25 October 2019
Moisture Protection of Insulation Material
Transformer manufacturers are well aware of the effect humidity has on insulation material, whether it is made of cellulose or aramid fibers. Changes in the water content of the material leads 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 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 forming during vapor phase drying. 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 material are very insidious, especially if they occur in the interior of 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 reaching equilibrium locally and expanding, causing 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 not allowing water molecules to make contact with cellulose. A widely used practice is the oiling of cellulose parts. Oil does not chemically interact with the cellulose, but forms a barrier between the cellulose and the ambient atmosphere. However, it is very important to consider that water will also migrate in oil, but at lower rate. For example, an oiled cylinder must not be exposed to a humid climate for a long duration. Furthermore, the oil layer slows the emission of the steam generated and the risk of blistering is even increased.
We recommend that our customers keep parts in their packaging for as long as possible. Plastic film can be penetrated by water molecules, but at a slower rate. Lab experiments and practical experience clearly demonstrates that an intact packing with plastic film and the addition of a desiccant is the best protection for sensitive materials.
Please contact us for further advice and information.