An analytical and genetic-algorithm-based design tool for brushless excitation systems of low-medium rated synchronous generators

The sizing procedures adopted for the everyday design of electrical machines are well known and consolidated. However, for brushless exciters of field wound, synchronous generators, there is significant room for improvement as the impact of the diodes’ commutations in the rotating bridge rectifier are often neglected. This paper deals with the development of a fast analytical, genetic-algorithm-based design tool for the excitation systems of salient-pole, field wound synchronous generators. As vessel for this study, the exciter of a particular 400kVAis considered. The proposed tool is focused on achieving exciter designs that minimize the voltage drop due to the commutation processes in the rotating diode rectifier, with minimum impact on the overall efficiency

Prediction of the voltage drop due to the diode commutation process in the excitation system of salient-pole synchronous generators

The commutation processes in uncontrolled diode rectifiers have been extensively studied and modelled. However, in some applications, such as electrical power generation, the effects of these processes are often neglected. In low to medium rated, field wound, synchronous generators, the excitation system makes up a significant percentage of the whole generating set. Thus, the voltage drop due to the diode commutations can be quite significant. It is therefore of critical importance that these are considered during all the design stages of the brushless excitation system of synchronous generators. In this paper, a detailed analysis of the commutation aspects related to the diode rectifier of a brushless exciter of a 400kVA synchronous generator is presented and an accurate voltage drop prediction model is proposed and validated

Aquilegia, Vol. 28 No. 3, May-June-July 2004: Newsletter of the Colorado Native Plant Society

https://epublications.regis.edu/aquilegia/1102/thumbnail.jp

Damper cage loss reduction and no-load voltage THD improvements in salient-pole synchronous generators

Salient-pole synchronous generators (SG) have a long history of utilization as reliable power generation systems. Important aspects of such generators include a high power-to-weight ratio, high efficiency and a low cost per kVA output [1]. Another critical aspect is the requirement for very low harmonic content in the output voltage. Hence, it is very important to be able to model and predict the no-load voltage waveform in an accurate manner in order to be able to satisfy standards requirements, such as the permissible total harmonic distortion (THD). Also, at steady-state conditions, parasitic voltages are induced in the damper bars which lead to a current flow with associated power losses and an increase in temperatures. This paper deals with an in-detail analysis of a 4 MVA SG, whose operation is studied and compared with experimental results for validation purposes. The same platform is then used to propose innovative solutions to the existing design and operational challenges of the machine aimed at reducing ohmic loss in the damper cage and improving the output voltage THD, without reverting to disruptive techniques such as rotor and/or stator skewing

Improved damper cage design for salient-pole synchronous generators

The benefits of implementing a damper winding in salient-pole, synchronous generators are widely known and well consolidated. It is also well known that such a winding incurs extra losses in the machine due to a number of reasons. In order to improve the overall efficiency and performance of classical salient-pole, wound field, synchronous generators that employ the traditional damper cage, an improved amortisseur winding topology that reduces the inherent loss is proposed and investigated in this paper. This is essential in order to meet modern power quality requirements and to improve the overall performance of such ’classical’ machines. The new topology addresses the requirements for lower loss components without compromising the acceptable values of the output voltage total harmonic distortion and achieves this by having a modulated damper bar pitch. As vessel for studying the proposed concept, a 4MVA, salient-pole, synchronous generator is considered. A finite element model of this machine is first built and then validated against experimental results. The validated model is then used to investigate the proposed concept with an optimal solution being achieved via the implementation of a genetic algorithm optimization tool. Finally, the performance of the optimised machine is compared to the original design both at steady state and transient operating conditions

Accelerating Development of SARS-CoV-2 Vaccines — The Role for Controlled Human Infection Models

The third coronavirus outbreak in the past 20 years, the SARS-CoV-2 pandemic has caused unprecedented morbidity, mortality, and economic disruption. Safe, effective, and deployable SARS-CoV-2 vaccines are urgently needed to mitigate the consequences of the pandemic and protect from future outbreaks. The accelerated response to Covid-19 includes investments in preclinical and clinical testing and manufacture of multiple vaccine candidates, with efficacy trials in the United States anticipated to start in July 2020

SARS-CoV-2 Variants in Patients with Immunosuppression

Patients with immunosuppression are at risk for prolonged infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In several case reports, investigators have indicated that multimutational SARS-CoV-2 variants can arise during the course of such persistent cases of coronavirus disease 2019 (Covid-19). These highly mutated variants are indicative of a form of rapid, multistage evolutionary jumps (saltational evolution; see Glossary), which could preferentially occur in the milieu of partial immune control. The presence of a large number of mutations is also a hallmark of the variants of concern — including B.1.1.7 (alpha), B.1.351 (beta), P.1 (gamma), and B.1.617.2 (delta) — which suggests that viral evolution in immunocompromised patients may be an important factor in the emergence of such variants. Since a large number of persons globally are living with innate or acquired immunosuppression, the association between immunosuppression and the generation of highly transmissible or more pathogenic SARS-CoV-2 variants requires further delineation and mitigation strategies

A theory of Plasma Membrane Calcium Pump stimulation and activity

The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is characterized by a high affinity to calcium and a low transport rate compared to other transmembrane calcium transport proteins. It plays a crucial role for calcium extrusion from cells. Calmodulin is an intracellular calcium buffering protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by increasing both the affinity to calcium and the maximum calcium transport rate. We introduce a new model of this stimulation process and derive analytical expressions for experimental observables in order to determine the model parameters on the basis of specific experiments. We furthermore develop a model for the pumping activity. The pumping description resolves the seeming contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation step and the observation that the pump binds two calcium ions at the intracellular site. The combination of the calcium pumping and the stimulation model correctly describes PMCA function. We find that the processes of calmodulin-calcium complex attachment to the pump and of stimulation have to be separated. Other PMCA properties are discussed in the framework of the model. The presented model can serve as a tool for calcium dynamics simulations and provides the possibility to characterize different pump isoforms by different type-specific parameter sets.Comment: 24 pages, 6 figure

HLA tapasin independence: broader peptide repertoire and HIV control.

Human leukocyte antigen (HLA) class I allotypes vary in their ability to present peptides in the absence of tapasin, an essential component of the peptide loading complex. We quantified tapasin dependence of all allotypes that are common in European and African Americans (n = 97), which revealed a broad continuum of values. Ex vivo examination of cytotoxic T cell responses to the entire HIV-1 proteome from infected subjects indicates that tapasin-dependent allotypes present a more limited set of distinct peptides than do tapasin-independent allotypes, data supported by computational predictions. This suggests that variation in tapasin dependence may impact the strength of the immune responses by altering peptide repertoire size. In support of this model, we observed that individuals carrying HLA class I genotypes characterized by greater tapasin independence progress more slowly to AIDS and maintain lower viral loads, presumably due to increased breadth of peptide presentation. Thus, tapasin dependence level, like HLA zygosity, may serve as a means to restrict or expand breadth of the HLA-I peptide repertoire across humans, ultimately influencing immune responses to pathogens and vaccines

Interventions Targeting Child Undernutrition in Developing Countries May Be Undermined by Dietary Exposure to Aflatoxin

Child undernutrition, a form of malnutrition, is a major public health burden in developing countries. Supplementation interventions targeting the major micronutrient deficiencies have only reduced the burden of child undernutrition to a certain extent, indicating that there are other underlying determinants that need to be addressed. Aflatoxin exposure, which is also highly prevalent in developing countries, may be considered an aggravating factor for child undernutrition. Increasing evidence suggests that aflatoxin exposure can occur in any stage of life, including in utero through a trans-placental pathway and in early childhood (through contaminated weaning food and family food). Early life exposure to aflatoxin is associated with adverse effects on low birth weight, stunting, immune suppression, and the liver function damage. The mechanisms underlying impaired growth and aflatoxin exposure are still unclear but intestinal function damage, reduced immune function, and alteration in the insulin-like growth factor axis caused by the liver damage are the suggested hypotheses. Given the fact that both aflatoxin and child undernutrition are common in sub-Saharan Africa, effective interventions aimed at reducing undernutrition cannot be satisfactorily achieved until the interactive relationship between aflatoxin and child undernutrition is clearly understood, and an aflatoxin mitigation strategy takes effect in those vulnerable mothers and children