What is the difference between impregnation and encapsulation?

Impregnation is the complete penetration of the process materials (generically called varnish or epoxy) into the windings of the transformers coils. This is often accomplished by using a Vacuum Pressure Impregnation (VPI) cycle.

Encapsulation is the complete encasement by the process materials (generically called varnish or epoxy) onto the windings of the transformers coils. In simple terms, the process materials have a thicker, more complete coating than an impregnated transformer. Encapsulation can be done by performing two or more VPI cycles or using a higher viscosity process material. Sometimes “potted” will be used for “encapsulated”.  A Cast Coil transformer would be considered to be both impregnated and encapsulated.

There is not an industry standard for encapsulation. A winding can be encapsulated, it can be impregnated or it can be encapsulated and impregnated.