Analysis of the Structural Mechanical Behaviour of the Stator End-Winding Region under Influence of Ageing Phenomena

Abstract:
In this study, the influence of ageing phenomena on the vibrational behaviour of the stator end-winding region is investigated. Because of changing requirements for the flexible operation of turbogenerators in conventional power plants, dynamic Lorentz forces and thermal cycles are stressing the stator end-winding region. As a consequence, characteristic ageing phenomena like cracks are visible in the electrical insulation system. These ageing phenomena are depicted in a 3D finite element analysis for this study. The resulting influence on steady state vibrations is analyzed and methods to detect the damages by a modal analysis are tested. It is found that ageing phenomena are influencing the vibration level locally by up to more than twice the reference value. Further, a reflection phenomenon within the stator end-winding region is occurring. Each bar, which is in contact to a weakened section, gets influenced in his individual vibrational behaviour. The detection of these failures is possible but difficult by modal testing. Damages like a decreased stiffness of a whole phase are noticeable due to decreased eigenfrequencies. A locally limited damage is visible in the mode shape results of 3D finite element analysis, but it is not possible to locate the damage exactly.