Jack David Flicker

A Comprehensive Review of Catastrophic Faults in PV Arrays: Types, Detection, and Mitigation Techniques

Three major catastrophic failures in photovoltaic (PV) arrays are ground faults, line-to-line faults, and arc faults. Although there have not been many such failures, recent fire events on April 5, 2009, in Bakersfield, CA, USA, and on April 16, 2011, in Mount Holly, NC, USA, suggest the need for improvements in present fault detection and mitigation techniques, as well as amendments to existing codes and standards to avoid such accidents. This review investigates the effect of faults on the operation of PV arrays and identifies limitations to existing detection and mitigation methods. A survey of state-of-the-art fault detection and mitigation technologies and commercially available products is also presented.

PV ground-fault detection using spread spectrum time domain reflectometry (SSTDR)

A PV ground-fault detection technique using spread spectrum time domain reflectometry (SSTDR) method has been introduced in this paper. SSTDR is a reflectometry method that has been commercially used for detecting aircraft wire faults. Unlike other fault detection schemes for a PV system, ground fault detection using SSTDR does not depend on the amplitude of fault-current and highly immune to noise signals. Therefore, SSTDR can be used in the absence of the solar irradiation as well. The proposed PV ground fault detection technique has been tested in a real-world PV system and it has been observed that PV ground fault can be detected confidently by comparing autocorrelation values generated using SSTDR. The difference in the autocorrelation peaks before and after a ground-fault in the PV system are significantly higher than the threshold set for ground-fault detection.

Electrical simulations of series and parallel PV arc-faults

Arcing in PV systems has caused multiple residential and commercial rooftop fires. The National Electrical Code® (NEC) added section 690.11 to mitigate this danger by requiring arc-fault circuit interrupters (AFCI). Currently, the requirement is only for series arc-faults, but to fully protect PV installations from arc-fault-generated fires, parallel arc-faults must also be mitigated effectively. In order to de-energize a parallel arc-fault without module-level disconnects, the type of arc-fault must be identified so that proper action can be taken (e.g., opening the array for a series arc-fault and shorting for a parallel arc-fault). In this work, we investigate the electrical behavior of the PV system during series and parallel arc-faults to (a) understand the arcing power available from different faults, (b) identify electrical characteristics that differentiate the two fault types, and (c) determine the location of the fault based on current or voltage of the faulted array. This information can be used to improve arc-fault detector speed and functionality.

PV faults: Overview, modeling, prevention and detection techniques

Recent PV faults and subsequent fire-hazards on April 5, 2009, in Bakersfield, California, and April 16, 2011, in Mount Holly, North Carolina provide evidence of a lack of knowledge among PV system manufacturers and installers about different PV faults. The conducted survey within the scope of this paper describes various faults in a PV plant, and explains the limitations of existing detection and suppression techniques. Different fault detection techniques proposed in literatures have been discussed and it was concluded that there is no universal fault detection technique that can detect and classify all faults in a PV system. Moreover, this digest proposes a transmission line model for PV panels that can be useful for interpreting faults in PV using different refelectomery methods.

Recommendations for CSM and R iso ground fault detector trip thresholds

PV ground faults have caused many fires in the U.S. and around the world. One cause of these fires is a “blind spot” in the ground fault ground fault fuse. As a result of this discovery, the Solar America Board for Codes and Standards identified a number of alternatives to ground fault fuses, but these technologies have limited historical use in the United States. This paper investigates the efficacy of two of these devices, isolation resistance monitoring (Riso) and current sense monitoring (CSM), in small (~3 kW) and large (>500 kW) arrays using both simulation and field data. The field data includes Riso and leakage current measurements of multiple PV systems, while the simulations include Riso and CSM measurements from various ground faults. From these results, it was found that the majority of leakage current is not from the modules, but from low inverter isolation-to-ground. Therefore appropriate thresholds to maximize detection area while minimizing nuisance tripping should be made based on the specific inverter isolation and switching noise rather than the configuration of the PV system.

Lifetime testing of metallized thin film capacitors for inverter applications

In order to understand the degradation mechanisms and failure precursors of metallized thin film capacitors (MTFC) used in photovoltaic (PV) inverters, we have carried out accelerated testing on MTFCs. By understanding the degradation mechanisms and precursors of imminent catastrophic failure, implementation of a prognostics and health management (PHM) plan can be used to optimize PV array operations and maintenance (O&M), decreasing cost per watt towards the US Department of Energy goals.

Miniature high voltage, high temperature component package development

With the next generation of semiconductor materials in development, significant strides in the Size, Weight, and Power (SWaP) characteristics of power conversion systems are presently underway. In particular, much of the improvements in system-level efficiencies and power densities due to wide-bandgap (WBG) and ultra-wide-bandgap (UWBG) device incorporation are realized through higher voltage, higher frequency, and higher temperature operation. Concomitantly, there is a demand for ever smaller device footprints with high voltage, high power handling ability while maintaining ultra-low inductive/capacitive parasitics for high frequency operation. For our work, we are developing small size vertical gallium nitride (GaN) and aluminum gallium nitride (AlGaN) power diodes and transistors with breakdown and hold-off voltages as high as 15kV. The small size and high power densities of these devices create stringent requirements on both the size (balanced between larger sizing for increased voltage hold-off with smaller sizing for reduced parasitics) and heat dissipation capabilities of the associated packaging. To accommodate these requirements and to be able to characterize these novel device designs, we have developed specialized packages as well as test hardware and capabilities. This work describes the requirements of these new devices, the development of the high voltage, high power packages, and the high-voltage, high-temperature test capabilities needed to characterize and use the completed components. In the course of this work, we have settled on a multi-step methodology for assessing the performance of these new power devices, which we also present.

Recommendations for isolation monitor ground fault detectors on residential and utility-scale PV systems

PV faults have caused rooftop fires in the U.S., Europe, and elsewhere in the world. One prominent cause of past electrical fires was the ground fault detection “blind spot” in fuse-based protection systems uncovered by the Solar America Board for Codes and Standards (SolarABCs) steering committee in 2011. Fortunately, a number of alternatives to ground fault fuses have been identified, but there has been limited adoption and historical use of these technologies in the United States. This paper investigates the efficacy of one of these devices known as isolation monitoring (or isolation resistance monitoring, Riso) in small (~3kW) and large (~700 kW) arrays. Unfaulted and faulted PV arrays were monitored with Riso technology and compared to SPICE simulations to recommend appropriate thresholds to the maximize the range of ground faults which could be detected while minimizing unwanted tripping. Based on analytical and computational models, it is impossible to determine a trip threshold that provides fire safety and negates unwanted tripping issues. This paper mathematically demonstrates that appropriate Riso trip thresholds must be determined on an array-by- array basis with sufficient leeway by system operators to adjust trip threshold settings for their particular usage cases.

An Irradiance-Independent, Robust Ground-Fault Detection Scheme for PV Arrays Based on Spread Spectrum Time-Domain Reflectometry (SSTDR)

A healthy photovoltaic (PV) array has a specific impedance between node pairs, and any ground fault changes the impedance values. Reflectometry is a well-known technique in electromagnetics, and it could be exploited to detect fault and aging-related impedance variations in a PV system. A fault detection algorithm using the spread spectrum time-domain reflectometry (SSTDR) method has been introduced in this paper. SSTDR has been successfully used for detecting and locating aircraft wiring faults. However, the wide variation in impedance throughout the entire PV system, which is caused by the use of different materials and interconnections makes PV fault detection more challenging while using reflectometry. Unlike other conventional ground-fault detection techniques specifically developed for PV arrays, SSTDR does not depend on fault-current magnitudes. Therefore, SSTDR can be used even in the absence of the solar irradiation, which makes it a very powerful fault-detection tool. The proposed PV ground-fault detection technique has been tested in a real-world PV system, and it can confidently detect PV ground faults for different configurations of PV arrays (single and double strings) and fault resistances (0.5, 5, and 10-

Failure modes and effect analysis of module level power electronics

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