Publications

Article 690.11 in the 2011 National Electrical Code® (NEC®) requires new photovoltaic (PV) systems on or penetrating a building to include a listed arc fault protection device. Currently there is little experimental or empirical research into the behavior of the arcing frequencies through PV components despite the potential for modules and other PV components to filter or attenuate arcing signatures that could render the arc detector ineffective. To model AC arcing signal propagation along PV strings, the well-studied DC diode models were found to inadequately capture the behavior of high frequency arcing signals. Instead dynamic equivalent circuit models of PV modules were required to describe the impedance for alternating currents in modules. The nonlinearities present in PV cells resulting from irradiance, temperature, frequency, and bias voltage variations make modeling these systems challenging. Linearized dynamic equivalent circuits were created for multiple PV module manufacturers and module technologies. The equivalent resistances and capacitances for the modules were determined using impedance spectroscopy with no bias voltage and no irradiance. The equivalent circuit model was employed to evaluate modules having irradiance conditions that could not be measured directly with the instrumentation. Although there was a wide range of circuit component values, the complex impedance model does not predict filtering of arc fault frequencies in PV strings for any irradiance level. Experimental results with no irradiance agree with the model and show nearly no attenuation for 1 Hz to 100 kHz input frequencies. © 2011 IEEE.

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This report describes the various methods and circuits that have been developed to detect an islanding condition for photovoltaic applications and presents three methods that have been developed to test those methods and circuits. Passive methods for detecting an islanding condition basically monitor parameters such as voltage and frequency and/or their characteristics and cause the inverter to cease converting power when there is sufficient transition from normal specified conditions. Active methods for detecting the island introduce deliberate changes or disturbances to the connected circuit and then monitor the response to determine if the utility grid with its stable frequency, voltage and impedance is still connected. If the small perturbation is able to affect the parameters of the load connection within prescribed requirements, the active circuit causes the inverter to cease power conversion and delivery of power to the loads. The methods not resident in the inverter are generally controlled by the utility or have communications between the inverter and the utility to affect an inverter shut down when necessary. This report also describes several test methods that may be used for determining whether the anti-islanding method is effective. The test circuits and methodologies used in the U.S. have been chosen to limit the number of tests by measuring the reaction of a single or small number of inverters under a set of consensus-based worst-case conditions.

The National Photovoltaic (PV) Program is sponsored by the US Department of Energy and includes a PV Manufacturing Research and Development (R and D) project conducted with industry. This project includes advancements in PV components to improve reliability, reduce costs, and develop integrated PV systems. Participants submit prototypes, pre-production hardware products, and examples of the resulting final products for a range of tests conducted at several national laboratories, independent testing laboratories, and recognized listing agencies. The purpose of this testing is to use the results to assist industry in determining a product's performance and reliability, and to identify areas for potential improvement. This paper briefly describes the PV Manufacturing R and D project, participants in the area of PV systems, balance of systems, and components, and several examples of the different types of product and performance testing used to support and confirm product performance.

The Photovoltaic Manufacturing Research and Development project is a government/industry partnership between the US Department of Energy and members of the US photovoltaic (TV) industry. The purpose of the project is to work with industry to improve manufacturing processes, reduce manufacturing costs, and improve the performance of PV products. This project is conducted through phased solicitations with industry participants selected through a competitive evaluation process. Starting in 1995, the two most recent solicitations include manufacturing improvements for balance-of-system (BOS) components, energy storage, and PV system design improvements. This paper surveys the work accomplished since that time, as well as BOS work currently in progress in the PV Manufacturing R and D project to identify areas of continued interest and product trends. Industry participants continue to work to improve inverters and to expand the features and capabilities of this key component. The industry also continues to advance fully integrated systems that meet standards for performance and safety. All participants included manufacturing improvements to reduce costs and improve reliability. Accomplishments of the project's participants are summarized to illustrate the product and manufacturing trends.

Photovoltaic (PV) power systems, like other electrical systems, may be subject to unexpected ground faults. Installed PV systems always have invisible elements other than those indicated by their electrical schematics. Stray inductance, capacitance and resistance are distributed throughout the system. Leakage currents associated with the PV modules, the interconnected array, wires, surge protection devices and conduit add up and can become large enough to look like a ground-fault. PV systems are frequently connected to other sources of power or energy storage such as batteries, standby generators, and the utility grid. This complex arrangement of distributed power and energy sources, distributed impedance and proximity to other sources of power requires sensing of ground faults and proper reaction by the ground-fault protection devices. The different dc grounding requirements (country to country) often add more confusion to the situation. This paper discusses the ground-fault issues associated with both the dc and ac side of PV systems and presents test results and operational impacts of backfeeding commercially available ac ground-fault protection devices under various modes of operation. Further, the measured effects of backfeeding the tripped ground-fault devices for periods of time comparable to anti-islanding allowances for utility interconnection of PV inverters in the United States are reported.

The balance-of-system (BOS) of a photovoltaic installation includes the array structure, trackers, ac and dc wiring, overcur-rent protection, disconnects, interconnects, inverters, charge controllers, energy storage and system controllers. The inverter (sometimes called power-conditioning subsystem (PCS), power conditioner or static power converter) is the key electrical power-handling component of a photovoltaic (PV) power system that has, ac loads. This paper will focus on the inverter and its related functions as the critical electrical BOS element in photovoltaic systems. An evolutionary summary for inverter hardware development, primarily in the US, will shed light on the paths that have been taken to arrive at today's state-of-the-technology. Recent developments, integrated packaging and opportunities for practical technology and hardware advancements will be presented. This paper will also touch on elementary battery issues as they relate to inverters and their control functions. Batteries are also critical, but often misunderstood, BOS components in stand- alone systems.

Article 690, Solar Photovoltaic Power Systems, has been in the National Electrical Code (NEC) since 1984. An NFPA-appointed Task Group for Article 690 proposed changes to Article 690 for both the 1996 and 1999 codes. The Task Group, supported by more than 50 professionals from throughout the photovoltaic (PV) industry, met seven times during the 1999 code cycle to integrate the needs of the industry with the needs of electrical inspectors and end users to ensure the safety of PV systems. The Task Group proposed 57 changes to Article 690, and all the changes were accepted in the review process. The performance and cost of PV installations were always a consideration as these changes were formed but safety was the number-one priority. All of the proposals were well substantiated and coordinated throughout the PV industry and with representatives of Underwriters Laboratories, Inc (UL). The most significant changes that were made in Article 690 for the 1999 NEC along with some of the rationale are discussed in the remainder of this article.

The National Electrical Code@ (NEC@) focuses primarily on electrical system installation requirements in the U.S. The NEC addresses both fire and personnel safety. This paper will describe recent efforts of the PV industry in the U.S. and the resulting requirements in the 1999 National Electrical Code-- Article 690 --Solar Photovoltaic Systems. The Article 690 requirements spell out the PV-unique requirements for safe installations of PV systems in the U.S.A. This paper provides an overview of the most significant changes that appear in Article 690 of the 1999 edition of the NEC. The related and coordinated efforts of the other standards- making groups will also be briefly reviewed.

The Photovoltaic Manufacturing Technology (PVMaT) project is a partnership between the US government (through the US Department of Energy [DOE]) and the PV industry. Part of its purpose is to conduct manufacturing technology research and development to address the issues and opportunities identified by industry to advance photovoltaic (PV) systems and components. The project was initiated in 1990 and has been conducted in several phases to support the evolution of PV industrial manufacturing technology. Early phases of the project stressed PV module manufacturing. Starting with Phase 4A and continuing in Phase 5A, the goals were broadened to include improvement of component efficiency, energy storage and manufacturing and system or component integration to bring together all elements for a PV product. This paper summarizes PV manufacturers` accomplishments in components, system integration, and alternative manufacturing methods. Their approaches have resulted in improved hardware and PV system performance, better system compatibility, and new system capabilities. Results include new products such as Underwriters Laboratories (UL)-listed AC PV modules, modular inverters, and advanced inverter designs that use readily available and standard components. Work planned in Phase 5A1 includes integrated residential and commercial roof-top systems, PV systems with energy storage, and 300-Wac to 4-kWac inverters.

In the Phase 2 project, Abacus Controls Inc. did research and development of hybrid systems that combine the energy sources from photovoltaics, batteries, and diesel-generators and demonstrated that they are economically feasible for small power plants in many parts of the world. The Trimode Power Processor reduces the fuel consumption of the diesel-generator to its minimum by presenting itself as the perfect electrical load to the generator. A 30-kW three-phase unit was tested at Sandia National Laboratories to prove its worthiness in actual field conditions. The use of photovoltaics at remote locations where reliability of supply requires a diesel-generator will lower costs to operate by reducing the run time of the diesel generator. The numerous benefits include longer times between maintenance for the diesel engine and better power quality from the generator. 32 figs.

Photovoltaic (PV) modules that provide only ac power give new dimensions to the use of, and utility interface of, PV systems because all of the dc issues are virtually eliminated. These AC PV modules offer the important advantage that customers may now purchase a PV system without hiring a design engineer. A qualified electrician will be able to install a complete PV system that performs as expected and meets local electrical codes. Simple installations of additional AC PV modules will be possible once the proper branch circuit wiring and protection have been installed. Codes and standards are currently being written to address the utility-interconnect issues for AC PV modules and other interactive inverters. An industry-supported Task Group has recently written and submitted proposals for changes to bring Article 690 of the 1999 National Electrical Code{reg_sign} (NEC{reg_sign}) up to the state-of-the-art for PV devices such as AC PV modules. This paper summarizes the proposed code changes and standards related to the evolving AC PV module technology in the United States. Topics such as the need for dedicated branch circuits for AC PV modules in residential applications are discussed and analyzed. Requirements for limiting the number of AC modules on a branch circuit and the listing requirements that make safe installations are discussed. Coordination of all standards activities for AC module installations, the building-integrated perspectives, and utility-interface issues is discussed.

The U.S. National Photovoltaic Program began in 1975 by supporting the development of terrestrial PV modules and hardware associated with grid-connected PV systems. Early PV-system demonstration programs were also supported and cost shared by the U.S. Department of Energy (DOE). A wide variety of PV systems were deployed, usually with utility participation. The early demonstration projects provided, and continue to provide, valuable PV system experience to utilities, designers and suppliers. As a result of experience gained, several important milestones in codes and standards pertaining to the design, installation and operation of photovoltaic (PV) systems have been completed. These code and standard activities were conducted through collaboration of participants from all sectors of the PV industry, utilities and the US DOE National Photovoltaic Program. Codes and standards that have been proposed, written, or modified include changes and additions for the 1999 National Electric Code{reg_sign} (NEC{reg_sign}), standards for fire and personnel safety, system testing, field acceptance, component qualification, and utility interconnection. Project authorization requests with the Institute of Electrical and Electronic Engineers (IEEE) have resulted in standards for component qualification and were further adapted for standards used to list PV modules and balance-of-system components. Industry collaboration with Underwriter Laboratories, Inc., with the American Society for Testing and Materials, and through critical input and review for international standards with the International Electrotechnical Commission have resulted in new and revised domestic and international standards for PV applications. Activities related to work on codes and standards through the International Energy Agency are also being supported by the PV industry and the US DOE. The paper shows relationships between activities in standards writing.