sato E, Minakata H, Tadakuma S.Maximum power tracking control method of a solarcell by approximating the I-V characteristics. Con-vention Record IEE Japan, Industry Applications So-ciety, p 191–194, 1998.

3. Nafeh A, Fahmy F, Mahgoub O, Abou El-Zahab E.Developed algorithm of maximum power trackingfor stand alone photovoltaic system. Energy Sources1998;20:45–58.

4. Hussein K, Muta I, Hoshino T, Osakada M. Maxi-mum photo-voltaic power tracking: An algorithm forrapidly changing atmosphere conditions. Proc InstElect Eng G 1995;142:59–64.

5. Watanabe T, Yoshida T, Ohniwa K. A maximumpower point tracking control method of a photovol-taic power generator with consideration of dynamiccharacteristics of solar cells. IEEJ Trans IA2003;123:863–869.

6. Miyatake M, Kouno T, Nakano M. Maximum powerpoint tracking method of photovoltaic generationsystem introducing line search method. Proc JapanIndustry Applications Society Conference, p 327–330, 2000.

Table 5. Comparison of generated energy duringinterval E

Fig. 14. Locus of operating point under decreasingillumination in P&O.

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AUTHORS (from left to right)

Daiki Tokushima (student member) graduated from Fukui National College of Technology (electrical engineering) in1997, completed the M.E. program in Electronic Systems Engineering in 1999 and the first stage of his doctorate at GifuUniversity (electronic and information engineering) in 2001, and is now in the second stage.

Masato Uchida (student member) graduated from Gifu University (electrical and electronic engineering) in 2001 and isnow in the doctoral program.

Satoshi Kanbei (student member) graduated in 2003 from Gifu University (electrical and electronic engineering) in 2003and is now in the doctoral program.

Hiroki Ishikawa (member) completed his doctorate at Gifu University (electronic and information engineering) in 1995and joined the faculty as a research associate. His research interests are characteristics analysis of current resonance convertersand induction motors, and SRM torque control. He is a member of IEEE.

Haruo Naitoh (member) completed his doctorate at the University of Tokyo (electrical engineering) in 1980 and joinedToshiba Corp. He was a visiting researcher at California Institute of Technology in 1984–85 and a visiting professor at VirginiaPolytechnic Institute in 1985–86. He has been a professor at Gifu University since 2000. He holds a D.Eng. degree.

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