VPD gas-phase drying technology popularization

Vapour Phase Drying (VPD) is an indispensable process for manufacturing high-grade oil-immersed transformers. Insulating materials such as paper, pressboard and spacers of the active part easily absorb moisture from air. Moisture will drastically weaken dielectric strength and trigger partial discharge. Under vacuum with kerosene vapour, the active part is heated evenly so that moisture trapped inside insulation can be fully removed. Cellulose insulation shrinks significantly after losing moisture during drying. For this reason, winding axial compression bolts shall only be pre-tightened before entering the drying tank. After VPD cycle and full cooling of the active part down to ambient temperature, final retightening shall be carried out according to specified torque to avoid winding loosening and displacement during operation.