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There were variations between different insulating papers, but all contained significant amounts of sulfur. The sulfur showed little migration and it is therefore less likely that transformer paper will contribute, in a significant degree, to the amount of sulfur found in transformer oil in service. Thermally upgraded qualities of transformer paper are treated with different nitrogen-containing compounds. Therefore, the clear increase of nitrogen content in oil exposed to thermally upgraded paper suggests that migration from transformer paper can contribute considerably to the amount of nitrogen-containing substances found in transformer oil in service.