A micro-computed tomography investigation of the breakdown paths in mica/epoxy machine insulation

X-ray computed tomography is a technology capable of visualizing a wide variety of materials in 3D. In this investigation, insulation samples were electrically aged and computed tomography imaging was used for analysis of the microstructure and breakdown paths. Samples consisting of mica tape wrapped around aluminum tubes and impregnated with epoxy resin were produced. Two samples were electrically aged at 40 kVrms AC and another two at 30 kVrms AC until electrical breakdown. The samples were then scanned and reconstructed using x-ray micro computed tomography. The reconstructed images were analyzed and used for rendering volumetric 3D-models of the breakdown channels in the insulation. The results of μ-CT investigation show the location of vaporized insulation as well as the directions of the breakdown paths. When inspecting the 3D models, it is possible to follow the breakdown from electrode to electrode. The results also show that the mica tape layers will act as barriers forcing the breakdown path to follow the epoxy resin layers. It is noted how the breakdown path takes a tortuous course around the mica tape layers and how resin pockets inside the insulation weaken the insulation. Conversely, an improved continuous coverage of the mica tape without wrinkles would improve the insulation properties.