A NUMERICAL ALGORITHM FOR DIRECT REAL-TIME ESTIMATION OF VOLTAGE PHASOR,FREQUENCY AND ITS RATE OF CHANGE

ABSTRACT

A new algorithm for direct estimation of voltage phasor, frequency and its rate of change, of a voltage signal distorted by a DC component and contaminated by random noise is presented. The method is based on the non-recursive Least Error Squares Technique (LS Technique). Using digitized samples of voltage at the relay location, the estimation of the voltage signal unknown parameters is improved by means of an update procedure. The algorithm is adapted to the actually estimated frequency iteratively, which improved the measurement accuracy, increased the measurement range and reduced sensitivity to the noise. To demonstrate the performance of the algorithm developed, computer simulated data record tests are presented. The proposed method seems to be particularly useful in the field of frequency relaying.     

Critical Grain Size/Preexisting Flaw Size Relations in Anisotropic Brittle Solids

A known solution for the critical condition of thermoelastic microcrack initiation is adopted to evaluate the extent of crack propagation after initiation and the arrest condition. It is shown that the critical temperature for crack initiation and the strength of thermoelastically anisotropic polycrystalline materials is not determined by the grain size only but also by the ratio of grain-to-flaw size. The variation of slopes in the strength-grain size plot is interpreted in terms of preexisting annular flaw-size change. The major implications of the analysis are discussed in relation to experimental results on a number of polycrystalline ceramics.     

Equations of perturbed motions and stability of state and semi-state systems

Problems in the construction of equations of perturbed motions for state and semi-state systems are discussed. It is shown that in most cases it is not possible to construct the equations of the perturbed motions, so that the classical approach to the stability analysis of a selected motion is not applicable. The necessary and sufficient stability conditions that provide the testing of stability of an arbitrary system motion, without the utilization of the equations of perturbed motions, are developed. The results obtained concern the original Lyapunov stability definition and contain as special cases some other existing stability results. Two examples are worked out: one of them concerns the analysis of a non-linear feedback system that does not possess a state model.     

Evaluation of contrasting techniques for X‐ray imaging of velvet worms (Onychophora)

Non‐invasive imaging techniques like X‐ray computed tomography have become very popular in zoology, as they allow for simultaneous imaging of the internal and external morphology of organisms. Nevertheless, the effect of different staining approaches required for this method on samples lacking mineralized tissues, such as soft‐bodied invertebrates, remains understudied. Herein, we used synchrotron radiation‐based X‐ray micro‐computed tomography to compare the effects of commonly used contrasting approaches on onychophorans – soft‐bodied invertebrates important for studying animal evolution. Representatives of Euperipatoides rowelli were stained with osmium tetroxide (vapour or solution), ruthenium red, phosphotungstic acid, or iodine. Unstained specimens were imaged using both standard attenuation‐based and differential phase‐contrast setups to simulate analyses with museum material. Our comparative qualitative analyses of several tissue types demonstrate that osmium tetroxide provides the best overall tissue contrast in onychophorans, whereas the remaining staining agents rather favour the visualisation of specific tissues and/or structures. Quantitative analyses using signal‐to‐noise ratio measurements show that the level of image noise may vary according to the staining agent and scanning medium selected. Furthermore, box‐and‐whisker plots revealed substantial overlap in grey values among structures in all datasets, suggesting that a combination of semiautomatic and manual segmentation of structures is required for comprehensive 3D reconstructions of Onychophora, irrespective of the approach selected. Our results show that X‐ray micro‐computed tomography is a promising technique for studying onychophorans and, despite the benefits and disadvantages of different staining agents for specific tissues/structures, this method retrieves informative data that may eventually help address evolutionary questions long associated with Onychophora.     

Toughening of Glasses by Metallic Particles

The role of elastic, thermoelastic, and interfacial properties in the toughening of a brittle matrix by metallic second-phase particles was studied. Two composites were studied: glass+partly oxidized Ni particles (thermal expansion coefficient of the glasses lower than, equal to, and higher than that of Ni) and glass+partly oxidized Al particles (thermal expansion and elastic moduli equal). Weak interfacial bonding between the nickel and its oxide and developed stress concentrations are the major toughness limitations found in the glass/Ni composites. When the thermal expansion coefficient and elastic modulus of the second phase are sufficiently greater than that of the glass matrix, a propagating crack will bypass the particles. When the thermal and elastic stresses are minimized and satisfactory bonding is achieved (glass/Al composites), a 60x toughness increase was realized.     

Fracture of LiF-MgF2 Precipitated Composites

The precipitation reaction in LiF-MgF₂ solid solutions was successfully used to toughen this composite. Incoherent, stable, rod-shaped MgF₂ precipitates were produced in the LiF matrix by quenching and aging treatments. They increased the fracture toughness of the system to >12 times that of undoped LiF single crystals. The maximum toughness was observed with 3 wt% MgF₂ after quenching and aging at 280°C for°300 min. The large toughness increase is attributed to pulling out of the incoherent MgF₂ rods and microcracking ahead of the primary crack.     

Non-linearities in semistate systems and exponential stability of response

The influence of non-linear elements on the exponential stability of a subclass of semistate systems is investigated. It is shown that some constraints on non-linearities can provide information on the behaviour of all phase coordinates if, initially, only the partial stability behaviour is known.     

Examination of the Thermal Equilibrium Assumption in the Microscopic Parabolic Heat Conduction Model Under the Effect of Two Types of Heating Sources

Abstract

The validity of using the microscopic parabolic heat conduction model under the effect of two types of volumetric heating sources is investigated analytically. The considered volumetric heating sources are a fluctuating periodic source and a unit step source of short duration. Also, the role of the electron gas thermal capacity term is investigated. Criteria are derived to show the necessity of using the microscopic parabolic heat conduction model. Other criteria are derived to show a justification for neglecting the electron gas thermal capacity term from the microscopic parabolic heat conduction model.