What Is Vacuum Drying of a Transformer?
Quick answer — Vacuum drying removes moisture and air from the insulation of the active part before it is oil-filled, so the dielectric strength is high and the risk of partial discharge and aging is low. The core-and-windings are heated under a vacuum to drive off the water, then the oil is added under vacuum. The dryness is verified by the moisture and the insulation tests.
Why is a transformer insulation vacuum-dried?
Why dry it
Moisture in the insulation lowers the breakdown voltage, causes partial discharge and accelerates aging. Removing it before oil filling is essential.
The process
The active part is heated (in a vacuum oven) so the water evaporates, and a vacuum is applied to lower the boiling point and remove the vapor, then the dry unit is filled with degassed dry oil under vacuum.
Verification
The dryness is checked by measuring the moisture (in the oil/paper), the insulation resistance and power factor, and the dielectric strength before the unit proceeds.
References & standards
- IEC 60076-1
- IEEE C57.12.90
Related
Related manufacturers
Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp
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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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