# Are oil-filled transformers ever run at a vacuum?

Larger oil-filled transformers are sometimes dehydrated by removing the oil and pulling a vacuum inside the chassis for an extended time. That requires that the chassis must be rated to hold a vacuum. If I read correctly, the dielectric strength of a vacuum is greater than that of the insulating oil. Given that, why not just run such a transformer at a vacuum all the time? Do the thermal benefits of the oil overwhelm the benefits of the insulation being perfectly dry all the time? If so, could you design a transformer that wouldn't have that property? Are there, in fact, transformers designed for vacuum operation?

• What if after some time the vacuum fails? – Lelesquiz Jun 21 '17 at 13:00

• @StephenCollings Probably quite a bit unless it was specifically designed with some other means than oil to conduct heat out of the windings and core and to dissipate it. Then you end up with the extra cost and size of a 'dry' transformer (and then some) plus a hermetically sealed housing.  – Spehro Pefhany Jun 21 '17 at 14:25