Due to the time to short may would like to offer in option for your consider scope as list
below.
· Lowering the transformer oil in main tank.
· Exchanging the filtering valve.
· Top up the transformer oil.
· Hot oil circulation in main tank.
· Oil testing criteria BDV > 60 kV IEC 156 Gap 2.5 mm.
· Duration of service transformer 32 hrs, after shutdown.
· ABB recommend to testing oil by BDV criteria shall be > 30 kV IEC 156 gap 2.5 mm.
If result lower may you will take a risk and strongly recommend to do the oil purify.