Microscopic study of a solid insulating material after breakdown under DC and AC voltages

The monitoring of the state of the insulation and it´s interpretation is an important and challenging task. In order to monitor the state of the insulation after application of various voltages under DC and AC conditions, the samples of the paper insulating material Leatherite paper was observed under Scanning Electron Microscope (SEM). The SEM (JEOL JSM-6480LV) was operated in low vacuum mode in order to observe the samples, which are difficult to view due to excessive surface charging. The experimental data were obtained by using a cylinder-plane electrode system with artificially created cavities punched at the centre of the sample. The SEM observations for the virgin samples of Leatherite paper (thickness of 0.13 mm) and the samples stressed with a voltage of 50%, 75% of breakdown voltage and on breakdown were taken under application of DC voltage. On comparison with a virgin sample, no indication of insulation deterioration was observed for the sample with the application of 50% of the breakdown voltage. However, at 75% of the breakdown voltage the center of the sample appears to be more whitish, indicating excessive charging of the sample. It indicates that the deterioration of sample has started. On breakdown, the sample clearly showed a puncture taking place right at the center as expected along with the whitish colour. The puncture was having an area of approximately 50 μm by 35 μm (1750 μm²). The puncture also implies that the center of the sample was highly stressed and the breakdown is mainly due to PD in cavities of the sample. Similar procedure was also adopted for the insulating samples of Leatherite paper under power frequency AC conditions and it is observed that the PD activity is more prominent under AC rather than DC and the puncture area of 180 μm by 100 μm (18000 μm²) was found.