In this paper results are presented from investigations on partial discharges (PD) in insulation systems, resembling the stator insulation in high voltage rotating machines. A model, simulating a stator winding in a slot, has been developed, consisting of simple rotating machine insulation test bars with epoxy/mica insulation, mounted between steel sheets forming a dot, in order to investigate the fundamental behaviour of PD in insulation defects in epoxy/mica insulation and the characteristics of the resulting electrical pulses. Stator slot couplers (SSC) were used to detect pulses coming from artificial defects implemented in the model as well as from the existing voids in the test bars' insulation. Some characteristic pulse shapes were found, which could be related partly to the defect type. The high frequency detection method, with directional couplers, made it possible to localize the defects thereby determining the pulse distortion in the chosen geometry. The influence of some parameters was investigated, such as the voltage applied, the location of the defect with respect to the sensor and the orientation of the defect in the slot. This information is essential for PD-detection systems where the noise/signal separation is based on the pulse shape, such as the SSC-technology used in the present investigations
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