Solar cells need to be wired to solar modules or solar generators in such a way as to safeguard the individual cells against damage from unusual operating conditions resulting from overload and/or overheating. In practical terms, this means that operation of any given cell in the first and third quadrants of the characteristic diode curve (see Figure 3.13), where the cell absorbs power rather than outputting it, should be avoided.
If, in the event of a malfunction (e. g. during shading), a solar cell can nonetheless be operated in the first and third power quadrants of the characteristic diode curve, measures must be taken to prevent the current passing through and output by the cell from becoming unduly strong. In order to see exactly how such scenarios should be handled, we need to take a closer look at the complete characteristic curves of actual solar cells.
As solar cell vendors do not provide technical data concerning solar cell operation in the first and third power quadrants, the Bern University of Applied Sciences PV Lab decided to investigate a number of commercial silicon solar cells and modules.
Figure 4.16 displays the characteristic curves that the PV Lab obtained for a monocrystalline solar cell with AZ « 102 cm2 surface area. The metering devices used for these investigations were the same as those used for voltage and current in a standard diode (appliance counting arrow system), i. e. the current indicated by these devices corresponds to Ґ in Figure 3.13. The first characteristic curve quadrant represents the non-conducting direction of the diode, the third quadrant represents the passband direction of the diode and the fourth quadrant represents the active solar cell areas to which power is output. On the other hand, the solar cell absorbs power in quadrants 1 and 3. In most cases, the diode reaches its avalanche point at a reverse voltage exceeding 15-25 V [4.4].