Corrosive Sulphur in Transformers - a Serious Problem?

CIGRE A2 Working Group publications provide a guide for long-term mitigation and risk assessment of sulphides in transformers.

In 2005 CIGRE set up Working Group [WG] A2.32 to investigate the Cu₂S problem.  In 2006 a paper published on the CIGRE SCA2 website¹ identified the main cause of the new type of failures to be an oxidation stability additive that oil refineries had begun to add to transformer oil, following changes to oil refining processes introduced from 1988 to the early 1990’s.

WG A2.32 published Technical Brochure #378 Copper Sulphides in TransformerInsulation ² in 2009. This document provides very extensive information on the Cu₂S failure mechanisms, its causes, practical failure, risk mitigation options and a new test method which could identify corrosive sulphur in oil, where this could previously not be detected by the old tests. This test has now been adopted as a new IEC standard 62535. 

WG A2.40 continued this important work and published Technical Brochure #625 Copper Sulphides – Long Term Mitigation and Risk Assessment in 2015. 3 This document also reported an additional type of failure, where the corrosive sulphur in the oil reacts with silver plated contacts and collector rings on tapchangers. The AgS builds up on the contacts and collector rings from where it can become dislodged and suspended in the oil during OLTC operations. AgS particles suspended in the oil have caused flash overs and serious damage to the OLTC and tapping windings on a number of Transformers.

AgS on OLTC contacts & collector rings

From the work of and the Technical Brochures published by WG A2.33 and A2.40 we now know that:

• For insulation to be contaminated by Cu₂S, it is a precondition that the oil is corrosive as determined by IEC 62535,  (DBDS >10 mg/kg in oil).
• The risk is increased significantly where temperatures of the oil or hot spots are > 80^oC, oxygen concentration in oil is low, but > 1000 ppm,   (typical in transformers with sealed conservators), dielectric stress level is high.
• Risk can be mitigated by exchanging corrosive oil with new non-corrosive oil, or adding of a metal passivator such as Irmaget 39, reducing the load and/or the temperature by adding forced cooling or starting forced cooling at a lower temperature.
• High voltage shunt reactors, generator step up transformers, HVDC transformer and rectiformers have the highest risk of failure, but other transformers are not immune to Cu₂S caused failures

References:

1.  R.Maina et.al. Dibenzyl Disulfide (DBDS) as corrosive sulphur Contaminant in used and unused mineral Insulating oils. Published    at  CigreSCA2 website 2006.
2.  Cigre Technical Brochure # 378 Copper Sulphides in Transformer Insulation (2009).
3.  Technical Brochure # 625 Copper Sulphides – Long Term Mitigation and Risk Assessment (2015)

These documents are available free to members on CIGRE’s global database at e-cigre (Click here)