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Moisture content, conditioning and shrinkage equilibrium moisture relations.
The fibrous cellulose material contains moisture. There is an equilibrium relation for the natural moisture content of cellulose fibres which will be established in atmospheric air containing a specific water vapour pressure, and another relation describing the equilibrium between the water content of impregnated pressboard or paper insulation and surrounding transformer oil.
Both relations are strongly temperature-dependent – higher moisture content at higher temperature.
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“Conditioning” in combination with testing
The electrical and mechanical properties of pressboard materials are influenced by the moisture content. The standard test prescriptions usually contain a specification how the samples shall be conditioned to a specified moisture content before the actual test is run.
Moisture content on delivery – storage in factories
The moisture content on delivery is limited by Standards, and a maximum value is guaranteed: 8% by weight for low-density material, 6% for high-density precompressed board, and 5% for thick laminates.The actual values for the high-density boards as they leave the manufacturing process are lower than “natural”. Therefore the material will pick up additional humidity after delivery, while it is stored and used in the transformers factories. This is particularly apparent in countries with subtropical climate. It may cause trouble with warping, if the humidification takes place in a non-uniform manner.
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Shrinkage properties
Shrinkage and swelling as a function of moisture content is different in different directions of the material (MD, CMD, thickness), and it is of course different for the different types of material. The standard test method first specifies conditioning of samples to a specified content of moisture. Their dimensions are measured. The samples are then dried down to a low moisture content by vacuum and high temperature, and the dimensions are measured again. The relatively small difference between the shrinkage of plane dimensions in MD and CMD directions is a characteristic of Figeholm high-density, pre-compressed board and laminate. This is important for the dimensional tolerances of fabricated parts in the transformers factory, e.g. for the roundness of flat annular components for winding and insulation and support.The specific shrinkage in thickness direction, perpendicular to the fibre web layers, is considerably larger than along the layers. Transformers manufactures have established benchmarks for the resulting overall compression of winding stacks under combined mechanical compression and factory drying. A low and consistent shrinkage of winding spaces is of course important for good winding tolerance, and for the stability of the manufactured windings in service.
The physics of drying pressboard parts
Everybody in the transformers business has learnt that it is of vital importance that the core-and-coil assembly get satisfactorily dried and impregnated with oil in the factory.
The physics of the drying process are, however, less well known and understood.
Moisture in fibrous material is contained or “bound” by different mechanisms. The largest part of the moisture in relatively humid material (“natural humidity level” – c.f. a previous section of this chapter) can be removed quite easily. The remainder requires more effort (higher temperature – vacuum).
Transporting the moisture out of the solid material is an example of a diffusion process. The driving force is the difference of water vapour pressure from inside the material to the surrounding. The parameter which mathematically characterizes the speed of moisture extraction is called “coefficient of diffusion”.
The important observation in regard to the drying of the pressboard material is that the diffusion coefficient is to much larger (diffusion so much speedier) for diffusion along the fibre web layers than for diffusion in perpendicular direction – towards the board sheet surfaces. This is particularly important for the design of press-plates of thick laminate.
The two available grades of laminated material differ as to the glue employed. One grade, HDLP, is laminated with polyester resin. The bonding layers are virtually impenetrable to moisture diffusion and oil impregnation. The processes will have to take place entirely along the layers.
The other grade, HDLC, is boded with casein glue. This material is penetrable during drying and impregnation, so some limited diffusion takes place perpendicular to the layers also. The glue is water-based, and the laminate, as delivered, has somewhat higher moisture content. The shearing strength of the casein bond is lower than for the polyester-bond. This results in lower bending strength of the laminate.
Both materials withstand high-temperature vapour-phase drying of the transformers before the impregnation.
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