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Author

Ward Bower

Bio: Ward Bower is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: Photovoltaic system & Grid-connected photovoltaic power system. The author has an hindex of 13, co-authored 41 publications receiving 970 citations.

Papers
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ReportDOI
01 Nov 2002
TL;DR: In this paper, the authors describe the various methods and circuits that have been developed to detect an islanding condition for photovoltaic applications, and present three methods to test those methods and circuit.
Abstract: 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 chosenmore » 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.« less

384 citations

Proceedings ArticleDOI
19 Jun 2011
TL;DR: The Distributed Energy Technologies Laboratory 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 DC AFCIs and Arc Fault Detectors (AFDs).
Abstract: 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 device's 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.

92 citations

Proceedings ArticleDOI
07 May 2006
TL;DR: The innovative micro-inverter addressed in this paper uses a unique proprietary circuit topology to reduce the numbers and sizes of capacitors, is currently rated at 150 W, and has been shown to be thermally robust and feasible.
Abstract: An extremely reliable micro-inverter is critical to the success of the AC PV Building Block and the AC PV Module concepts. [1,2,3,4] An innovative inverter design has been developed and prototyped in order to address some of the most critical issues associated with extending the mean time between failure (MTBF) and the total lifetime of the micro-inverter when it is integrated onto a PV module. The innovative micro-inverter addressed in this paper uses a unique proprietary circuit topology to reduce the numbers and sizes of capacitors, is currently rated at 150W, and has been shown to be thermally robust and feasible. Additionally, the smaller capacitors used in the design use advanced technology exhibiting much longer lifetime (30-year) in the anticipated thermal environment. The unconventional micro-inverter design considerations, layout, early findings from modeling and evaluation results are presented. Preliminary findings for a proposed redesign to increase the rating to 300W are presented. An initial assessment of a reliability requirement study for micro-inverters is also discussed. [5]

70 citations

Proceedings ArticleDOI
07 May 2006
TL;DR: In this work, a PLCC-based anti-islanding system that works with a commercially-available automatic meter reading system is explored and its effectiveness is discussed and conclusions about this method are offered.
Abstract: Many methods for detecting and preventing islanding of photovoltaic and other distributed energy resources (DERs) have been proposed. However, in general, all anti-islanding systems that are based in the inverter have a "non-detection zone" (NDZ), which is a range of loads that defeat that anti-islanding mechanism. One alternative is to use power line carrier communications (PLCC). In this work, a PLCC-based anti-islanding system that works with a commercially-available automatic meter reading system is explored. Initial experimental results on this PLCC anti-islanding system are presented. The effectiveness of PLCC-based islanding prevention is discussed, and conclusions about this method are offered.

58 citations

Journal ArticleDOI
TL;DR: The paths that have been taken to arrive at today's inverter technology are summarized and developments in integrated hardware, developments in packaging, advancements in manufacturing, and opportunities for the new millennium are presented.
Abstract: The balance-of-system (BOS) components in a photovoltaic (PV) installation include the array structure, trackers, ac and dc wiring, overcurrent 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 power system that is attached to ac loads. This paper focuses on the inverter and its related functions as the critical electrical BOS element in a photovoltaic system. The paths that have been taken to arrive at today's inverter technology are summarized and developments in integrated hardware, advancements in packaging, advancements in manufacturing, and opportunities for the new millennium are presented.

41 citations


Cited by
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01 Aug 2001
TL;DR: The study of distributed systems which bring to life the vision of ubiquitous computing systems, also known as ambient intelligence, is concentrated on in this work.
Abstract: With digital equipment becoming increasingly networked, either on wired or wireless networks, for personal and professional use alike, distributed software systems have become a crucial element in information and communications technologies. The study of these systems forms the core of the ARLES' work, which is specifically concerned with defining new system software architectures, based on the use of emerging networking technologies. In this context, we concentrate on the study of distributed systems which bring to life the vision of ubiquitous computing systems, also known as ambient intelligence.

2,774 citations

Journal ArticleDOI
TL;DR: In this paper, a topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications is presented, where the MIC topologies are classified into three different arrangements based on the dc link configurations.
Abstract: The annual world photovoltaic (PV) cell/module production is growing at almost an exponential rate and has reached 1727 MW in 2005. Building integrated PV (BIPV) projects are emerging as the strongest part of the PV market and grid interactive inverters are a key component in determining the total system cost. Module integrated converter (MIC) technology has become a global trend in grid interactive PV applications and may assist in driving down the balance of system costs to secure an improved total system cost. This paper concentrates on the topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications. The MIC topologies are classified into three different arrangements based on the dc link configurations. A systematic discussion is also provided at the end of the paper that focuses on the major advantages and disadvantages of each MIC arrangement. These are considered in detail and will provide a useful framework and point of reference for the next generation MIC designs and applications.

1,158 citations

Journal ArticleDOI
TL;DR: In this paper, the importance of grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance is investigated and emphasized.
Abstract: Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.

923 citations

Journal ArticleDOI
TL;DR: It is shown that versatile stand-alone photovoltaic (PV) systems still demand on at least one battery inverter with improved characteristics of robustness and efficiency, which can be achieved using multilevel topologies.
Abstract: This paper shows that versatile stand-alone photovoltaic (PV) systems still demand on at least one battery inverter with improved characteristics of robustness and efficiency, which can be achieved using multilevel topologies. A compilation of the most common topologies of multilevel converters is presented, and it shows which ones are best suitable to implement inverters for stand-alone applications in the range of a few kilowatts. As an example, a prototype of 3 kVA was implemented, and peak efficiency of 96.0% was achieved.

593 citations

Journal ArticleDOI
TL;DR: A new high-efficiency topology for transformerless systems is proposed, which does not generate common-mode currents and topologically guarantees that no dc is injected into the grid and has been verified in a 5-kW prototype with satisfactory results.
Abstract: The elimination of the output transformer from grid- connected photovoltaic (PV) systems not only reduces the cost, size, and weight of the conversion stage but also increases the system overall efficiency. However, if the transformer is removed, the galvanic isolation between the PV generator and the grid is lost. This may cause safety hazards in the event of ground faults. In addition, the circulation of leakage currents (common-mode currents) through the stray capacitance between the PV array and the ground would be enabled. Furthermore, when no transformer is used, the inverter could inject direct current (dc) to the grid, causing the saturation of the transformers along the distribution network. While safety requirements in transformerless systems can be met by means of external elements, leakage currents and the injection of dc into the grid must be guaranteed topologically or by the inverter's control system. This paper proposes a new high-efficiency topology for transformerless systems, which does not generate common-mode currents and topologically guarantees that no dc is injected into the grid. The proposed topology has been verified in a 5-kW prototype with satisfactory results.

561 citations