Local Load Research Articles

Extensive assessment of virtual synchronous generators in intentional island mode

Virtual Synchronous Generators (VSGs) are considered one of the most effective cutting-edge technologies for seamlessly integrating Renewable Energy Systems (RESs) into power grids. While various VSG control schemes have been proposed and analysed for different technical aspects, not many experimental demonstrations have been published about VSGs operating in Intentional Island Mode (IIM). Inversely, the IIM has become increasingly attractive, nowadays, to energize those remote areas of the world where entire districts are served by a small number of RESs operating off-grid. In this scenario, the main objective of this paper is to present an extensive performance assessment of the VSG operating in IIM. Different working conditions are considered, with special focus on: black start capability, grid-connected-to-IIM transition, grid reconnection, feeding of non-linear and asymmetric local loads, load sharing between multiple VSGs operating in parallel. In order to achieve this objective, an improved structure of VSG is implemented on 8kVA three-phase converters and significant experimental results are presented. Algorithms for black start capability and grid synchronization are illustrated in detail as well. Meaningful information is provided for engineers involved in activity of designing and testing islanded VSGs.

Математичні моделі локального нагрівання елементів електронних пристроїв

Linear and non-linear mathematical models for the determination of the temperature field, and subsequently for the analysis of temperature regimes in isotropic spatial heat-active media sub-jected to internal and external local heat load, have been developed. In the case of nonlinear boundary-value problems, the Kirchhoff transformation was applied, using which the original nonlinear heat conduction equations and nonlinear boundary conditions were linearized, and as a result, linearized second-order differential equations with partial derivatives and a discontinu-ous right-hand side and partially linearized boundary conditions were obtained. For the final linearization of the partially linearized differential equation and boundary conditions, the ap-proximation of the temperature according to one of the spatial coordinates on the boundary sur-faces of the inclusion was performed by piecewise constant functions. To solve linear bounda-ry-value problems, as well as obtained linearized boundary-value problems with respect to the Kirchhoff transformation, the Henkel integral transformation method was used, as a result of which analytical solutions of these problems were obtained. For a heat-sensitive environment, as an example, a linear dependence of the coefficient of thermal conductivity of the structural material of the structure on temperature, which is often used in many practical problems, was chosen. As a result, analytical relations for determining the temperature distribution in this envi-ronment were obtained. Numerical analysis of temperature behavior as a function of spatial co-ordinates for given values of geometric and thermophysical parameters was performed. The in-fluence of the power of internal heat sources and environmental materials on the temperature distribution was studied. To determine the numerical values of the temperature in the given structure, as well as to analyze the heat exchange processes in the middle of these structures, caused by the internal and external heat load, software tools were developed, using which a ge-ometric image of the temperature distribution depending on the spatial coordinates was made.

PRRSV-2 viral load in critical non-lymphoid tissues is associated with late gestation fetal compromise.

The impact of late gestation PRRSV-2 infection is highly variable within a litter, with a subset of fetuses displaying varying degrees of compromise following infection while others remain viable despite significant systemic viral load. To understand the underlying cause of this variation, we examined the susceptibility, distribution and impact of viral infection within non-lymphoid tissues. Samples of brain, heart, kidney, liver, lung, and skeletal muscle were obtained from fetuses of pregnant gilts at gestation day 86, and the presence and distribution of CD163+ cells within each tissue evaluated via immunohistofluorescence. Equivalent samples were collected from phenotypic extremes representing resistant, resilient and susceptible fetuses at 21 days following infection of pregnant gilts with PRRSV-2 at day 86 of gestation. Viral load and its impact in each tissue was evaluated by a combination of qPCR, in vitro viral recovery, and local expression of IFNG and CD163. Resting populations of CD163+ cells were observed in all six non-lymphoid tissues from healthy day 86 fetuses, though the apparent density and the morphology of positive cells varied between tissue. Viral RNA was detected in all six tissues derived from fetuses previously classified as highly infected, and infectious viral particles successfully recovered. Significantly more viral RNA was detected in heart, brain, lung and skeletal muscle of susceptible fetuses, relative to their viable counterparts. Infection was associated with an increase in the expression of CD163 in brain, kidney and lung. In addition, the presence of virus in each tissue coincided with a significant upregulation in the expression of IFNG, but the scale of this response was not associated with fetal susceptibility. Thus, PRRSV-2 is widely distributed across these susceptible non-lymphoid fetal tissues, and fetal outcome is associated with local viral load in critical fetal organs.

Indentation of sandwich beams: Comparison of Vlasov, Winkler, and shear theories with composite surfaces reinforced by CNTs and ANN model

The use of sandwich structures has spread to a wide range of fields today. As a result of their thin top and low stiffness, sandwich beams are very susceptible to local loads. One practical solution to increase the sandwich structure's indentation resistance is to add bending strength to the top. The structural properties of carbon nanotubes make them well-suited for use in polymer matrixes; their addition improves the attributes of polymers. Experimental and theoretical investigations of the sandwich structure's indentation behavior are presented in this article. As well, simulations of sandwich structures are conducted using Abaqus software. A tensile test of composite samples and Mori Tanaka's theory were used to determine the elastic modulus of epoxy resin reinforced with carbon nanotubes at different mass percentages. The elastic modulus of composites can be improved by adding carbon nanotubes up to 0.3 % by mass while the mechanical properties are reduced when more than 0.3 % of carbon nanotubes were added. Finally, the analytical solution and Abaqus modeling results for the indentation of the sandwich structure were compared with the results obtained from the experimental tests. A machine learning model is proposed for prediction of sandwich beams deflection. The results of the predictions were phenomenal, with MAE of less than 2 %. The computational costs have been compared with the experimental results, and the machine learning method is very efficient.

Numerical and experimental assessment of liquid metal embrittlement in externally loaded spot welds

Zinc-based surface coatings are widely applied with high-strength steels in automotive industry. Some of these base materials show an increased brittle cracking risk during loading. It is necessary to examine electrogalvanized and uncoated samples of a high strength steel susceptible to liquid metal embrittlement during spot welding with applied external load. Therefore, a newly developed tensile test method with a simultaneously applied spot weld is conducted. A fully coupled 3D electrical, thermal, metallurgical and mechanical finite element model depicting the resistant spot welding process combined with the tensile test conducted is mandatory to correct geometric influences of the sample geometry and provides insights into the sample’s time dependent local loading. With increasing external loads, the morphology of the brittle cracks formed is affected more than the crack depth. The validated finite element model applies newly developed damage indicators to predict and explain the liquid metal embrittlement cracking onset and development as well as even ductile failure.

Consensus based distributed secondary control for current sharing and voltage restoration considering local loads and constant power loads in dc microgrids

ABSTRACT Ensuring accurate current sharing and regulating output voltage is challenging in DC microgrids. Conventional droop control has limitations in achieving voltage regulation and accurate current sharing. Unequal transmission line resistances, local loads, and constant power load (CPL) add to the complexity. To tackle this issue, this paper presents a novel consensus based distributed secondary control that determines the resistance of transmission lines by injecting an external pulse to the reference voltage of each converter, and then measuring the resulting changes in output voltage and current. The proposed secondary control signal, denoted as ψi , enables simultaneous proportional current sharing and voltage restoration in the DC microgrid. To guarantee the stability of the proposed method, a comprehensive stability analysis of the system is performed. In contrast to similar methods, the proposed method enables achieving precise proportional current sharing and voltage restoration, even when faced with varying resistive loads and CPLs. Furthermore, the proposed method exhibits remarkable features including, high convergence speed, the capability to determine transmission line resistances instead of assuming them to be known, and robustness in plug-and-play scenarios and communication system failures. The proposed method is validated through simulations in MATLAB and experimental tests, affirming its effectiveness.

Reinforcement learning-based dynamic pruning for distributed inference via explainable AI in healthcare IoT systems

Deep Neural Networks (DNNs) have become the key technique to revolutionize the healthcare sector. However, conducting online remote inference is often impractical due to privacy constraints and latency requirements. To enable local computation, researchers have attempted network pruning with minimal accuracy loss or DNN distribution without affecting the performance. Yet, distributed inference can be inefficient due to the energy overhead and fluctuation of communication channels between participants. On the other hand, given that realistic healthcare systems use pre-trained models, local pruning and retraining relying only on the available scarce data is not possible. Even pre-pruned DNNs are limited in their ability to customize to the local load of data and device dynamics. The online pruning of DNN inferences without retraining is viable; however, it was not considered in the literature as most well-known techniques do not perform well without adjustment. In this paper, we propose a novel pruning strategy using Explainable AI (XAI) to enhance the performance of pruned DNNs without retraining, a necessity due to the scarcity and bias of local healthcare data. We combine distribution and pruning techniques to perform online distributed inference assisted by dynamic pruning when needed for highest accuracy. We use Non-Linear Integer Programming (NLP) to formulate our approach as a trade-off between resources and accuracy, and Reinforcement Learning (RL) to relax the problem and adapt to dynamic requirements. Our pruning criterion shows high performance compared to other reference techniques and ability to assist distribution by reducing resource usage while keeping high accuracy.

Degradation mechanisms of soil arching under a localized cyclic surface loading

Soil arching causes stress redistribution in many earth structures, such as pile-supported embankments and buried structures. A localized cyclic surface load (e.g., footing and traffic load) would weaken the arching effect and cause structural safety at potential risk. This paper presents a series of trapdoor tests using transparent soil to investigate the degradation mechanisms of soil arching subjected to a localized cyclic surface load. The particle image velocimetry (PIV) technique was adopted to monitor the inner soil deformations during test. The test results show that soil arching under localized cyclic surface loading first degraded locally on the trapdoor center and then progressed from the center to the entire trapdoor. The soil arching degraded faster within a lower backfill, on a wider trapdoor, and under a higher load frequency of localized surface loading. Owing to the volumetric expansion during trapdoor movement, the average vertical stress with soil arching increased faster under localized surface loading than that without soil arching. After full degradation of soil arching, the stress increment and vertical displacement contours with and without arching effect gradually tended to be similar. Finally, an empirical method was proposed to predict the soil arching ratio under localized cyclic surface loading.

Inspection of press joints based on the analysis of their deformation patterns under local thermal loading

The results of experimental studies of the possibility of using strain gauges under local pulsed thermal loading to evaluate the tension of the press connections of bearing rings with shafts are presented. Samples of press joints with tightness in the range from 38 to 118 microns were made. As a result of the study of the patterns of heat flow propagation in bearing rings and tension samples by contact method and means of thermal imaging, the possibility of separating deformations associated with the influence of temperature on the deformation measurement area and the temperature gradient in the heating area is shown. The methods of inspecting the press joint when exposed to a heater ring with a heat power capacity of 80 kJ and a temperature of 200°C and measuring the deformations of the ring and shaft by an opto-polarizing sensor with a base of 60 mm and a resolution of 2×10 — 7 relative deformations have been experimentally implemented. A correlation has been established between the time of reaching the maximum of local deformations of the shaft with the tightness of the press joints and the sign of deformations of the bearing ring with the tightness of the press fit, the gap between the ring and the shaft.

Unified power quality conditioner (UPQC) with voltage dips and over-voltages compensation capability

Unified power quality conditioners (UPQCs) allow the mitigation of voltage and current disturbances that could affect sensitive electrical loads while compensating the load reactive power. Diverse control techniques have been proposed to evaluate the instantaneous output voltage of the series active power filter of the UPQC but, in most cases, these controllers only can compensate a kind of voltage disturbance. This paper proposes a new digital controller for UPQCs which allows the load current harmonics and reactive power to be compensated at the grid side while avoiding the effect of voltage dips, over-voltages and voltage harmonics on the local loads. The controller characteristics can be established on-line without changing its basic structure. The performance of the proposed controller has been evaluated through simulation tests.

Multimedia fate simulation of mercury in a coastal urban area based on the fugacity/aquivalence method

Due to intensive industrial production and living activities, urban areas are the main anthropogenic mercury (Hg) emission sources. After entering the environment through exhaust gases, wastewater or waste residues, Hg can migrate and transform among different environmental compartments in various species, such as elemental mercury (Hg0), divalent mercury (Hg2+) and methylmercury (MeHg). Studies have yet to report on the multimedia behaviors of Hg in urban areas due to the complexity of the processes involved. In this study, the atmospheric Hg emission in Dalian, a coastal city in Northeast China, was estimated by an anthropogenic emission inventory, and a Level III multimedia model was constructed based on the fugacity/aquivalence method to simulate the fate of Hg in air, water, soil, sediment, vegetation and film. The total annual atmospheric emission was 9.91 t, of which coal combustion and non-coal sources accounted for 70.1 % and 29.9 %, respectively. Atmospheric emission and advection were dominated by Hg0, and aquatic emission and advection were dominated by Hg2+. The migration of air–vegetation, vegetation–soil and soil–air were three important pathways of Hg in urban areas. The model was validated by collecting local soil and vegetation samples and regional air, seawater and sediment monitoring data. The scenario simulation indicated that the local load would decrease to different extents with a 21.0 % reduction in atmospheric Hg emission by implementing the “coal-to-gas” measures. Our developed model can characterize the fate of Hg in coastal urban areas and provide a reference for control strategies.

Review of the islanding phenomenon problem for connection of renewable energy systems.

As distributed generators increase their importance on the electric power system, more and more parameters have to be controlled in order to assure the proper operation of the utility. One of the main problems encountered with this kind of generation is the potential formation of islands which could keep working in a normal way even if the utility grid has failed. Many methods have been developed to prevent this situation and they have been classified into three groups: passive, active and methods based on communication systems. This paper checks the validity of some of the active and passive anti-islanding methods. Some of them are shown to work properly with any kind of utility and local loads in the potential island. On the other hand, some others would not disconnect the power generator when the total power of the local load fits that of the generator

Experimental and theoretical study on the response of the X-frame CFRP sandwich beam under the local impulsive loading

The impact resistance of the carbon fibre reinforced X-frame sandwich beam and its foam reinforced structure was investigated experimentally and theoretically. To simulate the blast loading on the composite sandwich beams, the aluminum foam projectiles were utilised to impact the composite specimens. Under the local impulsive loads of the different intensities, the core shear failure, core crushing and skin fracture of the sandwich beams were observed by using a high-speed camera. A theoretical model for the dynamic response of the fibre-reinforced composite sandwich beam under the impact loads was constructed. Three phases, namely the local compression, the propagation of global bending/shearing wave and global stretching were considered in this theory. The theoretical model can provide the reasonable deflection history and failure modes of the sandwich beam compared with the experimental and simulated results. The impact pressure between the foam projectile and the beam were obtained by the theoretical and numerical predictions. The results indicated that the decreasing of the structural density can reduce the transmitted impulse. The optimal design charts were constructed by introducing the normalised variables based on the theoretical model. The minimum weight design revealed that the composite sandwich beams with low-density cores generally had the better specific impact resistance, while the foam-filled sandwich beams with the higher density had the outperformed impact resistance at the low impulse. It is expected that the theoretical model can provide a useful reference for predicting the impact resistance of the fibre-reinforced composite sandwich structures.

Preliminary steady state and dynamic analysis of a microgrid system

The objective of this paper is to analyse the steady state and dynamic behaviour of a MicroGrid system containing one microturbine generating Combined Heat and Power feeding some small local loads in islanded mode of operation. Future operating scenarios were also analysed and simulated, namely considering the installation of photovoltaic panels near consumers

Calculation of pre-stressed cable-stayed steel bridge with intersecting cable stays/Berechnung einer vorgespannten Schrägseilbrücke aus Stahl mit sich kreuzenden Schrägseilen

Compared to other structural systems, cable-stayed bridges are marked by their exceptional architectural appeal. However, these bridges have several disadvantages – their deformability under asymmetrical or local loads the relatively significant mass of stiffening girder and the pylons. A successful way of stabilizing the bridge’s initial form could be using additional stay cables and additional pylons. The article discusses the calculation method of an innovative cable-stayed steel bridge system consisting of intersecting cables and additional intermediate pylons. The system’s behavior is analyzed under symmetrical and asymmetrical loading. The calculation method results are compared to the numerical analysis using the FEM software.

Mass transport effects in gas-phase selective hydrogenation of 1,3-butadiene over supported Pd

The performance of selective hydrogenation reactions is affected by mass transfer limitations, induced by higher local metal loading and larger catalyst grain size.

New test-based detail categories for fatigue design of crane runway beams

Top-running overhead bridge cranes are frequently supported by welded crane runway beams whose wheel loaded constructional details, such as the welded flange-to-web connections and the continuous double fillet welds to fasten the crane rail, are subject to multiaxial fatigue. The fatigue stressing is composed by normal and shear stresses due to the local wheel load introduction and global bending. As the fatigue stressing exhibits a phase shift of the stress components, it is referred to as nonproportional. The currently applicable detail category of welded flange-to-web connections according to EN 1993-1-9 (2005) is theoretically derived and does not appropriately address the specifics of these connections. A classification of rail welds including a detail-related nominal stress formula is missing. A design proposal for both constructional details is presented that is based on fatigue tests on girders with travelling and stationarily pulsating wheel loads to simulate the nonproportional fatigue stressing. To derive new detail categories from the test results, a fatigue failure hypothesis is formulated modifying Findley’s critical plane approach for the evaluation of welded constructional details with partial penetration and weld root failure on the basis of notch stresses.

Neighborhood effects on tree growth in a Fagus sylvatica - Abies alba forest following an ice storm

Ice storms can cause substantial damage to tree crowns and lead to growth reduction. However, in uneven-aged stands, the growth of an individual tree may also increase due to crown release caused by the damage or mortality of neighboring trees. Three years after the devastating ice storm in 2014 in mixed uneven-aged Dinaric forests (Croatia), we cored 156 European beech (Fagus sylvatica) and 85 silver fir (Abies alba) trees across 20 permanent sample plots to study the post-storm growth response as a function of tree, stand, site, spatial arrangement and local competition factors. The ice storm damaged over 84% of trees on the sampled plots. Among the cored trees, 52.7% exhibited growth reduction, which on average amounted to −3.1% relative to the pre-disturbance average. Trees with less than 40% crown damage maintained their pre-disturbance growth rates or experienced only minor growth suppression. While 60% of beech trees suffered a growth reduction at an average rate of −7.2%, the average radial increment of fir after the storm was 14.0% higher compared to the pre-storm rate. A linear mixed-effects model suggests that the growth response can largely be explained by the focal and neighboring tree species identity, tree competition pressure, focal and neighboring tree damage, crown size, slenderness index and stoniness. Growth release was positively associated with fir, inverse distance-weighted crown damage of the nearest neighbor, shorter crowns, slenderness, less stony sites and less damaged trees. The analysis suggests that at the same level of local competition load, trees with a broadleaved and damaged nearest neighbor are more likely to experience growth release than those with a coniferous or undamaged nearest neighbor. This implies that uneven-aged stands with a substantial presence and mingling of both conifers and broadleaves are expected to be more resilient to ice storms and are less likely to suffer growth reduction.

Nominal stresses in continuous and intermittent rail welds of crane runway beams

Crane runway beams of hot-rolled I sections are frequently the supporting structures of top-running overhead bridge cranes. In case of light crane service, the crane rails are usually fastened to the top flange of the crane runway beams by fillet welds. A survey among steel construction companies revealed that continuous rail welds as well as intermittent rail welds are commonly used. The rail welds are subject to multiaxial fatigue since they are exposed to normal and shear stresses due to the local wheel load introduction and global bending. Currently, the design standards EN 1993-1-9 (2005) and EN 1993-6 (2007) neither provide an appropriate detail category nor a nominal stress formula for rail welds. The paper focuses on the calculation of the nominal stresses in rail welds as basis of the fatigue action effect within the nominal stress method. It describes the differences in the nominal stresses of continuous and intermittent rail welds because of the contact between rail and top flange that has to be assumed for intermittent rail welds. Based on work samples, the contact conditions of intermittent rail welds are characterized as ‚technical contact‘ (in contrast to ‚ideal contact‘) due to imperfections within the contact surfaces that are described in detail. A Finite Element model is presented that accounts for the ‚technical contact‘ between rail and flange. This model is analyzed in a parametric study to identify the decisive parameters on the nominal stresses in intermittent rail welds.

Reinforcement Learning-Based Integrated Control to Improve the Efficiency of DC Microgrids

The hierarchical control of the DC microgrid regulates the terminal voltages of the interfacing converter to achieve proportional load sharing and good voltage regulation at the DC bus. In doing so, the difference of the voltage at different nodes increases which results in higher circulating current and leads to higher losses. In this paper, a Reinforcement Learning Based Integrated Control (RLIC) is proposed which will minimize the circulating current and power losses in the transmission. The proposed RLIC consists of a primary and secondary controller. The primary controller is a robust sliding mode controller which receives the voltage references from secondary controller and regulates the terminal voltage and source current accordingly. The secondary control consists of a proportional integral control (PI), and a Deep Neural Network (DNN) surrogate model with implementation of Q-Learning as reinforcement method. A novel DNN based surrogate model uses the droop value from the PI controller and estimates the power loss and the local and global loading difference for a particular node, for a set of operating condition. This surrogate model is used by the Q-Learning based reinforcement technique which adjusts the droop constants and provide the voltage reference to primary controller to maintain load sharing and reduce the power losses, there-by leading to an improved overall efficiency. The proposed control structure is verified to improve efficiency while maintaining the load sharing and bus voltage regulation.