Fig.1 Block diagram of two-stage grid-connected PV system Fig.1 shows diagram of two-stage grid-connected PV system

II. MODEL OF PHOTOVOLTAIC

PV array is made of p-n junction semiconductors that convert sunlight into electricity. Photons are absorbed to produce electricity when incoming solar energy exceeds band gap energy of the PV module. The I-V characteristic also known as current voltage relationship is a complex non-linear function. Several mathematical models exist to describe behavior of the solar panel. In this paper the following model is used for describing the photovoltaic panel where, Ix is the short circuit current under Standard Test Conditions (STC), and Vx is the open circuit voltage under STC, and b is the characteristic constant of the panel and can be approximated by equation (3) with Vop is the voltage at which the maximum power is extracted from the panel and Pmax is the maximum power point of the panel, all under STC.The proposed inverter design is used an intelligent PV module concept that includes a DC-DC converter with MPPT controller.

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering

(An ISO 3297: 2007 Certified Organization)

Vol. 4, Issue 2, February 2015

III. CONTROL STRATEGY OF MPPT BASED ON DC-DCBOOST CONVERTER

A typical solar panel converts only 30 to 40 percent of the incident solar irradiation into electrical energy. Maximum power point tracking technique is used to improve the efficiency of the solar panel. According to Maximum Power Transfer theorem, the power output of a circuit is maximum when the Thevenin impedance of the circuit (source impedance) matches with the load impedance. Hence our problem of tracking the maximum power point reduces to an impedance matching problem. In the source side we are using a boost convertor connected to a solar panel in order to enhance the output voltage so that it can be used for different applications like motor load. By changing the duty cycle of the boost converter appropriately we can match the source impedance with that of the load impedance.

The Perturb & Observe algorithm states that when the operating voltage of the PV panel is perturbed by a small increment, if the resulting change in power ∆P is positive, then we are going in the direction of MPP and we keep on perturbing in the same direction. If ∆P is negative, we are going away from the direction of MPP and the sign of perturbation supplied has to be changed

Fig.2 Solar panel characteristics showing MPP and operating points A and B

Fig.2 shows the plot of module output power versus module voltage for a solar panel at a given irradiation. The point marked as MPP is the Maximum Power Point, the theoretical maximum output obtainable from the PV panel. Consider A and B as two operating points. As Fig.2 shown in the figure above, the point A is on the left hand side of the MPP. Therefore, we can move towards the MPP by providing a positive perturbation to the voltage. On the other hand,   point

B is on the right hand side of the MPP. When we give a positive perturbation, the value of ∆P becomes negative, thus itis imperative to change the direction of perturbation to achieve MPP.

B. DC-DC Converter

Switched mode DC-DC converter converts unregulated DC input voltage into regulated DC output voltage  at a specified voltage level. Switching power supplies offer much more efficiency and power density compare to linear power supplies . Basic converters that step up or step down voltage input contains elements like transistors, diodes, capacitor and inductors. Three basic converter topologies exist, they are buck (step-down), boost (step-up) and buckboost (step-up or step-down). In our proposed design boost topology is used because its free wheeling diode can be used for blocking reverse current and it efficiently amplify PV arrays output voltage into higher level. Fig. 4 shows the output response of boost converter, which clearly indicates that, the intelligent PV system with MPPT function increase the output power from solar PV drastically. Converters are controlled by pulse width modulation (PWM) duty cycle since the output of converter being determined by state of transistor switch. Thus optimum load impedance of PV module is achieved by varying duty cycle. 太阳能最大功率点追踪和逆变控制英文文献和中文翻译(2):http://www.chuibin.com/fanyi/lunwen_205049.html