PRESENTED BY:
T. Vennela

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ABSTRACT:
The electric power supplied by a photovoltaic power generation system depends on the solar radiation and temperature. Designing efficient PV systems heavily emphasizes to track the maximum power operating point.
This work develops a three-point weight comparison method that avoids the oscillation problem of the perturbation and observation algorithm which is often employed to track the maximum power point. Furthermore, a low cost control unit is developed, based on a single chip to adjust the output voltage of the solar cell array.
Index Terms: Photovoltaic, Perturbation and Observation Algorithm, Maximum Power Point Tracking
1. INTRODUCTION:
Photovoltaic (PV) generation is becoming increasingly important as a renewable source since it offers many advantages such as incurring no fuel costs, not being polluting, requiring little maintenance, and emitting no noise, among others. PV modules still have relatively low conversion efficiency; therefore, controlling maximum power point tracking (MPPT) for the solar array is essential in a PV system. The amount of power generated by a PV depends on the operating voltage of the array. A PV’s maximum power point (MPP) varies with solar insulation and temperature. Its V-I and V-P characteristic curves specify a unique operating point at which maximum possible power is delivered.
At the MPP, the PV operates at its highest efficiency. Therefore, many methods have been developed to determine MPPT. In MPPT, most control schema use the P&O technique because it is easy to implement. But the oscillation problem is unavoidable. This research developed an extended P&O technique- a three point weight comparison utilizing a boost converter to adjust the output voltage of the PV for tracking the MPP. This report introduces the basic principle of the PV system, traditional P&O and the proposed algorithm three weight comparison method, and finally conclusion.
2. Mathematical Model:
The building block of PV arrays is the solar cell, which is basically a p-n semiconductor junction, shown in figure 2.1. The V-I characteristic of a solar array is given by Eq. (1)
where V and I represent the output voltage and current of the PV, respectively; Rs and Rsh are the series and shunt resistance of the cell; q is the electronic charge; Isc is the light-generated current; Io is the reverse saturation current; n is a dimensionless factor; k is the Boltzmann constant, and Tk is the temperature in 0K.
In the above circuit, Isc is a current source which supplies current for the circuit comprising of semiconductor diode which is a solar cell, a resistance which is connected in shunt with the diode and a resistance connected in series with the diode.
Equation (1) was used in computer simulations to obtain the output characteristics of a solar cell, as shown in Figure 2.2. This curve clearly shows that the output characteristics of a solar cell are non- linear and are crucially influenced by a solar radiation, temperature and load condition. Each curve has a MPP, at which the solar array operates most efficiently.
3. Maximum Power Point Tracking:
Several techniques for tracking MPP have been proposed. Two algorithms are commonly used to track the MPPT- the P&O method has been broadly used because it is easy to implement. Figure 3 presents the control flow chart of the P&O algorithm. The MPP tracker operates by periodically incrementing or decrementing the solar array voltage. If a given perturbation of the PV, then the subsequent perturbation is generated in the same (opposite) direction. In the Figure 3, Set Duty Out denotes the perturbation of the solar array voltage, and Duty+ and Duty- represent the subsequent perturbation in the same or opposite direction, respectively.
4. Three-point Weight Comparison Method:
The P&O alogorithm compares only two points, which are the current operation point and the subsequent perturbation point, to observe their changes in power and thus decide whether increase or decrease the solar array voltage.

The P&O algorithm oscillates around the MPP, resulting in a loss of PV power, especially in cases of rapidly changing solar radiation. Therefore, the three point weight comparison is proposed to avoid having to move rapidly the operation point, when the solar radiation is varying quickly or when a disturbance or data reading error occur. Restated, the MPPT can be traced accurately when the solar radiation is stable and power loss is low.
The algorithm of the three-point weight comparison is run periodically by perturbing the solar array terminal voltage and comparing the PV output power on three points of the V-P curve. The three points are the current operation point (A), a point, B, perturbed from point A, and a point C, with doubly perturbed in the opposite direction from point B. Figure 4 depicts the nine possible cases. In these cases, for the points A and B, if the wattage of the point B is greater than or equal to that of point A, the status is assigned a positive weighting. Otherwise, the status is assigned a negative weighting. Of the three measured points, if two are positively weighted, the duty cycle of the converter should be increased. On the contrary, when two are negatively weighted, the duty cycle of the converter should be decreased.
In the other cases with one positive and one negative weighting, the MPP is reached or the solar radiation has changed rapidly and the duty cycle is not to be changed. Figure 4.2.
5. Configuration of the PV System:
Figure 5 shows the system configuration of the proposed PV system. This system consists of a solar array (75 W) with an open voltage of 21 V and a short circuit current of 4.6 A, an A/D and D/A converter, a 20 ohms/100 W, and a control unit on a single-chip. Figure 5 depicts the circuits of the boost converter connected from the output of the solar cell. The power flow is controlled by varying the on/off duty cycle of the switching. The average output voltages are determined by the Eq. (2).
Vout / Vin =1/(1-D) (2)
where Vout and Vin are the output and input voltage of the converter and D is the duty cycle of the switch S. The input power of the converter is equal to the output power of the converter if the converter is ideal, yielding the following equations.
Iout = Iin * (1-D) (3)
Figure 5.1: Configuration of the PV system
Figure 5.2 : Circuits of the boost converter
Rin = Vin / Iin = Vout / Iout * (1-D)^2 (4)
From Eq. (4), when the load (Rout), is fixed, the input resistance Rin can be controlled by varying the duty cycle. Therefore, the operating point of the solar cell can be controlled by the duty cycle.
A simulated solar source was established to compare results under the same environmental conditions for various results test cases. Figure 5.3 shows the configuration of the simulated solar source with a maximum energy of 32.68 mA/cm^2.
6. Simulation results:
A prototype MPPT system has been developed using the described method and tested in the laboratory. The PV array gives a 75 W maximum power, a 21 V open circuit voltage and a close-circuit current of 4.6 A at a solar energy of 1 kW/m^2 and a temperature of 250C. The PV array was simulated with two solar energy cases, 32.68 mA/cm^2 (case A) and 12.49 mA/cm^2 (case B) to test the proposed system under specific atmospheric conditions. Figures 6.1, 6.2 plot the V-I and V-P curves under the two cases at 650C.