Large Photovoltaic Arrays Research Articles

A fault diagnosis system for PV power station based on global partitioned gradually approximation method

As the solar photovoltaic (PV) power is applied extensively, more attentions are paid to the maintenance and fault diagnosis of PV power plants. Based on analysis of the structure of PV power station, the global partitioned gradually approximation method is proposed as a fault diagnosis algorithm to determine and locate the fault of PV panels. The PV array is divided into 16x16 blocks and numbered. On the basis of modularly processing of the PV array, the current values of each block are analyzed. The mean current value of each block is used for calculating the fault weigh factor. The fault threshold is defined to determine the fault, and the shade is considered to reduce the probability of misjudgments. A fault diagnosis system is designed and implemented with LabVIEW. And it has some functions including the data realtime display, online check, statistics, real-time prediction and fault diagnosis. Through the data from PV plants, the algorithm is verified. The results show that the fault diagnosis results are accurate, and the system works well. The validity and the possibility of the system are verified by the results as well. The developed system will be benefit for the maintenance and management of large scale PV array.

Power Optimization for Photovoltaic Microconverters Using Multivariable Newton-Based Extremum Seeking

Extremum seeking (ES) is a real-time optimization technique that has been applied to maximum power point tracking (MPPT) design for photovoltaic (PV) microconverter systems, where each PV module is coupled with its own dc/dc converter. Most of the existing MPPT designs are scalar, i.e., employ one MPPT loop around each converter, and all designs, whether scalar or mutivariable, are gradient based. The convergence rate of gradient-based designs depends on the Hessian, which in turn is dependent on environmental conditions, such as irradiance and temperature. Therefore, when applied to large PV arrays, the variability in environmental conditions and/or PV module degradation results in nonuniform transients in the convergence to the maximum power point (MPP). Using a multivariable gradient-based ES algorithm for the entire system instead of a scalar one for each PV module, while decreasing the sensitivity to the Hessian, does not eliminate this dependence. We present a recently developed Newton-based ES algorithm that simultaneously employs estimates of the gradient and Hessian in the peak power tracking. The convergence rate of such a design to the MPP is independent of the Hessian, with tunable transient performance that is independent of environmental conditions. We present simulation as well as the experimental results that show the effectiveness of the proposed algorithm in comparison with the existing scalar designs, and also to multivariable gradient-based ES.

Temperature recovery from degenerated infrared image based on the principle for temperature measurement using infrared sensor

Thermal camera has been applied in photovoltaic (PV) array monitoring to detect defects and hot spots. However, the infrared (IR) image is often degenerated and the temperature information displayed from the image can lead to erroneous interpretation. Hot spots will be obscured and a gradual change phenomenon will emerge when monitoring a large PV array. In this paper, the mechanism of IR image degeneration and gradual change phenomenon are studied and verified with experiments. The variations of atmospheric transmission and directional emissivity have been identified to be the cause of image degeneration. The sensitivity of atmospheric transmission and directional emissivity are defined to analyze the impact of these two factors on the temperature displayed. Based on this mechanism, a recovery method has been proposed to recover the real temperature from the degenerated IR image. Experiments have been conducted to test the effectiveness of the recovery method. In addition, the temperature sensitivity has been defined to analyze how atmospheric transmission and directional emissivity will affect the temperature difference displayed in the IR image. The proposed temperature sensitivity has been used as the criteria to assess the quality of IR image. Some rules of thumb are proposed to deploy the thermal camera in order to increase the quality of the IR image.

Enhanced Particle Swarm Optimization-Based Feeder Reconfiguration Considering Uncertain Large Photovoltaic Powers and Demands

The Kyoto protocol recommended that industrialized countries limit their green gas emissions in 2012 to 5.2% below 1990 levels. Photovoltaic (PV) arrays provide clear and sustainable renewable energy to electric power systems. Solar PV arrays can be installed in distribution systems of rural and urban areas, as opposed to wind-turbine generators, which cause noise in surrounding environments. However, a large PV array (several MW) may incur several operation problems, for example, low power quality and reverse power. This work presents a novel method to reconfigure the distribution feeders in order to prevent the injection of reverse power into a substation connected to the transmission level. Moreover, a two-stage algorithm is developed, in which the uncertain bus loads and PV powers are clustered by fuzzy-c-means to gain representative scenarios; optimal reconfiguration is then achieved by a novel mean-variance-based particle swarm optimization. The system loss is minimized while the operational constraints, including reverse power and voltage variation, are satisfied due to the optimal feeder reconfiguration. Simulation results obtained from a 70-bus distribution system with 4 large PV arrays validate the proposed method.

Bird use of solar photovoltaic installations at US airports: Implications for aviation safety

Several airports in the US have recently installed large photovoltaic (PV) arrays near air-operations areas to offset energy demands, and the US Federal Aviation Administration has published guidelines for new solar installations on airport properties. Although an increased reliance on solar energy will likely benefit airports from environmental and economic perspectives, bird use of solar installations should be examined before wide-scale implementation to determine whether such changes in land use adversely affect aviation safety by increasing risk of bird-aircraft collisions. We studied bird use of five pairs of PV arrays and nearby airport grasslands in Arizona, Colorado, and Ohio, over one year. Across locations, we observed 46 species of birds in airfield grasslands compared to 37 species in PV arrays. We calculated a bird hazard index (BHI) based on the mean seasonal mass of birds per area surveyed. General linear model analysis indicated that BHI was influenced by season, with higher BHI in summer than fall and winter. We found no effect of treatment (PV arrays vs. airfields), location, or interactions among predictors. However, using a nonparametric two-group test across all seasons and locations, we found greater BHI in airfield grasslands than PV arrays for those species considered especially hazardous to aircraft (species≥1.125kg). Our results suggest that converting airport grasslands to PV arrays would not increase hazards associated with bird-aircraft collisions.

Study on the Characteristics of Photovoltaic Large Array Based on the Double Diode Model

Research on the mathematical model and output characteristics of a large photovoltaic array has important significance for optimal allocation and maximum power point tracking. Based on the double-diode model of photovoltaic cells, this paper deduced the mathematical model of a large photovoltaic array and made the improvement to the model and parameter optimization. According to the principle of the maximum power point matching, the large photovoltaic array MATLAB simulation mode of the improved double diode was established by use of the Newton - Ravson algorithm to determine the values of Rs and Rp. The model has the advantages of simple structure, fast operation, and the output characteristics for various light irradiation and temperature levels could be simulated exactly only by use of some parameters from photovoltaic cell production businesses. The output current data of PV module under different temperature and illumination were measured by experiment. Compared with the simulation model, it is concluded that the precision of the model in low light and high temperature condition is better than that of the single diode model, and can be applied to various types of large photovoltaic array..

A simplified model of uniform shading in large photovoltaic arrays

This work presents a novel analytical approximation of the effect of inter-row shading on large photovoltaic (PV) arrays. Computation time is reduced orders of magnitude relative to full numerical simulations, allowing this method to be used in annual production estimation software – for instance, it is currently implemented in the National Renewable Energy Laboratory’s System Advisor Model program. A further advantage of the analytical approach is that, unlike numerical simulations, computation time does not increase with the size of the PV installation. Comparisons with full I–V curve simulations indicate that this simplified approach has typical error of 1% over multiple module fill factor, shade extent and shade opacity assumptions. Maximum error of 2-6% was found for simulations of crystalline silicon modules. Comparisons with experimental results show good agreement between the experiment and the model over a range of operating conditions, and intercomparison with prior modeling methods indicates a spread of possible model results, depending on model assumptions.

Are Manufacturing $I$– $V$ Mismatch and Reverse Currents Key Factors in Large Photovoltaic Arrays?

In this paper, two factors typical of large photovoltaic (PV) arrays are investigated: one is the current-voltage (I-V) mismatch consequent to the production tolerance; the other is the impact of reverse currents in different operating conditions. Concerning the manufacturing I-V mismatch, the parameters of the equivalent circuit of the solar cell are computed for several PV modules from flash reports provided by the manufacturers. The corresponding I-V characteristic of every module is used to evaluate the behavior of different strings and the interaction among the strings connected for composing PV arrays. Two real crystalline silicon PV systems of 8 times 250 kW and 20 kW are studied, respectively. The simulation results reveal that the impact of the I-V mismatch is negligible with the usual tolerance, and the insertion of the blocking diodes against reverse currents can be avoided with crystalline silicon technology. On the other hand, the experimental results on I-V characteristics of the aforementioned arrays put into evidence the existence of a remarkable power deviation (3%-4%) with respect to the rated power, linkable to the lack of measurement uncertainty in the manufacturer flash reports.

Maximum Power Point Tracking Scheme for PV Systems Operating Under Partially Shaded Conditions

Current-voltage and power-voltage characteristics of large photovoltaic (PV) arrays under partially shaded conditions are characterized by multiple steps and peaks. This makes the tracking of the actual maximum power point (MPP) [global peak (GP)] a difficult task. In addition, most of the existing schemes are unable to extract maximum power from the PV array under these conditions. This paper proposes a novel algorithm to track the global power peak under partially shaded conditions. The formulation of the algorithm is based on several critical observations made out of an extensive study of the PV characteristics and the behavior of the global and local peaks under partially shaded conditions. The proposed algorithm works in conjunction with a DC-DC converter to track the GP. In order to accelerate the tracking speed, a feedforward control scheme for operating the DC-DC converter is also proposed, which uses the reference voltage information from the tracking algorithm to shift the operation toward the MPP. The tracking time with this controller is about one-tenth as compared to a conventional controller. All the observations and conclusions, including simulation and experimental results, are presented.

Development of a suitable model for characterizing photovoltaic arrays with shaded solar cells

The aim of this study is to investigate the effects of non-uniform solar irradiation distribution on energy output of different interconnected configurations in photovoltaic (PV) arrays. In order to find which configuration is less susceptible to mismatch effects, a PV module model is developed. This model can take into consideration the effects of bypass diodes and the variation of the equivalent circuit parameters with respect to operating conditions. The proposed model can provide sufficient degree of precision as well as solar cell-based analysis in analyzing large scale PV arrays without increasing the computational effort. In order to produce more reliable and robust simulations, improved and extended algorithms are presented. Some results are discussed in detail and some recommendations are extracted by testing several shading scenarios.

Sun tracking by peak power positioning for photovoltaic concentrator arrays

This paper details the design, development, and evaluation of a microcomputer-based solar tracking and control system (TACS) capable of maintaining the peak power position of a photovoltaic (PV) array by adjusting the load on the array for maximum efficiency and changing the position of the array relative to the sun. At large PV array system installations, inverters are used to convert the dc electrical output to ac for power grid compatibility. Adjustment of the inverter or load for maximum array output is one function performed by the tracking and control system. Another important function of the system is the tracking of the sun, often a necessity for concentrating arrays. The TACS also minimizes several other problems associated with conventional shadow-band sun trackers such as their susceptibility to dust and dirt that may cause drift in solar alignment. It also minimizes effects of structural warpage or sag to which large arrays may be subject during the day. Array positioning is controlled by a single-board computer used with a specially designed input/output board. An orderly method of stepped movements and the finding of new peak power points is implemented. This maximum power positioning concept was tested using a small two-axis tracking concentrator array. A real-time profile of the TACS activity was produced and the data analysis shows a deviation in maximum power of less than 1% during the day after accounting for other variations.

A Novel Square Fresnel Lens Design for Illuminating Circular Solar Cells

A novel square Fresnel lens is described which illuminates a circular solar cell. The square lens can be close-packed in a large photovoltaic array to help minimize system cost. The novel design is accomplished by forming the Fresnel facets in the corners of the lens with different focal lengths than the central portion of the lens. Acrylic lenses were compression molded from a master cut according to the design and were determined to be 79.4% efficient in the solar cell spectral response range.