Charles J. Luebke

Photovoltaic DC Arc Fault Detector testing at Sandia National Laboratories

The 2011 National Electrical Code® (NEC®) added Article 690.11 that requires photovoltaic (PV) systems on or penetrating a building to include a listed DC arc fault protection device. To fill this new market, manufacturers are developing new Arc Fault Circuit Interrupters (AFCIs). Comprehensive and challenging testing has been conducted using a wide range of PV technologies, system topologies, loads and noise sources. The Distributed Energy Technologies Laboratory (DETL) at Sandia National Laboratories (SNL) has used multiple reconfigurable arrays with a variety of module technologies, inverters, and balance of system (BOS) components to characterize new Photovoltaic (PV) DC AFCIs and Arc Fault Detectors (AFDs). The devices detection capabilities, characteristics and nuisance tripping avoidance were the primary purpose of the testing. SNL and Eaton Corporation collaborated to test an Eaton AFD prototype and quantify arc noise for a wide range of PV array configurations and the system responses. The tests were conducted by generating controlled, series PV arc faults between PV modules. Arc fault detection studies were performed on systems using aged modules, positive- and negative-grounded arrays, DC/DC converters, 3-phase inverters, and on strings with branch connectors. The tests were conducted to determine if nuisance trips would occur in systems using electrically noisy inverters, with series arc faults on parallel strings, and in systems with inverters performing anti-islanding and maximum power point tracking (MPPT) algorithms. The tests reported herein used the arc fault detection device to indicate when the trip signal was sent to the circuit interrupter. Results show significant noise is injected into the array from the inverter but AFCI functionality of the device was generally stable. The relative locations of the arc fault and detector had little influence on arc fault detection. Lastly, detection of certain frequency bands successfully differentiated normal operational noise from an arc fault signal.

A study of ignition time for materials exposed to DC arcing in PV systems

This study examines the factors that influence the time to first ignition and burn through for materials, found in PV power systems, when exposed to DC arcing. Materials of interest include PV wire insulation, connectors, metal conduits, and insulation. The most important factors for the time to ignition are the power density absorbed and the material threshold. The most important factors for burn through are power density, heat released by polymers, thickness and flame retardant chemistry. Ignition occurs when the arc power and time of exposure are sufficient to produce localized heating that exceeds material thresholds. Data shows that arcs as low as 200 watts, at a radius of 10 mm, will ignite most plastics in 4 seconds. A radiation model is presented to calculate the absorbed power density. Times to the first ignition, re-flash, burn rate and flame retardants are factors in the prediction. The ratio of arc power density to the peak heat release rate of polymers is used in the burn through time prediction. A burn through time vs. arc watt correlation has been established. Burn through time is sensitive to the power density absorbed by the exposed material and includes geometry factors and material thresholds. Radiated power density is driven by arc power density. Arc power is determined by arc current, and arc gap. Ignition time & burn through estimates can be used to establish an AFCI trip curve to reduce the risk of fires due to arcing.

Field test results of DC arc fault detection on residential and utility scale PV arrays

It has been recognized that DC arcing faults pose a hazard in present photovoltaic (PV) systems. The 2011 National Electric Code [1] added a requirement for arc fault circuit protection and Underwrites Laboratories (UL) recently published an outline of investigation (UL1699B) to define the requirements for such a device. To satisfy the need for a PV arc fault detector (AFD) and test such a device in PV installations a method was developed for safely generating and recording DC series Arcing Faults within PV arrays. Field testing was performed on several residential and utility scale size PV systems. Results show that a DC Arc Fault Circuit Interrupter (AFCI) prototype was able to consistently detect and mitigate a series arcing fault under various operating conditions and without nuisance tripping. Analysis of electrical current waveforms confirmed that the spectral noise content is sufficient to detect arcing vs. non-arcing (baseline) conditions. This paper will include a general description of the arc generating apparatus used to safely insert a series arcing fault within a PV system, the test instrumentation used for measuring and recording arc voltage and current, and the results from testing a DC AFCI with various inverters, module technologies, PV array sizes and topologies, and under various illumination/operating conditions. The DC AFCI is able to detect and mitigate series arcing faults significantly faster than the ignition/burn through times established for PV insulation materials and within the trip times established by UL1699B [2]. Test results show that series arcing faults can be detected by a DC AFCI on a single string of either silicon or thin film type PV modules, and without a DC AFCI nuisance tripping when located on a non-faulted parallel string.