This underlines the importance of assessing both moisture and temperature of the insulation system. Temperature and vacuum are the main factors for drying speed and drying quality.
For optimized drying the transformer should be heated at the same time as vacuum is applied (as is done during the vapor phase process). With the combination of the low frequency heating (LFH) technique and hot oil spray or hot oil circulation, this can also be achieved on-site.
The low frequency voltage is necessary in order to reduce the applied voltage on the transformer when it is under vacuum. The reason is that the breakdown voltage of the insulation system is much lower under vacuum than under atmospheric pressure. This is also known as the Paschen law.
In order to prevent hotspots during drying (due to reduced cooling), the applied current for heating power transformers should not exceed 50% of the nominal current. Consequently, the copper losses will be no more than Ľ of the nominal losses and the risk of potential hot spots at points with higher resistance will be negligible.