Wrocław University of Science and Technology
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Termovision diagnostics of metal oxide surge arresters
Get PDFThe surge arresters diagnostics based on infrared temperature measurement was evaluated. The 110 kV metal oxide arresters with porcelain housings with two different varistor columns were used. The first one consisted of new varistor elements and the second one consisted of in field degraded varistors. The internal temperature was measured by means of two Tinytalk probes inserted in the varistor column at the bottom and at the top. The external temperature of the housing was measured by means of infrared camera. The maximum temperature of new varistors has grown about VC over the ambient temperature and the external temperature of the housing increased about 1°C only. The temperature of deteriorated varistors increased up to 15°C. Therefore, the arrester housing temperature increased about VC
A new approach to anti-fog design for polymeric insulators
Get PDFFirst results are reported of tests with a patterned texture, silicone rubber surface. Such texturisation is designed to reduce leakage current density and to increase creepage distance, with the aim of improving pollution performance of polymeric insulators. The tests are of two kinds: a) inclined-plane tests (IEC 587: 1984), to determine the erosion rate and tracking of rectangular samples of insulation materials when these are subjected to surface discharges. Such samples have been prepared both with a conventional plane surface and with a textured surface for comparative tests; b) clean-fog tests (IEC 507: 1991) that use an initially dry pollution layer of known salinity. Here, plane and textured samples have been used for comparative tests, where leakage current and discharge activity are monitored during the generation of the fog
New Criteria of Voltage Stability Margin for the Purpose of Load Shedding
Get PDFVoltage 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 (V^2 Y) 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
Performance of Hydrophobic Epoxy Insulators under Industrial Pollution
Get PDFCycloaliphatic epoxy resins have been used alongside silicones and ceramics for more than 30 years as insulation materials for outdoor applications. Insulators made of cycloaliphatic epoxies offer many advantages overer those made of ceramic or silicones. Standard cycloaliphatic epoxies are construction materials, however, they do not exhibit hydrophobicity as do silicones. A new generation of cycloaliphatic epoxy systems with enhanced hydrophobic properties, hydrophobic cycloaliphatic epoxy try to join the advantages of both cycloaliphatic epoxy resins and silicones The paper presents the laboratory and field test results of comercially available hydrophobic cycloaliphatic epoxy insulators. These insulators exhibit the hydrophobic properties under clean conditions. Unfortunately, when polluted by industrial dust the surface becomes hydrophilic. No hydrophobicity transfer was round even three months after surface contamination. Nevertheless, compared with the standard cycloaliphatic epoxy the hydrophobic cycloaliphatic epoxy offers some advantage under light industrial pollution or sea pollution
Performance of Composite Insulators Without Sheds under Light Pollution
Get PDFThe test of insulators without sheds or with small sheds number at Glogow pollution station was described. The performance of insulators with different leakage distance and position (horizontal or vertical) under light pollution is studied. The flashovers, leakage currents and surface erosion were monitored. The 75 kV test voltage is 4 kV higher than the maximum phase to ground voltage of 110 kV system. After long term testing the leakage distance of 80 cm was as withstand distance for horizontal rods and 100 cm for vertical rods. For practical proposes the security factor of 1.3 should be taken into account. Finally, under light pollution the 110 kV composite insulator, in horizontal position could be installed in the form of 100 cm long rods without sheds and vertical insulators with terminal distance of 100 em should have at least two additional sheds
Behaviour of modified insulator surface material under electric stress
Get PDFPerformance of electrical insulators is strongly dependent on their surface properties. In practice manufactures and utilities are using different means to improve them. New materials the insulators or coatings have been used in electric power systems. Also new technologies are tested to modifL insulators glaze, mainly plasma processes. During these processes some changes in the constitution of glaze has to be done and then a new hydrophobic layer is created. Some electrical and physical properties of modified (silicon rubber and fluorourethane coatings, plasma technology) and non-modified real insulators glaze have been presented. These investigations have shown that plasma processes are able to improve the surface properties of the glaze. Since the electrohydrodynamic behaviour of water droplets on the outdoor electrical insulation surface is very important, observations have been done by using a high-speed photo camera
Steep-front impulse voltage tests of composite insulators
Get PDFIt is very important to eliminate improper insulator designs and technological faults. The most sensitive parts of non-ceramic insulators (NCIs) are their microscopic and macroscopic interfaces. In this paper our experiences with the steep-front impulse voltage test are presented. According to the IEC standard, after the action of thermo-mechanical loads samples of NCIs are subjected to steep-front impulses with steepness s >= 1 kV/ns. Each impulse should cause an external flashover without any puncture of the tested NCI sample. A setup which makes it possible to produce steep-front impulse voltage with steepness in a range of 1-4 kV/ns has been constructed. Specially prepared samples of NCIs were tested by applying such impulses. The obtained results indicate that even 4 kV/ns pulses do not damage properly manufactured insulators. Moreover, the steepness is more selective than the standardised 1 kV/ns impulse for badly made insulators
Time-frequency analysis of non-stationary signals in power systems
Get PDFRepresentation of signals in time and frequency domain has been of interest in signal processing areas for many years, especially taking in the limelight time-varying non-stationary signals. The standard method for study time-varying signals is short-time Fourier transform (STFT) that is based on the assumption that for a shorttime basis signal can he considered as stationary. The spectrogram utilizes a short-time window whose length is chosen so that over the length of the window signal is stationary. Then, the Fourier transform of this windowed signal is calculated to obtain the energy distribution along the frequency direction at the time corresponding to the canter of the window. The crucial drawback of this method is that the length of the window is related to the frequency resolution. Increasing the window length leads to improving frequency resolution hut it means that the nonstationarities occumng during this interval will he smeared in time and frequency. This inherent relationship between time and frequency resolution becomes more important when one is dealing with signals whose frequency content is changing rapidly, Baraniuk (l),Choi and Williams (2)
Effect of Air Humidity on Charge Decay and Lifetime of PP Electret Nonwovens
Get PDFThe effects of air humidity on the charge decay and lifetime of polypropylene (PP) electret nonwovens manufactured by the pneumothermic method are presented. The PP fabric samples were corona-charged from a needle electrode under high voltage. The thermally-stimulated equivalent surface voltages were measured at a linear heating rate in a temperature range from 293 K to 395 K. The results of the measurements showed that the equivalent voltage depends on the structural properties of the PP nonwovens as well as the conditions of formation. The selected electret nonwovens were next placed in a chamber whose humidity was controlled within a range of 39% to 94%, and the decay of the electret charge was measured and analysed. The results show that conditioning the samples at a higher humidity influences the changes of the equivalent voltage and the lifetime of the electret fibre
Analysis of Three Phase Signal using Wigner Spectrum
Get PDFNew method of observation and diagnosis of inverter-fed induction motor drives is developed and tested. Unsymmetrical conditions concerning the machine impedances or valves operation are reflected in the spectrum of the current spacephasor. We estimate the spectrum of the space-phasor with the help of the Wigner- Ville distribution (WVD) and we obtain its time-frequency representation with excellent time and frequency resolution. The proposed method is tested with nonstationary multiple-component signals occurring during the fault operation of inverter-fed drives and transmission lines