The transport of copper through oil-impregnated paper insulation in electrical current transformers and bushings

In an investigation of OIP (oil impregnated paper) insulation removed from current transformers and bushings that have failed or have been taken out of service, the authors observed dark stains near the paper adjacent to copper braids that is used for capacitive grading. They investigate their cause and how they might affect the performance of the insulation. Analytical tests show that the stains were copper oxides, the type of oxide depending on the availability of oxygen.