MAXIMUM POWER POINT TRACKING FOR PHOTOVOLTAIC SYSTEMS

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INTRODUCTION
Renewable sources of energy acquire growing importance due to massive consumption and exhaustion of fossil fuel. Among several renewable energy sources, Photovoltaic arrays are used in many applications such as water pumping, battery charging, hybrid vehicles, and grid connected PV systems. As known from a (Power-Voltage) curve of a solar panel, there is an optimum operating point such that the PV delivers the maximum possible power to the load. The optimum operating point changes with the solar irradiation, and cell temperature. Therefore, on line tracking of the maximum power point of a PV array is an essential part of any successful PV system.
A variety of maximum power points tracking (MPPT) methods are developed. The methods vary in implementation complexity, sensed parameters, required number of sensors, convergence speed, and cost. This paper presents a simple MPPT scheme that does not require special measurements of open circuit voltage or short circuit current.



MPPT TECHNIQUES
The problem considered by MPPT methods is to automatically find the voltage VMPP or current IMPP at which a PV array delivers maximum power under a given temperature and irradiance. In this section, commonly used MPPT methods are introduced in an arbitrary order.
3.1. Fractional Open-Circuit Voltage
The method is based on the observation that, the ratio between array voltage at maximum power VMPP to its open circuit voltage VOC is nearly constant.
VMPP ≈ k1 VOC
This factor k1 has been reported to be between 0.71 and 0.78. Once the constant k1 is known, VMPP is computed by measuring VOC periodically. Although the implementation of this method is simple and cheap, its tracking efficiency is relatively low due to the utilization of inaccurate values of the constant k1 in the computation of VMMP.
3.2. Fractional Short-Circuit Current
The method results from the fact that, the current at maximum power point IMPP is approximately linearly related to the short circuit current ISC of the PV array.
IMPP ≈ k2 ISC
Like in the fractional voltage method, k2 is not constant. It is found to be between 0.78 and 0.92. The accuracy of the method and tracking efficiency depends on the accuracy of K2and periodic measurement of short circuit current.



PROPOSED MPPT METHOD
Most MPPT techniques attempt to find (search) the PV voltage that results in the maximum power point VMPP, or to find the PV current IMPP corresponding to the maximum power point. The proposed algorithm tracks neither the VMPP nor the IMPP. However, it tracks directly the maximum possible power PMAX that can be extracted from the PV.
The proposed algorithm is divided into two major parts: maximum power computation, and direct power control of the power drawn from the PV.
The flowchart of the proposed MPPT method is shown in Fig. 2. The algorithm gradually increases the computed value of PMAX and controls the power extracted from the PV to this value. If the actual power is well controlled within the tolerance band of the hysteresis controller, the partial tracking is succeeded and PMAX can be increased to greater value. But, if the power controller fails to track the PMAX, this means that the computed PMAX is greater than the maximum possible power of the PV. Therefore, a reduction (decreasing) in the computed PMAX must be done until the error between PMAX and PACT is limited between upper and lower limit.


CONCLUSION
The paper proposes a simple MPPT method that requires only measurements of PV voltage and current with the need to any environmental measurements (temperature, irradiance).The method is considered as a modified perturb and observe method. However, the principle difference between the proposed method and any other tracking method, is that the proposed method attempts to track and compute the maximum power and controls directly the extracted power from the PV to that computed value. While, any other method attempts to reach the maximum point by the knowledge of the voltage or the current corresponding to that optimum point. The proposed method offers different advantages which are: good tracking efficiency, relatively high convergence speed and well control for the extracted power thanks to the direct power control unit based on the ON/OFF hysteresis controller.