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

The treatment was terminated when the following oil parameters were achieved (as agreed with the client)

  • 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)

Identified Critical issues


Applied solutions

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