A review of stabilization methods for DCMG with CPL, the role of bandwidth limits and droop control

DC microgrids (DCMGs) integrate and coordinate various DC distribution generation units including various renewable energy sources and battery storage systems, and have been used in satellites, the International Space Station, telecom power stations, computer power supplies, electric aircraft, and electric ships. However, the presence of constant power loads (CPLs) can cause instability in DCMGs. Thus, this paper reviews the stabilization techniques that can resolve instability caused by CPLs, as well as various parameters of CPLs, such as bandwidth, and the frequency of the CPLs that can stabilize the DCMGs. It also discusses recent trends and future work in finding stability limits using the parameters of CPLs. It should be useful for directing research towards appropriate mathematical and experimental approaches for the stability of DCMGs with CPLs.

Sequential Cesium Incorporation for Highly Efficient Formamidinium-Cesium Perovskite Solar Cells

Although pure formamidinium iodide perovskite (FAPbI3) possesses an optimal gap for photovoltaics, their poor phase stability limits the long-term operational stability of the devices. A promising approach to enhance their phase stability is to incorporate cesium into FAPbI3. However, state-of-the-art formamidinium-cesium (FA-Cs) iodide perovskites demonstrate much worse efficiency compared with FAPbI3, limited by different crystallization dynamics of formamidinium and cesium, which result in poor composition homogeneity and high trap densities. We develop a novel strategy of crystallization decoupling processes of formamidinium and cesium via a sequential cesium incorporation approach. As such, we obtain highly reproducible and highly efficient solar cells based on FA1-xCsxPbI3 films, with uniform composition distribution and low defect densities. In addition, our cesium-incorporated perovskites demonstrate much enhanced stability compared with FAPbI3, as a result of suppressed ionic migration due to reduced electron-phonon coupling.

Dilatometry of Steel for the Production of 5.56 mm Calibre Rifle Barrels

During high rates of fire, the bore of the firearm barrel is exposed to high temperatures. This exposure induces structural changes in the barrel material, which is especially significant for the substrate of the galvanic chrome plating. The alloy steel grades used currently for firearm barrels, when exposed to heating above the ferrite stability limits, develop a phase transition with a discrete negative change in the material volume, which results in typical crazing in the bore. This effect is destructive to the galvanic chrome plating, leading to a loss of adhesion, which reduces the ballistic performance of the firearm, especially its muzzle velocity. This can be prevented by manufacturing barrels from steels having a limited range of phase transitions. The primary method for determining the presence of distinct volume changes in steel due to phase transition is dilatometry over a wide temperature range, which includes the interval within which the barrel bore is heated. This paper presents the dilatometry results for four steel grades, which included a steel grade currently used for firearm barrels, and an analysis of the effects of phase transition on the degradation of the barrel bore.

Stability Analysis of Power Hardware-in-the-Loop Simulations for Grid Applications

Power Hardware-in-the-Loop (PHiL) simulation is an emerging testing methodology of real hardware equipment within an emulated virtual environment. The closed loop interfacing between the Hardware under Test (HuT) and the Real Time Simulation (RTS) enables a realistic simulation but can also result in an unstable system. In addition to fundamentals in PHiL simulation and interfacing, this paper therefore provides a consistent and comprehensive study of PHiL stability. An analytic analysis is compared with a simulative approach and is supplemented by practical validations of the stability limits in PHiL simulation. Special focus is given on the differences between a switching and a linear amplifier as power interface (PI). Stability limits and the respective factors of influence (e.g., Feedback Current Filtering) are elaborated with a minimal example circuit with voltage-type Ideal Transformer Model (ITM) PHiL interface algorithm (IA). Finally, the findings are transferred to a real low-voltage grid PHiL application with residential load and photovoltaic system.

Investigation of Gyroscopic Effect on the Stability of High Speed Micromilling via Bifurcation Analysis

Catastrophic tool failure due to the low flexural stiffness of the micro-tool is a major concern for micromanufacturing industries. This issue can be addressed using high rotational speed, but the gyroscopic couple becomes prominent at high rotational speeds for micro-tools affecting the dynamic stability of the process. This study uses the multiple degrees of freedom (MDOF) model of the cutting tool to investigate the gyroscopic effect in machining. Hopf bifurcation theory is used to understand the long-term dynamic behavior of the system. A numerical scheme based on the linear multistep method is used to solve the time-periodic delay differential equations. The stability limits have been predicted as a function of the spindle speed. Higher tool deflections occur at higher spindle speeds. Stability lobe diagram shows the conservative limits at high rotational speeds for the MDOF model. The predicted stability limits show good agreement with the experimental limits, especially at high rotational speeds.

Finding Static Stability Limits: Comparison of Reactive Balance in Older People With and Without a History of Falls

Reactive balance is a highly relevant fall risk factor, but is rarely considered in clinical practice. Especially medio-lateral perturbations lead to a pronounced instability of the gait pattern. However, there is no consensus on a method for the assessment of individually challenging perturbation intensities to apply during walking. The aim of this study is to determine and compare the static stability-limits in older adults with and without a history of falls. Twelve older adults with (OAF; 75.6 ±3.66,9♀) and 19 older adults without a history of falls (OA; 77.5 ±4.99,12♀) were subjected to progressive-intensifying perturbations while standing on a perturbation treadmill. In addition, functional performance (Mini-BESTest), fear of falling (FES-I), and physical activity (kcal) were assessed Deflection of the treadmill-platform was randomized by timing and direction and was increased until the subject had to compensate with a step (stability-limit). The maximum deflection distance for each direction, as well as the FES-I score, mini-BESTest score, and activity level were evaluated for group differences using the t-test and Mann-Whitney-U test (α≤5%). There were no significant group differences in the mini-BESTest and between the maximum tolerated deflection distances. The OAF-subjects showed an increased FES-I score (median for OA=18.0 and OAF=22.0, p=0.032) and higher activity levels (median for OA=1974 kcal and OAF=3365 kcal, p=0.011). Despite an increased fear of falling, the older adults with a fall history showed a similar stability-limit, but higher activity levels. In future experiments these static stability limits should be tested during walking and evaluated via motion analysis.

Metastability and dynamic modes in magnetic island chains

The uniform states of a model for one-dimensional chains of thin magnetic islands on a nonmagnetic substrate coupled via dipolar interactions are described here. Magnetic islands oriented with their long axes perpendicular to the chain direction are assumed, whose shape anisotropy imposes a preference for the dipoles to point perpendicular to the chain. The competition between anisotropy and dipolar interactions leads to three types of uniform states of distinctly different symmetries, including metastable transverse or remanent states, transverse antiferromagnetic states, and longitudinal states where all dipoles align with the chain direction. The stability limits and normal modes of oscillation are found for all three types of states, even including infinite range dipole interactions. The normal mode frequencies are shown to be determined from the eigenvalues of the stability problem.

Spatial heterogeneity enhance robustness of large multi-species ecosystems

Understanding ecosystem stability and functioning is a long-standing goal in theoretical ecology, with one of the main tools being dynamical modelling of species abundances. With the help of spatially unresolved (well-mixed) population models and equilibrium dynamics, limits to stability and regions of various ecosystem robustness have been extensively mapped in terms of diversity (number of species), types of interactions, interaction strengths, varying interaction networks (for example plant-pollinator, food-web) and varying structures of these networks. Although many insights have been gained, the impact of spatial extension is not included in this body of knowledge. Recent studies of spatially explicit modelling on the other hand have shown that stability limits can be crossed and diversity increased for systems with spatial heterogeneity in species interactions and/or chaotic dynamics. Here we show that such crossing and diversity increase can appear under less strict conditions. We find that the mere possibility of varying species abundances at different spatial locations make possible the preservation or increase in diversity across previous boundaries thought to mark catastrophic transitions. In addition, we introduce and make explicit a multitude of different dynamics a spatially extended complex system can use to stabilise. This expanded stabilising repertoire of dynamics is largest at intermediate levels of dispersal. Thus we find that spatially extended systems with intermediate dispersal are more robust, in general have higher diversity and can stabilise beyond previous stability boundaries, in contrast to well-mixed systems.