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…