VPI (Vacuum Pressure Impregnation) is a process that our transformer coils undergo in which a special polyester resin coating is applied in interchanging cycles of pressure and vacuum. The coils are then cured overnight in an oven at 350F. The VPI process includes pressure and vacuum. This process allows for better penetration of the varnish in the deepest crevices of the transformer coil. It also offers an increased resistance to corona, an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged. Cast coil units are simply coils encapsulated in epoxy by a molding process while under vacuum. Although suitable for very harsh environments, they can sometimes be technically inferior to VPI. See below for brief comparison of VPI and Cast Coil.
Cast is the common method in Europe and parts of Asia but the U.S. has become savvy to VPI and over the years has switched their preference to VPI. Experience shows that VPI transformers are not only equivalent but, in most cases they are superior to cast coil transformers. And here’s another fun fact, VPI is usually around 10-20 percent less expensive … not a bad selling point!