This month’s Featured Technical Paper is from the Proceedings of the 2019 International Conference of Doble Clients. The paper is titled: An Investigation of Sulphides in Transformer Oil.
A 1982 vintage, free-breathing, auto transformer (245/144kV, 150 MVA) experienced two internal faults, one in 2012 and the second in 2017. After the first failure, the tap changer selectors were replaced, returning the original oil (filtered) to the transformer. The corrosivity was determined as low risk based on historical risk assessment factors (no excessive high ambient temperatures, light loads, enamel coated winding conductors, and high oxygen content in the oil). The second failure investigation discovered silver sulphide deposits on the new selector contacts after only five years of service. Lab analysis of the HV winding paper insulation confirmed presence of copper sulphide. This paper will focus on the potential sources of sulphides related to oil reclamation with a reactivated media process. As part of the transformer fleet asset management program, transformer mineral oil is refurbished based on oil quality results and prioritized by importance of the unit. Typically units are in service 25 to 30 years prior to oil quality limits triggering the first oil reclaim. A comparison of historical results from two different oil reclamation machines showed corrosivity correlation with one specific machine as well as a specific oil processing method. The process variation relates specifically to handling the volume of waste oil left over from the media reactivation process and the slow dissipation of residual heat after the columns of Fuller’s earth are reactivated. This paper will provide a summary of the investigation into the source of the corrosive sulphur and recommendations to reduce the risk of creating volatile sulphur compounds during oil reclamation utilizing a regenerative media process.
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