What is a heat run on a transformer, and why do we perform it?
Recently we were asked this question, and it’s a great one, because heat runs are a critical step in ensuring transformer reliability and long term performance.
When a transformer is under vacuum for an extended period, something interesting happens with moisture:
At room temperature, water begins to boil under vacuum.
As the vacuum continues, the pressure and energy drop further , lowering the temperature below 0°C and causing the water to freeze.
In the video below (courtesy of Leybold), you can see how water reacts under vacuum. Once frozen, that moisture can’t be removed during the final vacuum. To prevent this, we perform a heat run.
A heat run involves:
🔥 Placing the transformer under deep vacuum (typically < 1,000 microns)
🔥 Filling the unit ~60% with oil heated to ~65°C
🔥 Circulating that hot oil until specific targets are achieved (inbound oil temp or cold trap readings)
🔥 Draining the oil while maintaining vacuum before the final fill
By introducing hot oil under vacuum, the transformer retains heat even after draining, preventing moisture from freezing and allowing for deeper, more effective drying.
Benefits of a Heat Run:
-Moisture Removal – drives out residual water in insulation, reducing risk of PD, dielectric failure, and insulation aging.
-Improved Gas Removal – accelerates desorption of gases, preventing bubbles and discharges under stress.
-Stress Relief & Conditioning – relaxes mechanical stresses in cellulose and structural parts, improving long-term performance.
-Better Final Fill Quality – ensures stronger oil impregnation, leading to stable dielectric strength and reliability.
At MVA, we believe in sharing the “why” behind the process, not just the “what.”
The heat run may take time, but it’s a powerful step in extending transformer life and ensuring performance for decades to come.
Want to learn more about transformer vacuum filling, heat runs, oil filling, and moisture removal? Contact MVA to discuss how our transformer services help improve reliability, extend equipment life, and reduce the risk of premature failure.
