Stability Of Different Models Research Articles

A Unified Model of a Virtual Synchronous Generator for Transient Stability Analysis

A virtual synchronous generator (VSG) is prone to transient instability under a grid fault, which leads to the loss of synchronization between the new energy converter and grid, and threatens the operation safety of high-proportion new energy grids. There are a variety of control models in the existing VSG control, including active and reactive power models, which lead to their different transient stabilities. However, the evolution characteristics, correlation between different models of VSG, and the internal mechanism affecting transient stability have not been fully studied. To this effect, this paper analyzes their evolution characteristics based on the existing mainstream VSG control models and establishes a unified VSG model and its equivalent correspondence with other models. Then, the phase plane method is used to comprehensively analyze and compare the transient stability of the VSG unified model with other models. It is pointed out that the key factors affecting the transient stability of different models are three links of primary frequency regulation, reactive power regulation and reactive power tracking. Finally, the correctness of the established VSG unified model and the conclusion of transient stability analysis is verified by experiments.

Mechanical and modal analysis of different implant strategies for loss of three teeth with bone atrophy in the maxillary posterior region

Purpose: This study aimed to evaluate the stress distribution and secondary stability involved in five implant strategies, including implant-supported prostheses (ISP) and tooth-implant-supported prostheses (TISP), used for bone atrophy in the maxillary posterior region with teeth loss using finite element analysis, and to explore the more desirable implant methods. Methods: Five implant strategies were made to analyze and compare: M1, implant-supported prosthesis consisting of a short implant with a regular implant; M2, implant-supported prosthesis consisting of a tilted implant with a regular implant; M3, cantilever structure; M4, tooth-implant-supported prosthesis consisting of a short implant with a regular implant; M5, tooth-implant-supported prosthesis consisting of a regular implant, and M6, with only the natural teeth as a control group. Dynamic loading of the above models was performed in finite element analysis software to assess the stress distribution of the bone tissue and implants using the von Mise criterion. Finally, the secondary stability of different models was evaluated by modal analysis. Results: The maximum stress distribution in the cortical bone in M1(60 MPa) was smaller than that in M2(97 MPa) and M3(101 MPa), The first principal strain minimum was obtained in M2 (2271 μ ε ). M4 (33 MPa, 10085 Hz) with the best mechanical properties and highest resonance frequency. But increased the loading on the natural teeth. Conclusions: Short implants and tilted implants are both preferred implant strategies, if cantilever construction is necessary, a tooth-implant-supported prosthesis consisting of a short implant and a regular implant is recommended.

Building for tomorrow: Assessing the temporal persistence of text classifiers

Performance of text classification models tends to drop over time due to changes in data, which limits the lifetime of a pretrained model. Therefore an ability to predict a model’s ability to persist over time can help design models that can be effectively used over a longer period of time. In this paper, we provide a thorough discussion into the problem, establish an evaluation setup for the task. We look at this problem from a practical perspective by assessing the ability of a wide range of language models and classification algorithms to persist over time, as well as how dataset characteristics can help predict the temporal stability of different models. We perform longitudinal classification experiments on three datasets spanning between 6 and 19 years, and involving diverse tasks and types of data. By splitting the longitudinal datasets into years, we perform a comprehensive set of experiments by training and testing across data that are different numbers of years apart from each other, both in the past and in the future. This enables a gradual investigation into the impact of the temporal gap between training and test sets on the classification performance, as well as measuring the extent of the persistence over time. Through experimenting with a range of language models and algorithms, we observe a consistent trend of performance drop over time, which however differs significantly across datasets; indeed, datasets whose domain is more closed and language is more stable, such as with book reviews, exhibit a less pronounced performance drop than open-domain social media datasets where language varies significantly more. We find that one can estimate how a model will retain its performance over time based on (i) how well the model performs over a restricted time period and its extrapolation to a longer time period, and (ii) the linguistic characteristics of the dataset, such as the familiarity score between subsets from different years. Findings from these experiments have important implications for the design of text classification models with the aim of preserving performance over time.

Structural design and optimization of large-span three-centered cylindrical reticulated shell in a coal-fired power plant

Taking the dry coal shed of a coal-fired power plant as an example, the reticulated shell structure model of large-span three-centered cylindrical is established according to the specification requirements, and its design and optimization are carried out. It is to use the full stress design method to establish multiple comparison models by adjusting the thickness of the grid, to do the analysis and calculation for obtaining the comparison results of steel consumption, maximum vertical displacement and overall stability of different models, and then to determine the optimal span ratio of the large-span reticulated shell structure. All of these aim to draw relevant conclusions and provide reference for the design and optimization of similar large-span three-centered cylindrical reticulated shell engineering.

Development of Stem Volume Equation for Urban Trees of Abomey-Calavi in Southern Benin (West Africa)

Information on growing stock is important for understanding health assessment, environmental analysis, carbon storage estimation, and economic analysis of urban forest. The stand volume estimation enables the calculation of ecosystemic services value and growth stock of urban forests. However, most of volume models fitted for multiple species in tropical forests may not be suitable for urban trees. This study was conducted to develop generic volume models for urban trees in Abomey-Calavi at the southern Benin. A total of 1608 trees belonging to 80 plant species were measured for their diameter at breast height (DBH), stem height (h) and stem volume using non-destructive sampling methods. Using a nonlinear procedure, six volume models were constructed. Cross validation and Fit statistics like standard error of estimate (SEE), relative absolute error (RAE), root mean square error (RMSE), fit index (FI), Akaike information criterion (AIC) and Willmott’s agreement index (dw) were used to evaluate the efficiency and stability of different models. The six generic volume models developed in this study included both diameter and height. These models exhibited an absence of multicollinearity, with normal and homoscedastic residuals. Furthermore, they show high efficiency (IF > 0.997) and reduce of prediction errors (RMSE: 0.05388–0.06629 m³; RAE: 0.05186–0.06952), which ensuring stability in the estimates. However, the Model II was the best for predicting the stem volume of urban tree according to evaluation statistics and rank analysis. The models developed can provide stem volumes prediction with accurate estimations. Though, stem heights should be systematically measured. These models can contribute to assess the productivity of urban forests in order to pursue their sustainable management and planning.

Structure and magnetism of the(2×2)-FeO(111) surface

Based on ab initio calculations we determine the features and relative stability of different models proposed to describe the $(2 \times 2)$-FeO(111) reconstruction. Our results suggest that both wurtzite and spinel-like environments are possible, and their coexistence explains phenomena of biphase ordering. The surface phase diagram reflects a competition of charge and magnetic compensation effects, and reveals the important influence of the substrate on the final surface structure. Though antiferromagnetic couplings are dominant, frustration and the delicate balance of surface and bulk exchange interactions lead to a net surface magnetization that accounts for the large measured values.

Simulation of Well Wall Stress Distribution Model and Well Stability

Horizontal well has gained increasing importance in oil and gas development. Thus stress distribution models around the horizontal well attract more people than earlier. In the last few years, a lot of stress distribution models have been proposed by domestic and international scholars. Through numerical simulation analysis, the differences between five models have been analyzed. The classic model suits for qualitative characterization. Using D-P principle to judge well wall stability of different models instructs that the differences between models decrease with the increase of stability factor.

Stabilization and asymptotic behavior of a generalized telegraph equation

We analyze the stability of different models of the telegraph equation set in a real interval. They correspond to the coupling between a first-order hyperbolic system and a first-order differential equation of parabolic type. We show that some models have an exponential decay rate, while other ones are only polynomially stable. When the parameters are constant, we show that the obtained polynomial decay is optimal and in the case of an exponential decay that the decay rate is equal to the spectral abscissa. These optimality results are based on a careful spectral analysis of the operator. In particular, we characterize its full spectrum that is made of a discrete set of eigenvalues and an essential spectrum reduced to one point.

Structural, electronic and energetic properties of GaN[0 0 0 1]/Ga 2O 3[1 0 0] heterojunctions: A first-principles density functional theory study

We present first-principle calculations on the heterojunction between a wurtzite GaN(0 0 0 1) film and a monoclinic β-Ga2O3(1 0 0) substrate. The relative stability of different models of the GaN(0 0 0 1)/Ga2O3(1 0 0) interface was investigated and the most favorable interface consists of threefold- and sixfold-coordinated Ga. This interface structure gives rise to Ga-polarity in the GaN(0 0 0 1) epitaxial film. A detailed analysis of the electronic structure indicates that undistorted bonding at the interface stabilizes this structure between a wurtzite GaN(0 0 0 1) and a monoclinic β-Ga2O3(1 0 0).

Highly Strained Metastable Heterojunction between Wurtzite GaN(0001) and Cubic CrN(111)

We present first-principles calculations on the heterojunction between a wurtzite GaN(0001) film and a cubic CrN(111) substrate. Atomic and electronic structures of the interface in our heteroepitaxial compound semiconductor models are investigated with the use of four models with fixed stoichiometry . The relative stability of different models of the GaN(0001)/CrN(111) interface is examined as a function of the chemical potentials of Ga and Cr, and the most favorable interface consists of sixfold-coordinated Ga with three Ga–N and three Ga–Cr bonds. This interface structure gives rise to N-polarity in the GaN(0001) epitaxial film. The findings agree with previously reported experimental results. A detailed analysis of the bonding arrangements of the lowest energy interface indicates that hexagonal “bulklike” interatomic distances relax this highly strained metastable interface.

Structural, electronic, and energetic properties ofSiC[111]∕ZrB2[0001]heterojunctions: A first-principles density functional theory study

We present first-principle calculations on the heterojunction between a wurtzite GaN(0 0 0 1) film and a monoclinic β-Ga2O3(1 0 0) substrate. The relative stability of different models of the GaN(0 0 0 1)/Ga2O3(1 0 0) interface was investigated and the most favorable interface consists of threefold- and sixfold-coordinated Ga. This interface structure gives rise to Ga-polarity in the GaN(0 0 0 1) epitaxial film. A detailed analysis of the electronic structure indicates that undistorted bonding at the interface stabilizes this structure between a wurtzite GaN(0 0 0 1) and a monoclinic β-Ga2O3(1 0 0).