The first installations of the new hard epoxy-mica orpolyester-mica stator insulation systems in Bureau of Reclamationgenerators resulted in numerous problems that were not present with theolder soft asphalt-mica systems. One of the main attributes ofasphalt-mica systems was the tendency for these materials to expand andlock themselves into the stator slots. When the asphalt becomes warm andsomewhat plastic, it flows and causes the coil surface to bulge into theair passage slots. Because the binders in the hard systems remain rigid,the coils must be secured in the slots by other means to preventmovement. In the early years of hard insulation system development,loose coils became the major operation and maintenance concern. Manywindings wore out prematurely due to severe electrical and mechanicaldeterioration resulting from loosely installed coils. A factor causinglooseness in the slots involves further curing of some insulationsystems after installation, resulting in volumetric shrinkage of theinsulation. Manufacturers have addressed this problem by developingcoil-clamping systems that maintain slot pressures regardless ofvolumetric shrinkage. The Bureau of Reclamation has developedmaintenance procedures to detect loose coils and correct the situationbefore serious damage occurs. Some case histories involving the new hardwinding insulation systems are presented
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