Processing transformer insulation

Author: Lawrence Kirchner


Centuries ago, keeping your gun powder dry was critical to weapon functionality, and your own survival. In today’s world, keeping insulation dry is a matter of transformer survival.


Transformer insulation processing, also known as oil processing, is the practice of removing excess water from the transformer cellulose insulation. This is not only a routine procedure, it is also vital to the survival of a transformer. Here is why:

  1. Excess moisture in the insulation promotes early aging, ultimately leading to premature failure. Although transformers are inherently efficient devices, there are three key elements that cause a transformer to age: carbon based material (cellulose insulation), heat, and oxygen. The carbon based material refers to the transformer insulation. The heat is generated by the transformer, and the oxygen is found in the small amount of water imbedded in the insulation. When the amount of moisture is minimized, the aging process is delayed significantly.

    Although the insulation must be constantly monitored for water content, it serves an essential purpose within the transformer. Paper insulation grounds winding voltage and protects windings against severe short-circuit forces. Since the insulation looses it’s mechanical and electrical properties as it ages, reducing moisture is crucial to functionality.

    Insert Graphic/Chart related to: “It is estimated that doubling the recommended moisture content in the paper will speed up the aging process by a factor of two.”

  2. A surplus of H2O in the insulation can lead to Bubble Evolution, also leading to expedited collapse. Bubble Evolution refers to the water in a transformer that converts from a liquid to a vapor. This occurs within the “hottest spot” within the windings. Here, bubbles are generated when the moisture levels in the insulation are allowed to exceed industry recognized limits of approximately 0.5% by volume. The bubbles will then float to the top of the tank. During the assent, they could pass through an area of high dielectric stress and cause a flash. This flash has the potential to burn the surrounding insulation and lead to extreme malfunction.

  3. Last, there is a conductive issue within a transformer. Every day impurities in water cause it to become extremely conductive. Eventually the conductive properties of the water in the insulation will cause the insulation to conduct a lesser voltage winding, ending in failure.

Adding to the problem is a principal known as hygroscopicity. This principal refers to a material within the insulation that absorbs more moisture than any other object in the transformer.


How does the excess water make its way inside the transformer?


The most common source of water ingress is caused by failed or failing gaskets. A transformer should be regasketed every 12-15 years.


Even if gasket leaks are eliminated, a transformer cannot stay dry on its own. As stated above, if the aging process is left untreated, the life span of a transformer decreases exponentially. Transformer dryness must be monitored over time by measuring the amount of water in the oil. A great time to remove excess water in the insulation is when it is being regasketed.


Other sources of moisture ingress are defective nitrogen systems, defective bladders, and defective bushings.


How do we remove the water?


It must be boiled out. Aside from temperature, there is another factor which can cause water to boil: pressure. Water boils at a lower temperature when the pressure is lower.


The most common process used to dry transformer insulation is as follows:

  1. Pull full (under five torr) vacuum on the empty transformer tank (assuming the tank is capable of full vacuum, not all are).

  2. While under vacuum, pull enough oil into the unit to approximately cover all of the paper insulation. Typically, this is about half of the unit content.

  3. While heating, circulate the oil between the transformer and the rig until the transformer outlet temperature has reached its highest point. At this point, the insulation is as hot as the equipment can handle.

  4. Remove all oil from the unit.

  5. Pull vacuum on the empty tank containing the hot/wet insulation. The water will be converted to vapor and be extracted out of the transformer through the vacuum pump system.

  6. There are several techniques to determine moisture level of the insulation including the PIPER chart method and Dew Point method. The acceptable moisture content by volume throughout the industry is 0.5% for a new unit and 0.65% for an older unit.

  7. After achieving the desired moisture level, the unit can then be filled while under vacuum.

It is common practice to then circulate the entire contents of the unit through the processing equipment at least two passes while heating to re-impregnate the paper insulation with oil. The unit is then left to set for an additional 24 hours before applying operating voltage.


Enough about insulation, what about the oil?


Throughout the moisture removal process, oil passes through the equipment at least five times. The oil circuit provides a great opportunity to remove any impurities in the oil that may affect the dielectric properties of the unit.


This circuit generally includes inlet and outlet filters to catch microscopic particles such as paper fibers and carbon. After heating, the oil passes through a vacuum chamber which removes unwanted gasses and moisture.




Keep your transformers sealed and dry to extend the life of each unit in your fleet.