ÉTUDES EXPÉRIMENTALES DE MPPT D’UN SYSTÈME … · The photovoltaic power generation system is characterized by a simple structure, ... In sum of the above, ... and is multiplied

EXPERIMENTAL INVESTIGATIONS OF MPPT IN A SMALL SCALE PHOTOVOLTAICENERGY SYSTEM BASED ON EXTREMUM SEEKING CONTROL

Her-Terng Yau and Chen-Han WuDepartment of Electrical Engineering, National Chin-Yi University of Technology, Taichung, Taiwan

E-mail: [email protected]; [email protected]

ICETI 2012-TCSME_001_SCINo. 13-CSME-87, E.I.C. Accession 3545

ABSTRACTThis study carried out experimental validation based on the simulation analysis results of Yau and Wu [1],and compared the MPPT situations of three kinds of extremum seeking control. It was proved that thesliding mode extremum seeking control has better transient response and steady-state characteristic, andchanges the sunshine intensity to test the performance of algorithm, thus proving that the system can trackthe maximum power point rapidly in the environment with rapid atmospheric changes.

Keywords: photovoltaic (PV); extremum seeking control (ESC); sliding mode control.

ÉTUDES EXPÉRIMENTALES DE MPPT D’UN SYSTÈME ÉNERGÉTIQUEPHOTOVOLTAÏQUE DE PETITE ÉCHELLE BASÉ SUR LA RECHERCHE

D’UN CONTRÔLE EXTREMUM

RÉSUMÉDans cette étude on a réalisé des validations expérimentales basées sur les résultats d’analyses de simulation,et on a comparé des situations MPPT de trois types de recherche de contrôle extremum. Il a été prouvé quela recherche de contrôle extremum en mode glissant présente une meilleure réponse transitoire et un étatd’équilibre caractéristique. Les changements d’intensité de l’ensoleillement nous ont permis de tester laperformance de l’algorithme pour prouver ainsi que le système peut suivre le point de puissance maximalerapidement dans un environnement comportant des changements atmosphériques rapides.

Mots-clés : photovoltaïque ; contrôle extremum ; commande en mode glissant.

1001Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013

1. INTRODUCTION

As the energy consumption around the world increases, green energy becomes a topic of concern withrenewable energy attracting wide attention in energy development, among which solar power, wind power,hydraulic power, seawater temperature difference, tidal or tidal current power, biomass, and fuel cell are allkey projects.

The photovoltaic power generation system is characterized by a simple structure, easily extended generat-ing capacity, and high reliability, making it extensively used wherever there is sunlight. However, the outputpower of photovoltaic module is not a fixed value, it depends on the illumination of sunlight, atmospherictemperature, solar panel conversion efficiency, and solar angle. In order to use this energy effectively, theelectric power electronics must be combined with a robust and stable controller to allow the photovoltaicpower generation system outputting the maximum power in any atmospheric conditions and solar radiationchange. Previous studies have proposed various methods to overcome these problems [2].

Most of the solar cells do not work at the maximum power point for the nonlinear characteristic of solarcells, and the Maximum Power Point Tracking (MPPT) can be used to solve this problem. At present, themost used MPPT methods include: (1) perturbation and observation method (P&O), (2) incremental con-ductance method, (3) gradient method, (4) approximate straight line method, (5) voltage feedback method,(6) power feedback method, (7) extremum seeking control, (8) sliding mode control method [3–8].

The SMESC proposed in this study designs the MPPT controller based on the robust control theory so asto improve the system generating efficiency, system stability and robustness. Therefore, using the ESC inphotovoltaic power generation system can ensure system stability and good robustness under rapidly varyingatmospheric conditions. The ESC has been applied to MPPT of wind power generation at present, but only afew studies have used ESC in photovoltaic system to track the maximum power point. Since single ESC hasvery violent steady-state oscillation when the system is in a steady state, causing power loss, the steady-stateoscillation must be overcome.

Let us take a general view of the aforementioned traditional MPPT methods, based on the theoretical resultproposed in [1]. This sliding mode extremum seeking control algorithm can avoid the defect in traditionalrobust controller design that the system dynamic equation must be mastered completely, and can overcomethe defect of traditional MPPT methods without robustness. The experimental results proved the feasibilityof this method and the robustness and stability of system.

2. SYSTEM STRUCTURE

The V-I characteristic curve [1] of solar cell is related to the nonlinear characteristic of solar cell module, aswell as to the ambient temperature and solar illumination. Since the maximum power point varies with theexternal environment in practice, the operating point may be anywhere on the characteristic curve, and theoperating point generating the maximum power is the maximum power point. In addition, the short-circuitcurrent increases with the irradiance intensity.

The short-circuit current decreases with the irradiance intensity because the open-circuit voltage is un-likely to vary greatly with the irradiance intensity. As the ambient temperature influences the open-circuitvoltage of solar cell significantly, the open-circuit voltage of solar cell drops greatly when the temperaturerises, influencing the conversion efficiency. Therefore, it is not used in over-high temperature environmentto avoid the output voltage drop and system power reduction. In sum of the above, in order to attain thiscontrol objective, there must be an additional switched mode power converter between the load and PVarray to implement the MPPT, as shown in Fig. 1, so as to ensure that the solar cell always maintains themaximum power output to increase the generating efficiency of the system.

This study implemented the MPPT of photovoltaic power generation system based on the extremumseeking control algorithm. The MPPT was used to impel the solar panel to export maximum power with

1002 Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013

Fig. 1. System structure.

any weather change, temperature and solar radiation. This section briefly describes the system structureand controller design rule in [1], and designs a boost DC converter to make the output voltage meet therequirement for voltage in this study.

1. The system structure is shown in Fig. 1, including solar panel, boost DC converter and the controllerproposed by this study.

2. Solar panel: considering the system requirements and solar panel characteristics, the output power ofsolar panel is 30 W. The maximum open-circuit voltage of this solar panel is 21.5 V, the maximumshort-circuit current is 1.8A, the voltage at maximum power point is 18 V, and the current at themaximum power point is 1.71A.

3. Boost DC converter: this converter circuit consists of transistor, diode, inductor and capacitor. ThePWM signal generated by the controller controls the transistor on and off, and completes the MPPTfunction.

The maximum power tracking controller uses the dSPACE as the controller. The voltage and current signalswere captured and transferred to the computer. The PWM signal was obtained by the extremum seekingcontrol designed in this paper to drive the boost DC converter to implement voltage switching. The controllerused in this study was dSPACE DS1104, which is a real-time calculation system based on the calculationtechnology of Power PC. The control strategy can be validated by real test immediately after it is designedusing the calculation capacity of DS1104 and built-in input-output channel, Simulink functional equationbase of MATLAB (Real-Time Interface, RTI) and related man-machine interface software for test. Thespecifications are: 250 MHz CPU, 64-bit floating-point processor, 32 Mb SDRAM Global memory, 8 MbFlash memory, 5 timer interrupts, 5 ADC channels, 8 DAC channels.

3. BRIEF INTRODUCTION TO MPPT CONTROL METHOD

3.1. Extremum Seeking Control (ESC)Figure 2 shows the extremum seeking control block diagram [1, 9]. This is the simplest method, and thetarget is the maximum value in X-Y curve, i.e. the maximum power point of solar P-V curve, when thepresent position of balance point is determined by the Gradient Detector. The Switching Element was usedto determine whether the present signal should be changed, and then multiplied by integral gain to obtain a


Fig. 2. Extremum seeking control block diagram.

Fig. 3. Sinusoidal ESC block diagram.

new voltage. The results were compared with triangular wave to generate PWM signal to control MOSFET.In [1] the proof of stability is shown.

3.2. Sinusoidal Extremum Seeking Control (SinESC)Figure 3 presents the sinusoidal ESC block diagram [1, 10]. Sinusoidal ESC was applicable to nonlinearproblem. This method added excitation signal in the above ESC. If there is a micro perturbation in a stablesystem, the dynamic state of overall system would be influenced, thus, the disturbing Sinusoidal signal isadded in the systems dynamics. The expected control effect is obtained by using appropriate filter, so as tosearch for the maximum power point. This paper attempts to determine the maximum value in P-V curve.If the nonlinear curve (P-V curve) y = f (x) is an objective function, and has an extreme value, the MPPTcan be implemented by using integrator, filter, multiplier, adder and a sinusoidal oscillator.

The sinusoidal excitation signal is included as shown in Fig. 4. There is micro perturbation at the max-imum power point. The micro perturbation depends on the frequencies of sinusoidal signal and filter, butthe two frequencies cannot be higher than the frequency of the overall system. As shown in Fig. 4, whenthe slope signal of y = f (x) is negative, and is multiplied by a disturbing sinusoidal signal, the sinusoidalsignal turns to negative. Thus, the operating point is on the right of MPP. When the slope signal of y = f (x)is positive, and is multiplied by a disturbing sinusoidal signal, the sinusoidal signal turns to positive value,so the operating point is on the left of MPP. For detailed proofs the reader is referred to [10].

3.3. Sliding Mode Extremum Seeking Control (SMESC)Figure 5 presents a block diagram of the SMESC [1, 11].The above two methods use the Gradient Detector todetermine whether the signal should be changed; however, a very high switching frequency would resulted.


Fig. 4. Schematic diagram of sinusoidal excitation signal.

Fig. 5. Sliding mode ESC block diagram.

Fig. 6. Switching element.

There is energy loss, and the components may be damaged. Therefore, the Sliding Mode is used in thesystem to avoid an overly high switching frequency causing loss.

A switching function sign(σ) is shown in Fig. 6 and defined as

sign(σ) =

1; σ > 00; σ = 0−1; σ < 0

(1)

According to Yau and Wu [1], if we select the same Lyapunov function V (t) = σ2/2, we can find that

V̇ (t)< 0 (2)

Therefore, the stability of SMESC is proven.


Fig. 7. Block diagram of traditional extremum seeking control procedure.

Fig. 8. Measured graph of traditional ESC in uniform solar radiation 1000 W/m2: (a) power waveform, (b) voltagewaveform, (c) current waveform.


Fig. 9. Measured output power waveform of traditional ESC when irradiation increases from 400 to 1000 W/m2 andthen decreases to 400 W/m2.

4. EXPERIMENTAL RESULTS

This study implemented the hardware implementation of three different algorithms based on [1], and testedthe transient response and steady-state characteristic of algorithm and the robustness of algorithm when thesolar radiation changes, in order to prove that the algorithm can track the maximum power point accuratelyand rapidly in the case of rapid atmospheric changes. This section introduces the experimental results of themaximum power tracking in this paper. The block diagram of the process is shown in Fig. 7.

4.1. Experimental ResultsAfter the simulation analysis, based on [1], three different extremum seeking control algorithms are imple-mented in dSPACE controller respectively. Figures 8a–c are measured graphs in uniform solar radiation1000 W/m2 by traditional ESC. As the traditional ESC has violent oscillation in steady state, the MPPTeffect is not ideal, thus causing considerable power loss. The simulation analysis and measurement resultsshowed that the excessive steady-state oscillation is the defect in traditional ESC, so there must be a smalldisturbance quantity to overcome this problem.

In order to validate the weather change in [1] and to ensure that the algorithm can track the maximumpower point accurately for rapid atmospheric changes, this study proved the stability of algorithm by chang-ing the illumination. Figure 9 shows the measured power waveform of ESC when the irradiation increasesfor 400 to 1000 W/m2 and then decreases to 400 W/m2. As can be seen, the maximum power point canbe tracked accurately when the irradiation has changed. Moreover, the steady-state oscillation of traditionalESC causes severe power loss, thus, there must be an additional micro disturbing signal to improve itssteady-state characteristic.

Figures 10a–c are the measured graphs of SinESC in uniform solar radiation 1000 W/m2. When a mi-cro disturbance quantity is added in the traditional SinESC, the steady-state oscillation is more ideal thantraditional ESC, and the maximum power point can be tracked accurately. The system efficiency can beincreased, and the system can have stable performance in steady state.

The stability and robustness of SinESC are verified by changing the illumination. Figure 11 shows thatthe steady-state oscillation is more ideal than traditional ESC when the irradiation increases from 400 to1000 W/m2 and then decreases to 400 W/m2. As can be seen in the photovoltaic power generation system,the SinESC with additional disturbing signal is more applicable to MPPT than traditional ESC.

The traditional ESC and SinESC use the Gradient Detector for identification, but there will be some highfrequency switching phenomena causing power loss. The original Gradient Detector is changed to slidingmode control based on SMESC in this paper, so as to avoid high frequency switching causing component


(a)

(b)

(c)

Fig. 10. Measured graphs of SinESC in uniform solar radiation 1000 W/m2: (a) power waveform, (b) voltage wave-form, (c) current waveform.

losses, and the original sliding surface is replaced by three sliding layers, the chattering phenomenon occursas a result. Figures 12a–c show the actual patterns in uniform solar radiation 1000 W/m2 based on SMESC,the performance in steady-state oscillation is more excellent than traditional ESC and SinESC, and the powerloss can be reduced to increase the system efficiency. Figure 13 shows the measured power waveform ofSMESC when the irradiation increases from 400 to 1000 W/m2, and then decreases to 400 W/m2. Thesteady-state oscillation is more ideal than traditional ESC and SinESC.

After the test, the Root Mean Square Error (RMSE) values of three different ESC algorithms in uniformillumination and when illumination changes can be obtained based on Table 1. The maximum power pointis 30 W when the illumination is 1000 W/m2, the maximum power point is 15 W when the illumination


Fig. 11. Measured output power waveform of SinESC when irradiation increases from 400 to 1000 W/m2 and thendecreases to 400 W/m2.

Table 1. Comparison of RMSE values of various algorithms inuniform illumination and when illumination changes.

IrradianceAlgorithm 1000 W/m2 1000 W/m2 400 W/m2

value 400 W/m2 1000 W/m2

1000 W/m2 400 W/m2

ESC 4.273679 W 4.153322 W 1.850646 W1.562713 W 3.99554 W4.188391 W 1.273546 W

SinESC 1.955987 W 1.87441 W 1.494456 W1.30828 W 1.413136 W1.363297 W 1.184928 W

SMESC 1.223134 W 1.28549 W 1.443797 W1.300255 W 1.215399 W1.133664 W 1.047214 W

Table 2. Steady-state power maximum and minimum of various algorithms.Irradiance

Algorithm 1000 W/m2 1000 W/m2 400 W/m2

value → 400 W/m2 → 1000 W/m2

ESC Max 32.8 W 19.2 W 31.2 WMin 22 W 13.6 W 22.4 W

SinESC Max 30.8 W 16.4 W 30 WMin 26.4 W 12.8 W 24.4 W

SMESC Max 30.8 W 16 W 30.8 WMin 27.6 W 12.6 W 26.6 W

is 400 W/m2. Table 2 shows the steady-state power maximum and minimum of when the illuminationdecreases from 1000 to 400 W/m2 and the steady-state power maximum and minimum of 1000 W/m2

when the illumination increases from 400 to 1000 W/m2. Although the traditional ESC is easier to beimplemented and simpler among three algorithms, the implementation test found that this algorithm resultsin excessive power loss for excessive oscillation in the steady state, so that the system efficiency and thesteady-state response of system are both reduced. When the illumination changes, although the maximumpower point can be tracked, there is violent oscillation in the steady state, so a micro disturbing signal mustbe added in traditional ESC, and the steady-state oscillation of the system is less violent. According to the


Fig. 12. Measured graphs of SMESC in uniform solar radiation 1000 W/m2: (a) power waveform, (b) voltage wave-form, (c) current waveform.

implementation results of SinESC, it overcomes the violent oscillation of traditional ESC in a steady state,and the rise time is shorter than traditional ESC. In other words, the maximum power point is tracked faster,and can be tracked accurately when the illumination has changed. Finally, the performance of ESC basedon sliding mode in steady-state oscillation is better than SinESC. The maximum power point can be trackedrapidly when the solar radiation has changed. Many studies have applied ESC to wind power generationsystem, but seldom to photovoltaic power generation system. The SMESC proposed in this paper can beapplied to the MPPT of photovoltaic power generation system.


Fig. 13. Measured output power waveform of SMESC when irradiation increases from 400 to 1000 W/m2 and thendecreases to 400 W/m2.

5. CONCLUSIONS

Based on the simulation analysis and discussion of Yau and Wu [1], the MPPT algorithm of photovoltaicsystem was tested in this paper. The performance of three kinds of extremum seeking control proposedfrom circuit test, when the illumination changes, was implemented based on the ESC. The 30 W solarmodule was used to measure the MPPT performance in uniform irradiation and nonuniform irradiation. Theactual measurement results showed that the traditional ESC has violent oscillation in steady state, so theSinESC algorithm was proposed for improvement. An additional micro disturbance quantity avoided violentoscillation of the system in the steady state; however, there are still some high frequency switching lossesthat would reduce the system efficiency. Thus, the SMESC was proposed for modification. The concept ofsliding surface was converted into three different sliding layers, so as to avoid chattering phenomenon, andto reduce the system losses resulted from high frequency switching.

ACKNOWLEDGEMENTS

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for fi-nancially supporting this research under contract Nos. NSC 100-2628-E-167-002-MY3 and NSC 101-2622-E-167-012-CC3.

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ÉTUDES EXPÉRIMENTALES DE MPPT D’UN SYSTÈME … · The photovoltaic power generation system is characterized by a simple structure, ... In sum of the above, ... and is multiplied