A method for the evaluation of insulation systems for high voltage rotating machines

Partial Discharge Pattern of three important types of defects in insulation systems of high voltage rotating machines (the internal defect, the damaged semi-conductive screen and the defect on grading coating) were studied. As test object, model stator bars with artificial defects similar to practical ones on the ground wall and end-winding of bars were used. The internal defect in insulation was furthermore realized with a thermal ageing procedure on machine bars. The comparison of PD patterns, that were caused by different defect on stator bars presents clearly that the different defects generates different partial discharge types and that the defect type can be recognized and used for diagnosis of the insulation of high voltage rotating machines. For the interpretation of off-line and on-line measurements, the partial discharges were also measured at different temperatures in order to generate a basis for the on-site measurements, which can be only carried at given temperatures depending on the load conditions.