Evaluation of tape-based stress grading coatings by infrared thermography

In medium voltage rotating machines, stress grading (SG) coatings at the ends of coils are used to avoid partial discharges (PDs) on their surface. SG coatings are usually applied as a wrapped tape with certain length and thickness. These dimensions depend on the nominal voltage and the type of material used. The good performance of the SG coating depends mainly on its correct design and application. Commonly, problems become evident when the coils are tested for long term evaluation of insulation systems or once the coils are in service. During voltage endurance (VE) tests, when the voltage applied to the coils is about three or more times the nominal line-to-ground voltage, hot spots or partial discharges in the SG coatings led to early failures of these coatings and, with time, to the complete failure of the insulation system. If the stress grading composites are not correctly designed or applied, modification of the SG coatings would be required to pass the test. In this work, infrared thermography is used to analyze the manufacture quality of tape-based stress grading coatings. The hot spots that can make the systems fail either in service or in voltage endurance tests are related to the SG coating design. It is considered that by this inspection technique, problems with the manufacture process of the coating can be early detected and corrected.