Armando Fresquez

Differentiating series and parallel photovoltaic arc-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) 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.

Crosstalk nuisance trip testing of photovoltaic DC arc-fault detectors

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 noise-typically used for arc-fault detection-propagates 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 string-but in some cases the AFCI on both strings would trip.

PV inverter performance and reliability: What is the role of the IGBT?

The inverter is still considered the weakest link in modern photovoltaic systems. Inverter failure can be classified into three major categories: manufacturing and quality control problems, inadequate design, and electrical component failure. It is often difficult to deconvolve the latter two of these, as electrical components can fail due to inadequate design or as a result of intrinsic defects. The aim of the current work is to utilize the extensive background in both inverter performance testing and component reliability found at Sandia National Laboratories to assess the role of component failures in PV performance and reliability. Although there is no consensus on the least reliable component in a modern inverter system, the IGBT is often blamed for failures and hence this was the first component we studied. A commercially available 600V, 60A, silicon IGBT found in common residential inverters was evaluated under normal and extreme operating conditions with DC and pulsed biasing schemes. Although most of the sample devices were robust even under extreme conditions, a few of the samples failed during operation well within the manufacturer-specified limits. Additionally, we have begun in situ monitoring of IGBTs as well as other components within an operating 700 W, single-phase inverter. The in situ testing will guide future device-level work since it allows us to understand the conditions that are experienced by inverter components in a realistic operating environment.

Series and Parallel Arc-Fault Circuit Interrupter Tests

While the 2011 National Electrical Code%C2%AE (NEC) only requires series arc-fault protection, some arc-fault circuit interrupter (AFCI) manufacturers are designing products to detect and mitigate both series and parallel arc-faults. Sandia National Laboratories (SNL) has extensively investigated the electrical differences of series and parallel arc-faults and has offered possible classification and mitigation solutions. As part of this effort, Sandia National Laboratories has collaborated with MidNite Solar to create and test a 24-string combiner box with an AFCI which detects, differentiates, and de-energizes series and parallel arc-faults. In the case of the MidNite AFCI prototype, series arc-faults are mitigated by opening the PV strings, whereas parallel arc-faults are mitigated by shorting the array. A range of different experimental series and parallel arc-fault tests with the MidNite combiner box were performed at the Distributed Energy Technologies Laboratory (DETL) at SNL in Albuquerque, NM. In all the tests, the prototype de-energized the arc-faults in the time period required by the arc-fault circuit interrupt testing standard, UL 1699B. The experimental tests confirm series and parallel arc-faults can be successfully mitigated with a combiner box-integrated solution.

Characterizing fire danger from low-power photovoltaic arc-faults

While arc-faults are rare in photovoltaic installations, more than a dozen documented arc-faults have led to fires and resulted in significant damage to the PV system and surrounding structures. In the United States, National Electrical Code® (NEC) 690.11 requires a listed arc fault protection device on new PV systems. In order to list new arc-fault circuit interrupters (AFCIs), Underwriters Laboratories created the certification outline of investigation UL 1699B. The outline only requires AFCI devices to be tested at arc powers between 300-900 W; however, arcs of much less power are capable of creating fires in PV systems. In this work we investigate the characteristics of low power (100-300 W) arc-faults to determine the potential for fires, appropriate AFCI trip times, and the characteristics of the pyrolyzation process. This analysis was performed with experimental tests of arc-faults in close proximity to three polymer materials common in PV systems, e.g., polycarbonate, PET, and nylon 6,6. Two polymer geometries were tested to vary the presence of oxygen in the DC arc plasma. The samples were also exposed to arcs generated with different material geometries, arc power levels, and discharge times to identify ignition times. To better understand the burn characteristics of different polymers in PV systems, thermal decomposition of the sheath materials was performed using infrared spectra analysis. Overall a trip time of less than 2 seconds is recommended for the suppression of fire ignition during arc-fault events.

Determining the effect of temperature on microinverter inversion efficiency

Sandia National Laboratories is working to develop a set of test procedures and characterization models which may be used to describe the performance of AC modules and serve as a basis of product comparison. However, in measuring the module/microinverter system output, it is difficult to determine the effects of correlated parameters on system performance. In particular, we have found that the module temperature and the inverter temperature are highly correlated when the system is operating, and thus it is difficult to separate their effects on system power output. In 2014, we have conducted temperature testing on microinverters and we will show that the effect of temperature on microinverter performance is sufficiently small to justify omitting it from the final characterization model for AC modules.

Performance of utility interconnected photovoltaic inverters operating beyond typical modes of operation

The high penetration of utility interconnected photovoltaic (PV) inverters can affect the utility at the point of common coupling. Todays utility interconnection standards are evolving to allow voltage and frequency support, and voltage and frequency ride-through capability. With multi-MW-sized PV plants and multitudes of small commercial and residential systems coming online each year, the interconnection standards are allowing distributed energy resource equipment to provide reactive power to supplement existing voltage-regulating devices and ride-through voltage and frequency anomalies. These new interconnection requirements, coupled with the high dc-to-ac ratios, are becoming more common with declining PV module costs and are changing the modes of operation for utility-interconnected PV systems. This report investigates the effects these modes of operation have on the inverter performance, array utilization, and power quality while focusing on conversion efficiency.

Accelerated life testing of PV arc-fault detectors

As of 2011, the National Electrical Code® (NEC) has required arc-fault circuit interrupters (AFCIs) to be incorporated into photovoltaic (PV) systems to prevent fires. Some manufacturers are designing AFCIs to consist of arc-fault detectors (AFD) incorporated into inverters or combiner boxes in order to take advantage of the DC switching functionality of the existing hardware. Since AFCIs and AFDs are safety devices, it is critical to ensure the long-term functionality of AFD devices in these harsh environments. Sandia National Laboratories (SNL) has performed accelerated life tests on 10 arc-fault detectors. The devices were tested after being subjected to the thermal damage equivalent of 1.7-year increments in an inverter until 77.6 equivalent years of solder fatigue damage. 30% of the boards experienced component failures but there were no solder failures, indicating solder fatigue is not the primary failure mode. Based on these results, Sandia recommends an appropriate burn-in process be used for production of arc-fault prevention devices.