Wrocław University of Technology

About: Wrocław University of Technology is a education organization based out in Wrocław, Poland. It is known for research contribution in the topics: Laser & Fuzzy logic. The organization has 13115 authors who have published 31279 publications receiving 338694 citations.

New Criteria of Voltage Stability Margin for the Purpose of Load Shedding

Abstract: Voltage instability is a phenomenon which often contributes to the development of power system disturbances. While increasing load admittance, bus voltage decreases to such a degree that the apparent power (V2Y) does not increase. As a result, voltage collapses with all consequences resulting from it. During the large-scale power system disturbance, the last line of defense is the load shedding at the stations, where the stability margin becomes dangerously low. To do that, there is a need to use automatic devices which process local signals, detect the decreased margin, and activate the load shedding. As a criteria of operating such devices, the voltage level is adopted. The disadvantage of such an approach results from the fact that the relations between the voltage level and the stability limit depend very much on the load power factor. To overcome this weakness, this paper presents a criterion which is directly based on the definition of voltage stability. It calculates the derivative of apparent power against the admittance (dS/dY). It may be easily accomplished, because both the power and the admittance are measurable, and the changes of load occurs continuously, as a result of switching on and off the impedances, and/or actions of the transformer on load tap-changing devices.

Phase Transitions and Coexistence of Magnetic and Electric Orders in the Methylhydrazinium Metal Formate Frameworks

Abstract: We report the synthesis of four perovskite-type metal formate frameworks, [CH3NH2NH2][M(HCOO)3] (MHyM) with M = Mn, Mg, Fe, and Zn. These compounds exhibit two structural phase transitions. The first transition temperature depends weakly on a type of divalent metal and is observed at 310–327 K on heating. X-ray diffraction, DSC, and vibrational studies revealed that it has a second-order character. It is associated with partial ordering of the methylhydrazinium (MHy+) cations and change of symmetry from nonpolar R3c to polar R3c. Pyroelectric measurements suggest the ferroelectric nature of the room-temperature phase. The second, low-temperature phase transition has a first-order character and is associated with further ordering of the MHy+ cations and distortion of the metal formate framework. Magnetic susceptibility data show that MHyMn and MHyFe exhibit ferromagnetic-like phase transitions at 9 and 21 K, respectively. Since the low-temperature phase is polar, these compounds are possible multiferroic ...

Regular bipolar fuzzy graphs

Abstract: We introduce the concepts of regular and totally regular bipolar fuzzy graphs. We prove necessary and sufficient condition under which regular bipolar fuzzy graph and totally bipolar fuzzy graph are equivalent. We introduce the notion of bipolar fuzzy line graphs and present some of their properties. We state a necessary and sufficient condition for a bipolar fuzzy graph to be isomorphic to its corresponding bipolar fuzzy line graph. We examine when an isomorphism between two bipolar fuzzy graphs follows from an isomorphism of their corresponding bipolar fuzzy line graphs.

Chemical waves on spherical surfaces

Abstract: THE concentric-circular and spiral patterns exhibited by the Belousov–Zhabotinsky (BZ) reaction in thin films of solution are representative of spatiotemporal behaviour in a two-dimensional, planar excitable medium1–6. Here we report BZ chemical waves propagating on the two-dimensional surface of a sphere. A wave on the surface of a single cation-exchange bead, loaded with ferroin and bathed in BZ reaction mixture containing no catalyst, develops to form a rotating spiral. Unlike spiral waves in thin films of solution, which typically wind out to connect with a twin rotating in the opposite direction, these waves rotate from pole to pole in a single direction. The spiral winds outward from a meandering source at one pole, crosses the equator, and undergoes self-annihilation as it winds into itself at the other pole. This behaviour, which is not possible in a two-dimensional planar configuration, arises from qualitative (negative to positive) and quantitative changes in wavefront curvature as the wave traverses the spherical surface. These observations of a single spiral wave contrast with theoretical predictions7,8 of counter-rotating spirals in this geometry.

High-resolution spectrum-estimation methods for signal analysis in power systems

Abstract: The spectrum-estimation methods based on the Fourier transform suffer from the major problem of resolution. The methods were developed and are mostly applied for periodic signals under the assumption that only harmonics are present and the periodicity intervals are fixed, while periodicity intervals in the presence of interharmonics are variable and very long. A novel approach to harmonic and interharmonic analysis based on the "subspace" methods is proposed. Min-norm and music harmonic retrieval methods are examples of high-resolution eigenstructure-based methods. Their resolution is theoretically independent of the signal-to-noise ratio (SNR). The Prony method as applied for parameter estimation of signal components was also tested in the paper. Both the high-resolution methods do not show the disadvantages of the traditional tools and allow exact estimation of the interharmonic frequencies. To investigate the methods, several experiments were carried out using simulated signals, current waveforms at the output of an industrial frequency converter, and current waveforms during out-of-step operation of a synchronous generator. For comparison, similar experiments were repeated using the fast Fourier transform (FFT). The comparison proved the superiority of the new methods.