October 2012: Failure 2 weeks after a reclaim treatment with fuller earth (performed by a local operator).
Nameplate data of the transformer:
Transformer: grid transformer with OLTC
Power [MVA]: 425
Voltage [KV]: 500
Oil mass [Kg]: 58.000
Oil type: YPF65 paraffinic oil not inhibited
Analysis and diagnosis (by the holder)
The diagnosis of the holder, as a result of the field tests, presumed an high energy discharge in the main tank. Subsequently, after laboratory tests, was discovered a failure in the OLTC that produced a discharge in the transformer main tank.
The reclaim treatment with fuller earth made the oil aggressive towards the silver surfaces of the contacts.
Analysis, diagnosis and expertise by Sea Marconi
«At relatively high temperature, sulphur-containing oil molecules may decompose and react with metal surface to form metal sulphides» (IEC 60422 Ed. 4 – 2013 art. 5.17)
Sea Marconi calls this case as SCBP (Sulfur Combustion ByProducts) & Corrosive Sulfur.
“The oil regeneration process plant involves the high temperature reactivation of sorbent and fuller earth column by uncontrolled combustion. The Sulfur Compounds naturally present (DiBenzoThioPhene – DBTP, etc.) in the Oil and additives (DBDS, etc.) are decomposed in Sulfur Combustion ByProducts (such as corrosive H2S, Mercaptans, etc.) forming Sulphides CuS, Cu2S, Ag2S, etc.”. Ref. other case histories such as National Grid, ABB, etc.
|TCS DBDS Eq. [mg/Kg]||131|
|ASTM 1275 B||4C|
|TAN [mg KOH/g]||0,01|