A µCT Investigation of the Electrical Breakdown Mechanisms in Mica/Epoxy Machine Insulation

Abstract: Mica/Epoxy machine insulation is a composite material of the mineral mica, fibre-glass mesh and epoxy resin. The material is used in motors and generators rated for high voltage, where the material must withstand multiple stresses such as elevated temperatures, mechanical and electrical stresses. This is a novel investigation that focuses on the breakdown mechanisms related to electrical stress of the insulation. These include partial discharges and electrical treeing. The micro Computed Tomography (μCT) uses a series of X-ray images to visualise the interior. This provides a method for non-invasively examination of the insulation for any signs of material degradation caused by high electrical fields. The following thesis presents an initial study on how this material system interacts with the μCT, the production of 16 samples capable of being electrically aged and scanned in the μCT, with minimal interference, sample preparation and an analysis of the images produced from the μCT. The samples produced are electrically aged at different voltages after which they are scanned with the μCT. The images produced are then analysed for signs of material degradation using different software. Some of the samples did have a complete electrical breakdown and an attempt of correlating these breakdowns to the amount of folds in the insulation is done. It is found that samples with an aluminium core are suitable for μCT imaging. The visualisation of defects such as voids, resin rich parts or folds in insulation is also possible. The detection of material degradation due to electrical aging is not clear enough to draw any conclusions although some possible signs are found. For samples that had an electrical breakdown the result is much clearer. It shows the breakdown channels path inside the insulation, moving in between the mica tape layers. The samples with a complete breakdown also had tendencies that the breakdown channels followed the resin rich part in the sample. Even if no signs of electrical treeing can be seen in the samples during aging, the breakdown channels visualised still show what path the final treeing did take and how it did so by moving around the mica tape.