Vapor phase transformer drying – Part I

Vapor phase transformer drying – Part I

Vol. 3 Issue 2


Vapor phase drying is the most effective method for drying transformer insulation in a manufacturing setting. The process does not lend itself well to transformer drying in the field for a variety of reasons, including the difficulty of removing residual kerosene which can cause a potential change in transformer oil flash point. Several techniques are available for transformer insulation drying in both the field and in manufacturing. Vapor phase drying as part of transformer manufacturing is discussed in this paper.

Keywords: Vapor phase, transformer drying, vacuum chamber, VPD


Depending on the voltage class, it is desirable to suppress the water content of cellulosic-based materials to <0.5% and even as low as 0.1% (by weight) prior to shipment. Cellulosic Kraft materials such as high density pressboard, papers, tapes, crepe tubes and spacers are used in the paper/oil insulation system of the transformer. These are used to support windings and leads as well as to maintain dielectric clearances.

These materials are often provided by the supplier to the transformer manufacturer with a moisture content of approximately 5%. Because these materials are hygroscopic, they will take on some additional moisture through exposure to humid air during the manufacturing process. Efficient suppression of the moisture content from a starting point of >5% to less than 0.5% may involve the removal of many litres of water from the solid insulation during vapor phase drying.

A major consideration when comparing drying techniques is the time required to perform a complete, uniform drying of the entirety of the insulation system. Faster drying times reduce this step as a bottleneck in production and allow faster shipping time for the final product.

Hot air processing involves circulating hot air to raise the product temperature, followed by rapid lowering of pressure with vacuum pumps to evaporate water. Since heating must be completed before the vacuum step, the process takes more time.