Remember the ol’ transformer oil purification experiment from applied chemistry class? Thinking of it as you learn about moisture and oil contamination can be very useful.

Internal moisture in transformer oil tanks is not distributed uniformly, and can be concentrated in the most dangerous parts of the system. Water in the presence of oxygen, even at microscopic levels, is a contaminant that reduces the insulation properties of Oil. Water enters into the transformer oil over time, which reduces the oil’s dielectric (insulation) properties.  As the transformer becomes wetter, it decays even faster, which increases the risk of flashover (arcs/sparks), should the temperature rise to 90 c. Each time the moisture is doubled in a transformer, the life of the insulation is cut by one-half. This causes more electrical breakdowns than any other impurity.

The Solution: Oil Transformer Tank Dryout. This involves filtering and applying a vacuum to the transformer oil, typically in a rig that maximizes surface area.

• Perform a dryout when the transformer reaches 2.5% moisture by dry weight (m/dw) or 30% oil saturation, provided that the transformer tank is rated for vacuum.   
• Apply vacuum to a transformer after the dielectric oil has been removed. Note: make sure your gauge is NIST calibrated.
• A rule of thumb is that, when the transformer gets below 100 microns, the water has been sufficiently removed.  Note: this can take days! For this reason, having a digital vacuum gauge with bluetooth connectivity, and an app for remote monitoring, enables process monitoring over time.

For more detail about how moisture gets introduced to oil, the kinds of issues it causes and how to avoid them, click here for the Application Note.