The single PV modules are connected in both series and parallel, in the larger PV arrays. The series connected set of solar cells or modules is typically named a string. This combination of both series and parallel connections can lead to various problems in the PV arrays. One possible problem arises from the open circuit in one of the series strings. The current produced from the string connected in parallel (usually referred to as a block) then will have a lower current than the remainder of the blocks in the solar module. This is electrically the same as the case of a single shaded solar cell connected in series with various good cells, and the power from the whole block of solar cells is lost. For the Potential mismatch effects in larger PV arrays.
Although all the connected modules can be identical and the array does not suffer from any shading, mismatch and hot spot effects can still occur.
Mismatch effects in arrays and the bypass diodes
Parallel connections in addition to the mismatch effects can also lead to problems if the bypass diodes are not properly rated to handle the current produced by the total parallel connected array. As an example, in the parallel strings that have series connected modules, the bypass diodes of the series connected modules can become connected in parallel. Any mismatch in the series connected modules will lead to the current flowing through the bypass diode, thus leading to heating of this diode.
Yet, heating the bypass diode decreases the effective resistance. The majority of the current will now flow through the marginally hotter set of bypass diodes. These bypass diodes after that become hotter, so this further reduces their resistance and increases the current flow. Ultimately, nearly all the current produced can flow through one set of bypass diodes. If these diodes are not rated to handle the current produced by the parallel combination of modules, the diodes will burn out and allow further damage to the PV modules to happen. For the bypass diodes in paralleled modules, there are normally two bypass diodes for every 36 cell module.
Adding to the use of bypass diodes to prevent any mismatch losses, an extra diode, named blocking diode, can also be used to minimize the mismatch losses. The blocking diode, is usually used to prevent the module from loading the battery during night time by preventing current flowing from the battery through the PV array. With the parallel connected modules, every string required be connected in parallel must have its own blocking diode. This prevents the current flowing from one parallel string into the lower current string and also reduces the needed current carrying capability of the blocking diode, and thus helps in minimizing mismatch losses rising in the parallel connected arrays.