Power System Oscillations with Different Prevalence of Grid-Following and Grid-Forming Converters

The oscillatory behaviour of the power system is an aspect that is significantly affected by the increasing integration of converter-based generation sources. Several works address the impact of non-synchronous generation on the operation of the system from different points of view, but only a few studies focus on power-frequency oscillations with a prevalence of generation sources interfaced through power electronics. A lack of research can be found in particular in the comparative analysis of the two main control strategies for power converters, namely grid-following and grid-forming. The article aims to contribute to this direction, starting from a theoretical analysis of the two control structures and then examining the case study of an existing transmission system. The research provides a specific insight into the fundamental aspects related to synchronisation mechanism and inertial capabilities of both grid-following with synthetic inertia and grid-forming controls. The difference in the relationship between synchronisation unit and inertial capability is recognised as the fundamental aspect determining the different impacts on the oscillatory characteristics of the system. The observation derived in the theoretical analysis is then applied to an actual power system with a high predominance of converter-based generation, considering the Colombian interconnected national system as a case study

Evaluación de un intercambiador de calor de placas para el enfriamiento de ácido acético

In today heat exchange applications, the plate heat exchanger (PHE) is quite used in chemical en food industries. At the present work the assessment of a PHE to carry out the cooling of an acetic acid stream, using ethylene glycol as refrigerant agent was accomplished. It was obtained a value of 29.34 % for the percent over surface design (OS), a heat exchanger effectiveness (ε) value of 0.667, a number of heat transfer units (NTU) value of 1.316, while the pressure drop results for both the acetic acid (6.70 kPa) and ethylene glycol (82.06 kPa) were below the maximum permissible limit for the process (100 kPa). The increment of the acetic acid feed flowrate increased the values of the three parameters considered, while the increment of the acetic acid inlet temperature augmented three of the variables taken into account, except for the acetic acid pressure drop, which was reduced. The proposed PHE can be satisfactorily used for the evaluated heat transfer process.En las aplicaciones de intercambio de calor actuales, el intercambiador de calor de placas (ICP) es muy empleado en las industrias químicas y la alimenticia. En el presente trabajo se efectuó la evaluación de un ICP para llevar a cabo el enfriamiento de una corriente de ácido acético, utilizando etilenglicol como agente refrigerante. Se obtuvo un valor del porcentaje de sobrediseño de superficie (PSS) de 29,34 %, una efectividad de transferencia de calor (ε) de 0,667, un número de unidades de transferencia (NUT) de 1,316, mientras que las caídas de presión de tanto el ácido acético (6,70 kPa) como el etilenglicol (82,06 kPa) se encontraron por debajo del límite máximo permisible por el proceso (100 kPa). El incremento del caudal de alimentación del ácido acético aumentó los valores de los tres parámetros considerados, mientras que el incremento de la temperatura de entrada del ácido acético aumentó tres de las variables tomadas en cuenta, con excepción de la caída de presión del ácido acético, que se redujo. El ICP propuesto puede ser empleado satisfactoriamente para el proceso de transferencia de calor evaluado

Evaluación de un intercambiador de calor de placas para el enfriamiento de ácido acético

En las aplicaciones de intercambio de calor actuales, el intercambiador de calor de placas (ICP) es muy empleado en las industrias químicas y la alimenticia. En el presente trabajo se efectuó la evaluación de un ICP para llevar a cabo el enfriamiento de una corriente de ácido acético, utilizando etilenglicol como agente refrigerante. Se obtuvo un valor del porcentaje de sobrediseño de superficie (PSS) de 29,34 %, una efectividad de transferencia de calor (ε) de 0,667, un número de unidades de transferencia (NUT) de 1,316, mientras que las caídas de presión de tanto el ácido acético (6,70 kPa) como el etilenglicol (82,06 kPa) se encontraron por debajo del límite máximo permisible por el proceso (100 kPa). El incremento del caudal de alimentación del ácido acético aumentó los valores de los tres parámetros considerados, mientras que el incremento de la temperatura de entrada del ácido acético aumentó tres de las variables tomadas en cuenta, con excepción de la caída de presión del ácido acético, que se redujo. El ICP propuesto puede ser empleado satisfactoriamente para el proceso de transferencia de calor evaluado

Influence of environmental conditions and oxidation on the coefficient of friction using microalloyed rail steels

© IMechE 2020. In railway systems, certain atmospheric conditions – mainly related to temperature and relative humidity – lead to the creation of iron oxides which can affect the coefficient of friction between the wheel and the rail. This can result in increased wear of the rail, reducing its service life and thus increasing replacement costs. Pin-on-disc tests were conducted in a climate chamber to study the influence of environmental conditions and iron oxides on the coefficient of friction. The iron oxides generated on the surface of specimens extracted from microalloyed rail steel during wear tests were analysed using X-ray diffraction. The results show that hematite (α-Fe2O3) is the predominant iron oxide among all the oxides generated on the worn surfaces. It was further noted that the oxide layer resulting from the rise in both temperature and relative humidity does not significantly affect the average coefficient of friction for each steel samples tested. High relative humidity combined with high temperature leads to the formation of α-Fe2O3, which tends to increase the coefficient of friction. However, a boundary lubrication effect is observed at higher relative humidity due to a condensed water film, which reduces the coefficient of friction and counteracts the increase of the coefficient of friction expected due to the presence of hematite

Strength distribution of solar magnetic fields in photospheric quiet Sun regions

The magnetic topology of the solar photosphere in its quietest regions is hidden by the difficulties to disentangle magnetic flux through the resolution element from the field strength of unresolved structures. The observation of spectral lines with strong coupling with hyperfine structure, like the observed MnI line at 553.7 nm, allows such differentiation. The main aim is to analyse the distribution of field strengths in the network and intranetwork of the solar photosphere through inversion of the MnI line at 553.7 nm. An inversion code for the magnetic field using the Principal Component Analysis (PCA) has been developed. Statistical tests are run on the code to validate it. The code has to draw information from the small-amplitude spectral feature oppearing in the core of the Stokes V profile of the observed line for field strengths below a certain threshold, coinciding with lower limit of the Paschen-Back effect in the fine structure of the involved atomic levels. The inversion of the observed profiles, using the circular polarization (V) and the intensity (I), shows the presence of magnetic fields strengths in a range from 0 to 2 kG, with predominant weak strength values. Mixed regions with mean strength field values of 1130 and 435 Gauss are found associated with the network and intranetwork respectively. The MnI line at 553 nm probes the field strength distribution in the quiet sun and shows the predominance of weak, hectoGauss fields in the intranetwork, and strong, kiloGauss fields in the network. It also shows that both network and intranetwork are to be understood at our present spatial resolutions as field distributions of which we hint the mean properties.Comment: 10 pages, 6 figure

The WHaD diagram: Classifying the ionizing source with one single emission line

The usual approach to classify the ionizing source using optical spectroscopy is based on the use of diagnostic diagrams that compares the relative strength of pairs of collisitional metallic lines (e.g., [O iii] and [N ii]) with respect to recombination hydrogen lines (e.g., H{\beta} and H{\alpha}). Despite of being accepted as the standard procedure, it present known problems, including confusion regimes and/or limitations related to the required signal-to-noise of the involved emission lines. These problems affect not only our intrinsic understanding of inter-stellar medium and its poroperties, but also fundamental galaxy properties, such as the star-formation rate and the oxygen abundance, and key questions just as the fraction of active galactic nuclei, among several others. We explore the existing alternatives in the literature to minimize the confusion among different ionizing sources and proposed a new simple diagram that uses the equivalent width and the velocity dispersion from one single emission line, H{\alpha}, to classify the ionizing sources. We use aperture limited and spatial resolved spectroscopic data in the nearby Universe (z{\sim}0.01) to demonstrate that the new diagram, that we called WHaD, segregates the different ionizing sources in a more efficient way that previously adopted procedures. A new set of regions are defined in this diagram to select betweeen different ionizing sources. The new proposed diagram is well placed to determine the ionizing source when only H{\alpha} is available, or when the signal-to-noise of the emission lines involved in the classical diagnostic diagrams (e.g., H{\beta}).Comment: 10 pages, 5 figures, accepted for publishing in A&

Long-Term Effectiveness of a Smartphone App and a Smart Band on Arterial Stiffness and Central Hemodynamic Parameters in a Population with Overweight and Obesity (Evident 3 Study): Randomised Controlled Trial

Background: mHealth technologies could help to improve cardiovascular health; however, their effect on arterial stiffness and hemodynamic parameters has not been explored to date. Objective: To evaluate the effect of a mHealth intervention, at 3 and 12 months, on arterial stiffness and central hemodynamic parameters in a sedentary population with overweight and obesity. Methods: Randomised controlled clinical trial (Evident 3 study). 253 subjects were included: 127 in the intervention group (IG) and 126 in the control group (CG). The IG subjects were briefed on the use of the Evident 3 app and a smart band (Mi Band 2, Xiaomi) for 3 months to promote healthy lifestyles. All measurements were recorded in the baseline visit and at 3 and 12 months. The carotid-femoral pulse wave velocity (cfPWV) and the central hemodynamic parameters were measured using a SphigmoCor System® device, whereas the brachial-ankle pulse wave velocity (baPWV) and the Cardio Ankle Vascular Index (CAVI) were measured using a VaSera VS-2000® device. Results: Of the 253 subjects who attended the initial visit, 237 (93.7%) completed the visit at 3 months of the intervention, and 217 (85.3%) completed the visit at 12 months of the intervention. At 12 months, IG showed a decrease in peripheral augmentation index (PAIx) (−3.60; 95% CI −7.22 to −0.00) and ejection duration (ED) (−0.82; 95% CI −1.36 to −0.27), and an increase in subendocardial viability ratio (SEVR) (5.31; 95% CI 1.18 to 9.44). In CG, cfPWV decreased at 3 months (−0.28 m/s; 95% CI −0.54 to −0.02) and at 12 months (−0.30 m/s, 95% CI −0.54 to −0.05), central diastolic pressure (cDBP) decreased at 12 months (−1.64 mm/Hg; 95% CI −3.19 to −0.10). When comparing the groups we found no differences between any variables analyzed. Conclusions: In sedentary adults with overweight or obesity, the multicomponent intervention (Smartphone app and an activity-tracking band) for 3 months did not modify arterial stiffness or the central hemodynamic parameters, with respect to the control group. However, at 12 months, CG presented a decrease of cfPWV and cDBP, whereas IG showed a decrease of PAIx and ED and an increase of SEVR