Diamond Dotted Presspaper improves the mechanical strength of oil-filled transformer windings without reducing partial discharge levels. Hechin manufactures this paper by applying discrete dots of B-stage resin on both sides in a diamond pattern. The size and shape of the dots are carefully chosen to avoid interfering with vacuum drying and oil impregnation.
During B-stage, the resin dots are dry and tack-free, permitting storage and handling similar to normal presspaper. When the winding is heated for drying purposes, the adhesive dots melt and cure, thus creating permanent bonding sites unaffected by further heating cycles that may occur in service. The bonding strength is adequate to support effective gluing in unsupported coil design.
The pinhole-free multi-ply paper allows single layer application in LV foil windings. Temperature rise time in the drying oven is important, particularly during the final transition to the temperature at which the resin starts to melt. To obtain maximum bond strength, this transition should be completed as swiftly as possible. The drying requirements of the transformer normally provide for adequate curing of the resin.
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