Investigation of the electrical and chemical processes causing the failure event in a copper sulfide related transformer failure

Copper sulfide related failures of oil-filled plants have become more common around most parts of the world over the last couple of decades, which has led the industry to re-evaluate their asset risk analysis policy for mineral oil insulated power assets. Two main theories for the failure event suggested by the current state-of-the-art are thermal runaway and turn-to-turn disk electrical breakdown. This paper provides an over view of two possible failure scenarios, electrical breakdown and low degree of polymerization, and the likelihood of corrosive oil causing each scenario. Empirical DP studies have demonstrated that the corrosion process degrades the chemical cellulose chain bonds, where DP-life expectancy models demonstrated that the corrosion process reduces 33 % of the transformer life expectancy. The electrical breakdown strength experiments demonstrated that the Cu_xS deposits reduced the electrical breakdown strength of each Kraft paper layer. Finally the results are considered in the larger context of the transformer insulation life-expectancy and its probability of causing the failure event.