Deposition of conductive copper sulfide on paper insulation of power transformers was recognized as a serious risk that can result in equipment failure. The aim of this study was to investigate the influence of mineral insulating oil oxidation and the presence of metal passivator on copper sulfide (Cu 2 S) deposition. Oil oxidation was found to increase deposition of Cu 2 S on the insulating paper, as confirmed by IEC 61125 B and IEC 62535 tests. The presence of oxygen resulted in an increase of copper dissolution in the oil and facilitated progressive copper sulfide deposition on the insulating paper, as confirmed by ICP, AAS, and EDX measurements. The effectiveness of aminomethyl substituted toluylbenzotriazole (TTA) metal passivator in suppressing copper sulfide formation and deposition was studied using modified IEC 62535 test. The protective role of TTA bonded to the copper plate surface and absorbed in the insulating paper was observed in tests at 140 and 150 °C, even for highly oxidized oils.

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Copper sulfide deposits in the insulation systems of power transformers decrease the dielectric properties of the insulation and therefore constitute a significant risk for the operation of power transformers. Disulfides, oxidized sulfur compounds, thiols, and elemental sulfur, which could be present in insulating oil, have been recognized as the sources of reactive sulfur responsible for copper sulfide formation. Selective liquid−liquid extraction using N-methyl-2-pyrrolidone solvent was investigated for the purification of mineral insulating oils through the removal of compounds and precursors responsible for copper sulfide formation. The efficiency of the extraction process was evaluated using corrosive sulfur test IEC 62535, SEM/EDX measurements of paper before and after the IEC 62535 test, and measurements of the dibenzyl disulfide concentration in the oil using GC-ECD method. The oxidation stability of refined oils was evaluated using the IEC 61125 method. Precursors of copper sulfide deposits were completely removed from different mineral oils as a result of purification by extraction with N-methyl-2pyrrolidone and 1.0 wt % water as a cosolvent.

This article presents experimentally obtained moisture equilibrium charts for paper and pressboard insulations in mineral and natural ester transformer oils.