Sea Marconi employs its own method for diagnosing “DBDS corrosive sulfur – C1“, which involves::
interpreting visual signs on the transformer, in this case following an inspection of twin machines after a breakdown,
the symptoms and relative concentration of DBDS are identified by analysing the oil,
|
Recommended DBDS concentration
|
Reference standard
|
|
|---|---|---|
| For new insulating oils | “non detectable (< 5mg/kg)” | [IEC 60296 Ed. 4-2012, tab. 2, pag. 17] |
| for insulating oils already in operation – before energising | “non detectable (< 5 mg/Kg)” | [IEC 60422 Ed. 4-2013, tab. 3, pag. 24] |
| for insulating oils in operation – following energising | (< 5 mg/Kg)” If the concentration of DBDS is greater than the recommended threshold, a risk assessment must be carried out and mitigating actions applied; table 5, note d – these include a selective depolarisation treatment to effectively remove corrosive sulfur from the oil 11. 4. 4. |
[IEC 60422 Ed. 4-2013, tab. 5, pag. 31] |
| for insulating oils in operation | (< 10 mg/Kg)” –in this case too, mitigation techniques include selective depolarisation for effective removal from the oil 4. 2 page 25 | [CIGRE 378 fig. 9 pag. 31] |
case histories involving research into breakdowns in similar individual machines or types of machine (same oil, same manufacturer, same type of equipment, same operating profile, similar age) are stored in a database,
factors of uncertainty, the speed and the development over time (trend) of each diagnostic outcome are examined and monitored
according to the assessment of these key factors, specific problems are classified according to type and priority, and corrective actions (treatments) are indentified simultaneously by type and priority
The only way to assess the contamination of papers (see above – Causes) is by quantitative determination of the concentration of DBDS in oil correlated to the speed that the DBDS is converted into copper sulfide. Obviously, the higher the speed of conversion the higher the risk, and consequently the higher the priority to implement the necessary countermeasures.
Click to see a practical example
Constant load profile 7500 hours pa
Oil type = insulating mineral-based naphthenic
Oil mass = 50,000 kg
Age = year 2000
DBDS = 200 mg/kg in 2000, DBDS 150 mg/kg in 2005, indicating that 50mg/kg of DBDS were converted to copper sulfide!!!
DBDS = 120 mg/Kg in 2006 indicating that the problem has worsened considerably as the speed has increased on an annual basis from 10mg/kg to 30mg/Kg
This example enables implementation of the best maintenance strategy: for twin machines it is advisable to take adequate measures starting with the higher conversion speed of DBDS into copper sulfide.
With 50,000kg of oil in the transformer and DBDS at 200mg/kg, this means there are 10kg of DBDS in the transformer oil mass. After 5 years with contamination at 150mg/kg, this means there are 7. 5kg of DBDS in the transformer, and consequently 2. 5kg of DBDS have reacted with the copper components inside the transformer, forming up to around 1. 9kg of copper sulfide. This is not evenly distributed; it builds up in the hotter parts of the transformer.
If there is a hot spot (eg T2, ie a temperature between 300° and 700°C, diagnosed by analysing dissolved gases and interpreted according to IEC 60599), there will clearly be greater amount of copper sulfide formation in this area (Arrhenius law), which represents a weak point from the point of view of electrical insulation, and therefore the point that has the highest probability (within the shortest time) of developing into an electrical fault with a power arc.
N.B. In the presence of oils with added passivators (egIrgamet 39), their degradation rates must be assessed in correlation to the speed of DBDS degradation.
Irgamet is typically added to oil in concentrations of 100mg/kg, however it is observed that after about a year the concentration is reduced by up to 90%.
Irgamet is thermally unstable and in the presence of hot spots not effective against the corrosive action of DBDS.
