Connecting a power electronic converter to a generator implies new and different voltage load stress to the insulation and field grading systems. One main concern is the implications on the field grading varnish or tape at the end-winding. The winding design and voltage levels are different in a hydro generator compared to high power motors connected to a frequency converter. This task therefore includes electromagnetic modelling and analysis of the voltage distribution inside the winding and experimental characterization of the field grading material as function of applied voltage and frequency. Laboratory models of the end-winding will be developed and the models will be verified by surface electric field measurements. The long term effect of significant harmonic distortions on the field grading and the insulation systems will be assessed on selected models.