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Szczecin, Poland

Wankowicz J.,Energetyki Instytut | Bielecki J.,Energetyki Instytut | Szrot M.,Energo Complex | Subocz J.,ZUT | Malewski R.,Malewski Electrical
43rd International Conference on Large High Voltage Electric Systems 2010, CIGRE 2010

Recently, HV bushing failures were reported, which destroyed large network transformer. These transformers were produced some thirty years ago by the local manufacturer who then designed the internal insulation with a large safety margin, but 400 kV and 220 kV bushings were imported. These oil-type bushings were designed for approximately 30-year technical life. Most of them are still in a relatively good condition but some have developed on-coming insulation fault. Simultaneous replacement of all, nearly 400 bushings is impossible, considering their long delivery time and high cost. An effort was initiated to develop diagnostic procedures and a monitoring system to identify the bushings to be replaced first. Such diagnostics shall be based on a known mechanism of insulation degradation. One of them is due to penetration of atmospheric moisture into the bushing through aged sealing gaskets and gradual increase of water content in the bushing core. In such case the highest concentration of moisture is expected at the outer surface of the core. Another cause is the manufacturing technology of oil-type bushings, which are heated in vacuum chamber to extract moisture trapped in thin paper layers separated by co-axial aluminum foils. The moisture is first evaporated from the paper layer edges, but if the vacuum is applied too fast then the evaporation becomes very intense. The latent heat of evaporation (enthalpy) is absorbed, and the temperature distribution along the bushing axis drops near the core edges. This local temperature drop prevents an efficient extraction of moisture trapped in the middle section of the paper layers. In principle, the manufacturing technology requires a slow lowering of the pressure in the oven, but it may take a few weeks to dry 400 kV bushing. An attempt to shorten this time results in leaving a wet paper pocket usually near the bushing mid-length. Acceptance tests that involve measurements of capacitance and tan δ do not reveal such small pocket of wet paper, since they show an average of the whole cellulose volume. Different diagnostic procedures used to assess bushing insulation condition were evaluated in view of an effective measurement of moisture content in cellulose. Records of polarization and depolarization current can be processed to reveal excessive moisture content. A comparison of frequency-resolved measurement of bushing capacitance and tan δ, as well as tan δ versus test-voltage characteristic, allow to identify bushings with wet-cellulose insulation. AC leakage current recording has been implemented in many bushing monitoring-systems for long time. However, such monitoring provides a reliable warning when the internal fault has reached an advanced stage. Conventional measurement methods of partial discharges in the bushing mounted on transformer in substation are hampered by a high-level of electromagnetic perturbations. However, the developing fault can be revealed by detection of the high-frequency electromagnetic field radiated from the bushing. Source

Experiences in moisture contents assessment in paper-oil insulation of transformers with advanced ageing processes have been presented. It was found that joint measurement RVM+PDC increases quality of assessment, especially for transformers with non-homogenous morphology of insulation. For water contents in cellulose higher that 3% it is necessary to take into consideration for analysis results of oil tests. The measurement RVM+PDC after considering oil tests allows to identify deposits on the active part of transformers. Source

Szrot M.,Energo Complex Sp. z o.o. | Subocz J.,ZUT
Przeglad Elektrotechniczny

Relaxation processes in oil impregnated pressboard with various moisture contents have been analysed on the basis of universal Jonscher dielectric response law and multi-parameter Debye and Curie-Schweindler model. It was found that for moisture contents on the level of 3% there is the change of polarisation and relaxation mechanisms and nonlinear effects starts to grow, which is related to ion mass transport in macroscopic range. As an effect in highly moisturized pressboard it is expected to obtain large errors in moisture contents assessment with RVM and FDS methods. Source

Biel W.,Katedra Hodowli Trzody Chlewnej | Maciorowski R.,ZUT
Zywnosc. Nauka. Technologia. Jakosc/Food. Science Technology. Quality

The research material consisted of grains of six wheat cultivars (Triticum aestivum L.) harvested in 2007 and 2008 (spring cultivars: Torka, Korynta, Cytra, Zadra; winter cultivars: Fregata and Markiza). The analyses comprised the contents of: dry mass, total protein, crude fat, fibre, total carbohydrates, fibre fraction, mineral compounds, amino acids, and nutritional parameters of protein (CS, EAAI, and PER). The Korynta cultivar was characterized by the highest content of protein (176 g/kg d.m.), whereas the Markiza cultivar. - by the lowest content of protein (151 g/kg d.m.). The content of crude fibre in the grains of spring cultivars was lower than in the grains of winter cultivar. The grains of Fregata and Markiza winter cultivars were characterized by the highest content of lignin (ADL). A high content of exogenous amino acids was found in the grains of Markiza cultivar and this was reflected in a high nutritive value computed on the basis of CS and EAAI parameters and of the predicted PER value. In all the samples investigated, lysine appeared to be the first amino acid to limit the value of protein value (CSMH, CSWE). Source

Holowaty J.M.,ZUT | Wichtowski B.,ZUT
Structural Engineering International: Journal of the International Association for Bridge and Structural Engineering (IABSE)

The long-term use of steel bridges leads to changes in the mechanical properties of their structural steels. This issue is demonstrated in a study of cast steels from four railway truss bridges built in 1875. The tests for each bridge involved an analysis of the chemical composition, determination of hardness and toughness, along with yield strength and ultimate strength. The tests made it possible to calculate the current mechanical properties of steels and to estimate their fracture toughness. One of the bridges in the study was built from low-carbon steel and the others from high-carbon steel, which was not thought to be used at that time. An assessment of the current mechanical properties of the steels allowed the bearing capacities of the bridges to be estimated and, following this, technical designs for their upgrading and repair to be drawn up. Source

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