In a larger PV array, individual PV modules are connected in both series and parallel. A series-connected set of solar cells or modules is called a "string". The combination of series and parallel connections may lead to several problems in PV arrays. One potential problem arises from an open-circuit in one of the series strings. The current from the parallel connected string (often called a "block") will then have a lower current than the remaining blocks in the module. This is electrically identical to the case of one shaded solar cell in series with several good cells, and the power from the entire block of solar cells is lost. The figure below shows this effect.
Parallel connections in combination with mismatch effects may also lead to problems if the by-pass diodes are not rated to handle the current of the entire parallel connected array. For example, in parallel strings with series connected modules, the by-pass diodes of the series connected modules become connected in parallel, as shown in the figure below. A mismatch in the series connected modules will cause current to flow in a by-pass diode, thereby heating this diode. However, heating the by-pass diode reduces the effective resistance. Most of the current will now flow through the slightly hotter set of by-pass diodes. These by-pass diodes then become even hotter, further reducing their resistance and increasing the current flow. Eventually almost all the current may flow through one set of by-pass diodes. If the diodes are not rated to handle the current from the parallel combination of modules, they will burn out and allows damage to the PV modules to occur.
In addition to the use of by-pass diodes to prevent mismatch losses, an additional diode, called a blocking diode, may be used to minimize mismatch losses. A blocking diode, shown in the figure below, is typically used to prevent the module from loading the battery at night by preventing current flow from the battery through the PV array. With parallel connected modules, each string to be connected in parallel should have its own blocking diode. This not only reduces the required current carrying capability of the blocking diode, but also prevents current flowing from one parallel string into a lower-current string and therefore helps to minimize mismatch losses arising in parallel connected arrays.