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São Paulo, Brazil

Koreeda T.,Voith Hydro Ltda | Koreeda T.,University of Sao Paulo | Matos J.,University of Sao Paulo
Journal of Thermal Analysis and Calorimetry | Year: 2011

The study of mica-epoxy-based composite has a great role in high voltage machines industry. Beside electric properties, this composite should present compatible mechanical and chemical, in this case thermal characteristics to insulate properly the conductor strand, avoiding short-circuits, and leading electrical current with minimal losses. Improvement of the quality is possible through the knowledge of raw materials and system. This study aims to list a complete thermal characterization of mica composite, its components, epoxy resin, anhydride methylhexahydrophtalic, mica tape and zinc naphthenate, and thermoanalytical interactions between them. These data shows intrinsic properties of the system, which is so relevant to its great electrical and thermal performance. Thermal analysis allows the detailed study of curing process and thermal decomposition, predicting and suggesting mechanisms, beside future and possible optimization to the system. Composite system glass transition (T g) was obtained through an important and very respected methodology, presenting the value of T g = 138 ± 2 °C, finally characterizing the material. © 2011 Akadémiai Kiadó, Budapest, Hungary.

Neto A.D.,Voith Hydro Ltda | Jester-Zuerker R.,Voith GmbH | Jung A.,Voith GmbH | Maiwald M.,Voith GmbH
IOP Conference Series: Earth and Environmental Science | Year: 2012

It is well known that part-load operation of Francis turbines is dominated by time-dependent phenomena. A high swirl level at the turbine runner outlet constitutes a complex inlet boundary condition for the draft tube flow at this condition. The draft tube flow simulation using a quasi-steady approach of typical two-equation turbulence modelling (k-ω SST) can provide good agreement between measurements and computational calculation for the optimum point. Global main structures of the flow as well as secondary flows can be well predicted. For part load condition this hypothesis is quite questionable due to different time scales which are associated to the different effects that can be observed at this condition. Steady state simulations do not predict the physics of the flow accurately. This work presents an evaluation of the part-load simulation results when applying a hybrid RANS-LES turbulence modelling approach using the finite volume method. Firstly, steady state results employing the k-ω SST turbulence model are presented. Then transient calculations employing both k-ω SST and SAS SST, a 2nd generation hybrid turbulence model, are also presented. In this model, a spatial filtering based on the grid size is not the main criteria applied to determinate the smallest calculated scales of the flow, but the instabilities of the flow field itself are the trigger for the reduction of eddy viscosity production. The effects of the mesh resolution on the solved turbulent structures are also evaluated. The numerical results are compared to time-averaged velocity profile data obtained by LDA (Laser-Doppler-Anemometry) measurements at the inlet of the draft tube diffuser of an nq=55 1/min Francis turbine. Velocity components in flow direction and transversal to it are evaluated. The investigation shows that the steady state results present considerable deviations when compared to those measurements. The transient calculations using the unsteady k-ω SST approach shows some improvement, but the tendencies are similar to steady state calculations. Calculations employing the SAS SST model shows overall better concordance between simulations and measurements. The results calculated with a finer grid model do not present significant improvement on the velocity profiles prediction when compared with the coarser grid one, although allowing the calculation of smaller eddies. © Published under licence by IOP Publishing Ltd.

Koreeda T.,Voith Hydro Ltda | Koreeda T.,University of Sao Paulo | Matos J.,University of Sao Paulo | Goncalves C.S.,Voith Hydro Ltda
Journal of Thermal Analysis and Calorimetry | Year: 2011

Cure kinetics study of epoxy resin composite, employed as stator bars insulation system, was evaluated through differential scanning calorimetry using the dynamic methodology, different heating rates. These experiments provided some important information about the system as activation energy (E a = 65.4 kJ mol-1), glass transition (T g) values on different curing stages including the final cured material information and, besides that, it enables the comparison of these data with new materials under development. The activation energy value allows the determination of different energy needs of the system under evaluation, specially temperature for the material cure, improving the possibility of comparison between different insulation systems in use in the high voltage insulation business. The composite conversion degree based on the cure enthalpy (ΔH cure) at different time of cure was also subject of analysis and from that it was possible to comprehend the cure pattern which allows the cure state prediction of further samples of this type of material and the more accurate evaluation of similar samples acquired directly from stator bars. © 2011 Akadémiai Kiadó, Budapest, Hungary.

Da Silva M.J.,Voith Hydro Ltda | Pereira F.,Voith Hydro Ltda | Lamas T.E.,Voith Hydro Ltda | Gonclves C.S.,Voith Hydro Ltda | And 4 more authors.
33rd Electrical Insulation Conference, EIC 2015 | Year: 2015

The evaluation of high-voltage groundwall insulation aged under long-Term operating conditions is addressed using three sets of Roebel bars removed from large hydrogenerators after several decades of service. To cover an important cross-section of the possible operating regimes for typical hydros, the units involved in the study were selected to represent (i) the air-cooled Pumped-storage, (ii) the air-cooled base-load and (iii) the water-cooled base-load service regimes. Experimental techniques were used to permit assessment of the insulation condition under visual, electrical and morphological aspects. Contrary to what is usually assumed, the results suggest that the pumped-storage operating regime, with its repeated thermal-mechanical cycles of start-stops may create only minimum aging to the groundwall insulation while, in some particular cases, units operating at relatively low temperatures and under mild base-load conditions, may be found to exhibit a more severe insulation degradation in the long run. © 2015 IEEE.

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