A string diode prevents a string module from being operated under reverse current (forward direction in solar cells) in the event of a malfunction attributable to shading, failure or the like. A string diode should present an inverse voltage that is appreciably higher than the maximum operating open-circuit voltage of the string (e. g. 2 ■ VOCA-STC) and that can indefinitely conduct at least 1.25 times the module short-circuit current ISC-STC of the string – although it should be noted that the factor for Alpine PV plants is around 1.5 rather than 1.25. If one side of a solar generator string is grounded, the diode should always be installed on the ungrounded side. A functioning string diode provides completely reliable protection against reverse current, including in cases of wire failure or a severe malfunction in a string module. However, as string diodes are prone to atmospheric power surges, relatively strong currents such as those exceeding 4-6 A need to be cooled, and always provoke a voltage drop and thus power loss – although this is of minor importance in the presence of relatively high system voltages. Nowadays, string diodes are mainly used in relatively large PV installations with numerous parallel strings and operating voltages exceeding 100 V. A proven and low-cost solution for string diodes is to integrate into them rectifier bridges which are well insulated and are available for up 35 A and 1400 V applications. A single-phase bridge of this type is sufficient for two strings, and a triphase bridge is sufficient for three strings.
Since, as noted in Section 188.8.131.52, (a) full protection against reverse current is unnecessary and (b) string diode cooling elements for large-scale module currents are prohibitively expensive, the tendency for some time now has been to dispense with these diodes if possible, except in special cases such as test installations that are frequently rewired. Dispensing with string diodes simplifies the system, enhances system reliability and also marginally improves efficiency, particularly for low system voltages. In cases where a stand-alone system is needed in order to obviate nocturnal battery discharge by the PV generator of a reverse current diode, this can also be achieved by installing only one such diode near the charge controller or through the use of a charge controller that performs this function.