Wrocław University of Science and Technology
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Kontrola stanu izolacji w sieciach z izolowanym punktem neutralnym jako środek ochrony przeciwporażeniowej w górnictwie. Opracowanie i wykonanie stanowiska do badań centralnych zabezpieczeń upływowych.
No full textCentralne zabezpieczenie upływowe jest przeznaczone do ochrony przed niebezpiecznymi skutkami upływu prądu elektrycznego do ziemi w kopalnianych trójfazowych sieciach elektroenergetycznych prądu przemiennego i izolowanym punktem zerowym o napięciu znamionowym do 1000 V i częstotliwości znamionowej 50 Hz. Ogranicza ono prawdopodobieństwo porażenia prądem elektrycznym, powstania pożaru, zainicjowania wybuchu gazu względnie pyłu węglowego w stanach zagrożeń w eksploatacji sieci lub urządzeń elektrycznych, którym źródłem są uszkodzenia izolacji doziemnej. Zabezpieczenie kontroluje całą sieć oddziałową, wskazuje aktualną wartość rezystancji izolacji, reaguje na każdy rodzaj zwarcia i powoduje natychmiastowe wyłączenie prądu elektrycznego w przypadku obniżenia wartości rezystancji do wartości krytycznej (pomiarowej). Zabezpieczenie może być instalowane w osłonach kopalnianych urządzeń energoelektrycznych (np. w stacjach transformatorowych). Centralne zabezpieczenie upływowe charakteryzuje się następującymi cechami: -układ elektryczny zabezpieczenia jest zbudowany z elementów przekaźnikowo-tranzystorowych, -zabezpieczenie ma samoczynną kontrolę izolacji sieci w stanie beznapięciowym i pod napięciem, -daje możliwość kompensacji prądów pojemnościowych sieci, jest łatwe i dogodne w eksploatacji i konserwacji
Measurements of interfacial charges in layered PTFE using step electroacoustic technique
No full textThe step-electroacoustic (SEA) technique was applied to measure interfacial charges in layered PTFE. Acoustic signals were measured from both sides of the sample and time-domain analysis was applied to eliminate the effect of reflections of acoustic waves at inner surface calibrated curves of space charge. electric field and potential in three layered specimen have been shown. (C) 2001 Elsevier Science B V. All rights reserved
Power quality evaluation using advanced spectrum estimation methods
Get PDFThe authors show that the use of high-resolution spectrum estimation methods instead of Fourier-based techniques can improve the accuracy of measurement of spectral parameters of distorted waveforms encountered in power systems, in particular the estimation of the power quality indices (such as inter/harmonic groups and subgroups) . The comparison of the frequency and amplitude estimation error, based on numerical simulations is presented. Presentation of selected power quality indices is then followed by comparison of estimation error in the case of application of FFT-based algorithms and parametric methods. Investigated waveforms are typical for dc arc furnace plant. MUSIC and ESPRIT high-resolution methods are used to analyze waveforms in a supply system of a DC arc furnace. Reduction of variance of results is achieved by using robust averaging procedure (winsorized mean)
Diagnostics of high voltage metal oxide arresters procedure errors
No full textA technique has been described which can determine ac resistive current directly or by obtaining the watt loss of the test specimen and dividing by the true rms (trms) value or the applied voltage. The procedure errors with standard voltage dividers and digital systems have been calculated. Special attention was paid to the voltage shift. A new method for the direct resistive current measurement was proposed. The effect of small amount of harmonics was shown
Ultrasaturation Phenomenon in Power Transformers—Myths and Reality
Get PDFIn this paper, the ultrasaturation phenomenon of power transformers during their energization is studied. It is shown that under special conditions, the currents observed after transformer switching on do not contain enough restraining information (e.g., second harmonic), which may lead to protection maloperation. This paper concentrates on a thorough explanation of the problem and possible causes of ultrasaturation. Theoretical investigations are supported and illustrated with simulation studies performed both with MATLAB and Electromagnetic Transients Program–Alternative Transients Program. The outcomes of this research can further be used as hints for substation operation personnel as well as for the development of new protection stabilization criteria, which is not discussed further in this paper
Calculation of Current Transformer Transient Errors
Get PDFTransient saturation of current transformers may cause maloperation of protective devices. Therefore there to a need for a method which enables the calculating of transient transformation errors
Distance Digital Algorithm Immune to Saturation of Current Transformer
Get PDFSaturation of current transformers due to slowly decaying primary current D.C. components cause errors in reproduction of the current fundamental harmonic. Most of the relays are more or less sensitive to the errors. If the saturation occures before the given relay operated. the operation may be delayed up to about 2 time constants of the D.C.component. In some, although rare cases, transient CT errors may cause unselective operation of the relay. Whether the current transformer saturates and when it occures depend mainly on the accuracy limit factor and on the residual flux in the transformer core. However bearing in mind the very high expected values of short circuit currents and the long D.C. time constants one may conclude, that the design of CT-s which never saturate would end in bulky and expensive units. Therefore most of the protective CT-s which are in service saturate during severe transients. It is a duty of protection engineers to design the relays in such a way, that the errors caused by the saturation neither cause maloperation, nor bring about excessive delay
Behaviour of zinc oxide surge arresters under pollution
No full textThis paper presents results of pollution tests with AC voltages which were carried out with a multi-unit zinc oxide arrester. Zinc oxide surge arresters are exposed to heavy thermal and electrical stresses if the porcelain housings of the arrester units are polluted. Together with other environmental stresses such as temporary over-voltages or high temperatures this can lead to thermal runaway of the metal oxide arrester. In the past in pollution research of MOAs a lot of attention was paid to the coupling in of currents at the flanges of a multi unit arrester with the porcelain housings non-uniformly polluted. This has also been taken into account in the ANSI testing standard for pollution tests of MOAs. Although there is no generally agreed standard for pollution testing of MOAs most pollution tests are carried out according to the test procedures given in IEC 507 which only have been proved to be suitable to test the pollution performance of standard insulators
Behaviour of Insulators and Surge Arresters at Glogow Pollution Test Station
Get PDFThe paper reports on the pollution performance of different insulators and surge arresters at Glogow test station from 1981 up to 2004. The environmental protection progress and technology modernization caused a significant decrease of dust and gas emission at a nearby copper smelting plant. As a result, the pollution severity of the area changed from pollution level IV (very heavy) in 1980s to pollution level T (light) after 2000. The flashover voltage was measured on porcelain post insulators under natural conditions in 1987 and, after insulator wetting with a water spray, in 2002-2004. The electrical strength of porcelain insulators was compared with insulators coated with a silicone rubber layer and with insulators coated by fluorourethane layer StaClean. The aging of insulator glazes and corrosion flanges are described. The external and internal charge records and temperature on metal oxide surge arresters with porcelain housing are also presented
Real-Time Determination of Power System Frequency
Get PDFThe main frequency is an important parameter of an electrical power system. The frequency can change over a small range due to generation-load mismatches. Some power system protection and control applications, e.g., frequency relay for load shedding, load-frequency controller, require accurate and fast estimation of the frequency. Most digital algorithms for measuring frequency have acceptable accuracy if voltage waveforms are not distorted. However, due to nonlinear devices, e.g., semiconductor rectifiers, electric arc furnaces, the voltage waveforms can include higher harmonics. The paper presents a new method of measurement of power system frequency, based on digital filtering and Prony’s estimation method. Simulation results confirm, that the proposed method is more accurate than others, e.g., than the method based on the measurement of angular velocity of the rotating voltage phasor