| The structure is complex and end-windings are long in end region of largegenerator. For most large turbo-generator has2-pole, the axial distance of twosides of end region is about1/3the total length. The impact of end region shouldnot be ignored. Stator end-windings stretch along involutes in end region. Bothcurrent and flux density distributions are complex in space. Flux leakagemagnetic potential changes with time and space. Therefore, the unwantedparasitic magnetic leakage leads eddy current to be induced in the metal parts.With the increasing capacity of turbo-generator, the electromagnetic loadincreases. The increased electromagnetic load leads magnetic flux leakage andthe eddy current loss of the structural components to increase in end region. Itmay also result in local overheating, regional stress increasing and other issues.Sometimes it endangers the safe operation of generator.Resisting and gathering magnetic flux are two methods which are usuallyused to weaken the magnetic leakage. The former method is based on resistingmagnetic flux by eddy current induced in metal screen, and the later method isbased on gathering magnetic flux to magnetic screen where the magneticresistance is low. The eddy current in metal screen could resist magnetic leakageflux, which force part of the flux leakage to change path. The geometric shape,size and position of components in end region and the inherent properties ofmetal material all have an impact on the distribution of eddy current loss.Therefore, accurate calculation of the magnetic field and temperature distributionand loss of the metal parts is essential to the design for the end region.In this dissertion, a3-D geometry model of the end region for large capacity water-hydrogen–hydrogen cooled turbo-generator was modeled. A mathematicalmodel of nonlinear transient eddy current field was solved by using3-D FEM.The distribution of the magnetic field and the eddy current loss of metal parts inthe end region are obtained under no-load and rated-load conditions. Theinfluence of copper screen on the magnetic field distribution of the clampingplate was analyzed in this paper. Comparative analysis of the loss in differentcase, with and without metal screen, is given to describe the importance ofcopper screen. To reduce end screen temperature, a hollow metal screen isproposed. Using the results of the loss obtained in the magnetic field as the heatsource, the temperature field was solved based on the basic theory of fluid heattransfer. The calculated thermal results were compared with the test data to verifythe accuracy of loss calculation. All of these could provide a reference for furtherstudy and optimizing the structures of turbo-generator in end region. |
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