Wrocław University of Science and Technology
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Advanced Signal Processing Methods for Evaluation of Harmonic Distortion Caused by DFIG Wind Generator
Get PDFIntensified growth of wind power generation emphasizes the need for detailed evaluation of disturbances caused by wind generators. The paper presents a physical model of doubly fed induction generator (DFIG) connected to low voltage distribution grid. Harmonic distortions caused by wind generator under various operational conditions regarding wind speed, active and reactive power have been investigated. Fourier transform, ESPRIT and Prony analysis have been applied simultaneously to evaluate distortion level
Generation of Short Acoustic Waveforms Using Piezoelements
No full textPolymeric (PVDF), ceramic (PZT), crystalline (LiNb03) and composite (PZT/PS) piezoelectric elements were used as actuator, for generation of short standard waves Such kind of acoustic waveforms are useful for space charge distribution measurements in dielectric by pressure propagation method. The pulsed (t pulse = 5 ns) and step (t risetime = 1 ns) voltages were applied to thin and thick piezoelectric samples from pulse cable-generator. The monopolar and bipolar acoustic pulses with nanosecond duration as well as steplike waveforms and signals with linear rise in time were obtained. The acoustic waveforms were observed by measuring voltage signal generated in another 9 pm thick PVDF sensor. The simple model has been proposed as an attempt to understand the generation of acoustic signals
Capacitance Changes in Degraded Metal Oxide Varistors
Get PDFZnO based ceramics show a highly nonlinear current-voltage (I-V) characteristic due to intergranular layers at the boundaries of ZnO grains. This research indicates that the junctions formed on the border between ZnO grains and the intergranular phase are responsible for the creation of potential barriers which control the flow of current through the bulk of such a ceramic. Having polycrystalline nature and a large number of intergranular barriers, and so a high energy absorption capability, ZnO ceramics are used as active components in overvoltage protection devices. But as a result of charge transport the height of the barriers decreases and varistor degradation occurs. The degradation seems to affect mainly the prebreakdown region of the I-V curve and it results in increased leakage current. This paper presents a part of research aimed at assessing the effect of ageing on the capacitance characteristic of ZnO varistors
Changes in varistors properties after radial field test
Get PDFZnO varistors are used in commercial surge arresters in high voltage application in the area of power engineering. Metal oxide arresters (MOA) operated in polluted areas may be subjected to an environmental stresses causing an internal partial discharges (PD), increased varistor temperature, increased leakage current, which may lead to “thermal run-a-way”. Internal PD, increased temperature and increased current may occur due to the uneven voltage distribution created by the surface leakage currents. It may be achieved when different voltage distribution between varistor column and porcelain housing becomes, especially when the stable dry zone occurs. The author, in this work, establishes uneven voltage distribution immersing arrester in tap water. This paper present changes in varistors properties after radial field test. It was shown, that changes in currentvoltage characteristic can be result from degradation of insulating layer covering the varistors as result of the strong partial discharges created between internal surface of the porcelain housing and attentive surface of the varistors column
Probes for spot measurement of surface conductivity on polluted insulators
Get PDFThe surface conductivity of insulators in the field is usually very non-uniformly distributed. In this case the integral surface conductivity is not an appropriate parameter of pollution severity. A better evaluation can be achieved on the basis of local conductivity measurements by means of special probes, e.g. the rod probe or tissue strip probe. In this paper the form factors of different probes are given, and the influence of wet-contamination area, the meniscus and the pollution layer thickness on the measured value of surface conductivity are shown. It was found that the surface conductivity on polluted silicone rubber measured by a tissue strip probe is proportional to the Equivalent Contamination Deposit Density ECDD
Real-Time Modelling and Digital Implementation of Fuzzy-Logic Controllers
Get PDFThis paper is concerned with the numerical optimization of the Fuzzy Logic Controller (FLC) for both its efficient simulation and easy real-time implementation. The basic concept of the FLC based on generalized T-operators approach has been presented. Several assumptions regarding the mathematical formalization of the FL,C have been made leading to its numerical simplification. They include: Goguen formulas for T-operators, "Center of Gravity" method for defuzzification and the overlapping between not more than two membership functions for the controllers fuzzy output. Under these assumptions, the paper presents the FLC non-fuzzy output as the ratio of two weighted sums of certain functions of the inputs reducing the computational burden of the PLC algorithm at least few tens times comparing with the definition-based approach. The Fuzzy Logic Power System Stabilizer (PSS) has been chosen as an application example. Results of simulation studies demonstrate the efficiency of both the presented optimization method in terms of numerical simplicity and the designed PSS in terms of improving the power system stability
Fuzzy Set Approach to Transformer Differential Relay
Get PDFIn the process of decision making by a transformer differential relay often occurs a measure of uncertainty. It is so, because the differential current may be caused by other reasons than internal faults. Because of that, to improve reliability the relays are comparatively slow, because the correct decision requires at least one full cycle of current, and sometimes the delay is much longer. To improve it one may use several criteria and apply the so called `a fuzzy set approach'. Fuzzy sets have become a very powerful mathematical tool to describe quantitatively uncertain values and relations between them. They are perfect to solve the decision making problems in cases when symptoms are uncertain and the situation unclear. If applied to the protective relays they make the best compromise between the speed and selectivity of the protection
Special Insulator for Installation of Optical Phase Conductor on 15 kV Line
No full textThanks to the recent improvements in the technology of glass fibres and the development of digital data transmission techniques, optical fibres have made their way into power distribution and industry. Several kinds of optical cables are installed on high voltage networks. In most cases, they are embedded in the overhead ground wire. Some of them are installed in medium voltage distribution networks. Then the optical cable is either self-supporting or embedded in the conductor. The latter solution requires a special insulator-separator. Results of test carried out on such insulator-separators made of polycarbonate, silicon rubber-thermosetting, silicon rubber-RTV, polymer concrete are presented
Model order selection criteria: comparative study and applications
Get PDFArtykuł przedstawia kryteria i porównanie metod redukcji modelu procesu. Przedstawiono i porównano różne kryteria bazujące na dekompozycji macierzy korelacji według wartości własnych: AIC, MDL i MIBS. Porównania dokonano na sygnałach harmonicznych odpowiadających układowi niestacjonarnemu. A practical application of information theoretic criteria is presented in this paper. Eigenvalue decomposition of the signal correlation matrix based AIC, MDL and MIBS criteria are investigated and used for online estimation of time varying parameters of harmonic signals in power systems
Application of Evolutionary Algorithms with Adaptive Mutation to the Identification of Induction Motor Parameters at Standstill
Get PDFIn this paper the application of evolutionary algorithms to the identification of induction motor equivalent circuit parameters at standstill is presented. In order to improve the time efficiency of the identification procedure, the adaptive mutation mechanism is introduced to the evolutionary algorithm. Few versions of the adaptive mutation mechanisms are investigated and evaluated in simulations. By employing the adaptive mutation the significant reduction of the processing time has been obtained while the required accuracy of the algorithm has not been deteriorated. The results of simulation are verified and confirmed in the experimental tests