High-performance current differencing transconductance amplifier and its application in precision current-mode rectification

A CMOS-based circuit for realization of high-performance current differencing transconductance amplifier (CDTA) is demonstrated. The proposed circuit offers the advantages of a wide frequency bandwidth and very small input terminal impedance. The results of circuit simulations and an application example are given to illustrate the advantages of the proposed circuit for precise high-frequency signal rectification.     

Measurement and analysis of harmonic distortion in power distribution systems

Power distribution systems often exhibit different harmonic behaviours at different locations. It is not usually possible to completely characterise the distortion at a particular site, however, due to insufficient data about the measured system and instrumentation inability to evaluate or register complete set of measurement quantities. In this paper existing power quality standards are analysed and new harmonic distortion indices are derived. A new index describing the total harmonic distortion in current measurements, TRD, is suggested in order to simplify and standardise harmonic measurements. Another index representing the increase of harmonic voltage distortion with TRD, equivalent harmonic impedance z_eq, is proposed as an efficient way of characterising a particular site. The framework described here can be used to predict future harmonic behaviour at any previously measured point. Finally, extensive harmonic measurements of a real distribution system were performed to prove the usefulness of the proposed indices.     

Testing Positiveness of Polynomials

Many problems in mathematics, logic, computer science, and engineering can be reduced to the problem of testing positiveness of polynomials (over real numbers). Although the problem is decidable (shown by Tarski in 1930), the general decision methods are not always practically applicable because of their high computational time requirements. Thus several partial methods were proposed in the field of term rewriting systems. In this paper, we exactly determine how partial these methods are, and we propose simpler and/or more efficient methods with the same power.     

3D nonlinear transient field‐circuit modeling of inductive fault current limiters

Due to increasing levels of fault currents, fault current limiters (FCLs) are expected to play an important role in the protection of future power grids. Inductive saturable FCLs are particularly interesting due to their inherent reaction on the fault. Many different configurations have been proposed in literature. Being difficult or impossible to create accurate analytical models of some FCL configurations, the development of finite element (FE) models for inductive FCLs is required. This paper presents a 3D nonlinear transient FE modeling technique applied to two inductive FCLs, namely the so‐called open‐core and three‐leg single‐core FCLs. The models have been validated by comparing simulation results with lab measurements. Results show excellent agreement. The models constitute a valuable tool for design, optimization, and verification of inductive FCLs. Copyright © 2015 John Wiley & Sons, Ltd.     

Hybrid modelling and emulation of mem‐systems

The recently published memristor emulator is based on a digital potentiometer, which is controlled by a microprocessor according to a programmed algorithm. After completing the emulator with suitable mutators, it is possible to emulate also memcapacitors and meminductors. This paper describes a novel approach to the modelling and emulation of general mem‐systems without the necessity of utilizing a digital potentiometer and additional mutators. Copyright © 2011 John Wiley & Sons, Ltd.     

Wind speed parameters sensitivity analysis based on fractals and neuro-fuzzy selection technique

Fluctuation of wind speed affects wind energy systems since the potential wind power is proportional the cube of wind speed. Hence precise prediction of wind speed is very important to improve the performances of the systems. Due to unstable behavior of the wind speed above different terrains, in this study fractal characteristics of the wind speed series were analyzed. According to the self-similarity characteristic and the scale invariance, the fractal extrapolate interpolation prediction can be performed by extending the fractal characteristic from internal interval to external interval. Afterward neuro-fuzzy technique was applied to the fractal data because of high nonlinearity of the data. The neuro-fuzzy approach was used to detect the most important variables which affect the wind speed according to the fractal dimensions. The main goal was to investigate the influence of terrain roughness length and different heights of the wind speed on the wind speed prediction.     

Digital fracture surfaces and their roughness analysis: Applications to cement-based materials

Roughness numbers employed in morphological analyses characterize height irregularities of solid surfaces, which are also utilized in fractographic studies. In principle, roughness numbers are computed as height differences between the measured height profile and the reference level that has to be implemented into the profile. Positioning reference level is not an unambiguous computational operation and the result depends on the type of optimization procedure as well as the functional pattern used. A wrong position or an inconvenient pattern results in wrong roughness numbers, which devalue morphological analyses. The Fourier series has proved to be a reliable functional pattern capable of optimum positioning within the digitalized height profiles. The whole procedure is illustrated with fracture surfaces of hydrated cement pastes, whose fracture surfaces have been a subject of fractography analyses and morphological studies for several past decades.