Optimization of Integrated Power Conditioning PV Parameters

Ahmed Hassan1, Ahmed Shawky1, Mohamed Orabi1, IEEE Senior Member, Jaber A. Abu Qahouq2, IEEE Senior Member, and Mohamed Z. Youssef3, IEEE Senior Member

1APEARC, Aswan University, Aswan 81542, Egypt

2 Electrical and Computer Eng. Dept., The University of Alabama, Tuscaloosa, Alabama 35487, USA 3Bombardier Transportation, Kingston, Canada

orabi@ieee.org

Abstract - Photovoltaic systems should be managed to produce the maximum power under all operating conditions. The Manager is called Maximum Power Point Tracker MPPT. In this paper, the MPPT manager is attached into each PV cell in order to solve the problems of the PV shadow effect. The ripple correlation control algorithm has been used as the control action of the MPPT. The proposed system borderlines have been studied here. Then, the suitable operating ranges have been defined. Also, the inverse minor loop gain theorem is used to study the stability regions of the system.

Index Terms – Dc-Dc converters; Boost converter; Photovoltaic; Stability; Efficiency.

I. INTRODUCTION

Renewable energy resources become the best ways to reduce the oil, gas demand and its resultant pollution. This energy comes from natural resources such as sunlight, wind, rain, tides and geothermal heat. It has not any exhaust, noise or danger on wildlife. Therefore, nowadays there is an increasing trend globally to increase the renewable resources in electricity production. Photovoltaic systems become widespread among the renewable energy systems, also, a large initial investment and limited life time span of photovoltaic array make it necessary for the customer to extract the maximum power from the PV system. In additional, the latest technologies in semiconductor physics try to optimize the PV system where, nowadays its cost reduced compared with the past and its energy go higher. This makes the PV system to be more attractive where the cost of watt per cell has been decreased.

On the other hand, PV system is still facing some problems like partial shading, hotspot effect, mismatching effect, non uniform irradiation effect, and the multi peak phenomenon [1, 2]. A number of PV cells are connected in series and parallel

to form a solar array to produce the desired voltage and current level. In the series connection when numbers of cells are connected in series. The solar array is adversely affected by non equal illumination and one of these cells is shaded where it may be shaded by neighboring construction; this condition is called partial shading [3-4]. In addition in series connection every cell carries the same current. The partial shading makes the shaded cell produce low photon current or acts as a resistor, so it will consume power instead of generating power. The power consumed in the shaded cell will appear in the form of temperature increasing. The temperature increasing can damage the cell or at least reduce its time life, this problem is called hotspot effect [6-7]. In case of series connection the systems assume that all these cells are identical, but each cell different from the other. Each cell has its own characteristic, so it has a MPP different from the other, this is the mismatching problem. In addition to the mismatching problem there is another problem produces the same action which called the non uniform irradiation problem. This problem means that each cell will receive different irradiation from the other, so its behavior will be different.

Nowadays a bypass diode is used to eliminate the shaded panel by connecting in parallel with each other. The bypass diode can protect the shaded panel, but in the other hand it causes the multi peak phenomenon. The multi peak phenomenon means that the P-V characteristic curve has not one peak of power. The MPPT will see the P-V characteristic with multi peak of power. This problem confuses the MPPT operation. Figure 1 shows the I-V characteristics of the PV cell at different levels of shading [3-5].

There are two methods for overcoming the previous PV problems. The first one by eliminating the shaded cell where this decreases the non uniform factor of shading and irradiation and the other method is by controlling each cell alone with a MPPT. The first method is so easy, but it affects the system efficiency and stability. The second method is the

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Figure 5: The control circuit algorithm flow chart.

Figure 5: The proposed boost converter circuit.

When the system operates in region 1, the output signals of the two differentiators are in phase. The outputs of the two differentiator circuits are applied to an XOR circuit. The output of the XOR will be logic 0. This means that the duty cycle of the signal that drives the power switches should be decreased to reach the maximum power point. Then the output of the XOR circuit is applied to an integrator circuit that produces the average value of the input signal, which indicates the location of the operating point. The output signal of the XOR circuit is compared to a generated saw tooth signal in order to produce a train of pulses. Then, the

duty cycle is determined by the generated DC signal, and then applied to the power MOSFET transistors.

In the case of the operating point is in region 2, the output signals of the two differentiator circuits are going to be out of phase. The output signal of the XOR circuit is logic 1. This means that the duty cycle of the signal that drives the power switches should increase in order to get closer to the maximum power point. The output signal of the PWM is not powerful enough to drive the power switches. So the output of the PWM is applied to a gate drive circuit which drives the power MOSFETs, and make sure that the two power switches will not turn on at the same time.

III. BORDER STUDY OF THE PROPOSED SYSTEM

The MPPT should track the cell maximum power under all environmental conditions employing the converter circuit. The main affected conditions on the delivered power from any PV cell are the received irradiation, temperature and loading condition. These parameters determine the actual delivered power. As the target system is a series connection for the power conditioning converters as shown in Fig. 2, thus their passing (loading) current should be the same. Note that this current is representing the current fed inverter. Therefore, this current value is designed based on the study of the borders of the system. Lower received irradiation can limit the available obtained current at the output and so limit the overall system operation. Where, it can force the proposed system to operate at defined regions.

By another way, the proposed converter circuit is a boost converter to increases the output voltage. This means that the input current of the converter circuit (delivered from the cell) should be higher than the output current (load current). Therefore, the cell must deliver a current higher than the load current. On the other hand, at high load current the duty cycle cannot track the MPP especially in lower irradiation as the cell current decreases with low received irradiation. This issue called load limitation.

A border studding has done for the system in order to identify the optimum load current that the system can deliver under the irradiation change. From the previous discussion it is clear that the system cannot operate in a very wide range of irradiation change with achieving MPPT. As a result the duty cycle cannot manage the converter and track the MPP under low irradiations with high load current. Figure 7 shows the duty cycle versus the load current at different irradiations. It is shown that, at high load current there is a limitation on the irradiation where the system cannot operate at lower level irradiation (e.g. at i = 4A the possibe irriadition where the system can operate will equal to IR = and the system cannot operate at lower irradiation). This limitation may

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The next step after getting the open loop transfer function with the load impedance and the source impedance is to get the closed loop TF. Figure 12 shows the feedback of the converter which sensing the input voltage then comparing it with the reference voltage which comes from the MPPT to get the error signal. is the gain of the sensor. The error signal is the input of the compensator to get the duty cycle which controls the converter input voltage equal to the reference voltage. In order to get the closed loop TF, the duty cycle d(s) is replaced by ( ) = ( ( ) − ) (6) Equation (5) can be rearranged to be in the form of; ( ) = ( ) + ( ) + ( ) (7) So input voltage equation is obtained as; ( ) = ( ) + ( ) + (8) The inverse minor loop gain is the suitable technique to study the stability of the system from the relation between the input impedance of the converter Z and the output impedance of the source (PV cell) [9-10]. From the previous equations the stability regions can be determined from the term (1 +Z Z ) which makes the equation undefined if it equal zero.

The condition at which this term is equal zero is Z Z =−1 which means that Z and Z are equal in magnitude with 180 difference in phase.

Figure 12: The converter with the closed loop feedback.

The main conclusion is that the stability regions can be determined by equations of the input impedance of the

converter and the output impedance of the PV cell. In addition these impedances can be measured experimentally using a frequency analyzer and then comparing them together to know the stability regions.

V. CONCLUSION

In this paper, a border study of a MPPT has been presented in order to know the suitable operating regions at which the duty cycle can manage the converter to do its purpose. The regions of maximum and minimum efficiency have been defined in order to select the point of optimum efficiency which can cover optimum irradiation ranges. The parameters of the stability are also defined from the stability study of the PV cell and the converter impedances. The inverse minor loop gain theorem has been used to perform the stability study in this paper.

ACKNOWLEDGMENT

This work is sponsored in part by the Egyptian Science and Technology Development Funds (STDF) under STDF project # 1954 and in part by the U.S. - Egypt Science and Technology Joint Fund in cooperation with U.S. Department of Agriculture (USDA) under USDA Project # 58-3148-0-204. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the funding agencies.

REFERENCES

[1] Hassan. A, Abdelkarem. E, Ahmed. M.E, Orabi. M, Yuncong Jiang, Qahouq. J.A.A. ; “Single-cell photovoltaic with integrated converter” in The 33rd IEEE International Telecommunications Energy Conference (INTELEC), 2011

[2] Yuncong Jiang, Qahouq, J.A.A,Hassan, A,Ahmed, M.E,Orabi, M.”Energy Efficient Fine-grained approach for Solar Photovoltaic Management System”, in the 33rd IEEE International Telecommunications Energy Conference (INTELEC), 2011.

[3] Esram, T. Kimball, J.W. Krein, P.T. Chapman, P.L. Midya, P,” Dynamic Maximum Power Point Tracking of Photovoltaic Arrays Using Ripple Correlation Control”, IEEE Transactions for Power Electronics ,vol 21, no 5 ,September 2006

[4] .P. T. Krein, "Ripple correlation control, with some applications," in Proc.1999 IEEE Int. Symp. Circuits Syst., 1999, pp. 283-286.

[5] T. Esram and P. L. Chapman, "Comparison of photovoltaic array maximum power point tracking techniques," Vol 22, IEEE Transactions on Energy Conversion, June 2007.

[6] C. R. Sullivan and M. J. Powers, "A high-efficiency maximum power Point tracker for photovoltaic arrays in a solar-powered race vehicle," in Proc. 24th Annu. IEEE Power Electron. Spec. Conf., 1993, pp. , 574-580

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[7] C.-T. Pan, 1.-Y. Chen, c.-P. Chu, and Y.-S. Huang, "A fast maximum power point tracker for photovoltaic power systems," in Proc. 25th Annu. Conf. IEEE Ind. Electron. Soc., 1999, pp. 390-393.

[8] T Suntio , J Leppäaho. ; J Huusari,. ; Nousiainen, L. “Issues on Solar-Generator Interfacing With Current-Fed MPP-Tracking Converters” in the IEEE Trans. On Power Electronics, Sept. 2010.

[9] J Leppäaho.; J Huusari; L Nousiainen; T Suntio,“Dynamics of current-fed converters and stability-assessment of solar-generator interfacing” in The International Power Electronics Conference (IPEC), 2010.

[10] T Suntio; J Puukko; L Nousiainen, T Messo,.J Huusari,“Change of paradigm in power electronic converters used in renewable energy applications” in The IEEE 33rd International Telecommunications Energy Conference (INTELEC), 2011.

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