Experimental Investigation of the Spatial and Temporal Evolution of the Tangential and Normal E-Field Components along the Stress Grading System of a Real Stator Bar

This paper presents results based on direct experimental measurements of tangential (E t ) and normal (E n ) E-field components along the stress grading system (SGS) of a real stator bar (Roebel type) for different AC 60 Hz applied voltages. These measurements were made with a new electro-optic system allowing for the study of both spatial distributions of two E-field components along the bar and their temporal evolution at critical points. The results obtained allowed us to calculate the correlation between the distribution of E n and E t along the SGS. In particular, it was demonstrated that the E n distribution presents a characteristic minimum, which can be used to identify the zone of partial discharge inception. Moreover, it was possible to observe an enlargement of the E t component distribution followed by a saturation in magnitude with the applied voltage increase. Moreover, the results have demonstrated that the waveform of the E n component is mostly affected by the SG material used, producing a greater distortion in its waveform than those obtained for the E t component. The more significant distortion was obtained at the end of the outer corona protection (OCP) material, corresponding to the first maximum of the E n component and characterized by the appearance of a third harmonic of large amplitude.