Sea Marconi employs its own diagnostic metric for diagnosing “DBDS corrosive sulfur – C2? criticality:
visual signs on the transformer are interpreted, in this case following an inspection after failure of twin machines,
through analysis of the oil, symptoms are identified, namely the specific indicators (TCS – Total Corrosive Sulfur), and related concentrations
|
Recommended DBDS value
|
Reference standard
|
|
|---|---|---|
| For new insulating oils | “non detectable (< 5 mg/Kg)” | [IEC 60296 Ed. 4-2012, table 2, page 17] |
| for insulating oils in operation – before energisation | “non detectable (< 5 mg/Kg)” | [IEC 60422 Ed. 4-2013, table 3, page 24] |
| for insulating oils in operation – following energisation | (< 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, table 5, page 31] |
| for insulating oils in operation | (< 10 mg/Kg)” – in this case selective depolarisation to remove effectively from oils is also one of the mitigation techniques 4. 2 page 25 |
[CIGRE 378 Fig. 9 page 31] |
the database is used to study family or subjective case histories in the search for failures in similar machines (same oil, same manufacturer, same type of equipment, same operating profile, similar age),
factors of uncertainty, speed and evolution over time (trend) for each symptomatic indicator are examined and monitored
on the basis of assessment of these key factors, specific criticalities are classified according to type and priority, and type and priority of corrective actions (treatments) are identified at the same time
The only way to evaluate contamination of paper (see causes above) is by determining the amount of Total Corrosive Sulfur in the oil correlated with the speed of conversion of these compounds into copper sulfide. Obviously, the higher the speed of conversion the higher the risk, and consequently the greater the priority of action to implement the necessary countermeasures.
Click to see a practical example
Constant load profile 7500 h/year
Oil type = mineral oil with naphthenic base
Oil mass = 50,000 kg
Age = year 2000
DBDSeq = 200 mg/kg in 2000 DBDSeq 150 mg/kg in 2005 means 50 mg/kg of DBDSeq were converted into copper sulfide!!!
DBDS = 120 mg/kg in 2006 means that the criticality worsened considerably because the speed 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.
50,000 kg of oil in the transformer and DBDS at 200mg/kg means having 10 kg of DBDS in the transformer oil mass. After 5 years with contamination at 150 mg/kg, this means that 7. 5 kg of DBDS remain in the transformer, and consequently 2. 5 kg of DBDS have reacted with the copper components inside the transformer, forming up to around 1. 9 kg of copper sulfide. These are not uniformly distributed, but accumulate in the hottest areas of the transformer.
If there is a hot spot (e. g. T2), it is clear that in that area the speed of formation of copper sulfide is higher (Arrhenius equation), which determines a weak point from the point of view of electrical insulation and thus that with the highest probability of evolving (in less time) into an electrical failure with the power arc.
N.B. In the presence of oils with added passivators (e. g. Irgamet 39) their degradation rate must be evaluated in correlation with the rate of degradation of corrosive sulfur compounds.
Irgamet is typically added to oil with concentrations of 100 mg/kg but it has been seen that after about one year its concentration has decreased to 90%.
