Ward Bower

Innovative PV Micro-Inverter Topology Eliminates Electrolytic Capacitors for Longer Lifetime

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 150 W, 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 proposed redesign to increase the rating to 300 W are presented. An initial assessment of a reliability requirement study for micro-inverters is also discussed

Photovoltaic DC Arc Fault Detector testing at Sandia National Laboratories

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 devices 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.

Array Performance Characterization and Modeling for Real-Time Performance Analysis of Photovoltaic Systems

Improvements in the methods used for photovoltaic (PV) system design, performance rating, and long-term monitoring are needed by the rapidly growing industry, as well as by the U.S. Department of Energy in evaluating progress by solar technology development initiatives. This paper describes an improved model for rating and monitoring PV array performance, discusses initial results from an outdoor laboratory designed to assist industry in optimizing system components and integration, and provides a brief discussion of the system performance metrics currently being used by the PV community

Inverters—critical photovoltaic balance-of-system components: status, issues, and new-millennium opportunities†

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 todays inverter technology are summarized and developments in integrated hardware, advancements in packaging, advancements in manufacturing, and opportunities for the new millennium are presented.

Investigation of ground-fault protection devices for photovoltaic power system applications

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.

Simulation and Experimental Study of the Impedance Detection Anti-Islanding Method in the Single-Inverter Case

The impedance detection method of islanding prevention is often touted as one that possesses no nondetection zone in the single-inverter case. However, simulation results and experiments indicate that this is not necessarily true; the effectiveness of impedance detection is highly dependent on the specific implementation of the method. This paper discusses a simulation and experimental study to examine the origins of this nondetection zone, and a simple modification that appears to significantly reduce the size of the nondetection zone

Analysis of grounded and ungrounded photovoltaic systems

Grounding has always been a subject of controversy during discussions of electrical systems. Grounding techniques and requirements, like language, vary from region to region and country to country. Optimized grounding for personnel protection does not optimize the fire safety of a system and grounding for fire safety does not optimize personnel safety. Grounding to provide protection for equipment requires a third set of requirements. Photovoltaic (PV) power systems are current sources and require different grounding techniques than conventional voltage sources. Distributed leakage paths, multiple fault paths, and new roles for fuses and circuit breakers are among a few of the new issues that need careful consideration. This paper presents and analyzes the grounding issues associated with PV energy sources. Grounding configurations, faults, personnel safety, fire safety, and surge protection are addressed.

Creating dynamic equivalent PV circuit models with impedance spectroscopy for arc fault modeling

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.

Certification of photovoltaic inverters: the initial step toward PV system certification

There is no complete photovoltaic product (component or system) certification program in effect today in the United States. Photovoltaic (PV) modules and inverters are listed for safety (using standards UL1703 and UL1741, respectively), and certification for environmental qualification of PV modules is conducted. However these do not provide critical performance information such as PV module energy rating, inverter performance characteristics, or system performance. Domestic and international standards organizations have begun waiting requirements for photovoltaic system certification that are aimed primarily at small stand-alone applications. The module and balance-of-system industries often provide inconsistent or insufficient specifications and data to designers and customers to allow adequate comparison or a true prediction of performance for installed systems. This paper describes an industry consensus process to establish necessary testing protocols for certification of inverters.

Balance-of-system improvements for photovoltaic applications resulting from the PVMaT Phase 4A1 program

The Photovoltaic Manufacturing Technology Program (PVMaT) began in 1990 as a cost-shared partnership among the photovoltaic industry and the Unites States National Photovoltaic Program. It has been conducted in several phases that were staggered to support technology evolution in the industry. Phase 4A goals broadened the scope of PVMaT resulting in a proposal solicitation that was divided into two parts: (1) Phase 4A1-product-driven system and component technology-with goals to improve system integration, improve component efficiency, and support manufacturing and system or component integration to bring together all elements for a PV product; and (2) Phase 4A2-product-driven PV module manufacturing technology-which addressed PV cell and module manufacturing to maximize the flexibility of applications and to reduce costs for PV products. Of the thirteen awards made in Phase 4A, eight were in 4A1. Developments through these subcontracts include advanced system integration, new and innovative inverter products for a broad range of PV applications and product modifications intended to result in improved reliability and reduced manufacturing costs. This paper summarizes the research, development and progress under phase 4A1.