Analysis of Synchronous Generators’ Local Mode Eigenvalues in Modern Power Systems

New energy sources, storage facilities, power electronics devices, advanced and complex control concepts, economic operating doctrines, and cost-optimized construction and production of machines and equipment in power systems adversely affect small-signal stability associated with local oscillations. The objective of the article is to analyze local oscillations and the causes that affect them in order to reduce their negative impact. There are no recognized analyses of the oscillations of modern operating synchronous generators exposed to new conditions in power systems. The basic idea is to perform a numerical analysis of local oscillations of a large number of synchronous generators in the power system. The paper represents the local mode data obtained from a systematic analysis of synchronous generators in the Slovenian power system. Analyzed were 74 synchronous generators of the Slovenian power system, plus many additional synchronous generators for which data were accessible in references. The mathematical models convenient for the study of local oscillations are described first in the paper. Next, the influences of transmission lines, size of the synchronous generators, operating conditions, and control systems were investigated. The paper’s merit is the applicable rules that have been defined to help power plant operators avoid stability-problematic situations. Consequently, boundaries were estimated of the eigenvalues of local modes. Finally, experiments were performed with a laboratory-size synchronous generator to assess the regularity of the numerically obtained conclusions. The obtained results enable the prediction of local oscillations’ frequencies and dampings and will be useful in PSS planning.

Influence of Hole Localization on Local and Global Dynamic Response of Thin-Walled Laminated Cantilever Beam

In this study, we discuss the effects of the diameter and position of a hole on the dynamic response of a thin-walled cantilever beam made of carbon-epoxy laminate. Eigen-frequencies and corresponding global and local eigen-modes were considered, where deformations of the beam wall were dominant, without significant deformation of the beam axis. The study was focused on the circumferentially uniform stiffness (CUS) beam configuration. The laminate layers were arranged as [90/15(3)/90/15(3)/90]T. The finite element method was employed for numerical tests, using the Abaqus software package. Moreover, a few numerical results of the structure’s behaviour, with and without a hole, were verified experimentally. The experimental eigen-frequencies and the corresponding modes were obtained using an experimental modal analysis, comprising the LMS system with modal hammer. We found that the size and location of the hole affected the eigen-frequencies and corresponding modes. Furthermore, even a small hole in a beam could significantly change the shape of its local modes. The numerical and experimental results were observed to have high qualitative compliance.

Effective damping of local low frequency oscillations in power systems integrated with bulk PV generation

High penetration of renewable sources into conventional power systems results in reduction of system inertia and noticeable low-frequency oscillations (LFOs) in the rotor speed of synchronous generators. In this paper, we propose effective damping of LFOs by incorporating a supplementary damping controller with a photovoltaic (PV) generating station, where the parameters of this controller are coordinated optimally with those of a power system stabilizer (PSS). The proposed method is applied to damp local electromechanical modes by studying a system comprising a synchronous generator and a PV station connected to an infinite bus. The PV station is modeled following the instructions of the Western Electricity Coordinating Council. The problem is modeled as an optimization problem, where the damping ratio of the electromechanical modes is designed as the objective function. Constraints including upper and lower limits of decision parameters and damping ratio of other modes are considered by imposing penalties on the objective function. Different optimization algorithms are used to pursue the optimal design, such as political, improved gray wolves and equilibrium optimizers. The results validate the effectiveness of the proposed controller with PSS in damping local modes of oscillations.

Knowledge and Social Beliefs of Malaria and Prevention Strategies Among Itinerant Nomadic Arabs, Fulanis and Dagazada groups in Chad: A Mixed Method Study

Background: Nomadic populations in Chad are at increased risk of contracting malaria because of their lifestyle. Being highly mobile they are often excluded from disease control programs, and access to preventive measures and treatment is more difficult. Effective malaria control interventions take account of local modes of transmission, patterns of care-seeking behavior and community perceptions of cause and prevention practices. There is currently little information about malaria knowledge and perceptions among nomadic groups in Chad, or their awareness of malaria control interventions and this study sought to address this knowledge gap. Methods: A mixed methods study, including a cross-sectional survey with men and women (n = 78) to determine the level of knowledge and use of malaria prevention strategies among Arabs, Peuls and Dagazada nomadic groups. Three focus group discussions were conducted with women to explore their representation of malaria and knowledge of preventive methods. Key informant interviews were held with leaders of nomadic groups (n = 6) to understand perception of malaria risk among itinerant communities.Results: nomads are aware of the risk of malaria, recognize the symptoms and have local explanations for the disease. Reported use of preventive interventions such as Seasonal Malaria Chemoprophylaxis (SMC) for children and Intermittent Preventive Treatment (IPT) of malaria in pregnancy was very low. However, 42.3% of respondents reported owning at least one LLIN and 60% said they slept under an LLIN the night before the survey. In case of a malaria episode, nomads seek clinicians, informal drug sellers in the street or market for self-medication, or traditional medicine depending on their financial means. Interviews with nomad leaders and discussions with women provide key themes on: (i) social representation of malaria risk and (ii) social representation of malaria and (iii) perspectives on malaria prevention and (iv) malaria treatment practices.Conclusion: The nomadic groups included in this study are aware of risk of malaria and their level of exposure. Local interpretations of the cause of malaria could be addressed through tailored and appropriate health education. Except for LLINs, malaria prevention interventions are not well known or used. Financial barriers lowered access to both mosquito nets and malaria treatment. Reducing the barriers highlighted in this study will improve access to the healthcare system for nomadic groups, and increase the opportunity to create awareness of and improve uptake of SMC and IPT among women and children.

CLASSIFICATION OF BUCKLING MODES IN STIFFENED FUNCTIONALLY GRADED COMPOSITE TUBES SUBJECT TO BENDING

Bamboo poles, and other one-dimensional thin-walled structures are usually loaded under compression, which may also be subject to bending arising from eccentric loading. Many of these structures contain diaphragms or circumferential stiffeners to prevent cross-sectional distortions and so enhance overall load-carrying response. Such hierarchical structures can compartmentalize buckling to local regions in addition to withstanding global buckling phenomena. Predicting the buckling mode shapes of such structures for a range of geometric parameters is challenging due to the interaction of these global and local modes. Abaqus finite element software is used to model thousands of circular hollow tubes with random geometric parameters such that the ratios of radius to periodic length range from 1/3-1/7, the ratio of wall thickness to radius varies from 1/4-1/10. The material used in this study is a type of bamboo, where the Young’s and shear moduli are point-wise orthotropic and gradually increase in magnitude in the radial direction. Under eccentric loads with varying eccentricity, the structures can buckle into a global mode or local modes within an internode, i.e. periodic unit. Moreover, the local modes may contain only one wave or multiple waves in the circumferential direction. As expected, numerical results show that the global mode is more likely to occur in small and thick tubes, whereas the local modes are observed in larger tubes with a smaller number of circumferential waves present in thicker walls. Also, greater eccentricity pushes the local mode domains towards smaller tubes. An efficient classification method is developed herein to identify the domains of each mode shape in terms of radius, wall thickness and eccentricity. Based on linear discriminant analysis, explicit boundary surfaces for the three domains are defined for the obtained data, which can help designers in predicting the mode shapes of tubular structures under axial bending.

Anti-Inflammatory Therapeutic Approaches to Prevent or Delay Post-Traumatic Osteoarthritis (PTOA) of the Knee Joint with a Focus on Sustained Delivery Approaches

Inflammation plays a central role in the pathogenesis of knee PTOA after knee trauma. While a comprehensive therapy capable of preventing or delaying post-traumatic osteoarthritis (PTOA) progression after knee joint injury does not yet clinically exist, current literature suggests that certain aspects of early post-traumatic pathology of the knee joint may be prevented or delayed by anti-inflammatory therapeutic interventions. We discuss multifaceted therapeutic approaches that may be capable of effectively reducing the continuous cycle of inflammation and concomitant processes that lead to cartilage degradation as well as those that can simultaneously promote intrinsic repair processes. Within this context, we focus on early disease prevention, the optimal timeframe of treatment and possible long-lasting sustained delivery local modes of treatments that could prevent knee joint-associated PTOA symptoms. Specifically, we identify anti-inflammatory candidates that are not only anti-inflammatory but also anti-degenerative, anti-apoptotic and pro-regenerative.

Kinship networks of seed exchange shape spatial patterns of plant virus diversity

By structuring farmers’ informal networks of seed exchange, kinship systems play a key role in the dynamics of crop genetic diversity in smallholder farming systems. However, because many crop diseases are propagated through infected germplasm, local seed systems can also facilitate the dissemination of seedborne pathogens. Here, we investigate how the interplay of kinship systems and local networks of germplasm exchange influences the metapopulation dynamics of viruses responsible for the cassava mosaic disease (CMD), a major threat to food security in Africa. Combining anthropological, genetic and plant epidemiological data, we analyzed the genetic structure of local populations of the African cassava mosaic virus (ACMV), one of the main causal agents of CMD. Results reveal contrasted patterns of viral diversity in patrilineal and matrilineal communities, consistent with local modes of seed exchange. Our results demonstrate that plant virus ecosystems have also a cultural component and that social factors that shape regional seed exchange networks influence the genetic structure of plant virus populations.

Coordinated control among PSS, WTG and BESS for improving Small-Signal Stability

The goal towards attaining a sustainable future has led to the rapid increase in the integration of converter control based generators (CCBGs). The low inertia response characteristics of CCBGs and the weak tie lines in interconnected systems pose a huge threat to Small-Signal Stability (SSS). Adequate damping of low-frequency oscillations (LFO) is pivotal in ensuring the maximum power transfer through the critical transmission corridors. These operational issues become more serious with the significant reduction in system inertia as a result of the high penetration of CCBGs. Therefore, appropriate control techniques are an absolute requirement for preventing LFOs from limiting the penetration of CCBGs in interconnected networks. This may also eventually lead to revisions in grid codes mandating CCBGs to provide auxiliary damping control. But, the progressive addition of multiple damping controllers for specific target modes can lead to the drifting of eigenvalues (EVs) associated with other electromechanical modes (EMs) in the system. This is due to the adverse interactions between multiple damping controllers in the uncoordinated control approach and may result in deteriorating SSS. Therefore, this paper proposes a simultaneous coordinated control among Battery Energy Storage System (BESS), Wind Turbine Generators (WTG) and Power System Stabilizer (PSS) for enhancing SSS in networks with high wind penetration by considering both inter-area (IA) and local modes. The performance of the proposed coordinated control is corroborated using IEEE 68 bus system for multiple operating scenarios for which the critical modes in the system have the lowest damping index (DI). The effectiveness of modulating the active power, reactive power and simultaneous modulation of both active and reactive power injected by BESS along with a dual-channel Optimized WTG Damping Controller (DOWDC) and PSS is evaluated. The impact of the different coordinated control strategies on voltage dynamics is also investigated. The simulation results validate the better performance of the proposed coordinated control over uncoordinated control approaches.

Hepatitis C virus Modelling In Vitro and In Vivo

Experiments have shown that hepatitis C virus (HCV) infections in vitro disseminate both distally via the release and diffusion of cell-free virus through the medium, and locally via direct, cell-to-cell transmission. To determine the relative contribution of each mode of infection to HCV dissemination, we developed an agent-based model (ABM) that explicitly incorporates both distal and local modes of infection. The ABM tracks the concentration of extracellular infectious virus in the supernatant and the number of intracellular HCV RNA segments within each infected cell over the course of simulated in vitro HCV infections. By constraining our ABM parameters using experimental data, we found that direct, cell-to-cell infection accounts for 98% (85%–100%, 95% credible region) of infection events, making it the dominant mode of HCV dissemination in vitro. Yet, while HCV spread via cell-free virus contributes little to the total number of infection events in vitro, it plays a critical role in enhancing cell-to-cell HCV dissemination by providing access to distant, uninfected areas, away from the already established large infection foci. Mathematical modelling of HCV load decay under antiviral therapy has allowed for the determination of antiviral efficacy and other important parameters. Current mathematical models (MMs) of HCV infection are based on a set of ordinary differential equations (ODEs) and assume that infectious cell lifespans are exponentially distributed over time, meaning that every infected cell has an equal probability of dying at any time. Here, we introduce a new MM which: (1) allows for a realistic eclipse delay between the moment of cell infection and the release of new virus; and (2) considers both exponential and (log)normal distributed durations for infectious cell lifespan, wherein cells are assumed to continuously producing virus. Application of this MM to HCV load data for patients undergoing antiviral therapy leads to different conclusions when predicting parameter values.