The lifetime of transformer oils without anti-oxidation additive depends on operational conditions and is usually 3 to 5 years. Life time of oil in switch gear varies from several dozen hours to 3 – 5 years (again, subject to how often switches are operated).

Transformer oils reach their limits within 5 to 10 years, whereas life time of switch gear varies widely. An effective anti-oxidation additive in the oil significantly extends its lifetime.


Transformer oil life time can be extended by continuous regeneration of oil through a thermo-siphon filter attached to the side of the transformer.

Thermo-siphon filters are designed to extend transformer oil lifetime. The system is connected to the transformer tank with two pipelines and plate valves. The process involves thermo-siphon principle: the hotter oil from the top layers passes through radiator coolers on its way down. The filter is attached parallel to the radiators. Therefore, the oil also passes through the filter from top to bottom and is continuously filtered.

A filter like this, as well as an adsorber, is used to extend transformer oil lifetime. Thermo-siphoic filters are used to automatic regeneration of oil within the transformer to slow down the process of oil aging, therefore extending oil and transformer lifetime.

Thermo-siphon filters are used in transformers of 160 kVA and above to extend transformer oil lifetime.

New anti-oxidation additives (amidopyrine and antipyrine), along with alumina oxide, have shown good performance when used in thermo-siphon filters. Besides, there are many additives on the market; just 0.01% of additive (by weight) can significantly reduce acidity of the oil and extend the oil lifetime by 200 – 300%.

The tank of an airtight transformer is design to eliminate contact of oil with air entirely. This maximizes transformer oil life time and keeps the transformer in good condition.