Ground-wall insulation system analysis combining advanced imaging techniques and numerical simulation

• Defects ground-wall insulation composite-material of rotating machines identified by high resolution X-ray tomography.
• Electric field within the composite material is simulated.
• Regions where the electric stress exceeds the critical threshold for partial discharge initiation are identified.
• The proposed work-flow can suggest modifications of the material manufacturing and application process.

High resolution X-ray computed tomography is used to reconstruct the interior of a sample of an insulation system based on mica-paper tapes impregnated with polyester imide resin that is used in ground-wall insulation of a.c. motor – and generator – windings rated 11 kV and above. The tomographic volume is post-processed to identify the presence of minute voids, air-bubbles and very low-density zones that can bring-on partial discharges and material deterioration critical for the operative life of the rotating machines. The tomographic volume is segmented into different materials and a computational mesh is built on it. The electrostatic field is simulated under suitable boundary conditions at the copper conductors and at the ground side. The numerical model allows predicting the maximum intensity of the electric field in critical zones and spotting points above the 2.2 kV/mm threshold, as derived from experimental partial discharge inception voltage carried out on the coil, yielding an indirect pre-evaluation of the insulating material that can guide the improvement of the manufacturing process (i.e. impregnation taping).