Evaluation of Lightning Impulse Test by Frequency Response Analysis

In this work are presented the basis for improving the interpretation of transformer lightning impulse test and the development of a graphical user interface system, which allows comparisons of time domain data and frequency response. The frequency response is obtained from deconvolution of voltage and neutral current records. A quantitative comparison of frequency response is performed using the techniques applied to displacement detection through Frequency Response Analysis, such as correlation and spectral deviation. The system is implemented using 8 bit digitizers to acquire the voltage and neutral current records. The quantization error and reliability of the frequency response obtained is handled through the use of the coherence function and tolerance bands. The system is thoroughly tested applying a lightning impulse test to a dry type distribution transformer, simulating an interdisc fault with a spark gap. Failure detection is confirmed

Direct Metal Laser Fabrication of Cu Slabs from Powder Precursor: Surface Depth of Melt and Furnace Temperature Issues

A DMLF processing unit based on a raster-scanned 80 W CO2 laser beam has been developed to process single layers of metallic powder precursor. The process chamber provides atmosphere control (high vacuum and inert gas refill) besides temperature elevation up to 700 o C. In this work, copper powder precursor is confined inside a refractory steel mask surrounded by an aluminum oxide jacket that acts as insulator. The powder layers can have thicknesses of 0,5 and 1 mm. An infrared pyrometer measures in real time the temperature at one location in the surface of the powder bed. Scan speed, scan step, and furnace temperature have been varied to find combinations of such parameters that render surface melting and maximum densification. Partially melted samples were produced and their mass density was measured. Micro-hardness and surface roughness were also measured along the resolidified surface, the first rendering an average of 80,6 HV compared to the 90-105 HV of oxygen free copper, while the second resulting in an 8 μm Ra value. Maximum melt of depth achieved is ~0,15 mm followed by a sintered layer.Mechanical Engineerin

Matroid toric ideals: complete intersection, minors and minimal systems of generators

In this paper, we investigate three problems concerning the toric ideal associated to a matroid. Firstly, we list all matroids $\mathcal M$ such that its corresponding toric ideal $I_{\mathcal M}$ is a complete intersection. Secondly, we handle with the problem of detecting minors of a matroid $\mathcal M$ from a minimal set of binomial generators of $I_{\mathcal M}$. In particular, given a minimal set of binomial generators of $I_{\mathcal M}$ we provide a necessary condition for $\mathcal M$ to have a minor isomorphic to $\mathcal U_{d,2d}$ for $d \geq 2$. This condition is proved to be sufficient for $d = 2$ (leading to a criterion for determining whether $\mathcal M$ is binary) and for $d = 3$. Finally, we characterize all matroids $\mathcal M$ such that $I_{\mathcal M}$ has a unique minimal set of binomial generators.Comment: 9 page

On Ramsey numbers of complete graphs with dropped stars

Let $r(G,H)$ be the smallest integer $N$ such that for any $2$-coloring (say, red and blue) of the edges of $K\_n$, $n\geqslant N$, there is either a red copy of $G$ or a blue copy of $H$. Let $K\_n-K\_{1,s}$ be the complete graph on $n$ vertices from which the edges of $K\_{1,s}$ are dropped. In this note we present exact values for $r(K\_m-K\_{1,1},K\_n-K\_{1,s})$ and new upper bounds for $r(K\_m,K\_n-K\_{1,s})$ in numerous cases. We also present some results for the Ramsey number of Wheels versus $K\_n-K\_{1,s}$.Comment: 9 pages ; 1 table in Discrete Applied Mathematics, Elsevier, 201

The Demographic Transition in Colombia: Theory and Evidence

The demographic transition from high to low mortality and fertility rates was one of the most important structural changes during the twentieth Century in most Latin American economies. This paper uses a simple economic framework based on Galor and Weil (2000) for understanding the main forces behind this structural transition; namely, increases in the returns to human capital accumulation driven by continuous advances in productivity led families to reduce the number of offspring and increase the level of investment in their education. As a result, the economy transits from a stage of stagnation subject to Malthusian forces to a stage of sustained economic growth, where increases in productivity lead to improvements in living standards. We use available data for Colombia between 1905 and 2005 to test the main predictions of the model with time series analysis, finding empirical evidence in their favor.Economic Growth, Demographic Transition, Colombia. Classification JEL: C32; J11; N36; O40; O54.