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In high-voltage transmission and distribution systems, the dielectric strength of transformer oil is directly linked to insulation safety and operational stability. Over time, moisture—typically introduced through ambient humidity or released from insulation materials—along with dissolved gases such as hydrogen and methane, and solid particles, gradually accumulate in the oil. This leads to a reduction in dielectric performance and increases the risk of partial discharge, particularly in systems rated at 110 kV and above.

Vacuum purification technology is widely applied to address these challenges. Such systems operate under reduced pressure conditions (typically ≤ −0.08 MPa) to facilitate the separation of dissolved gases and free water from the oil. Integrated heating units generally maintain oil temperature within the range of 40–65°C to enhance moisture evaporation while avoiding thermal degradation of the oil. Multi-stage filtration systems, commonly with filtration precision between 1–5 μm, are used to remove particulate contaminants.

In engineering practice, system design and operation often follow standards such as IEC 60422 to ensure that treated oil meets requirements for dielectric strength, moisture content, and cleanliness. For transformers operating at 110 kV and above, systems with continuous online purification capability are typically recommended to reduce outage-based maintenance and support stable long-term operation.