MPPT for PV Arrays

For a set level of illumination, photovoltaic (PV) arrays generate different amounts of electrical energy depending on the current being drawn from them. Typical volt – age/current profiles from a PV cell are shown in Figure 6.51, where each curve represents a particular combination of light intensity and temperature. Clearly, the maximum power is extracted from the cell when the product of cell current and voltage is maximized. For each curve, these points are labeled Pmpp

Therefore, as has been the case with other energy-harvesting methods, in order to extract maximum energy from the transducer, an optimal load resistance must be connected in the form of a power converter emulating that resistive load as its input impedance. Setting the load impedance is relatively simple as long as the op­timal load is known. The difficulty with PV, and perhaps all harvester technologies, is that finding the optimal load as operating conditions change can be difficult. This difficulty arises from the fact that the maximum power point is dependent on the temperature, the irradiation level, and the age of the solar cell. Some estima­tion could be made by measuring the temperature and irradiation level, but the usual method employed in large-scale PV installations is to continually hunt for the maximum power point, modifying the duty cycle of a power converter to ensure

that the maximum power is extracted. The perturb and observe (P&O) method has to date been the preferred technique for hunting for the MPP. The duty cycle of a power converter is continually perturbed, and if a change in duty cycle increases the power output, the duty cycle is again changed in the same direction. If the change in duty cycle produces a reduction in output power, the duty cycle is next perturbed in the opposite way and as such the maximum power point of the solar array is attained.

Figure 6.52 shows a typical output power versus output voltage curve of a PV array when operated under static conditions. If the initial output voltage of the ar­ray results in an output power at point A, an increment of Д1 will move the operat­ing point to the MPP position. However, at point B, decreasing the output voltage by Д2 will result in optimal operation of the PV array.