INTRODUCTION
Copper sulfide deposition on cellulosic materials such as
insulating paper and pressboard in oil-immersed transformers
and reactors is widely known to be attributed to corrosive
sulfur in mineral oils [1-3]. Dibenzyl disulfide (DBDS) was
found to be one of the prime compounds of corrosive sulfur
[3-5]. Various tests for confirming the presence of corrosive
sulfur in mineral insulating oils were proposed [6-8].
Recently, IEC 62535 test was established particularly to
investigate the possibility of the copper sulfide deposition in
mineral insulating oils [9]. A mechanism has been proposed
in which copper sulfide deposition consists of two reactions
via DBDS-Cu complex [10]. The latest research on
mechanism and suppression of the copper sulfide deposition
was reviewed in CIGRE WG A2-32 [11].