Different types of parallel arc-faults on the DC side of a PV array.

Different types of parallel arc-faults on the DC side of a PV array.

Sandia National Laboratories/Tigo Energy researchers recently submitted an application for a US patent related to research on arc-fault detection and circuit interruption in solar photovoltaic (PV) systems. Sandia’s patent-pending methods, filed as Identifying an Arc-Fault Type in Photovoltaic Arrays, allow AFCIs to distinguish between series and parallel faults.

The 2011 National Electrical Code® requires PV DC series arc-fault protection, but does not require parallel arc-fault protection. As a result, manufacturers are creating arc-fault circuit interrupters (AFCIs) that only safely de-energize the arcing circuit when a series arc-fault occurs. Because AFCI devices often use the broadband AC noise on the DC side of the PV system for detection (and series and parallel arc-faults create similar frequency content), it is likely an AFCI device will open in the event of either arc-fault type.

Fast Fourier transform of string current during parallel and series arc-faults with a 3 kW inverter. Data smoothed with 400 Hz rectangular sliding-average.

Fast Fourier transform of string current during parallel and series arc-faults with a 3 kW inverter. Data smoothed with 400 Hz rectangular sliding-average.

In the case of parallel arc-faults, opening the AFCI will not extinguish the arc and may make the arc worse, potentially creating a fire. Due to the fire risk from parallel arc-faults, Tigo Energy and Sandia studied series and parallel arc-faults and confirmed the noise signatures from the two arc-faults types are nearly identical. As a result, their paper, “Differentiating Series and Parallel Photovoltaic Arc-Faults,” presents three alternative methods for differentiating parallel and series arc-faults along with suggestions for arc-fault mitigation of each arc-fault type.

Once the arc-fault type is determined, series arc-faults can be de-energized by opening the string at any point. Many parallel arc-faults can be de-energized by opening connectors between the modules and all parallel arc-faults can be de-energized by shorting the modules or the strings. Unfortunately, the shorting solution requires shorting the input capacitor of the inverter and repairs to the array would require shading the modules or working at night. Additional analysis of different system configurations and appropriate AFCI responses during parallel arc-fault events is recommended.