Reverse Current Flow in Solar PV Systems: Detection and Prevention

Reverse Current Flow in Solar PV Systems: Detection and Prevention

Reverse current (a.k.a. backfeed) is one of the quiet failure modes in PV arrays . It can overheat conductors, stress bypass diodes , damage modules, and in worst cases start fires. This guide explains why reverse current happens, how to detect it early , and how to design it out —with worked examples and calculations you can reuse in design reviews and field audits. What is reverse current? In a PV array with parallel strings , a faulted or heavily shaded string can be driven backwards by the healthy strings. Instead of delivering power to the bus, the weak string absorbs power —turning into a heater. Reverse current can also occur: From a battery back into the PV array (nighttime backfeed) if blocking/control fails. From an external DC source (e.g., DC-coupled wind, rectifiers) sharing the same bus. Due to wiring mistakes (polarity reversal) during commissioning. Key risks: Thermal runaway in cells or junction boxes (bypass diode overstress). Conductor overheating (I²R losses). Nuisance trips or m…