ABSTRACT
Copper sulfide deposition on cellulosic insulating materials in oil-immersed transformers was
investigated by heating tests stipulated by IEC 62535 with controlled atmospheres to which mineral
insulating oils were exposed. Both oxygen in the atmosphere and 2,6-di-tert-butyl-p-cresol (DBPC)
in the oil were found to accelerate the copper sulfide deposition on the insulating paper when the oil
contains dibenzyl disulfide (DBDS). The weight of copper sulfide deposition on the insulating paper
was found to increase when the oxygen concentration was increased up to 20 volume %. X-ray
absorption near edge structure (XANES) analysis was applied to study the chemical bonding state of
copper atoms in copper deposits on the insulating paper for the first time. It can be clarified the
copper deposits on the insulating paper are mainly Cu2S and small amounts of CuO are contained.
Index Terms — Copper, copper compounds, oxygen, oil insulation, transformer oil,
sulfur compounds, paper insulation, transformers.