Abstract :
Economy of space, long service life and utmost security against electrical breakdown are all expected of a winding insulator. Rotor slot insulators of large turbogenerators are heavily stressed mechanically. The separate requirements for both high thermo-mechanical and electrical strength are fully met by glass fiber reinforced epoxy resins. If one sets a requirement, however, for stiffness, and maximum dielectric strength during mechanical deformation, a requirement which is closer to real operating conditions, then glass fiber reinforced plastics (GFRP) no longer have the best properties. An adjusted test method simulates the actual facts of local compression of the winding copper against the insulation and the slot wall or against the insulation and a cavity - e.g. vent ducts. Generally speaking, no demonstrable success has been achieved with insulators improved only in their dielectric strength. Micro-cracks grow in the deformed rigid insulation and these become electrical weak points. Lamination technique, however, allows the combination of different insulating materials over a wide range, each having parts of the sum of the required properties. Multi-layered hybrid laminate compositions of GFRP with included homogenous plastic film and compressible protective layers give optimal results. In comparison with the slot liners of standard GFRP, the mechanically stressed and deformed "sandwich" construction shows considerably increased safety with regard to electrical breakdown.
