Results of investigations having the purpose to find out possible correlations between the internal structure of ground-wall insulation of ac rotating machines and dissipation factor (tan) and Partial Discharge (PD) measurement results, are presented in this paper. After testing the complete coils, portions of ground-wall insulation were removed and analyzed using X-ray computed tomography. The 3D digital structure were reconstructed stacking the tomographic slices and specialized image processing tools were used both to visualize the void distribution and to estimate their morphological attributes. Histograms representing the number of distributed micro-voids as a function of their volume have been adopted to verify how the magnitude and density of micro-voids are reflected in the tan and PD test results.
