Moisture Protection of Transformer Insulation Materials

Transformer manufacturers are well aware of the effect that humidity has on insulation materials, whether they are made of cellulose or aramid fibers. Changes in the water content of the material lead to shrinkage and swelling, respectively.

During the winding process, winders have noticed deformation between the strips at the upper end of the vertically wound cylinder. In extreme cases, the cylinder assumes a “flower-like shape”. This is an outcome of the dried cylinder absorbing water, causing the cylinder’s circumference to swell. The strips prevent it from forming evenly, and therefore, it is only effective in the space between the strips.

But the effects can be more dramatic and go beyond mere dimensional or geometrical changes. For example, blisters form during the vapor phase drying process. The evaporating water cannot escape fast enough, the resulting vapor pressure exceeds the ply strength of the material, and a void is formed.

Cracks in laminated materials are particularly insidious, especially if they occur within the material and remain undetected. This can happen when relatively dry laminated material is exposed to increased ambient humidity. The material will absorb humidity from the cutting edge until it reaches equilibrium locally, causing the material to expand and creating mechanical stress between the area of the edges and the dry interior of the material. This results in internal, invisible cracks. Needless to say, cracks in structural parts will affect its mechanical strength.

How can these negative effects be avoided? The answer is by preventing water molecules from making contact with cellulose. A widely used practice is the application of oil to cellulose parts. Oil does not chemically interact with cellulose, but instead forms a barrier between the cellulose and the surrounding atmosphere. However, it is essential to consider that water will also migrate in oil, albeit at a slower rate. For example, an oiled cylinder should not be exposed to a humid climate for an extended period. Furthermore, the oil layer slows the emission of the steam generated, and the risk of blistering is increased.

We recommend that our customers keep parts in their packaging for as long as possible. Water molecules can penetrate plastic film, but at a slower rate. Laboratory experiments and practical experience clearly demonstrate that an intact package with plastic film and the addition of a desiccant provides the best protection for sensitive materials.

Please get in touch with us for further advice and information.