A Quantile‐based Test for Symmetry of Weakly Dependent Processes

This article considers the problem of testing for symmetry of the marginal distribution of weakly dependent, stationary random processes. A quantile‐based test for symmetry is proposed, which is easy to implement, requires no moment assumptions and has a standard asymptotic distribution. The finite‐sample properties of the test are assessed by means of Monte Carlo experiments. An application to financial time series is also discussed.     

ON THE DETERMINATION OF THE NUMBER OF REGIMES IN MARKOV-SWITCHING AUTOREGRESSIVE MODELS

Abstract. Dynamic models with parameters that are allowed to depend on the state of a hidden Markov chain have become a popular tool for modelling time series subject to changes in regime. An important question that arises in applications involving such models is how to determine the number of states required for the model to be an adequate characterization of the observed data. In this paper, we investigate the properties of alternative procedures that can be used to determine the state dimension of a Markov-switching autoregressive model. These include procedures that exploit the ARMA representation which Markov-switching processes admit, as well as procedures that are based on optimization of complexity-penalized likelihood measures. Our Monte Carlo analysis reveals that such procedures estimate the state dimension correctly, provided that the parameter changes are not too small and the hidden Markov chain is fairly persistent. The use of the various methods is also illustrated by means of empirical examples.     

Detection and classification of winding faults in windmill generators using Wavelet Transform and ANN

This paper exploits the Wavelet Transform (WT) analysis along with Artificial Neural Networks (ANN) for the diagnosis of electrical machines winding faults. A novel application is presented exploring the problem of automatically identifying short circuits of windings, which often appear during machine manufacturing and operation. Such faults are usually resulting from electrodynamics forces generated during the flow of large short circuit currents, as well as forces occurring when the machines are transported. The early detection and classification of winding failures is of particular importance, as these kinds of defects can lead to winding damage due to overheating, imbalance, etc. Application results and investigations of windmill generator winding turn-to-turn faults between adjacent winding wires are presented. The ANN approach is proven effective in detecting and classifying faults based on WT features extracted from high frequency measurements of the admittance, current, or voltage responses.     

Folding of Trp-cage Mini Protein Using Temperature and Biasing Potential Replica��Exchange Molecular Dynamics Simulations

The folding process of the 20 residue Trp-cage mini-protein was investigated using standard temperature replica exchange molecular dynamics (T-RexMD) simulation and a biasing potential RexMD (BP-RexMD) method. In contrast to several conventional molecular dynamics simulations, both RexMD methods sampled conformations close to the native structure after 10–20 ns simulation time as the dominant conformational states. In contrast, to T-RexMD involving 16 replicas the BP-RexMD method achieved very similar sampling results with only five replicas. The result indicates that the BP-RexMD method is well suited to study folding processes of proteins at a significantly smaller computational cost, compared to T-RexMD. Both RexMD methods sampled not only similar final states but also agreed on the sampling of intermediate conformations during Trp-cage folding. The analysis of the sampled potential energy contributions indicated that Trp-cage folding is favored by both van der Waals and to a lesser degree electrostatic contributions. Folding does not introduce any significant sterical strain as reflected by similar energy distributions of bonded energy terms (bond length, bond angle and dihedral angle) of folded and unfolded Trp-cage structures.     

在单级式光伏逆变器中应用1700V碳化硅开关器件的效益研究

在光伏电能变换系统中应用碳化硅(SiC)器件可以大幅降低成本,从而提高产品的竞争力。一方面开关频率的增加导致滤波元件变得更小,另一方面将最大系统电压增加到1500V可以消除在三相系统中的升压级。本文将在采用1700V碳化硅开关器件的一个单级三相电路中探讨上述可能性。     

Template-assisted large-scale ordered arrays of ZnO pillars for optical and piezoelectric applications

Spatially separated ZnO pillars, typically 300 nm in diameter and 2 microm in height, are fabricated via a template-directed approach that leads to long-range hexagonal order. The templates of Au nanodisk arrays are obtained by using metal membranes as a lithography mask. The growth of ZnO pillars is performed in a double-tube system through vapor diffusion-deposition. The growth mechanism of the pillars is studied in detail and is proposed to be a combination of vapor-liquid-solid and vapor-solid models. The piezoelectric and optical properties of single pillars are characterized using piezoresponse force microscopy and micro-photoluminescence spectroscopy, respectively. The pillars show strong excitonic emissions up to room temperature, which indicate a relatively low defect density and good crystalline quality. The obtained piezoelectric coefficient d(33) is (7.5+/-0.6) pm V(-1), which is to our knowledge the first reported value for a single nanopillar.     

Optimal sizing and limitations of passive filters in the presence of background harmonic distortion

In this paper, the optimal sizing and limitations of shunt, passive, single-tuned harmonic filters for industrial loads are examined. The investigation takes into account not only harmonic current sources due to nonlinear loads, but also the presence of background voltage harmonic distortion. First, an analytical approach in simplified cases is developed, which leads to closed form expressions. These expressions help in gaining a deeper insight into the role of the filter branches and the influence of the various parameters of the system. They also show, both quantitatively and qualitatively, the limitations of passive filters with respect to harmonic control. Next, the general case of this constrained optimization problem is formulated and solved using genetic algorithms. The constraints ensure acceptable solutions in accordance with Standard recommendations, utility regulations and filter parameter variations. The proposed objective function to be maximized is a modified version of the power factor and ensures the optimal solution among acceptable ones. Through extensive simulations of an example system for a variety of voltage and current harmonic levels, potentialities and limitations of filter branches are thoroughly investigated. Practical conclusions drawn can be considered as guidance on engineering judgment for harmonic control in any specific application.     

A transformation method for the solution of power switching circuits based on network topological concepts

In this paper, a general and systematic method for the analysis of varying topology power semiconductor circuits is presented. the changes of the conduction state of the semiconductor switching devices are handled by successive modifications of the tree of the circuit graph. These tree modifications are systematically reflected on a square transformation tensor. On the basis of well known network topological concepts, this generalized transformation tensor can be constructed in a relatively simple way. This tensor constitutes a flexible and powerful tool to assemble automatically the necessary on-switch current and off-switch voltage equations required for any conduction pattern. These manipulations are accomplished with a step-by-step modification procedure of the equations describing the circuit in the most previous conduction state. the basic steps of an algorithm suitable for the practical implementation of the analysis of any power switching network on a digital computer are described, and an example is used to demonstrate the effectiveness of the proposed method.