Situation
Twin transformers installed in a quarry:
Manufacturer: xxx
Year: 1980
Power [MVA]: 250
Voltage [kV]: 400
Oil mass [kg]: 41,000
Problem
Following a problem associated with excessive gas formation diagnosed by another supplier, the latter asked Sea Marconi to drain the oil, keep the transformers under vacuum for a specified period, then refill them with oil and carry out oil treatment.
The procedure presented severe logistical constraints as the transformers were located in a quarry. Sea Marconi was able to exploit the modest size and modularity of its decontamination units for the oil treatment.
Sea Marconi solution
Sea Marconi implemented the following measures:Draining the oil from the transformer, refilling the transformer under vacuum and oil treatment
Details of stages in the process
- Installation of the site and piping connections
- draining and preparation of the transformer for vacuum
- applying the vacuum to the transformer
- keeping the transformer under vacuum
- refilling under vacuum
- checking the conservator and purging
- treating the oil by circulating it through Sea Marconi decontamination units
- sampling the oil for testing at the end of the process
- disconnection and uninstalling the site
Criteria for terminating the treatment
- water in the oil < 5mg/kg at 20°C (sampling by aspiration in the treatment unit);
- oil dielectric strength > 60 kV;
- amount of water extracted < 0.25 litre/24 hours;
- temperature 70°C during the final treatment.
Details of treatment
- Vacuum achieved:< 5 mBAR
- Oil circulation flow rate:4000 l/h (T1), 4500 l/h (T2)
- Average oil temperature at the top of the tank:74.3°C (T1), 74.4°C (T2)
- Average oil temperature at the bottom of the tank:53°C (T1), 55°C (T2)
- Extracted water:3.1 litres (T1), 2.5 litres (T2)
- Dielectric strength:75.1 kV
- Water content in oil:6mg/kg (T1), 7mg/kg (T2)
Duration of treatment: 3 days (T1) and 4 days (T2)
