Transformer oil: breakdown voltage from 14 to 70 kV

A transformer oil service company wanted to know what cartridge filtration can do for oil that would otherwise have been condemned. A batch of used mineral transformer oil was therefore filtered in a controlled test at the manufacturer of Delta Xero.

The oil’s starting point failed both of the standards transformer oil is assessed against (BS 148:2020 and IEC 60422:2013): the water content was 350 mg/kg, more than eight times the 40 mg/kg limit, the breakdown voltage was 14 kV against a minimum requirement of 30 kV, and there was visible sediment in the oil.

The oil was run twice through a DX1001T with a single filter element, in an open drum and with no attempt to protect it from moisture and dust in the air along the way. Samples were taken before filtration, midway and after each pass, and analysed by VPS Power, a UKAS-accredited laboratory.

After the second pass the water content was 28 mg/kg and the breakdown voltage 70 kV, more than twice the minimum requirement. The laboratory assessed all the parameters that filtration can affect as satisfactory for continued service under IEC 60422.

The test was time-limited and took place in an open drum; it does not document results from a closed system in service. But it shows that oil with water, sediment and a failed breakdown voltage could be brought back within specification with a single filter element and two passes.