Characterizing Pv Arcing Conditions with Impedance Spectroscopy and Frequency Response Analysis

Photovoltaic arc faults have led to a number of rooftop fires that have caused significant property damage and threatened the safety of building occupants. In response, Article 690.11 was added to the United States 2011 National Electrical Code, requiring new PV systems on or penetrating a building to include a listed arc fault protection device. Many proposed arc fault detectors utilize frequency content on the PV string to remotely determine if an arc fault is occurring. A frequency response analyzer was used to measure the AC frequency and impedance response of an 80 W polycrystalline module with degradation similar to those which have caused arc fault. The module had intermittent electrical connectivity in the junction box because of a failed solder bond, and the quality of the connection could be manipulated by adjusting the external wire orientation. By reorienting the wire, the impedance profile changed and the frequency response transitioned between high-pass, low-pass, and no filtering behavior. This indicated (1) certain module conditions can filter arc fault noise frequencies, and (2) the frequency response or impedance spectroscopy of PV components could be used to identify conditions that cause arc faults in PV systems. The implications for the former are serious because if the detection frequencies used by the fault circuit interrupter (AFCI) are sufficiently filtered, the device will not trip and the arc will not be extinguished. Finally, infrared images were taken of the intermittent module to identify the area of damage and corroborate the impedance spectroscopy results.