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Vacuum Pressure Impregnation: VPI vs. VPE

A common transformer construction method is Vacuum Pressure Impregnation, usually referred to as VPI. Another insulation term is VPE which can have two meanings, most commonly Vacuum Pressure Epoxy. VPE can also mean Vacuum Pressure Encapsulation. To better clarify this discussion, VPE Epoxy will be referred to as VPEx and VPE Encapsulation will be referred to as VPEn.

Impregnation verses Encapsulation: Encapsulation typically refers to a thicker coating of insulation than is provided by impregnation. There is no industry specific definition of either impregnation or encapsulation. These terms are left to the manufacturer to define.

Typically epoxy resins have a higher viscosity than the polyester resins usually used in VPI designs. Therefore a transformer which has undergone a VPI process with an epoxy resin will have a thicker insulation coating and may be considered to be both VPEx and VPEn. A transformer which undergoes several VPI cycles with a polyester resin may also be considered VPEn because the thicker coating “encapsulates” the windings.

The question of “is VPI, VPEx or VPEn the best choice” is difficult to answer.

  • The quality of the VPI, VPEx or VPEn processes can vary widely between manufacturers. 
  • The epoxy and its properties used in the VPEx process can vary widely between manufacturers.  Blanket statements and comparisons with the epoxy can’t be accurately made. 
  • The thickness and quality of VPEn insulation application can also vary. 
  • To add additional issues with comparing VPI, VPEx or VPEn, a manufacturer would also have to compare these technologies with cast coil technology. 
  • Cast coil technology and systems also vary substantially between U.L. and IEC approved designs. 

So this leaves the specifier with a better understanding of the product differences but no clear direction. This article would recommend that the specifiers talk to one or more transformer manufacturers who would have specific application experience. It may be determined that multiple technologies may be suited for an application.

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