Development of a Methodology for Improving Photovoltaic Inverter Reliability

In evaluating the energy-generation potential of a photovoltaic (PV) energy system, the system is usually assumed to work without interruptions over its entire life. PV energy systems are fairly reliable, but as any complex system, they may fail. In PV systems, the inverter is responsible for the majority of failures, and most inverter failures are blamed on the aluminum electrolytic capacitors typically used in the dc bus. This paper investigates the effects of common failure modes on the reliability of PV inverters and suggests a model framework for decomposing the inverter into subsystems for more detailed study. The challenges of statistical analysis based on small data sets are discussed, and simulations are performed to illustrate the proposed model using a simple decomposition into subsystems of the inverter used in the 342-kW PV system at the Georgia Tech Aquatic Center.     

Determining the relative effectiveness of islanding detectionmethods using phase criteria and nondetection zones

Islanding of a utility-interactive photovoltaic (UIPV) system occurs if the UIPV system continues to power a section of the utility system after that section has been disconnected from the utility source. Since islanding creates hazards for personnel and equipment, UIPV systems are required to detect and prevent it. It is desirable to have a simplified method of determining which islanding detection methods (IDMs) are most effective. In this paper, a previously described method for finding the nondetection zones (NDZs) of IDMs is experimentally verified. This method is used to determine the NDZs of several common IDMs. These results indicate that, of the IDMs discussed in this paper: (1) Sandia Frequency Shift (SFS) is most effective; and (2) the worst-case loads are low-power loads that are near resonance at the line frequency and have a large capacitance and small inductance (a high value of the quality factor Q)     

New method for reactive power and energy measurement

The properties of an electronic reactive power and energy measurement method based on a frequency-controlled power-to-pulse-rate conversion are analyzed. The reactive power and energy measurements can be efficiently performed by using a simple integrating circuit as a phase shifter, and frequency-controlled analog-to-pulse-rate converter. A low value of the output pulse rate should not be considered as an inherent disadvantage of the method. By introducing a phase-locked loop in order to achieve high pulse-rate output, the benefit of high resolution is added to the method. The improved electronic structure for the measurement can be implemented in the design of reactive power and energy standards. For precision var-hour meters based on the proposed method, it is possible to have an efficient error-checking and adjustment procedure. The proposed method allows reactive power and energy measurement to be made in single- and poly-phase, symmetrical and unsymmetrical, and balanced and unbalanced networks     

Nondominated sorting genetic algorithm for optimal phasor measurement placement

This paper considers a phasor measurement unit (PMU) placement problem requiring simultaneous optimization of two conflicting objectives, such as minimization of the number of PMUs and maximization of the measurement redundancy. The objectives are in conflict since the improvement of one of them leads to the deterioration of another. Instead of a unique optimal solution, it exists a set of best tradeoffs between competing objectives, the so-called Pareto-optimal solutions. A specially tailored nondominated sorting genetic algorithm (NSGA) for a PMU placement problem is proposed as a methodology to find these Pareto-optimal solutions. The algorithm is combined with the graph-theoretical procedure and a simple GA to reduce the initial number of the PMU's candidate locations. The NSGA parameters are carefully set by performing a number of trial runs and evaluating the NSGA performances based on the number of distinct Pareto-optimal solutions found in the particular run and distance of the obtained Pareto front from the optimal one. Illustrative results on the 39- and 118-bus IEEE systems are presented.     

The reform of the urban land use system in Eastern Europe: A suggestion for belgrade

This paper provides suggestions to reform the urban land use system in Eastern European cities, using the city of Belgrade as an example. The main features of the current system are examined, as well as their effects on urban development. The suggested reform is based on: (a) privatizing urban land; (b) introducing markets for urban land; and (c) redefining the role of local government. Methods to privatize the land are selected, and general policy issues related to this and to the introduction of a market for urban land are discussed. A framework for government intervention is suggested.An earlier version of this paper was presented at the 31st European Congress of the RSAI, Lisbon, Portugal, August 1991.     

Characterization of lactic acid bacteria isolated from Bukuljac, a homemade goat's milk cheese

The Bukuljac cheese is traditionally homemade cheese, produced from heat-treated goat's milk without the addition of any bacterial starter culture. The presence of lactic acid bacteria (LAB) in Bukuljac cheese has been analyzed by using a polyphasic approach including microbiological and molecular methods such as rep-PCR with (GTG)5 primer. Lactobacillus paracasei subsp. paracasei represents a dominant strain in the microflora of analyzed cheese. Out of 55 Gram-positive and catalase-negative isolates, 48 belonged to L. paracasei subsp. paracasei species. Besides lactobacilli, five Lactococcus lactis subsp. lactis and two Enterococcus faecalis were found. Results of PCR-denaturing gradient gel electrophoresis (DGGE) of DNA extracted directly from the fresh cheese revealed the presence of Leuconostoc mesenteroides. Only lactobacilli showed a high proteolytic activity and hydrolyzed alpha(s1)- and beta-caseins. They are also producers of diacetyl. In addition, 34 out of 55 isolates, all determined as lactobacilli, showed the ability of auto-aggregation. Among 55 isolates, 50 also exhibited antimicrobial activity.     

A fair method for centralized optimization of multi-TSO power systems

This paper addresses the problem of centralized optimization of an interconnected power system partitioned into several regions controlled by different transmission system operators (TSOs). It is assumed that those utilities have agreed to transferring some of their competencies to a centralized control center, which is in charge of setting the control variables in the entire system to satisfy every utility’s individual objective. This paper proposes an objective method for centralized optimization of such multi-TSO power systems, which relies on the assumption that each TSO has a real-valued optimization function focusing on its control area only. This method is illustrated on the IEEE 118 bus system partitioned into three TSOs. It is applied to the optimal reactive power dispatch problem, where the control variables are the voltage settings for generators and compensators. After showing that the method has some properties of fairness, namely freedom from envy, efficiency, accountability, and altruism, we emphasize its robustness with respect to certain biased behavior of the different TSOs.     

A senior design course bridging power systems and powerelectronics: power quality impacts of electric vehicle charging

A new capstone design course, which combines elements of power electronics and power systems analysis, has been introduced at Georgia Tech as part of the newly revised curriculum. The paper explains the relationship of the new course with respect to the rest of the power curriculum, and how the course material and teaching style respond to various ABET requirements for an integrated design experience     

Green power: status and perspectives

The main objective of the so-called green power marketing is to provide selective customers of electric energy with choices to purchase electric energy from sustainable, environmentally friendly sources. This paper presents a rationale for marketing of green power provides a brief overview of some of the existing efforts in the United States and abroad, and attempts to identify the conditions and future trends for survival of renewable energy in the energy marketplace. Our goal is primarily to provide a representative sample of the current technology status, rather than a comprehensive coverage of worldwide initiatives in this area     

Control of voltage stability using sensitivity analysis

The effects of static compensation on the voltage stability boundary are investigated. For a class of voltage instabilities which correspond to static bifurcations of load flow equations, minimum singular values of the Jacobian matrix and total generated reactive power are calculated as indicators of stability margin, and sensitivity methods are used for reactive support allocation. Improvement in stability margin under progressive loading was investigated on a 39-bus test system for different allocations and amounts of reactive support with reactive generation capabilities taken into account