1,720,977 research outputs found
An experimental investigation into the behaviour of stray fluxes and currents in and around the core frames of turbogenerators
No full textThe aim of this research was to provide information to designers of turbogenerators to enable them to assess the effects of making various electrical connections to the core frame. In particular they will be interested in the effects on the efficiency of the machine and on the risk of serious damage to the stator core. These experiments were made using a small physical model to represent a turbogenerator. This model was designed and built specifically for this investigation. The reasons for building a new model and the design of this model are described. Measurements of stray fluxes are presented showing both the effects of changing the working voltage or current in the model windings and the effects of connecting various conducting strips to the frame. Although it is damage to the stator core and losses in the frame and the core which are of greatest interest, these cannot easily be measured. Instead attention is focused on stray fluxes - it being argued that many of the less well understood losses can be directly associated with one type of stray flux, and that most damage can be associated with heating or with eddy currents driven by stray fluxes. Qualitative predictions are made of the effects on stray fluxes and the associated losses that would result from the presence or absence of the following electrical connections to the frame: axial conducting strips between the core and the frame, shorting straps connecting these axial strips together, and the electrical connection of the core to the frame. In addition to the above options which formed the original subject of this investigation, a number of other changes are discussed which might produce a reduction in core end saturation and losses. Of particular interest is the effect of reversing the pitch direction of the stator winding. It is often assumed that this makes no difference to the magnitudes of the flux densities or to the losses within the machine. However, it is shown that there is a difference and a preferred pitch direction for a synchronous generator is identified. In addition to making qualitative predictions based on our arrangements and/or a theoretical understanding of the behaviour of stray flux, proposals are made for simplified numerical models of the frame which might be interfaced to the powerful numerical models which are now available for core end design.</p
Accurate measurement of losses in three core armoured cables
No full textIt is widely acknowledged within the cable community that the existing IEC 60287 recommendations for determining the losses in three core, SL type armoured cables are not representative of the real cable system. This leads to more frequent requests for measurements to be taken on real cable systems to determine the losses of the system as built. It is important to consider carefully the different causes of error in such measurements; this paper presents an overview of experience gained from conducting measurements across a range of difference cable sizes
Simulating surface charge dynamics
No full textSurface charge dynamics can play an important role in electric field distributions in both AC and DC insulation systems. In this paper the theory behind surface charge dynamics is introduced with a particular focus on the calculation of surface divergence; an operator which is used to determine movement of charge constrained to a surface. A detailed discussion of the model implementation is provided and validated against an analytical solution. Surface charge dynamics in AC and DC insulation systems are then investigated. Assuming the measurement data in the literature is representative it is demonstrated that surface currents have the capability to significantly alter electric field distributions in voids, even over the timescales of an AC cycle
Analytical thermal rating method for cables installed in J-tubes
No full textOne possible thermal pinch point along the route of a wind farm export circuit is a J tube, commonly used to provide mechanical protection to cable sections between the sea floor and the offshore platform. Current ratings for such cable sections are not covered by the scope of IEC 60287, while the existing publications covering such systems have limitations. This paper presents an updated 2D analytical method and a 3D extension for the rating of J tubes with short air section lengths. Continuous rating comparisons have been made against a 3D finite element model which shows a 4.5% variation in rating from the 2D analytical model for air section lengths greater than 10 m, rising to 13% for short air section lengths. With the addition of longitudinal heat transfer within the new 3D analytical approach this variation decreases by 2.5%. Both methods proposed can be solved readily using conventional spreadsheet tools and are broadly compliant with the IEC 60287 methodology
Design study of a high temperature superconducting generator with YBCO windings
No full textThis paper considers the merits of using a HTS rotor winding in a low-speed high-torque synchronous generator and the possibility of building a small-scale demonstrator to investigate the practicality of building such a machine. Pareto optimization results are presented for both full size and demonstrator size machines with YBCO rotor windings. This optimization used a variation of the Kriging method to reduce the number of FEM models required. A number of design issues and some sensitivity analysis are also discussed
A flexible model to calculate buried cable ampacity in complex environments
No full textThe ampacity of buried cables is significantly influenced by the thermal properties of the burial environment. When these thermal properties are not homogeneous it is usually necessary to utilize simulations with a relatively high computational cost that may also use commercial software. In this paper an alternative approach is proposed using conformal maps. Temperature is calculated in an annular domain which is a conformal mapping of the half plane space. Circumferential dependence is captured by expanding temperature as a Fourier series, a finite difference solver then determines temperature components radially. The model is as flexible as any two-dimensional slice model of heat transfer through thermal conduction. Two case studies are considered: three land-based cables in planar configuration and a submarine export cable. The thermal properties of both burial environments are based on conditions which may be encountered in the field and exhibit a high level of stratification. Using a finite element analysis simulation as a benchmark, typical percentage differences in cable ampacities were 0.5%-1%. In addition to accuracy and flexibility the low computational cost of the proposed approach allows for large parameter sweeps, which may be required in a design phase, without requiring commercial software
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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