5 CONCLUSIONS
Copper sulfide deposition on the insulating paper was
remarkably accelerated by the coexistence of oxygen in the
atmosphere and DBPC in the insulating oil when the oil
contains DBDS. Furthermore, it was found that the copper
deposits on the insulating paper were mainly Cu2S and
small amounts of CuO are contained. It has been
considerable risk of conductive copper sulfide deposition
on the insulating papers, when insulating oils contain both
DBDS and DBPC in the existence of oxygen in the
transformers. Therefore, specification of the transformers
and selection of the insulating oils are important to suppress
the copper sulfide deposition on the insulating papers.
ACKNOWLEDGMENTS
The authors would like to acknowledge Mr. K.
Taninouchi, Mr. T. Kawashima, Mr. F. Kato, Mr. J.
Tanimura and Mrs. N. Hirano of Mitsubishi Electric
Corporation for their support and great advice