Technical Notes on Coil Insulation and Vacuum Pressure Impregnation (VPI) Process

Core Summary:
Insulation treatment determines the service life of transformers. Modern processes typically adopt a "non-wetting" approach combining "less-tape wrapping + Vacuum Pressure Impregnation (VPI)."

Key Process Parameters:

1. Material Selection:

• Reinforcement Materials: Mica tape combined with glass fabric.

• Bonding Agent: Uses epoxy resin with latent curing agent (e.g., boron trifluoride complex). This adhesive is stable at room temperature but cures rapidly when heated.

2. Impregnant (Impregnating Resin):

• Uses anhydride curing agent + cycloaliphatic epoxy resin. This combination has low viscosity, good permeability, and excellent water resistance.

3. VPI Process Flow:

• Vacuum Drying: Removes moisture and air from inside the coil.

• Vacuum Resin Filling: Resin is drawn into the depths of the insulation layer under negative pressure.

• Pressure Impregnation: Pressure (e.g., 7 kg/cm²) is applied to force resin into microscopic air gaps.

• Curing: Rotational curing at high temperature (approx. 150°C) to minimize resin loss.