Reverse current: It can only occur when modules are connected in parallel and the open circuit terminal voltage (open circuit voltage) of the individual parallel strings is different. In normal operation, this is avoided adequately when the strings are of the same length.
Short circuit in one or more modules causes the open circuit terminal voltage of a module string to be significantly lower than the open circuit terminal voltage of other parallel strings. The internal diode structure of the solar cells causes reverse current to flow through the faulty generator string that, depending on the strength of the current, may lead to excessive heating or destruction of the modules in this string.
The following faults may lead to reduction of the open circuit terminal voltage of a generator string and subsequent reverse current in parallel-connected systems
- Short circuit in one or more modules
- Short circuit in one or more cells in a module
- String diodes connected in series with the individual strings prevent any reverse current in the corresponding string.
- String fuses connected in series to the individual strings can limit the reverse current in the corresponding string to the permitted maximum. The losses at the string fuses are significantly lower than those of string diodes.