There are major drawbacks to this approach. MPPT is performed by the central inverter on the entire array. The central inverter can also be known as a string inverter because the panels are connected in a series string.
If one or more of the panels is partially shaded, its power output will be less, its maximum power point will change and the single central (or string) inverter has no way of knowing this. The maximum available power from the entire array will not be extracted by the central inverter. MPPT needs to be performed on each panel individually.
7. Effects of shading
Shade on a solar panel has a major negative effect. Shade is due to the panel surface covered or partially covered with dust, dirt, leaves, a flock of birds flying overhead and the presence of clouds. The panel power is reduced. When the panel is part of a series string the entire string output is affected. To prevent a shaded panel from reducing the performance of the entire string, a bypass diode or a Cool Bypass Switch is connected across the panel as shown in Figure 10
. Under normal conditions the output current from each string flows through the panels connected in series. If one panel is shaded its current source reduces. The full string current will then flow through the panel Rp as shown in Figure 2
. Rp is a high resistance and the panel will severely overheat. To prevent this situation, bypass diodes placed in parallel with the panel offer a path for string current to flow, preventing a hotspot.
An alternative to Schottky diodes is the Cool Bypass Switch. The SPV1001 Cool Bypass Switch consists of a MOSFET that is switched on and off by a control circuit which provides a path for current to flow around the shaded panel. Compared to a Schottky, the Cool Bypass Switch has less leakage current during the OFF time and lower voltage drop during the ON time thus improving efficiency.