A flexible AC distribution system device for a microgrid

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Learning to read and spell english words by Chinese students

This study examined the structural relationships between (a) the latent independent constructs of orthographic and lexical knowledge and phonological sensitivity and (b) the effect of these constructs on the latent construct of literacy manifested by reading aloud and spelling regular and exception English words in 156 Cantonese-speaking Chinese students (M age = 10.8 years) who were learning English as a second language in Hong Kong. Three carefully designed and item-analyzed indicators subserved the construct of orthographic and lexical knowledge, and another three indicators subserved the construct of phonological sensitivity. Our hypothesis of greater contribution of word-specific orthographic and lexical knowledge than phonological sensitivity to learning to read and spell English words in these Chinese children was supported by results from multiple regression, principal component analyses and especially by structural equation modeling. The various goodness-of-fit indexes showed the appropriateness of the indicators in measuring the latent constructs as well as the relationships among these constructs.published_or_final_versio

Low-power wind energy conversion system with variable structure control for DC grids

This paper presents a discussion on the use of variable structure control, i.e., sliding mode control, for improving the dynamic control performance of a low-power wind energy conversion system (WECS) that is connected to a DC microgrid. The sliding mode control is applied to the wind turbine system to extract the maximum possible power from the wind, thus achieving the state of maximum power point tracking to reach the maximum power generation (MPG), and also applied to the power converter to reach the maximum power injection (MPI) to the load. The amount of energy extractable from a dynamically changing wind using the WECS with sliding mode control is compared with that of the classic PI controller. Simulation results show that for a dynamically changing wind, more energy can be harvested with the sliding mode control as compared to the PI control. © 2014 IEEE.published_or_final_versio

Nonlinear Dynamic Power Tracking of Low-Power Wind Energy Conversion System

This paper addresses the use of variable structure control (i.e., sliding mode (SM) control) for improving the dynamic performance of a low-power wind energy conversion system (WECS) that is connected to a dc grid. The SM control is applied to simultaneously match 1) the maximum power generation of the wind turbine system from the wind with 2) the maximum power injection of the grid-connected power converter into the grid. The amount of energy extractable from a dynamically changing wind using the WECS with SM control is compared with that of classic PI control. Both the simulation and experimental results show that more energy can be harvested with the SM control as compared to the PI control for any dynamically changing or random wind conditions

DC electric springs: an emerging technology for DC grids

There is widespread attention on integrating renewable energy sources, such as the solar power, to DC distributed power systems and DC microgrids. The voltage stability and the power quality issues are of concern if a large proportion of power sources in these DC power systems are generated by intermittent renewable energy sources. This paper presents an electric active suspension technology known as the DC electric springs for stabilizing and improving the quality of the power distributions in DC power grids. The basic operating modes and characteristic of a DC electric spring under different types of serially-connected non-critical loads will first be introduced. Then, various potential issues that affect the power quality of the DC power systems, namely the bus voltage instability, voltage droop, system fault, and harmonics, are briefly addressed. Laboratory-scale experiments validated that the aforementioned quality issues can be mitigated using the proposed DC electric spring technology. © IEEE.published_or_final_versio

Coordinated control and energy management of distributed generation inverters in a microgrid

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Smart distribution network device for microgrid applications

This paper focuses on the design of a smart distribution network device for improving the power quality and reliability of the overall power distribution system that the microgrid is connected to. The control design employs a newly developed Model Predictive Control algorithm which optimizes the steady-state and the transient control problems separately to achieve a faster computational time for large power systems. Kalman filters are also employed as state observers to identify the required signals needed for monitoring purposes as well as to extract the harmonic spectra of the grid voltage and the load currents. The design concept is verified through different test case scenarios to demonstrate the capability of the proposed device and the results obtained are discussed. © 2012 IEEE.published_or_final_versio

Endoscopic biliary drainage for severe acute cholangitis

Background. Emergency surgery for patients with severe acute cholangitis due to choledocholithiasis is associated with substantial morbidity and mortality. Because recent results suggested that emergency endoscopic drainage could improve the outcome of such patients, we undertook a prospective study to determine the role of this procedure as initial treatment. Methods. During a 43-month period, 82 patients with severe acute cholangitis due to choledocholithiasis were randomly assigned to undergo surgical decompression of the biliary tract (41 patients) or endoscopic biliary drainage (41 patients), followed by definitive treatment. Hospital mortality was analyzed with respect to the use of endoscopic biliary drainage and other clinical and laboratory findings. Prognostic determinants were studied by linear discriminant analysis. Results. Complications related to biliary tract decompression and subsequent definitive treatment developed in 14 patients treated with endoscopic biliary drainage and 27 treated with surgery (34 vs. 66 percent, P>0.05). The time required for normalization of temperature and stabilization of blood pressure was similar in the two groups, but more patients in the surgery group required ventilatory support. The hospital mortality rate was significantly lower for the patients who underwent endoscopy (4 deaths) than for those treated surgically (13 deaths) (10 vs. 32 percent, P<0.03). The presence of concomitant medical problems, a low platelet count, a high serum urea nitrogen concentration, and a low serum albumin concentration before biliary decompression were the other independent determinants of mortality in both groups. Conclusions. Endoscopic biliary drainage is a safe and effective measure for the initial control of severe acute cholangitis due to choledocholithiasis and to reduce the mortality associated with the condition.published_or_final_versio

Mining and Analyzing the Italian Parliament: Party Structure and Evolution

The roll calls of the Italian Parliament in the XVI legislature are studied by employing multidimensional scaling, hierarchical clustering, and network analysis. In order to detect changes in voting behavior, the roll calls have been divided in seven periods of six months each. All the methods employed pointed out an increasing fragmentation of the political parties endorsing the previous government that culminated in its downfall. By using the concept of modularity at different resolution levels, we identify the community structure of Parliament and its evolution in each of the considered time periods. The analysis performed revealed as a valuable tool in detecting trends and drifts of Parliamentarians. It showed its effectiveness at identifying political parties and at providing insights on the temporal evolution of groups and their cohesiveness, without having at disposal any knowledge about political membership of Representatives.Comment: 27 pages, 14 figure