Wrocław University of Science and Technology
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Temperature monitoring of metal oxide surge arresters
Full text linkVaristor overtemperature above the ambient is an important state parameter of metal oxide surge arresters. The temperature monitoring using passive SAW sensors enables realisation of a surge counter function, an energy monitor, monitoring of electrical ageing and pollution stress. For temperature measurements during pollution tests of metal oxide arresters the not so advanced, TINY TALK sensors could be used. This method of temperature measurement was also applied in the field for temperature control of arresters tested at the pollution station near Głogów, Poland. The preliminary results during the first year of monitoring are presented and compared with results of similar measurements conducted in Germany close to the seacoast
Franklin rods - sharp or blunt ?
Full text linkThis paper gives the historical background of 250 year old question: should the Franklin rod tip be sharp or blunt ? The earlier works, the new field results from USA and own laboratory experiments are presented. The switching impulse tests were carried out with two different shapes of high voltage electrodes: rod or sphere and without additional dc bias voltage. In the arrangement with rod as high voltage electrode the smaller breakdown voltages were measured with sharp grounded electrode with negative impulse polarity. At the positive polarity the breakdown voltages do not depend on the shape of grounded electrode. The tests with spherical high voltage electrode showed, that breakdown voltages under negative polarity are practically the same for three different grounded electrodes. In spite of that, the remarkable differences in time to breakdown were noted
TSC transformer with asymmetric source and load
Full text linkThe paper analyses unconventional connection of a three-phase transformer with asymmetric source and load. In the first step no-load condition of a three-phase transformer is calculated. Then no-load Yy a Yd transformer with asymmetric source and Yd transformer with 10Ω load on secondary circuit were modelled in EMTP-ATP programmes
Entrainment of quartz in flotation tests with MIBC frother
No full textDuring passing gas bubbles through suspensions containing hydrophobic or/and hydrophilic particles, some mechanical carryover of particles to the froth layer always takes place. The entrainment of mineral particles is influenced by many parameters, mostly by the particle size and the concentration of the frother. The paper deals with influence of methylisobutylcarbinol (MIBC), a common and standard laboratory flotation tests frother, on the mechanical carryover of quartz, a typical gangue mineral component of ores. Entrainment tests were carried out in a laboratory Mechanobr flotation machine with quartz particles suspended either in water or frother solution. The mechanical recovery of quartz in the frother increased with time and reached plateau after about 20 minutes of flotation. The plateau level of quartz recovery depended on the frother concentration. Initially it is observed an increase of the plateau with frother concentration until it reaches the highest plateau level at about of 32 % of quartz recovery. The shape of flotation kinetics, the plateau level of the quartz entrainment after long (about 20 min) bubbling time and at sufficiently high concentration (above about 510-4 M MIBC) can be used as parameters to characterize the ability of frothers to entrainment of particles. In the studied system of quartz-MIBC-Mechanobr flotation cell at 20 min of flotation and 510-4 M of MIBC, the maximum entrainment is 32 %. These parameters can be useful for evaluation of the mechanical recovery of particles in flotation systems. Surface chemistry of frother interaction with minerals in relation to mechanical recovery of particles will be discussed
Degradation and destruction of ZnO varistors caused by current pulses
Full text linkAlthough the modern metal oxide elements are resistant to degradation caused by continuous operating voltage the degradation or destruction caused by current impulses, especially lightning currents, is still a serious problem. This phenomenon was the subject of the current study where single zinc oxide elements and 6 kV arresters manufactured in Poland were tested with 8/20 ms impulse currents. During our investigation the temperature and changes in current-voltage characteristics were recorded. These results indicate that even varistors that were stressed with high currents could reach saturation state in which following impulses did not cause any further degradation. Both single varistor elements and 6 kV arresters showed only little ageing if stressed with 0,5 kA impulses. The 6 kV arresters composed of 10 elements were able to withstand smaller currents than single varistors. Our findings suggest that a combination of sparking effects and less than optimal contacts are the cause varistor degradation
WYNIESIENIE MECHANICZNE ZIAREN PODDANYCH FLOTACJI WYŁĄCZNIE SPIENIACZEM
Full text linkZ przeglądu dotychczasowych badań oraz badań opisanych w tym artykule, wynika że wzrost stężenia spieniacza powoduje nasilenie flotacji mechanicznej; ponadto istnieje pewne stężenie graniczne, przy którym krzywa zależności wychód stężenie spieniacza osiąga plateau. Każdy spieniacz flotacyjny można scharakteryzować jedną liczbą charakterystyczną dla niego przy odpowiednim stężeniu tego spieniacza. Określenie wartości stężenia granicznego maksymalnej flotacji mechanicznej jest bardzo pożądane, gdyż może to być bardzo użyteczny parametr do opisu wyniesienia mechanicznego. Znaleziono związek pomiędzy stężeniem stosowanych odczynników pianotwórczych i wyniesieniem mechanicznym kwarcu. Potwierdzono, że przy długim czasie flotacji występuje tzw. graniczna flotacja mechaniczna, czyli największa możliwa do uzyskania w danym układzie flotacyjnym
Leakage Current on Porcelain and Silicone Insulators Under Sea or Light Industrial Pollution
Full text linkAbstract—The leakage current on heavily polluted silicone rubber insulators is usually smaller than on similarly polluted porcelain insulators. In this paper, results are presented that show that under light contamination, the currents on the two types of insulators can have similar values. Results are also presented from two different sites that show that under certain conditions the currents on silicone insulators can be higher than on porcelain. The phenomenon is explained, referring to the superior washability of the porcelain and moisture absorption of the pollution layer on the silicone rubber insulators during times of high humidity
OUTDOOR INSULATORS AND HOUSINGS BREAKDOWNS CAUSED BY POLLUTION
Full text linkA heavy pollution on outdoor insulators causes electrical discharges, flashovers, surface erosion or tracking. In extremely cases the dielectric breakdowns occur. In this paper the breakdowns of four insulators has been shown: porcelain bushing insulators on rectifier set ups of electrostatic precipitators, the porcelain fuse cutouts, a porcelain housing of metal oxide surge arrester and polymer concrete insulators. The analysis of other damages of condenser bushings and instrument transformers published in the literature has led us to a hypothesis that a part of these failures could be initiated or accelerated by external pollution
Artificial aging of polymer insulation surface in a clean fog chamber and its evaluation
No full textIt is reported about electrical performance of RTV silicone rubber coatings and epoxy insulators under accelerated aging in a clea fog chamber. The new type of solid layer consists of silicon dioxide and a filter agent Perfil. The research was made to study the behaviour of light polluted indoor insulations under wet conditions (condensation). The currents and partial discharges were registered. The evaluation of aging was made by measurement of contact angle and voltage-current characteristics of condensed water deposits on the insulators surface
Impact of Characteristics of Block-Transformers on Inrush Currents and Voltage Distortions at the Connection Point of Wind Parks
Full text linkIn the procedure of connecting large wind parks (WP) to the external power grid, TSO's or DSO's request is that WPs in operation do not excessively affect the power quality in the connection point. In the process of determining WP operating procedures it is necessary to include switching operations in the WP main feeder. The transformers connected near the wind generator units are then energized and a transformer inrush current appears, followed usually by voltage distortions. Those currents may cause nuisance tripping of protection devices and power quality problems in the WP connection point. For illustration, the connection to a real power grid of a large WP with twelve generators and block transformers is simulated. Considered network short circuit power is comparable to typical parameters of the transmission or distribution medium-voltage networks. Simulated inrush currents depend on the characteristics of connected devices in the network. Results of our study can be useful for correct protection relay device setting and determining optimal WP exploitation procedures