Wrocław University of Science and Technology
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Application of higher-order spectra for signal processing in electrical power engineering
Get PDFDuring recent years higher order statistics (HOS) have found a wide applicability in many diverse fields, e.g.: biomedicine, seismic data processing, harmonic retrieval and adaptive filtering. In power spectrum estimation, the signal under consideration is processed in such a way, that the distribution of power among its frequency is estimated and phase relations between the frequency components are suppressed. Higher order statistics known as cumulants, and their associated Fourier transforms, known as polyspectra, reveal not only amplitude information about a signal, but also phase information. If a non-Gaussian signal is received along with additive Gaussian noise, a transformation to higher order cumulant domain eliminates the noise. These are some methods for estimation of signal components, based on HOS. In the paper we apply the MUSIC method (Multiple Signal Classification), both for the correlation and the 4th order cumulant, to investigate the state of asynchronous running of synchronous machines. When the investigated signal is distorted by a coloured noise the exact results can be achieved by applying cumulants
Wpływ oscylacyjnie tłumionych udarów napięciowych na zjawiska starzeniowe w ceramice warystorowej.
Get PDFWarystory z tlenku cynku (ZnO) są ceramiczymi materiałami polikrystalicznymi, charakteryzującymi się silnie nieliniową charakterystyką napięciowo-prądową. Przy niskich napięciach zachowują się jak mało stratne izolatory – w prądzie przewodzenia dominuje składowa pojemnościowa, a składowa rezystancyjna stanowi nie więcej niż 10% całkowitego prądu. Przy wysokich napięciach warystor staje się przewodnikiem – rezystancja warystora gwałtownie maleje, co skutkuje silnym wzrostem prądu przewodzenia. Te właściwości warystora wykorzystuje się w urządzeniach ochrony przeciwprzepięciowej zarówno urządzeń elektronicznych, jak i elektroenergetycznych urządzeń wysokonapięciowych. W eksploatacji warystory ZnO mogą być narażone na różnego typu oddziaływania, które mogą powodować zmiany ich właściwości elektrycznych
Prediction of CT saturation period for differential relay adaptation purposes
Get PDFA new approach to CT saturation assessment is presented in the paper. Detailed algorithms for decaying DC component measurement and CT saturation prediction are proposed and their accuracy is discussed. The information on CT saturation period is further used for adaptation of the generator / transformer differential protection. Improved stabilization for external faults is achieved by appropriate temporal adjustment of the differential curve for the time period of CT saturation. The protection developed was tested with both EMTP-generated and real world registered current signals
Time-frequency analysis of non-stationary phenomena in electrical engineering
Get PDFThis paper serves the idea of applying joint time-frequency representations in electrical engineering. Main directions of researches are concentrated around Cohen’s class of transformation which gives some possibilities of adaptation for analysed signal by choosing appropriate kernel function. Additionally, novel approach delivered by S-transform is also introduced. In order to investigate the methods several experiments were performed using simulated phenomena of switching on the capacitor banks in distribution system. Firstly some aspect of S-transform application were present. Then the influence of different kernel functions were investigated when Wigner-Ville, Choi-Williams and Zhao-Atlas-Marks distributions were applied. Obtained results were supplemented by comparison to classical spectrogram. Proposed methods allowed to track instantaneous frequency as well as energy with better time-frequency precision than classical spectrogram. It leads to applications in diagnosis and power quality area
A multi-criteria differential transformer relay based on fuzzy logic
Get PDFThis paper presents a digital relay scheme for the primary protection of power transformers. A multi-criteria algorithm is developed based on fuzzy sets for the decision making part of the scheme. The cost of wrong decision-making and the amount of information inflow are used along with several standard criteria to improve the reliability of the protection. It is shown that the proposed scheme, if properly tuned, can enhance the sensitivity and selectivity of the digital relay and mitigate problems associated with conventional relay schemes
Measurement of IEC Groups and Subgroups by using Advanced Spectrum Estimation Methods
No full textIEC Standards characterize the waveform distortions in power systems with the amplitudes of harmonic and interharmonic groups and subgroups. These groups/subgroups utilise the waveform spectral components obtained by using a fixed frequency resolution DFT; while this choice represents a good compromise among different aims such as the need of good accuracy, simplification and unification. In some cases the power system waveforms are characterized by spectral components that the DFT with fixed frequency resolution can not capture with enough accuracy. This paper investigates the possibility of a group/subgroup evaluation using advanced spectrum estimation methods: the adaptive Prony, ESPRIT and root-MUSIC methods. The paper presents also the results of applications of these methods to test waveforms and to waveforms obtained from simulations of a real DC arc furnace plant
Classification of low voltage distribution networks by means of voltage distortion.
Get PDFDetermination of power quality becomes more and more important in the future. Low voltage networks are usually large and very complex. Therefore the calculation of power quality parameters by modeling as equivalent network is hardly possible in practice. That’s why new methods for efficient and exact estimation of power quality parameters in low voltage networks are necessary. The presented method is based on the fact that networks of similar structure have a similar behavior in power quality. The Points of Common Coupling (PCC) are divided into different classes, where each class consists of PCC’s with similar characteristics. This way the method allows the estimation of power quality levels based on the class a PCC is assigned to. The paper demonstrates the method for the 5th voltage harmonic as an example power quality parameter. The above-mentioned classification of substations is based on probabilistic neural networks
Spectrum estimation of non-stationary signals in traction systems
No full textThe growing complexity of the AC and DC traction systems in terms of both new technologies and automation requires a careful control of the Power Quality disturbances. Among the disturbances, the waveform distortions are of special concern because of the widespread use of power static converters for both locomotive drives and auxiliary services. Unfortunately, the spectral components of these converters can be highly time-varying both in amplitude and frequency with consequent difficulties in detecting the waveform components and defining proper distortion indices. In this paper the Prony method was tested for this purpose and a novel approach for the analysis of traction non-stationary signals, based on the “subspace” methods, is proposed: the “rootMusic” harmonic retrieval method which is an example of highresolution eigen structure-based methods. Both methods can allow an accurate and useful estimation of amplitude and frequency of spectral components with their changes in time
Optimized robust averaging of event-related potentials
No full textAveraging is one of the most commonly used procedures in Neuroscience and is the basic procedure for obtaining event-related potentials (ERP). Averaging or the estimation of the location of data can be done in different ways, although only the arithmetic mean is routinely used in the current practice of ERP research. In this paper, a number of location estimators (mean, median, trimmed mean, Winsorised mean and recently introduced trimmed L-mean) are reviewed. A new robust location estimator tanh, which allows the data-dependent optimization, is proposed for averaging of small number of trials. The performance of the location estimators, expressed by the signal-to-noise ratio (SNR) of averaged waveforms, is compared for different number of epochs randomly drawn from a set of real auditory evoked potential (EP) data. Procedure of multi-averaging is developed to optimize the choice of the location estimator with respect to SNR or other criterion of the performance. Experiments with real auditory EPs demonstrate significant achievable gain compared to the mean averaging. The procedure of multi-averaging can be applied to a wide range of experimental data, when an objective criterion of the quality of the averaged data can be calculated
Bipolar space-charge problem for semiconductors and insulators
Get PDFIn this work we make use of classical analysis for electrical conduction with one or two boundary conditions. The space charge is defined by free holes and electrons as well as by trapped electrons. Different interactions between carriers such as carrier recombination and generation are considered. We have determined the conditions in which a metal-solid-metal system can act as an n-p junction. Also, we have determined the conditions in which the current-voltage dependence can be strongly non-linear and discontinuous