Publications

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Ground faults in photovoltaic (PV) systems pose a fire and shock hazard. To mitigate these risks, AC-isolated, DC grounded PV systems in the United States use Ground Fault Protection Devices (GFPDs), e.g., fuses, to de-energize the PV system when there is a ground fault. Recently the effectiveness of these protection devices has come under question because multiple fires have started when ground faults went undetected. In order to understand the limitations of fuse-based ground fault protection in PV systems, analytical and numerical simulations of different ground faults were performed. The numerical simulations were conducted with Simulation Program with Integrated Circuit Emphasis (SPICE) using a circuit model of the PV system which included the modules, wiring, switchgear, grounded or ungrounded components, and the inverter. The derivation of the SPICE model and the results of parametric fault current studies are provided with varying array topologies, fuse sizes, and fault impedances. Closed-form analytical approximations for GFPD currents from faults to the grounded current carrying conductor-known as %E2%80%9Cblind spot%E2%80%9D ground faults-are derived to provide greater understanding of the influence of array impedances on fault currents. The behavior of the array during various ground faults is studied for a range of ground fault fuse sizes to determine if reducing the size of the fuse improves ground fault detection sensitivity. The results of the simulations show that reducing the amperage rating of the protective fuse does increase fault current detection sensitivity without increasing the likelihood of nuisance trips to a degree. Unfortunately, this benefit reaches a limit as fuses become smaller and their internal resistance increases to the point of becoming a major element in the fault current circuit.

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To improve fire safety in PV systems, Article 690.11 of the 2011 National Electrical Code (NEC) requires photovoltaic (PV) systems above 80 V on or penetrating a building to include a listed arc-fault protection device. Many arc-fault circuit interrupter (AFCI) devices are currently being listed and entering the market. Depending on the manufacturer, AFCIs are being deployed at the module-level, string-level, or array-level. Each arc-fault protection scheme has a different cost and arc-fault isolation capability. Module-level and string-level AFCI devices tout the ability to isolate the fault, identify the failed PV component, and minimize the power loss by selectively de-energizing a portion of the array. However, these benefits are negated if the arcing noisetypically used for arc-fault detectionpropagates to parallel, unfaulted strings and cause additional AFCI devices on the PV array to trip. If the arcing signature crosstalks from the string with the arc-fault via conduction or RF electromagnetic coupling, the location of the arc-fault cannot be easily determined and safe PV generators will be disconnected. Sandia National Laboratories collaborated with Texas Instruments to perform a series of nuisance trip scenarios with different PV configurations. Experimental results on a 2-string array showed arc detection on the faulted string occurred an average of 19.5 ms before unfaulted stringbut in some cases the AFCI on both strings would trip. © 2012 IEEE.

The 2011 National Electrical Code® Article 690.11 requires photovoltaic systems on or penetrating a building to include a DC arc-fault protection device. In order to satisfy this requirement, new Arc-Fault Detectors (AFDs) are being developed by multiple manufacturers including Sensata Technologies. Arc-fault detection algorithms often utilize the AC noise on the PV string to determine when arcing conditions exist in the DC system. In order to accelerate the development and testing of Sensata Technologies' arc-fault detection algorithm, Sandia National Laboratories (SNL) provided a number of data sets. These prerecorded 10 MHz baseline and arc-fault data sets included different inverter and arc-fault noise signatures. Sensata Technologies created a data evaluation method focused on regeneration of the prerecorded arcing and baseline test data with an arbitrary function generator, thereby reducing AFD development time. © 2012 IEEE.

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) which only safely de-energize the arcing circuit when a series arc-fault occurs. Since 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. 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 National Laboratories studied series and parallel arc-faults and confirmed the noise signatures from the two arc-faults types are nearly identical. As a result, three alternative methods for differentiating parallel and series arc-faults are presented along with suggestions for arc-fault mitigation of each arc-fault type. © 2012 IEEE.

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